Oakley Sound Systems One of Three Voltage Controlled Oscillator
Contents
1. The third lead is a 6 way interconnect This is made up to be 90mm long This will connect the MAIN 1 headers on the main and socket boards You can use some cable ties to bundle the wires in each interconnect together This is best done once all the interconnects are in position The Molex Alternative to MTA For those of you who want to use the cheaper Molex system the following information may be useful A quick note on the female plugs these are sometimes called housings since they aren t plugs themselves but merely housings for the individual crimp terminals Terminals have to be bought in packs of one hundreds but this is OK because they are not expensive These are normally designed to be crimped but they can be easily soldered with care Make each wire the correct length I normally strip back the wire by just 2 to 3mm Place all the bare wire into the crimp on a heatproof surface I use 12mm MDF board to protect my bench top which although not at all burn proof will take plenty of heat from a soldering iron without major damage Rest a pair of pliers on top of the wire to hold it in place Slip the crimp under the wire so that the wire s insulation butts up to the edge of the terminal Then solder in place Sometimes I find I need to gently squash the crimp part of the terminal so that it will fit into the housing This is easier to do before you solder it although it can be done after with care Do not use the water wash2. LM13700 transconductance amp LM2903 dual comparator LM723 voltage regulator LT1013CP low drift dual op amp SSM2210P matched NPN pair TL074 quad bi fet op amp TL072 dual bi fet op amp Trimmers 100K multiturn trimmer 10K multiturn trimmer 2K multiturn trimmer 1K horizontal carbon trimmer 1K cermet trimmer 22K horizontal carbon trimmer 47K horizontal carbon trimmer Pots 50K Linear Spectrol 248 pot Oakley Spectrol pot brackets Spectrol nuts C11 12 2 23 22 C13 16 C3 C10 C4 C14 C9 C15 5 7 17 6 19 24 26 1 C18 20 21 C8 25 D2 4 D3 Q1 2 4 5 You can use the BC549 too Q3 U6 U8 U7 Ul U2 U3 U4 U9 U5 TUNE V OCT HFT SHP T PSU SHP S SYM S SYM T DEPTH WIDTH TUNE 2 TUNE 1 on main board EXP LIN SYNC on pot board 7 off for the above 7 off for the above Miscellaneous SPDT toggle switch PWM type 1 off 4 way 0 156 MTA header PWR 1 off 3 way 0 1 header Oakley Buss optional connector 1 off 4 way 0 1 MTA header MAIN 2 Main PCB and socket PCB 2 off 4 way 0 1 MTA housing MAIN 2 cable 2 off 5 way 0 1 MTA header POTS Pot PCB and socket PCB 2 off 5 way 0 1 MTA housing NSE cable 2 off 6 way 0 1 MTA header MAIN 1 Main PCB and socket PCB 2 off 6 way 0 1 MTA housing MAIN 1 cable 2 off 8 way jumper lead LINK interconnect 1 off Leaded ferrite beads L1 L2 You may well want to use sockets for the ICs I would recommend low profile turned pin t
3. about 2 5V If the operating point is not exactly half the peak value of the sawtooth output of U5 then the triangle wave will have discontinuities in it This leads to a slightly brighter or harsher sound than the perfect textbook triangle waveform SHP T is a trimmer that allows precise enough adjustment in the operating point to eliminate these glitches The created rectified wave is filtered before amplifying and level shifting by U6 The SYM T trimmer sets the DC offset of the wave output to match that of the sawtooth output The final output is again 10V p p The sine wave shaper circuit shown on page two of the schematics acts upon the triangle wave output The rounded peaks of the sinewave are created by deliberately overdriving the inputs of one half of U8 a dual operational transconductance amplifier OTA These devices are normally used as VCAs but in this VCO it is merely acting as a soft clipping unit The non linearities of the OTA s input stage being utilised to squash the top and bottom peaks of the triangle wave input The gain of the shaper is fixed with R32 and R48 SHP S adjusts the amount of overdrive Since U8 gives a current output it must be turned into a voltage and this is done by U9 pins 5 6 7 R18 provides a special negative feedback path within the shaper circuit This increases the non linearity of the whole stage at the critical peaks of the output waveform SYM S provides compensation for the OTA
4. maximum peak voltage of the sawtooth waveform the capacitor has less charge to loose before the output of U5 reaches zero Thus the frequency is higher than it would have been Using a resistance in series with C14 we have a voltage drop developed across the resistor that increases with frequency due to the increased current through the exponential convertor This method was first postulated by Sergio Franco and is usually called Franco compensation because of this This works very well but it does mean that the amplitude of the sawtooth waveform decreases slightly with increasing frequency In the first two VCO issues I made this resistor variable I was using the Franco resistance to compensate for both reset time and errors in the exponential convertor In the issue 4 VCO this resistance was fixed by R37 and is compensating for reset time only The exponential convertor causes errors due to bulk emitter resistance or Rbe in the NPN pair The SSM2210 is a very good matched pair with low values of Rbe but it still has an effect at very high frequencies of the VCO Rbe is not a real resistor but it acts as if a resistor was connected in series with the emitter of the transistor In the early issue VCO boards this loss of high frequency performance was cancelled out by the effect of the Franco resistor But really this is a bit of a cludge since the Franco is designed to compensate for a fixed timing error the FET reset time whils
5. not need cutting to size They are already at the correct length The very first issue 4 VCO module This one has been built using MTA interconnects Note the use of cable ties to keep the interconnection wires tidy Connections The first thing that will need doing is the interconnection between the pot and main board This is done with a 55mm or 85mm long 8 way jumper connection Simply thread all eight wires into six holes of the LINK pads from the underside of the pot board The other end of the jumper can then be gently coerced into the LINK pads of the main board Solder each end of the jumper You must solder the pot board from the top of that board and the main board from the bottom of that board That s it that the two boards joined up Wow that was easy If you are using the recommended MTA interconnections this section will be very easy too All the wiring between the sockets and the main board is done with one 4 way jumper one 5 way jumper and one 6 way jumper Here you will be using either the MTA system or the slower but cheaper Molex system Make up the 4 way interconnect first This should be made from wires 100 mm long Make sure you get pin 1 going to pin 1 on the other housing pin 2 to pin 2 etc This cable will connect to the headers called MAIN 2 on each board The second lead is a 5 way interconnect This is made up to be 145 mm long This should connect the POTS headers on the socket board and the pot board
6. of it its easy I can do it in about one minute but I ve had a lot of practice Now leave it on for 30 minutes and then check the scaling Adjust if necessary HFT Turning the HFT will affect the pitch across the whole range of the VCO but just like the V OCT trimmer you need to make sure you can get one octave spread across one octave of your keyboard This time though we are superfine tuning the interval at the top end of your keyboard only If you only have a small keyboard use the Coarse pot to get the VCO really high Anything over 2kHz high whistle is fine Again ignore pitch its the interval we are wanting to get right Once you have set up the perfect octave then check down at the lower end that everything is still responding to 1 V octave It is Good If it isn t go back and repeat the adjustments above Remember if you have skimped on the V OCT trimming no amount of tweaking of the HFT will get it to play in tune TUNE This sets the range over which your VCO acts Set the Coarse pot to its minimum setting and set the Fine tune pot to its middle position Play the lowest note on your keyboard Now adjust the TUNE trimmer until the note that you can hear is equivalent to your lowest note minus three semitones or so That is if your bottom note is a C then adjust TUNE so that you get a A Now use your Coarse Tune pot to set the correct note You may want to compare it to another keyboard module to make sure that you have se
7. ordering procedure which also works very well The panel itself is made from 3mm thick black anodised aluminium The fpd panel can be edited including changing the colour with the Frontplatten Designer The program available on the Schaeffer web site but it should be noted that the program is for Windows only The pot spacing on the PCB is equivalent to the vertical spacing of the MOTM series of modules but the socket and horizontal spacings are very different Trimmers In the UK we call them presets The first thing you need is a trimmer tool Screwdrivers are a pain especially with multiturn trimmers Spectrol and others make trimmer adjusters for less than a pound But don t use the adjuster as a normal screwdriver you ll break it There are quite a few trimmers on the PCB Lets run through them all in turn PSU Set this to give 10 00V with respect to local ground at TP1 Before your adjust the V OCT trimmer you need to make sure the effects of the HFT trimmer are minimised To do this switch the module off and measure the resistance between the HFT trimmer s pins 2 and 3 Pin 1 is the square one and pin 2 is the middle one Turn the trimmer until you get the lowest value you meter will read It ll probably go right down to zero ohms The other trimmers are best set up in the modular with your chosen power supply It is also a good idea to let it warm up for at least fifteen minutes V OCT Use this to generate a perf
8. pot brackets are used the PCB can be firmly supported to the front panel The output levels for the sawtooth sine and triangle are standardised at 5V ie 10V peak to peak The pulse wave output is also 10V peak to peak but the peak levels vary with pulse width The Oakley VCO uses an interesting technique to maintain the average voltage over one cycle to zero volts This new circuit essentially adds an offset to the pulse output to compensate for the non zero average voltage for any pulse wave that isn t a square wave Ordinarily one would see average DC values varying from 5V to 5V as the PW is swept from one end to the other In the new VCO this average level is kept at zero This means that for narrow pulses you now have a wave that goes from just below OV down to just below 10V up For square waves you have the usual 5V up and 5V down For wide pulses you have a wave that is just below OV up and just below 10V down The reason for this is that fast pulse width modulation no longer adds thumping to the audio output ie fast EG sweeps of PW will sound great Although no switch is provided to turn this feature off you can simply omit one resistor and it will function like other VCOs All outputs have an output impedance of roughly 1K The pulse output is also switchable between centre modulated and edge modulated forms of pulse width modulation Most VCOs will only offer edge modulated in which only one edge o
9. probably beyond the scope of this document but in a nut shell it means this The linear input will not act with a strict volt per hertz relationship That is for every 1V the output frequency will not rise by a fixed amount But it will act so that audio rate FM will create a constant depth of modulation on any given frequency controlled by the usual 1 V octave inputs This means that the for audio rate FM effects clangs bells and the like you can get very good results with the linear FM input R3 has been chosen to set the operating frequency of the VCO at approximately 1kHz when the voltage at the base of the left hand transistor is zero The exponential convertor works at its most accurate when the voltage across the two bases is zero And since the human ear is particularly sensitive to frequency changes at around 1kHz it is good to make the most accurate part of the VCO at this frequency The output of the exponential convertor is a current It is this current that controls the core of the oscillator Contrary to many people s ideas the core of a VCO is typically a linear CCO That is a current controlled oscillator A doubling of current to or from in this case the CCO will produce a doubling of output frequency The core of this VCO is a traditional sawtooth oscillator C14 is the timing capacitor that is charged by the output of the exponential convertor Since one end of C14 is tied to 5V the voltage at the other side of C14 moves
10. see an A B or C suffix used eg BC549C This letter depicts the gain or grade of the transistor actually hfe of the device The One of three VCO is designed to work with any grade device although I have used BC550 throughout in my prototypes The positive temperature coefficient PTC resistor is worthy of a mention Meggitt make a 3000ppm K type that is a very good part It is available from Farnell and the part number is 732 278 However the actual theoretical value of compensation should be closer to around 3500ppm K when used in the circuit that I have chosen The Meggitt part thus falls slightly short of this and may cause the VCO to lose V oct tracking over the full keyboard range unless the ambient temperature is pretty stable It is therefore preferable to get something closer to the ideal value The ones I recommend are 1W wirewound resistors with a nominal TC of 3500ppm K You can buy these from us KRL Bantry in the US or from Senso s Vintage Planet www senso dds nl index html These are sold as equivalents to the old Tel Labs Q81 series Once fitted to the VCOs they give better tuning stability and allow the modular to warm up quicker They are also physically larger than the Meggitt type which means they are more stable in air drafts too The PTC resistor is fitted to the board so that it lies across the top of the SSM2210 A bit of thermal compound eg Thermpath could be used to keep the two devices in co
11. towards zero volts as current is sucked out of it The higher the current the faster the voltage drops This voltage is sniffed by one half of dual op amp U5 which produces a replica of this voltage at its output Q4 aids the output of U5 as well as providing a suitable offset for the triangle generator circuit see later U7 pins 1 2 3 is a comparator and it is watching the output of U5 When the voltage across C14 reaches 5V the output of U5 pin1 normally at 15V suddenly flies upward towards 10V This sudden level change passes through C13 and turns on JFET Q3 This rapidly shorts out C14 and the voltage across the capacitor drops to zero and both of its pins are at 5V once again This means that the voltage at the input of U8 is also at 5V and the charging process begins again C10 controls the time that the FET is on The sync input enables an external voltage to trip the comparator early This will cause premature shorting of the capacitor locking the fundamental of the VCO to an incoming external sawtooth signal However the fall time of the VCO s saw waveform will still be set by the input CV to the VCO Thus you will not get traditional sawtooth waveforms but half completed sawteeth The sonic affects of these are marvellous especially if you sweep your VCO with an envelope generator whilst locked to another VCO at fixed frequency The controlled VCO is often called the slave while the fixed V
12. was eradicated With the older VCOs any two slave VCOs tended to sync to each other as well as the master Interesting effect though I swapped the positions of the power supply regulator with the CV summer circuitry This enabled me to move the two tuning multiturn trimmers to the edge of the board The old issue 1 board had these parts in the centre of the PCB and they were impossible to trim when the board was fitted into the Oakley Orbital monosynth Moving the trimmers to the edge means that we can now use side adjustable trimmers in the Orbital to facilitate easy tuning with the boards in situ This does not affect the modular set up in any way in that we will continue to use top adjustable trimmers as before As it turns out no issue 2 boards ever made it into the Orbital project The pitch spacing of the polyester capacitors was changed from 0 3 to 5mm This was in line with other Oakley boards Added separate triangle and sine wave offset trimmers This meant that switching to centre on the PWM source switch would not affect the mark space ratio of the square wave output Issue 3 Boards had the following features added Sinewave purity increased Two high frequency tracking compensation networks instead of just one The standard Sergio Franco technique used on the earlier issues is now fixed to compensate only for the FET discharge time Dave Rossum s technique for compensating for the bulk emitter resistance Rbe of the NPN
13. CO is called the master The incoming sync signal is first buffered by Q1 a simple emitter follower This is shown on the second page of the schematics R11 C1 and C2 provide decoupling to prevent power supply noise from accidental triggering of the VCO The buffered signal is then passed to a simple differentiator based around C9 back on page one again This part in conjunction with D3 and R24 only allow only fast moving rising edges of the input waveform to reset the VCO core In theory this should allow any pulse wave to be used as sync waveforms but the best sync sounds can be obtained when a falling sawtooth master signal is used This last statement is very important if you want to use sync effectively Use sawtooth outputs from Oakley VCOs to sync other Oakley VCOs MOTM VCOs produce rising ramps not falling sawtooth waveforms So you if you want to use a MOTM VCO as the master to sync the slave Oakley VCO then you must invert the MOTM s output first You could use an Oakley MultiMix to do the inversion The suggested layout of the Oakley VCO includes a depth control for the sync input This will allow you to create partial synching effects This effect is very difficult to describe in words and has to be heard to be believed But very very complex timbres can be produced this way At high frequencies the VCO can go a little flat due to the finite time it takes to reset C14 and errors in the exponential convertor By lowering the
14. Oakley Sound Systems One of Three Voltage Controlled Oscillator Module PCB Issue 4 User s Guide V4 7 Tony Allgood B Eng PGCE Oakley Sound Systems CARLISLE United Kingdom COARSE EXP FM PWM i er 1 0CT EXP FM LIN FM 000 SINE PULSE 0000 OAKLEY MODULAR SYSTEMS VOLTAGE CONTROLLED OSCILLATOR Introduction This User Guide is for the Issue 4 board set only Check that you have an Issue 4 board The issue number is written at the bottom left hand corner of the main board The Oakley One of Three is a voltage controlled oscillator module that was inspired in part by the VCO design of the later MiniMoogs In this module I decided to create a VCO that sounded good and performed well enough to keep up with the digital oscillators of modern day synthesisers This is the fourth issue of this popular project and module The new issue mainly contains mechanical updates and still features the same powerful analogue sound that won the last three issues much acclaim The VCO features sawtooth pulse triangle and a low distortion sine output The duty cycle of the pulse output may be controlled directly by a PCB mounted pot and also an external voltage the sensitivity being controlled by another PCB mounted pot The frequency of the VCO can be controlled by two PCB mounted pots one for fine the other for coarse adjustment These four pots are designed to be fitted to the main board and when special
15. able flux solder in this application You must use either good old fashioned ersin or rosin flux based solders or the newer so called no clean types I actually prefer the rosin based ones for this because I find they flow better Once you have soldered it wait a bit for it to cool and then push it into the housing until it clicks If it doesn t go in then take it out and bend the crimp slightly backwards Now try again Power Connections The power socket is 0 156 Molex MTA 4 way header Friction lock types are recommended This system is compatible with MOTM systems Power Pin number 15V 1 Module GND 2 Earth PAN 3 15V 4 The PAN pad on the PCB has been provided to allow the ground tags of the jack sockets to be connected to the powers supply ground without using the modules OV supply Earth loops cannot occur through patch leads this way although screening is maintained Of course this can only work if all your modules follow this principle Front Panel On the website I have included a FPD database of the suggested 3U front panel layout Actual panels can be obtained from Schaeffer Apparatebau of Berlin Germany The cost is about 25 for the panel VAT and the postage is extra so it usually helps to order a few panels at the same time All you need to do is e mail the fpd file to Schaeffer in Germany or Frontpanel Express in the US and they do the rest You can also use the Frontplatten Designer program s own online
16. ad spacing is 0 2 or 5mm Do not chose cheap and nasty ceramic types usually high K obtainable from some surplus places These can lead to a noisy audio output Alternatively you can use polyester parts in place of the larger values of ceramics C14 is a close tolerance 2 or better polystyrene capacitor or better The PCB is laid out for EXFS HR series made by LCR Farnell and others sell these They are good components at a good price The matched pair SSM2210P can be difficult to trace although Farnell and RS Ltd do carry stock You can use the LM394 or MAT 02 instead You could also match your own pair from discrete transistors but thermal matching is very hard to do U2 and U3 are both good dual op amps The LT1013CP from Linear Technology is an ideal part I used to recommend the OPA2277 but we now know of one case where a particular batch of OPA2277 caused oscillation to the 5V reference voltage The LM1458 is a real oldie and some places don t stock it any more In the UK Farnell and RS sell the little chap for peanuts The LM13700N may be substituted with either the older 13600 still made by JRC and the now defunct Phillips NE5517 All ICs are dual in line DIL or DIP packages These are generally but not always suffixed with a CP or a CN in their part numbers For example TL072CP Do not use SMD SM or surface mount packages The BC550 or BC549 devices are discrete low noise transistors Quite often you
17. and down You do not have to use the other washer that the switch kit comes with Now solder a short length of uninsulated solid core wire to each of the three tangs You can use some resistor lead clippings if you wish Make sure the wire leads stick up at right angles from the panel surface Now refit the socket PCB with all of its sockets back onto the front panel Carefully align the three wires through the three solder pads in the PCB Now you can then flip the panel over and fit all the sockets washers and nuts Once this is done you can solder the three switch leads onto the PCB Clip off any excess lead length that sticks out above the solder Mounting the Board Mounted Pots NOTE This procedure is rather different to that of the Omeg Piher pots you may have used on older Oakley boards The first thing to do is to check your pot values Spectrol do not make it that easy to spot pot values Your pot kit should contain Value Marked as Quantity Location 50K linear M248 50K M 4 off Main PCB 50K linear M248 50K M 3 off Pot board Fit the pot brackets to the pots by the nuts supplied with the pots You should have two nuts and one washer per pot Fit only one nut at this stage to hold the pot to the pot bracket Leave off the washer for later Make sure the pot sits more or less centrally in the pot bracket with legs pointing downwards Tighten the nut up carefully being careful not to dislodge the pot position I use a small pair of plie
18. e same as vertical spacing of the MOTM modular synthesiser There are detailed instructions later in the document about how to build the boards The whole project takes around 3 hours to build and test Oakley VCO Issue Changes For those interested the Oakley VCO has evolved slowly over the last nine years Issue 1 The start the VCO is based primarily on the third series Minimoog VCO with plenty of additions eg in built 10V reference sync different exponential convertor and summing stage sinewave output pulse width selection output buffering Omeg pots Temperature compensation was generally provided by a 3000ppm K 900mW film resistor This was later upgraded to the KRL wirewound types for last ten or so boards that were sold Issue 2 Boards had the following features added Added 10V reference to go with the 10V internal reference voltage already on issue 1 This stabilises absolute pitch drift so that supply rail potential doesn t affect the pitch of the VCO All pitch pots use the 10V references The range of the TUNE trimmer was reduced and this was coupled with a reduction in the quiescent operating frequency Added SYNC input buffer circuit This has the advantage of providing a high impedance input The old issue had a 10K input resistance which caused a slight drop in amplitude to the other master VCO connected to the SYNC input An additional benefit is that unwanted cross coupling between two or more slave VCOs
19. e with fresh solder will actually make the hole easier to suck clean Sometimes people like to substitute parts in place of my own recommendations Feel free to do this but remember that there is normally a good reason why I have selected that particular part If you do find that say changing an op amp with another one makes an improvement please do let me know either via the Oakley Synths list or directly to me All resistors should be flat against the board surface before soldering It is a good idea to use a lead bender to preform the leads before putting them into their places I use my fingers to do this job but there are special tools available too Once the part is in its holes bend the leads that stick out the bottom outwards to hold the part in place This is called cinching Solder from the bottom of the board applying the solder so that the hole is filled with enough to spare to make a small cone around the wire lead Don t put too much solder on and don t put too little on either Clip the leads off with a pair of side cutters trim level with the top of the little cone of solder Once all the resistors have been soldered check them ALL again Make sure they are all soldered and make sure the right values are in the right place The diodes can be treated much like resistors However they must go in the right way The cathode is marked with a band on the body of the device This must align with the vertical band on t
20. ecoupled by C20 to C23 These prevent stray high frequency pulses from entering or leaving the unit by the power supply lines A LM723 voltage regulator IC is used to provide the 10V This is an old IC design some 30 years but it is a superb device and has very low noise The only problem is that you have to set the output manually PSU is a trimmer to set the voltage to be exactly 10 00V The PCB has been laid out for a cermet trimmer for long term stability Additional power supply decoupling is provided to each set of op amps and ICs This prevents any reset pulses from travelling down the power supplies and soft synching the other VCOs in your system To find the next part of the power supply we need to look at the first sheet of the schematics Two sections on this page generate the stable 5V and 10V required by the VCO U2 pins 5 6 7 provides the 5V supply by simply buffering half the 10V rail An LT1013 is specified to provide a very stable output U3 pins 1 2 3 is a simple inverter circuit that produces a very stable 10 00V supply for the tune pots and trimmers Together with the stable 10 00V supply this prevents any perturbations in the 15V supply rails from affecting the tuning of the VCO In reality the tolerance of the 22K resistors may mean that the output is not exactly 10 00V This is perfectly OK since we are not interested in the actual value so much but the stability Those 22K resistors are metal film and th
21. ect 1 V octave scaling This trimmer will need to be adjusted along with the fine and coarse pots on the front panel You will need a scope or a digital frequency counter or the best of all a guitar chromatic tuner Some people use another keyboard or a calibrated VCO and listen to the beats but that can take longer Now comes the tricky bit Plug your midi CV convertor or 1 V oct keyboard into the 1 V octave input of the VCO Play a lowish note on the keyboard then go two octaves higher Adjust V OCT until the interval is EXACTLY two octaves We are only setting the interval and not the actual frequency It does not have to be a perfect C when C is being pressed on the keyboard It could be an F or whatever The key thing groan is that we are setting the musical gap between the notes If you do need to alter the pitch of the VCO to help you use the front panel controls only Leave the TUNE trimmer until later For any interval if you find the higher note is flat then turn the V OCT trimmer to make it flatter still This actually reduces the range between the two notes Conversely if you find your interval is greater than an octave turn the trimmer to make the top note even higher I always adjust V OCT on the high note of any interval and only adjust the front panel Tune pot on the lower This will probably require some patience and plenty of twiddling of the front panel controls as well But you will get there Once you get the hang
22. eir value will not change appreciably over the years or at differing temperatures The VCO s pitch is determined by a variety of sources Two 1V octave CV inputs and two pots on the front panel and a trimmer to set initial frequency U2 pins 1 2 3 is built as a voltage summer It takes the voltages from the five sources and adds them together The gain of the summer is set by the input resistors and R44 and the V OCT trimmer For the two 1 V octave inputs the gain is roughly 0 7 The V OCT trimmer is adjusted to give a rise of one octave in output frequency when the keyboard input voltage goes up by 1V The output of the CV summer is then fed to the exponential convertor via R29 and R30 The exponential convertor is based around on half of U3 5 6 7 and the matched NPN pair of U4 Its output is a current that is proportional to the exponent of the voltage applied at the base of the left hand transistor of U4 The circuit gives the VCO a sensitivity of roughly 18mV octave so R29 and R30 reduce the output of the CV summer to this level However it is worth noting that R30 is a temperature sensitive resistor The resistance of R30 will go up with temperature at a rate of 0 33 for every degree Celsius This should counteract the temperature effects produced by the semiconductor junctions in U4 To get the best temperature stability R30 is mounted right on top of U4 This way the temperature of the two devices should be the same It is possib
23. f the pulse wave is affected by the pulse width pot or modulating CV The Oakley VCO allows you to modulate either one edge edge or both centre In fixed pulse width applications this generally makes no audible difference However when used with fast moving modulating CVs the difference in timbre is apparent A high impedance synchronisation input is provided to prematurely reset the VCO waveform With this you can force the VCO s operating frequency to that of an external sawtooth signal say from another VCO module This input is level sensitive so hard sync is possible with inputs of about 3V or above Inputs below this will cause only occasional synchronisation leading to interesting harmonic structures Please note that to cause hard sync effects the master sync signal must be a sawtooth falling ramp shape waveform of 10V peak to peak The slave VCO will not sync to a ramp waveform which typically has a rising ramp and does not feature the fast rising edge the VCO needs to lock on to These ramp waveforms are erroneously called sawtooth by some manufacturers However a simple inverting circuit will be sufficient to turn the waveform around the right way so it can be used as a sync master The VCO supports the standard 1 V octave exponential voltage to frequency relationship However a linear control input is also provided for constant depth frequency modulation Temperature compensation is performed by using a matched trans
24. he board In other words the point of the triangular bit points towards the cathode of the diode When all the diodes are in place double check all are pointing the right way The polyester film capacitors are like little blue boxes Push the part into place up to the board s surface Little lugs on the underside of the capacitor will leave enough of an air gap for the water wash to work Cinch and solder the leads as you would resistors The ceramic capacitors are strange flat plates made from pot Be careful with these and make sure you have bought the ones with 0 2 lead spacing Forcing the smaller 0 1 ones into these larger pads will break them Another thing to watch out for is the identification markings on these capacitors For example n47 is actually 470pF The smaller electrolytic capacitors are very often supplied with 0 1 lead spacing My hole spacing is 0 2 This means that the underside of these radial capacitors will not go flat onto the board This is deliberate so don t force the part in too hard The capacitors will be happy at around 0 2 above the board with the legs slightly splayed Sometimes you will get electrolytic capacitors supplied with their legs preformed for 0 2 Smm insertion This is fine just push them in until they stop Cinch and solder as before Make sure you get them in the right way Electrolytic capacitors are polarised and may explode if put in the wrong way No joke Oddly the PCB lege
25. he output waveform should be a square wave The output of the comparator swings from 15V to ground or zero volts The output of the comparator is out of phase with the audio input Q2 inverts this signal so that the generated pulse wave will go high when the saw or triangle wave is high It would be possible to generate the correct phase by simply wiring the comparator s inputs pins differently Thus you wouldn t have to invert the output since it would already be the right way around Earlier issues of the VCO in fact did this However this caused a less than perfect output waveform The positive feedback provided by R40 which is needed for good comparator action would cause the sawtooth or triangle wave to be superposed to the output waveform This was considered not to be a problem but with the new issue I decided to do things differently And perhaps more correctly What follows after the comparator is I think new to modular VCO design The inverted output of the comparator is now summed with two voltages in U9 pins 12 13 amp 14 One comes from the 15V supply via R74 The other is the output of the CV summer The former allows the 15V 0V output swing of the comparator to be normalised between 5V and 5V The other allows the output to be offset against zero volts as the pulse width is varied When the CV summer has zero volts at its output the generated pulse waveform is a square wave No additional offset is added and the wavef
26. he sockets will be fitted on the top of the board and therefore be soldered on the bottom of the board You may well find your own way of soldering the sockets but the way I do it is as follows Fit all your sockets into one of the boards The bevel edge should align with the picture on the board legending Do not solder them at this stage Take your front panel and align this over the sockets Now carefully place your front panel with PCB and sockets upside down onto your bench or kitchen table The holes where the sockets will be should hang over the edge of the bench so that the sockets aren t forced back up through the holes You ll also probably need a small counter weight to stop the panel from falling over the edge Now allowing the PCB to rest flat on top of the sockets you can begin to solder all the pins to the board Once all the sockets are soldered you now need to turn your attention to the toggle switch Now it would be great if we could have a PCB mounted switch but we haven t and as far as I can see nobody makes one quite like what we need So we have to kludge a little here All the same you ll still end up with a nice neat solution Remove the socket PCB from the panel and fit your switch in the correct hole Use the one of the nuts and the spring washer on the inside of the panel Use the second nut to secure the switch to the panel It doesn t matter which way the switch goes in so long as the toggle moves up
27. istor pair in the exponential convertor and a high quality temperature sensitive resistor The Oakley CV gate bussing You really should think about it if you have a medium sized Oakley system Using the Oakley Dizzy board it allows Keyboard CV and Gate signals to be piped around the back of the modular s case along with the power supply rails Any VCO and VCF can be connected to the KEY CV line and this will save you having to patch KEY CV to every module that needs it Inserting any patch lead into the 1V OCT socket will override the CV bus line connection The gate signals are treated similarly to the CV line but for use with the ADSRs and other EG modules The Oakley CV gate buss uses a common three way 0 1 header to carry the two signal lines A third as yet unused connection is also present for future expansion The new VCO issue supports the Oakley CV gate buss natively Previous issues had the CV buss being connected to the module via a wire tail attached to the 1 V octave socket s normally closed NC lug The new socket board features an optional three way header that can be fitted to allow direct connection to the CV gate buss on an installed Oakley Dizzy system If not required the header can simply be omitted altogether and a solder link put in its place Or the header can be fitted and a simple two way jumper like those used on computer motherboards can be fitted to allow to the buss to implemented at a later date The Is
28. le to get an exponential response from a single transistor but that has problems as Vbe the junction voltage changes with ambient temperature The temp co resistor cannot compensate for this change in the transistor s operating current So the now classic circuit with two perfectly matched transistors and an op amp U4 and U3 is used Changes in the Vbe for one half of the transistor pair are mirrored in the other The op amp then matches the current in the first transistor with the same current in the other one So the collector current in the first transistor will effectively control the collector current in the second And it is the current drawn by the second transistor that controls the frequency of the VCO The op amp method also has another bonus it allows an additional current to be injected into the inverting pin of U3 This current will directly control the output current of the exponential convertor R15 allows an input CV to control this current We now have a linear frequency modulation input whose sensitivity is set by R15 Connecting this input to the wiper of a pot allows you to control the sensitivity of this input directly from the VCO s front panel Note that this input is not a true linear input control This VCO and most other modular VCOs don t have that capability What we have here is a constant modulation index input This is linear modulation but not linear control The precise description of this statement is
29. line which is updated periodically by myself This can be found at www oakleysound com parts pdf VCO issue 4 Parts List A quick note on European part descriptions To prevent loss of the small as the decimal point a convention of inserting the unit in its place is used eg 4R7 is a 4 7 ohm 4K7 is a 4700 ohm resistor 6n8 is a 6 8 nF capacitor Resistors All 5 carbon 1 4W or better Components designated as 1 should be 0 25W 1 metal film or better 22R R50 42 41 60 7 75 100R Rll 390R R19 23 470R 1 R37 IK R55 62 24 56 39 67 1K 3500ppm K Temp co R30 2K R17 25 2K2 R5 2K2 1 R22 5K1 R68 5K6 R65 6K2 R59 6K2 1 R21 8K2 R16 69 10K R80 78 27 70 66 48 46 49 35 10 34 10K 1 R2 57 58 18K R79 22K R82 73 9 71 22K 1 R4 6 33K R72 77 76 39K R47 39K 1 R29 47K R12 33 54 56K R63 62K R18 81 68K R74 68K 1 R44 100K R36 38 13 15 61 52 51 100K 1 R1 32 150K R64 180K 1 R28 220K R8 270K R53 390K R14 20 390K 1 R40 680K 1 R3 45 750K R83 1M R26 84 43 IM 1 R31 Capacitors 100nF 63V polyester 100pF Low K ceramic 10uF 25V elect 18pF Low K ceramic InF 100V polyester InF 1 polystyrene 220pF low K ceramic 22uF 25V elect 4u7 35V elect 470pF low K ceramic Discrete Semiconductors 1N4148 signal diode BAT42 Schottky diode BC550 NPN transistor J112 J FET Note there is no D1 Integrated Circuits LM1458 dual bi polar op amp
30. nd marks the positive side with a although most capacitors have the marked with a stripe Obviously the side marked with a must go in the opposite hole to the one marked with the sign Most capacitors usually have a long lead to depict the positive end as well IC sockets are to be recommended especially if this is your first electronics project Make sure if you need to wash your board that you get water in and around these sockets And that any water is thoroughly dried out before you power up Sockets harbour little pools of water in their pins which can lead to some very odd effects Several sharp taps face down onto a towel will loosen off any remaining globules of liquid The transistors are all in the same type of packaging and therefore look the same Only the numbers on the side will allow you to tell them apart Match the flat side of the device with that shown on the PCB legend Push the transistor into place but don t push too far Leave about 0 2 Smm of the leads visible underneath the body of transistor Turn the board over and cinch the two outer leads on the flip side you can leave the middle one alone Now solder the middle pin first then the other two once the middle one has cooled solid Sometimes transistors come with the middle leg preformed away from the other two This is all right the part will still fit into the board However if I get these parts I tend to straighten the leg
31. ntact You ll need to carefully bend the legs of the TC to make sure it lies flat on the surface of the 2210 and still fits through its solder pads The JFET is the J112 this is very common and is the same FET die as the original MiniMoog But take note the Mini used the E112 and that has a different pin out Some US customers have found the J112 quite difficult to trace However it can be found at Radio Shack The board mounted pots are Spectrol 248 series with Oakley pot brackets You could use any type you want but not all pots have the same pin spacing Not a problem of course if you are not fitting them to the board We carry a stock of all the pots you need as the VCO pot kit One thing to note if you are buying knobs for these pots The older Oakley modules used Omeg pots which come with a 6mm shaft while the Spectrol ones have a 6 35mm shaft The toggle switch is a standard SPDT single pole double throw switch The types I use are made by APEM in France and have a flat toggle RS Components Ltd 320 916 and Farnell 1082301 sell them in this country at a very good price L1 to L2 are leaded ferrite beads These are little axial components that look like little blackened resistors They are available from most of the mail order suppliers Find them in the EMC or Inductor section of the catalogues Farnell sell them as part number 9526820 The multiturn trimmers are the ones that have the adjustment on the top of the box S
32. o the new socket board Latest PCBs are Ni Au finish and are RoHS compliant Power Supplies The design requires plus and minus 15V supplies These should be adequately regulated Although quite large perturbations in the supply will not cause any pitch changes waveform linearity and DC offset will be affected by an unregulated supply The current consumption is about 30mA per rail Power is routed onto the PCB by a four way 0 156 MTA or Molex type connector You could of course wire up the board by soldering on wires directly The four pins are 15V ground panel 15V The panel connection allows you to connect the metal front panel to the power supply s ground without it sharing the modules ground line More about this later Circuit Description The VCO circuit can be thought of as several little subsections connected together Lets look at these little sections separately The first thing to look at is the power supply This is shown on the bottom of the second sheet of the schematics The VCO could have been powered by 15V but I wanted the VCO to sound like a minimoog To do this I have used the same voltage levels within the VCO core as the original Thus we need to generate stable 10V and 5V supplies I still used the 15V supply for most of the op amps as this would not have any affect on the waveforms Power enters the board on a 4 way Molex connector It is filtered initially by two small ferrite inductors and d
33. of support is the Oakley Sound Forum at Muffwiggler com Paul Darlow and I are on this group as well as many other users and builders of Oakley modules If you can t get your project to work then Oakley Sound Systems are able to offer a get you working service If you wish to take up this service please e mail me Tony Allgood at my contact e mail address found on the website I can service either fully populated PCBs or whole modules You will be charged for all postage costs any parts used and my time at 25GBP per hour Most faults can be found and fixed within one hour and I normally return modules within a week The minimum charge is 25GBP plus return postage costs If you have a comment about this builder s guide or have a found a mistake in it then please do let me know But please do not contact me or Paul Darlow directly with questions about sourcing components or general fault finding Honestly we would love to help but we do not have the time to help everyone individually by e mail Last but not least can I say a big thank you to all of you who helped and inspired me Thanks especially to all those nice people on the Synth diy Oakley Synths and Analogue Heaven mailing lists and those at Muffwiggler com Tony Allgood at Oakley Sound Cumbria UK This update September 2010 No part of this document may be copied by whatever means without my permission
34. orm from U9 s output moves between 5V and 5V As the pulse width changes then the output of the CV summer moves away from zero This is added to the waveform and the pulse output waveform moves above or below zero volts Analysing this dynamically changing output reveals that the average output voltage is actually always zero This is because the offset added compensates for the altering amounts of time the waveform spends in the high or low states The summing circuit also inverts the waveform so once again we create the wrong phase A simple op amp invertor circuit U9 pins 9 10 amp 11 turns the pulse wave the right way round again If you do not want this dynamically varying output signal then all you need to do is omit R71 The source of the audio source for the comparator comes from either the triangle or sawtooth waveforms This selection is another unique feature of the Oakley One of three VCO Both will sound the same with fixed pulse widths But they do sound different when the width CV is modulated quickly Components Most of the parts are easily available from your local parts stockist I use Rapid Electronics RS Components and Farnell here in the UK The VCO module was designed to be built mostly from parts obtainable from Rapid Electronics and ourselves The resistors can be 5 carbon 0 25W types except where stated However I would go for 1 0 25W metal film resistors throughout since these are very cheap n
35. owadays If you do want to use 5 types the please note that some of the resistors have to be 1 or better types Failure to use good quality parts in these locations will affect the tuning stability of the VCO These critical parts are indicated in the parts list Some of you may want to use 0 1 precision resistors in these critical positions These are quite pricey and may be best to use in the most critical positions only These would be R32 R44 R29 It is my opinion that this is not worth doing and that 1 types are perfectly adequate All the electrolytics abbreviated to elect in the parts list should be 25V or 35V except where stated and radially mounted Don t chose too high a voltage The higher the working voltage the larger in size the capacitor A 220V capacitor will be too big to fit on the board 35V is a good value to go for The pitch spacing of all the non polar capacitors is 5mm 0 2 This may differ from some of the older Oakley boards you have built The polyester capacitors are actually metallised polyester box types These have a lead pitch of 5mm 0 2 and come in little plastic boxes in a variety of colours Typical working voltages will be 63V 100V or even 400V They key thing to look out for is physical size and lead pitch Most of Rapid s are either 63V and 100V and all will fit on the board The ceramic capacitors should be low K ceramic plates These are sometimes called COG types The le
36. pair is now added with variable depth controlled by the new HFT trimmer Constant zero average output voltage from the pulse wave This is achieved by considerable changes to the older simple design The board has four Spectrol pots instead of three Omeg ones This makes the board less cramped since the board is now longer The solder pads for the jack sockets now are on standard sized solder pads 0 1 headers are still provided for those of you who like them although the main reason for keeping these is the Orbital The new board layout required a new modular faceplate to be made for the module Issue 4 Boards had the following features added Major mechanical changes The module is now constructed from three PCBs and features solderless interconnects This facilitates easy construction The board set can still be used with the issue 3 front panel design Spectrol 248 pots are now used throughout the module Change to the high frequency tracking HFT circuitry to include Rene Schmitz s improved take on the Rossum method of compensating for Rbe Change to the way the fine tune pot s CV was handled The single metal film 10M resistor was replaced by a simple resistor network 10M 1 resistors are getting hard to come by these days Replaced the now deleted CA3080 from the sine shaping circuitry The new board simply uses one half of an LM13700 device Easy to fit to the Oakley CV gate buss using an optional three way header fitted t
37. pectrol and Bourns make these Some types are 22 turns while others are 25 turns Either will do They should have three pins that are in a line at 0 1 pitch Don t chose the 10 turn ones with the adjustment on the end they won t fit on the PCB The four trimmers SYM S SYM T SHP S and SHP T are standard sealed carbon units These are adjusted from the top and as such are called horizontally mounted types Piher and other companies make suitable types Lead spacing is 0 2 for the track ends and the wiper is 0 4 above a line joining the other two pads I use Iskra ones available from Rapid The PSU trimmer is a special single turn cermet type This type of trimmer has a different PCB footprint than the other types Lead spacing is 0 2 for the track ends and the wiper is 0 2 away above a line joining the other two pads The eight way jumper interconnect is a one piece assembly bought ready made from several places I buy mine as an eight way 55 mm or 85mm long piece These come as pre stripped and even tinned with solder too Make sure you get the 0 1 2 54mm pitch variety The Rapid part number is 22 1650 for the 55mm and 22 1655 for the 85mm one For the 0 1 interconnections I use the 26 awg MTA parts These are made by Amp now part of the massive Tyco empire To use these effectively you need a special tool to poke the wires into the special housings The housing contains specially shaped contacts that cut th
38. rough the insulation of the wire so you don t need to do any stripping Just simply push down on the wire with the tool to lock it into place in the housing You can also use Molex strip and crimp systems More about this one later on Name Farnell Part number Amp s part number Four way housing 1098712 640442 4 Four way header 588593 640456 4 Five way housing 1098713 640442 5 Five way header 588600 640456 5 Six way housing 1098714 640442 6 Six way header 588611 640456 6 Handtool 589494 59803 1 Input and output sockets are now board mounted in the standard module assembly You could of course use any types if you would like to wire your sockets up with individual wires I use the Switchcraft 112A 1 4 sockets in all my ready made modules These excellent parts are also used on the Moog and MOTM modulars The version you need to fit in the socket board is the 112APC This part is stocked by most suppliers Both Rapid and Farnell sell is at a reasonable price Rapid also sell a Far Eastern clone of the 112APC part that is considerably cheaper but doesn t look as nice Please note that the standard 112A will not fit easily into the boards as it has solder tags Finally if you make a change that makes the circuit better do tell the Oakley synths mailing list or myself directly Any updates are added to the current user guide as quick as possible UK builders should know that there is a Oakley Preferred Parts List on
39. rs to tighten the nut Do not over tighten Now doing one pot at a time fit each pot and bracket into the appropriate holes in the PCBs Solder two of the pins attached to the pot bracket Leave the other two pins and the three pins of the pot itself Now check if the pot and bracket is lying true That is all four pins are through the board and the bracket should be flat against the board s surface If it is not simply reheat one of the bracket s soldered pads to allow you to move the pot into the correct position Don t leave your iron in contact with the pad for too long this will lift the pad and the bracket will get hot When you are happy with the location you can solder the other two pins of the bracket and then the pot s pins Do this for both pots and snip off any excess wire from the pot s pins at this point The Spectrol pots are lubricated with a light clear grease This sometimes is visible along the top of the mounting bush of the pot body Try not to touch the grease as it consequently gets onto your panel and PCB It can be difficult to get off although it can be removed with a little isopropyl alcohol on a cotton wool bud You can now present the front panel up to the completed boards to check that they fit If it does I then mount the boards up proper You need to add the washer between the panel and the nut Again do not over tighten and be careful not to scratch your panel Note that the pot shafts of the pots will
40. s out by squashing gently all the three of them flat with a pair of pliers The flat surface of the pliers parallel to the flat side of the transistor There are three types of trimmer used in this project In two of these types we are actually using two different values Look very carefully at each one before soldering them into place they are very difficult to remove if you put them in the wrong place If your multiturn pot is marked as 104 then this is a 100 000 ohm or 100K pot 103 is a 10K pot These multiturn pots can go in any way around The last part except for the pots you will probably put on is the temp co resistor This is to be fitted so that it lies across the top of U4 The leads of the resistor should be bent so that it goes in the pads provided Put a small blob of heatsink compound on the top of U4 Then push the resistor onto it Clean up any squeezed heatsink compound from around the resistor and make sure none of it goes on the pins of U4 Now you can solder the resistor in place The 0 1 headers are fitted on the main board so that pin is towards the right hand side of the board The clip on the side of the header will match with the picture on the PCB legend and should be towards the top of the board The 0 156 header should go into the board so that the plastic lock is on the left hand side I would make the main circuit board in the following order resistors IC sockets small non polar capacitors diodes
41. s own offset voltage This offset voltage means that the OTA will soft clip asymmetrically By adding a small voltage of the opposite polarity to one of the input pins we can cancel the effects of the offset The sine wave created by this process does not give us a perfect textbook example of a sine wave However the wave is very low in harmonics and for musical uses it is adequate The final output and the final circuit block is the pulse conditioner This is also found on page two of the schematics The first section of this is the CV summer and is based around U9 pins 1 2 3 This circuit simply sums together the PWM pulse width modulation CV input and the voltage from the wiper of the Width pot The output of the CV summer will go from around 5V to 5V when being controlled by the Width pot alone The pulse wave generator itself is based around the second half of the comparator U7 pins 5 6 amp 7 This compares either the triangle wave or sawtooth wave output with a voltage set by the output of the CV summer If the waveform is higher than the voltage from the CV summer the comparator s output goes low If it is lower the output goes high Thus the output is either low or high and spends very little time in between This creates a rectangular waveform where the proportion of time spent high or low is controlled by the WIDTH pot and or the external CV If the width pot is set to its middle position and no input CV is applied t
42. sue 4 PCB set The main PCB is 108 mm depth x 143 mm height in size All three boards use double sided copper traces and have through plated holes The solder pads are large and are easy to solder and de solder if necessary They have a high quality solder mask on both sides for easier soldering and have clear legending on the component side for easier building If you are building the standard design there are no components mounted off the boards All components including sockets and pots are soldered directly to the boards Previously many Oakley modules have had the sockets switches and extra pots wired to the board by individual wires This module allows all the socket wiring to be done via the socket PCB and two MTA solderless or Molex connections If you are building this module in the standard Oakley format this new system will reduce assembly time and possible wiring errors Some people will wish to use this Oakley design in a non standard format such as fitting it to another manufacturer s rack or one of their own invention This is perfectly easy to do Simply do not use the socket board and wire the main board to the sockets as per usual I have provided space for four of the control pots on the main PCB whilst the other three pots are fitted to their own board If you use the specified pots and brackets the PCBs can be held firmly to the panel without any additional mounting procedures The pot spacing is 1 625 and is th
43. t Rbe causes an error related to frequency Over the limited range of an audio VCO this is usually perfectly OK and Franco is used in many commercial synth VCOs as the only high frequency compensation technique For issue three I decided to go one better The newer issue VCOs use a fixed Franco resistor for the reset time and also use the Rossum technique to compensate for the exponential generators error Dave Rossum founder of E Mu systems in the 1970s was an important player in the development of the SSM chips The SSM2030 VCO chip which was not as good in comparison to the later Curtis CEM3340 was groundbreaking when it came out Dave s method of HF compensation for the SSM2030 involved gently pulling the base of the first transistor of the NPN pair lower when the collector current through the second NPN gets bigger In the issue 3 VCO I slavishly copied Dave s idea as presented in Electronotes and indeed the Prophet V many years ago However due to work done recently by Ren Schmitz this simple method can be improved upon with the same number of components As in the original Rossum idea a fixed emitter resistor R17 can be used to effectively measure the emitter current since the voltage on its bottom end will fall as collector current rises Previously we took this voltage passed it through a diode to compensate for the base emitter voltage drop Vbe of the matched pair and then fed a small proportion of the resultant voltage back
44. t it to the right octave SHP T Play an A an octave above middle C Adjust SHP T so that the triangle output sounds smooth You will know when it is right If you have a scope adjust SHP T so that the sharp steps disappear from the output SYM T Set the PWM TYPE switch to EDGE Listen to the pulse output at any frequency that s comfortable to listen to Now turn the PULSE WIDTH pot to give you a square wave This will be somewhere near the middle of its travel A square wave sounds hollower and less buzzy than the pulse wave so it should be easy to find Now set the PWM type switch to CENTRE It ll probably change the tone of the output signal Adjust the SYM T trimmer so that you can again hear the square wave Flick between the two modes to confirm that you have a square wave in each switch position SYM S and SHP S Plug the sine output into an amplifier and use your ears for this one Set the VCO to make a lowish note around 200Hz will do Adjust SHAPE until the sine output sounds pure Its shouldn t be too buzzy or too hollow Then go back and adjust the SYM S trimmer to get it really pure You may need to bounce back and forth between the two trimmers until you get a good sound Its not essential to get this right just set it so you get a nice sounding output It s easier to do than to explain Now you have set up your VCO and you are ready to go Final Comments If you have any problems with the module an excellent source
45. to the base of the left hand transistor This works well enough However Ren s idea improves on this on two counts Firstly the diode in the pure Rossum method doesn t exactly compensate for the Vbe because the current flowing in the diode and the base emitter junction are different In the new circuit both semiconductors see the same current as they are fed from identical resistive sources R17 and the HFT trimmer are both the same value Therefore the voltage drop across D2 and Vbe should track each other reasonably well even over a change in ambient temperature Secondly the new method feeds the base of the left hand transistor with more or less the same resistance That is the extra resistance due to the HFT circuitry the left hand transistor base sees is pretty much fixed at the value of R20 only Thus altering the HFT trimmer should not result in a change in the overall operating frequency of the VCO as it did in the old method Ideally the diode should be matched with the transistor array However HFT compensation due to Rbe is only a small consideration at audio frequencies so we need not get too bogged down with the higher orders of errors Going back to the VCO core The sawtooth output from U5 Q2 is amplified by the other half of US pins 5 6 7 before being sent to the output pad The final output is roughly 5V to 5V ie 10V p p The triangle shaper is essentially a full wave rectifier whose operating point is
46. transistors electrolytic capacitors and connectors Then the final water wash You can now fit the trimmers using no clean or ordinary rosin flux solder Do not fit the pots at this stage The mounting of the pots requires special attention This will be covered later in this User Guide Populating the Socket Board You have one socket board to populate and the method is a little unusual On the board the first things to solder are the headers These are fitted to the BOTTOM of the board and are soldered from the top side This is obviously opposite to what you are normally used to The legending is on the top of the board and the bottom of the board is marked as such in copper on the underside Fit both the headers so that pin 1 is the square pin The friction lock on the header should correspond to the legend on the top ie the opposite side of the board If you are intending to fit the Oakley CV gate buss to your synthesiser system then you will need to fit a three way header in the location CV BUSS This is to be fitted on the bottom of the board like all the other headers If you have no intention of ever using the CV gate buss then you should link out pins and 2 of the three way header with a piece of wire You can use a resistor lead clipping to do this job It is possible to fit the header and then use a two way jumper to temporarily link pins 1 and 2 together This way you can add the Oakley Buss at a later date very easily T
47. ypes as these are the most reliable You need two 14 pin DIL sockets and seven 8 pin DIL sockets Populating the Main Circuit Board Oakley PCBs are supplied with a RoHS compliant finish This is a high quality finish but does possess slightly different soldering characteristics to the traditional lead based HASL finish Handle the boards with care and avoid touching the solder plating since this can cause premature tarnishing of the finish Shelf life is hard to predict but we recommend soldering in all the components less than one year from when you receive your board Neither I nor Paul Darlow are responsible for any accidents caused whilst working on these boards It is up to you to use your board responsibly and sensibly Occasionally people have not been able to get their Oakley projects to work first time Some times the boards will end up back with me so that I can get them to work The most common error with most of these was parts inserted into the wrong holes Please double check every part before you solder any part into place Desoldering parts on a double sided board is a skill that takes a while to master properly If you have put a component in the wrong place then the best thing to do is to snip the component s lead off at the board surface Then using the soldering iron and a small screwdriver prize the remaining bit of the leg out of the hole Use wick or a good solder pump to remove the solder from the hole Filling the hol
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