Overdrive-II Builder`s Guide
Contents
1. You need to cut your new cable into two small pieces that will allow the switch to be connected to the board If you are using the suggested front panel then you will be fitting your switch so that the toggle moves up and down Lay the module flat so that the front panel is facing away from you and the board is facing upwards lying flat on the work surface Your fitted switch will have two sets of three contacts and these will now be lying horizontally The wiper contacts of the switch are the two middle pins one above the other Let us wire the bottom set of contacts first These are to be connected to the pads on the board named S1A Solder both wires of one of the twisted pairs to the pad S1A There are two pads so that each wire is connected to its own solder pad The other end of your pair should now be soldered to the switch But you have three contacts and only two wires in your pair Solder one of the wire ends it doesn t matter which to the right hand tag of the bottom row of contacts Then solder the remaining one to the middle tag Now let us wire the top set of contacts These are to be connected to the pads on the board named S1B Solder both wires of the other twisted pair to the pad S1B Again there are two pads so that each wire is connected to its own solder pad The other end of this twisted pair should now be soldered to the switch This time we are using a different part of the switch So solder one of the wire ends again it2. MTA connector PWR L1 and L2 in conjunction with C24 26 27 and 30 provide high frequency filtering and decoupling They essentially act to keep the power supply as free from noise as possible The main overdrive and distortion circuitry is actually run from a lower 5V supply This is generated in the usual if somewhat overkill fashion of using two three terminal regulators U3 and US These generate a very stable quiet low noise split 5V rail which the more sensitive parts of the overdrive circuitry can run R28 and C22 and their negative equivalent R29 and C33 provide additional filtering and isolation from main 15V rails D11 to D14 provide discharge paths during power up and down cycles which could potentially harm the ICs on the circuit The input signal is split into two one pathway going to the overdrive circuitry the other going to the INPUT BLEND switch The input blend switch determines whether the harmonic multiplier circuit receives its audio input from the input directly or the output of the overdrive circuit We will take a look at the overdrive circuitry first This is top part of the schematic shown on page 1 The input signal is attenuated by an inverting amplifier block based around U2b pins 6 5 and 7 This circuit reduces the input signal to around a quarter and provides the following sections with a constant source impedance Without this part of the circuit the overdrive core and the blend pot may affect the source
3. doesn t matter which to the middle tag of the top row of contacts Then solder the remaining one to the left hand tag You can now use a couple of cable ties to hold the two twisted pairs together for neatness The second switch is relatively easy to wire up I use solid core tinned copper wire instead of insulated wire Simply wire each solder tag of the switch to the empty solder pad directly below it I normally bend the wire at one end into a hook and place the straight end into the PCB pad s hole I then loop the hooked end around the switch tang and squash the hook into place before soldering it The solder pad on the board can then be soldered from the underside and the excess wire on snipped off The in blend switch can be wired with either insulated wire or solid core wire as shown above 14 Final Comments If you have any problems with the module an excellent source 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 wi
4. middle section of the Serge Wavemultiplier and the late Jiirgen Haible s Wavefolder The signal is split into two one going into an inverting op amp U6a of gain 6 and the other to a simple summing amplifier based around U4a U6a has two 2 7V zener diodes in its feedback loop This restricts the final output of U6a to 2 7V or thereabouts The zeners do not act very strongly so the final output is gently squeezed as opposed to being clipped off sharply at 2 7V Remember though that the signal is being amplified by 6 at this stage so it only requires the input signal to reach 0 25V before the output shows signs of being clipped Being an inverting amplifier the output of U6a is not only restricted but also inverted This is important as the signal is now added to the original input signal by the inverting summing circuit of U4a Note the resistor values of R9 and R8 The restricted output from U6a some six times higher than it was is now reduced by a factor of two thirds by the summing amplifier So the overall gain of the signal going through U6 a and U4a is 4 The original signal is inverted in the summing stage and it has a gain of 2 The overall effect is therefore a signal gain of 2 at signal levels below 0 25V and 2 at all signals above 0 25V This change in the polarity of the gain that is dependant on the signal level produces the required wavefolding action D1 and D2 provide additional restriction of the output first s
5. reverse parallel with the resistative feedback path These diodes act to limit the voltage across the feedback resistor These are shown in the schematic as D5 and D6 They can be switched in and out of the circuit with S1A the soft hard selection switch Because they act upon the feedback voltage and not the output signal directly they do not behave as a traditional clipping circuit In the latter any signal is simply limited to a set maximum or minimum voltage This circuit is more subtle than that The gain of the op amp non inverting amplifier is set by the resistances within the feedback loop but because of the way it works this is always more than or equal to one That is the output level is never less than the input The exception to this is at very high frequencies when the op amp reaches its operating limits Ignoring the diodes for now Ula has its passband gain set to a minimum of 11 via R2 and R3 The gain pot allows this gain to increase still further by increasing the feedback resistance With the gain pot turned up full the maximum passband gain of the op amp without the diodes is 111 A diode will behave in such a way as to limit the positive voltage across it to no more than 0 6V It does this by effectively changing its resistance depending on the voltage it has across it Two diodes connected in reverse parallel head to tail tail to head will seek to limit the voltage across them to 0 6V D5 and D6 are connected to act like
6. Oakley Sound Systems SU Oakley Modular Series Overdrive IlI Distortion and Harmonic Multiplier module Builder s Guide V2 0 4 Tony Allgood Oakley Sound Systems CARLISLE United Kingdom Introduction This is the Project Builder s Guide for the issue 1 Overdrive II 5U module from Oakley Sound This document contains a basic introduction to the board a full parts list for the components needed to populate the board a list of the various interconnections and a circuit description For the User Manual which contains a basic overview of the module a summary of its operation and a personal perspective from the author please visit the main project webpage at http www oakleysound com overdrv htm For general information regarding where to get parts and suggested part numbers please see our useful Parts Guide at the project webpage or http www oakleysound com parts pdf For general information on how to build our modules including circuit board population mounting front panel components and making up board interconnects please see our generic Construction Guide at the project webpage or http www oakleysound com construct pdf The issue 1 Overdrive II PCB On the printed circuit board I have provided space for the five main control pots If you use the specified 16mm Alpha pots and matching brackets the PCB can be held very firmly to the panel without any additional mounting procedures The pot spacing on this board is d
7. V 3 2 Module GND 4 3 15V 5 4 Not connected 6 5 5V is not used on this module so location 3 pin 2 is not actually connected to anything on the PCB If fitting the PWR header you will also need to link out pins 2 and 3 of PSU This connects the panel ground with the module ground Simply solder a solid wire hoop made from a resistor lead clipping to join the middle two pads of PSU together 11 Wiring the sockets If you have used Switchcraft 112 sockets you will see that they have three connections One is the earth tag one the bevelled edge One is the signal tag which will be connected to the tip of the jack plug when it is inserted its marked with a small T The third tag is the normalised tag or NC normally closed tag and is marked T S presumably standing for tip switched The NC tag is internally connected to the signal tag when a jack is not connected This connection is automatically broken when you insert a jack In the Overdrive II module we will be using the NC lug on the INPUT socket to short out the input line to ground when there is no jack inserted Connect with a piece of insulated wire the following tags on each socket to the respective pad on the PCB Socket Name Tag type PCB pad name INPUT Signal IN INPUT NC GND INPUT Earth PNI OUTPUT Signal OUT OUTPUT Earth PN2 There is no connection made to the NC tag on the output socket I have used black heatshrink tubing to act as a strain relief fo
8. amps U4 and U6 The fold pot acts as a simple volume control with U7a serving as a standard voltage follower or buffer circuit U7a ensures that the following wavefolding circuits are fed from a very low output impedance which would not be the case had we used the voltage available directly at the wiper of the fold pot There are two wavefolding stages in this harmonic multiplier More stages were considered but for this particular module two was considered enough Wavefolding can be done in a variety of ways but all rely on the basic principle of bending down the waveform peaks This essentially means that the circuit s gain like the overdrive circuitry described above has a gain that varies with the input signal level Unlike the overdrive circuit that simply reduces the gain at higher input signals the wavefolder reverses the gain That is a positive going waveform is turned into a negative going one If the input signal is large enough this mirroring of the waveform can even exceed the original signal level If this new signal is now fed into another wavefolder circuit block then the mirrored parts of the waveform are mirrored once again Each time there is a wavefolding action the sound radically changes and higher order harmonics are heard U6oa and U4a make up the first wavefolding section As far as I know this is the first time that wavefolding has been done this way The circuit will produce a different sound than other methods such as the
9. for details Miscellaneous Leaded axial ferrite beads L1 L2 MTA156 4 way header PSU Oakley MOTM power supply MTA 100 6 way header PWR Synthesizers com power supply Other Parts Required Switchcraft 112APC 1 4 sockets Two off mounted on panel Five 20mm knobs Around 2m of insulated multistrand hook up wire for the switch and socket connections 10 Connections Power connections MOTM and Oakley The PSU 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 Pin 1 on the I O header has been provided to allow the ground tags of the jack sockets to be connected to the powers supply ground without using the module s 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 It s worth filling the empty holes of the PWR pads with solder Power connections Synthesizers com The PWR power socket is to be fitted if you are using the module with a Synthesizers com system In this case you should not fit the PSU header The PWR header is a six way 0 1 MTA but with the pin that is in location 2 removed In this way location 3 is actually pin 2 on my schematic location 4 is actually pin 5 and so on Power Location number Schematic Pin number 15V 1 1 Missing Pin 2 5
10. ifferent to many of our older 5U modules instead of 1 625 it is 1 375 Used in conjunction with smaller 20mm diameter knobs this still allows for an attractive module design and finger friendly tweaking The design requires plus and minus 15V supplies The power supply should be adequately regulated The current consumption is around 25mA Power is routed onto the main PCB by either our standard four way 0 156 MTA156 type connector or the special five way Synthesizers com MTA100 header The four pins are 15V ground earth panel ground 15V The earth 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 The main PCB has four mounting holes for M3 bolts one near each corner These are not required for panel mounting if you are using the four 16mm pot brackets The board size is 71mm deep x 158mm high No socket board is available for this project Since the module only uses two sockets it was not considered worthwhile to use a special board for this purpose Aled o qoooTW t R Ama ca i alpen a A ord Cee tee SE c 3 a aem eee e 3 ela 88 8 S23 11 3aTwcesno ONNOS AJT The prototype Overdrive II fitted to a natural finish Schaeffer MOTM format panel Circuit Description The module is powered in the conventional way from a split rail of 15V This comes in to the module via the 0 156
11. path it also switches in via S1B two more reverse parallel diodes D9 and D10 In conjunction with current limiter R19 these two diodes act as a traditional diode clipper and truncate the voltage across them to around 0 6V This is the hard mode and two different types of clipping are now possible in this mode The diode clipping is apparent with high input signals with low gain or low input signals with high gain However at high gains with medium to high input levels both the op amp output limiting and diode clipping work together to give you a more pronounced effect Immediately after the diode clipping circuit there is some additional high frequency filtering based around the actions of R26 and C21 This low pass filter reduces the harshness and gives a rounded tone to the sound Coupled with the bass end roll off of C5 the overall frequency response over the distortion sections is focussed on the mid range part of the audio spectrum The tone control is based around the classic TS and SD 1 design and is built around U1b Its a simple circuit that works well in practice and produces enough tonal variation to be very useful At the lowest setting of the Timbre pot C4 acts in conjunction with R19 to produce a single pole low pass filter At the high end of the tone pot s travel C4 now mostly acts in conjunction with the feedback resistor R22 and R5 and produces a shelving high pass filter This boosts high frequencies and produces a much
12. r the wiring to the sockets 12 Wiring the Switches You have two switches to wire up One of them is relatively straightforward and uses a SPDT switch the other uses a DPDT switch and is slightly more complex Let s get the hard one out of the way first This is an APEM double pole double throw on on toggle switch Note the twisted wire pairs connecting the PCB to the switch The first thing you need to do is to make up some twisted pairs of wires These will be used to connect each pole of the hard soft switch to the board A twisted pair is exactly that two wires twisted together to form a simple cable The best way to make a twisted pair is to use a hand drill or slow moving battery powered drill Take two one metre lengths of coloured insulated multistrand wire and tie them to something fixed like a door handle Did he say door handle I did but you can be professional and use a bench vice if you wish Straighten the wire out and push the two free ends into a chuck of the drill Now hold the drill so that the wire is horizontal and fairly taught Both wires should be the same length if not make sure they are Now let the drill slow spin and the wires will twist together neatly Don t make it too tight but don t put too few turns on it either Practice makes perfect as they say You should be warned that when you release the wire it tends to curl up a bit but this is fine and you should have a nice twisted pair to work with 13
13. recommended 47R R28 R29 220R R5 IK R3 R22 R21 1K5 R19 3K3 RI 10K R11 R2 11K R17 R10 R8 15K R27 22K R24 R13 27K R23 33K R4 R26 R20 R7 R9 R16 68K R14 100K R12 R25 110K R18 360K R6 R15 Capacitors 100nF axial multilayer ceramic C8 C17 C26 C16 C15 C12 C27 C9 C14 C11 C10 C13 10pF 2 5mm COG ceramic C7 C20 47pF 2 5mm COG ceramic C18 C6 47nF 100V polyester C21 150nF 63V polyester C25 220nF 63V polyester C4 C5 470nF 63V polyester C1 C3 C2 C19 2u2 63V electrolytic C22 C29 C23 C28 C30 C24 Discrete Semiconductors 1N4148 silicon diode D5 D6 D9 D10 D11 D12 D13 D14 2 7V 400mW zener diode D1 D2 D3 D4 D7 D8 Integrated Circuits 78L05 5V 100mA regulator U3 79L05 5V 100mA regulator U5 OP275G dual op amp U1 U2 TL072CN dual FET op amp U4 U6 U7 There is some scope for using different op amps to those I have suggested However the ones I have chosen are stable and sound good IC sockets are to be recommended You need five 8 pin DIL sockets Potentiometers Pots All pots Alpha 16mm PCB mounted types 10K linear TIMBRE 47K or 50K linear BALANCE BLEND FOLD 100K log DRIVE Four 16mm pot brackets Switches One single pole ON ON SPDT toggle switch is required for the IN BLEND selection One double pole ON ON DPDT toggle switch is required for the HARD SOFT selection Both switches are mounted on the panel and wired to the board with fly wires see later
14. sharper sound This is particularly apparent in the distortion mode since this part of the circuit produces much more higher harmonics due to the more severe clipping affect The output of the tone control stage is fed to the other end of the blend pot This is a simple arrangement that lets the user apportion the mix of distorted and straight through signal The relatively high impedance output of the wiper of balance pot would not be able to drive the next part of the circuit so it is buffered and amplified by a simple inverting amplifier U2a The output of U2a is called DRIVE_OUT on the circuit diagram Drive_out is fed to two places one to the in blend switch and two to the balance pot The latter allows the overdriven signal to be mixed with the output of the harmonic multiplier circuit much in the same way as the blend pot U7b provides the buffering of the wiper voltage so that the output impedance of the module is set only by R21 This 1K resistor found on many modules limits the output current preventing instability with long cable connections and damage to U7b It also allows you to directly connect the output to other module outputs within a multiple module This serves then as a crude mixer where the resulting output is a sum of the two connected modules Although once done quite a lot in the old days it is not terribly good practice as not all new modules feature such a resistor The harmonic multiplier is based around two dual op
15. signal s integrity and cause unwanted distortion if you were using the Overdrive module in parallel with another module U2b s output is then split one side going to the blend pot which will provide the dry signal and the other side going into the overdrive s core circuitry The core of the overdrive module is based around the same circuit as used in many guitar overdrive foot pedals To my knowledge this type of design first appeared in the Ibanez Tubescreamer TS 808 pedal but it also appears in other commercial pedals including the Boss OD 1 and SD 1 and other later Ibanez pedals The TS 808 pedal has become something like the equivalent of the TB 303 in the guitar world and the original green units sell for a great deal of money However the basic circuit is actually quite simple and it is one that I have used in various home made pedals for some time There are great deal of DIY TS 808 clones out there and many of them will talk about the huge differences in using different diodes and types op amps I found that for synthesiser use at least the actual sonic differences due to the actual devices used are not quite as obvious as internet lore has decreed However I will leave it to the builder to experiment here and I am sure that the Oakley forum would be a great place to discuss your findings The key in the basic TS overdrive circuit is the usage of a standard non inverting high gain op amp stage Ula pins 1 2 3 with two diodes in
16. tage of wavefolding These two diodes gently clip the output of U4a to 3 3V This prevents very loud input signals from overfolding This is what I call what happens when the folded parts of the waveform become so much greater than the input signal as to drown out the effects of the folding The second stage of wavefolding is done with a similar circuit to the first This time we do not need the overfolding diodes as it only needs to be done in the first stage The second stage output from U6b is AC coupled into the balance pot by C19 C19 prevents any DC signals from reaching the balance pot and causing unpleasant crackling as the pot is turned Overdrive II issue 1 Parts List For general information regarding where to get parts and suggested part numbers please see our useful Parts Guide at the project web page or http www oakleysound com parts pdf The components are grouped into values the order of the component names is of no particular consequence A quick note on European part descriptions R is shorthand for ohm K is shorthand for kilo ohm R is shorthand for ohm So 22R is 22 ohm 1K5 is 1 500 ohms or 1 5 kilohms For capacitors luF one microfarad 1000nF one thousand nanofarad 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 1 0 25W or 0 4W metal film resistors are
17. thin 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 and Analogue Heaven mailing lists and at Muffwiggler com Tony Allgood at Oakley Sound Cumbria UK August 2012 No part of this document may be copied by whatever means without my permission 15
18. this So as the output level of Ula rises either by turning up the gain pot or by applying a bigger input signal so does the feedback voltage Once the feedback voltage gets to beyond 0 6V or below 0 6V the diodes start to conduct and the rise in output level is tempered However the gain of Ula cannot fall to below unity so the diodes only appear to act on the amplified signal Thus what we get at the output is a clipped amplified signal plus the original signal superimposed on it This is the TS sound and it has great tonal characteristics I ought to add that a great part of the overdrive sound is also due to frequency shaping C5 rolls off the gain at lower frequencies C6 and the slew rate of the op amp act to curtail high frequency components It is these that set the width of passband that area in which the op amp acts as an amplifier In addition C3 will effectively block any DC signals to the amplifier The Oakley Overdrive has a major difference that the original TS doesn t have We can switch out the diodes completely and allow Ula to become an unlimited high gain amplifier But of course everything has limits and this time the output is limited by the supply voltage to Ula This is set by the 5V power supply and this causes the output to clip at around 4V for positive excursions and 4V for the negative The op amp cannot produce any output signal higher than this However when S1 switches out the two diodes in the feedback
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