Noise/Filter Builder`s Guide
Contents
1. Final Comments If you have any problems 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 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 and Analogue Heaven mailing lists and those at Muffwiggler com Tony Allgood at Oakley Sound Cumbria UK November 2010 updated March 2013 No part of this document may be copied by whatever means without my permission 192. aA BR WN 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 the PSU header This connects the panel ground with the module ground Simply solder a solid wire hoop to join the two middle pads of PSU together 13 Building the Noise Filter module using the Sock8 board This is the simplest way of connecting all the sockets to the main board The Sock8 board should be populated in the way described in our construction guide found on the project webpage There are only two headers UPR for upper which is six way and LWR for lower which is eight way Both headers are fitted to the bottom side of the board You need to make up two interconnects The six way one should be made so that it is 80mm long The eight way should be made to be 130mm ze 4 e o 7 Gh ILE WE b ad l b e a gt The prototype Noise Filter module showing the detail of the board to board interconnects Here I have used the Molex KK 0 1 system to connect the Sock amp to the main PCB 14 Building the Noise Filter module by wiring the sockets manually If you have bought Switchcraft 112A sockets you will see that they have three connections One is the earth or ground tag One is the signal tag which will be connected to the tip of the jack plug when it is inserted The third tag is the normalised tag or NC normally
3. closed tag 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 Once fitted to the front panel the ground tags of each socket can be all connected together with solid wire I use 0 91mm diameter tinned copper wire for this job It is nice and stiff so retains its shape A single piece of insulated wire can then be used to connect those connected earth tags to pin 1 of LWR Pin 1 is the square solder pad All the other connections are shown in the tables below UPPER Pin Pad name Socket Connection Lug Type Pin LP OUT Connect to LPF OUT Signal lug Pin 2 Not connected Pin 3 Not connected Pin4 HP OUT Connect to HPF OUT Signal lug PinS WHITE OUT Connect to HPF IN NC lug Pin6 HP IN Connect to HPF IN Signal lug LOWER Pin Pad name Socket Connection Lug Type Pin Panel ground Connects to all sockets Ground lugs via wire frame Pin2 PINK OUT Connect to PINK NOISE Signal lug Pin3 WHITE OUT Connect to WHITE NOISE Signal lug Pin4 IR OUT Connect to INFRA RED Signal lug PinS PINK OUT Connect to LPF IN NC lug Pin6 LP IN Connect to LPF IN Signal lug Pin7 module ground Connect to CV IN NC lug Pin8 CV IN Connect to CV IN Signal lug 15 Testing testing 1 2 3 Apply power to the unit making sure you are applying the power correctly Reversal of the power s polarity will usually destroy all the op amps in an instant If you can monitor current ch
4. done with either insulated multistrand wire or solid core wire This is an APEM on off on toggle switch Note the spring washer of the switch is on the inside of the panel The switch can then be tightened from the rear reducing the chances of scratching the front panel Note also the position of the THAT2180LC chip The notch on the chip s case is towards the top of the board 12 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 LWR 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 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 Missing Pin 5V Module GND 15V Not connected 1 NnBWN
5. output that allows C2 to be charged up by the op amp U5 The VCA s current output is dependant on two things one the current through R15 and the voltage at the VCA s control pin pin 3 In fact the current through R15 is multiplied by the mathematical logarithm of the voltage at pin 3 Now pin 3 is controlled by the front panel pot and any connected external CV This way the speed of the integrator can be manually and automatically adjusted The voltage at pin 3 varies from around 290mV lowest frequency to 90mV highest frequency when controlled by the front pot alone The gain of the VCA goes up the more negative the voltage However if the gain of the VCA is allowed to go too high then the whole filter will become unstable and an oscillation is seen at the output of U6b pins 5 6 7 The actions of the resistor network R7 R4 and R3 in conjunction with D2 act as a voltage clamp on the control voltage This prevents excessively negative voltages from reaching pin 3 and thus prevent the VCA from being opened too far and causing the circuit to oscillate The current through R15 is dependant on the voltage output of op amp U5b pins 5 6 7 This op amp is a differential amplifier and this particular circuit subtracts the input signal coming into the circuit via R31 with the output of the integrator coming in via R23 This now means that the integrator has to respond to both the input signal and its own output The upshot of this is
6. that shown on the PCB legend Push the transistor into place but don t push too far For now there is no need to solder it in place If you don t like the sound of just pushing the transistor into the unsoldered holes in the board you can use a turned pin socket I buy these as 20 or 32 way SIL sockets and then simply cut them down to size p J gt A gb Ayn re zga AMRS e ery Mitts 928 e Anaea SS oca einen E E ano 9 AY ay Q1 fitted into a three way section of a SIL turned pin socket 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 legs out by squashing gently all the three of them flat with a pair of pliers The flat surface of the pliers jaws is parallel to the flat side of the transistor Once the transistor is inserted the circuit will take time to settle After a few seconds you should hear the tell tale hiss of a reverse biased transistor junction You are listening out for the best quality noise sound Don t get too concerned with overall loudness as some of the quieter ones can actually sound better You may need to alter the 17 LEVEL trimmer so that you get an even tone If the LEVEL trimmer is set to high then the noise pre amps may clip and that won t sound very nice either There s no hard and fast rules here just enjoy the process If you have
7. OG ceramic 470p 2 5mm COG ceramic 2n2 polyester film box 33nF polyester film box 100n polyester film box 330nF polyester film box luF polyester film box 2u2 polyester film box 22uF 35V electrolytic 2u2 35V electrolytic Discrete Semiconductors 1N4148 signal diode BC547 see note below C4 C5 C6 C7 C8 C9 C10 C11 C16 C20 C21 C22 C23 C14 C34 C12 C3 C1 C2 C31 C15 C19 C33 C30 C17 C26 C32 C27 C28 C29 C13 C18 C24 C25 D1 D2 QI Q1 is the noise transistor I tried many different types of transistor in this circuit I found BC548s and BC547s to be best I have no idea why they should sound better than the BC549s or any other I tried but they did I didn t like the sound from any of the BC550s I tried at all I suggest buying five BC547s and try each one in turn See separate section later on how to do this Integrated Circuits TL072CP dual FET op amp THAT2180LC voltage controlled amp LF412CN dual precision FET op amp 1458 dual op amp U3 U4 U5 U6 U9 U1 U2 U7 U8 Please ensure that the THAT2180 VCA chips are seated into the board correctly The notch at the top of the chip represents pin one square pad on PCB and is fitted so that it is towards the top of the board Trimmers 20K 6mm horizontal 50K 6mm horizontal Pots 47K linear 16mm Alpha pot ADJ LEVEL LPF HPF CVDEPTH 10 Alpha pot brackets Switch SPDT on off on Miscellaneous Leaded Ferrite be
8. Oakley Sound Systems SU Oakley Modular Series Noise Generator and Dual Filter PCB Issue 4 Builder s Guide V4 0 3 Tony Allgood Oakley Sound Systems CARLISLE United Kingdom amp HPF FREQ LPF FREQ S mi cy DEPTH gt HPF IN HPF OUT LPF IN LPF OUT INFRA RED O WHITE NOISE PINK NOISE ina OAKLEY NOISE FLTER The suggested panel layout for the MOTM format of the Oakley Noise Filter module Introduction This is the Project Builder s Guide for issue 4 of the Noise Filter 5U module from Oakley Sound This document contains a basic introduction to the board a description of the schematic a full parts list for the components needed to populate the boards and a list of the various interconnections For the User Manual which contains an overview of the operation of the unit the history of the various board issues and all the calibration procedures please visit the main project webpage at http www oakleysound com noise 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 Noise Filte
9. a scope then you can monitor what s going on by looking at the white noise output You ll find that the best noise sources are the ones with an even voltage swing about 0V Bad sounding devices tend to have more positive voltage spikes Once you have found a nice sounding transistor switch off and solder the little chap in There are just two trimmers to be set before you are finished As we have seen the LEVEL trimmer allows you to trim the output of the white noise and also the pink noise Monitor the white noise with an oscilloscope Adjust LEVEL until the peak signal is around 6V If you don t have a scope then just adjust it until you get the roughly the same volume as one as your modular s VCOs The ADJ control can be set in a variety of ways What we need to do is to set the average value of the IR output to zero One way is to use a scope and try and get equal swings either side of zero volts Another way is to use your Multimix output LEDs Adjust until you get equal green and red times Beware the infra red output is very random and very slow Be prepared to spend around a minute in waiting for the output to settle to the new ADJ position If you find you cannot get the IR output to get away from either supply rail then please re adjust the LEVEL trimmer to give a higher level of noise to the IR circuitry Too little pink noise will mean you haven t got enough signal to flip the comparator U8a 18
10. ads Knobs to fit 6mm shafts Molex or MTA 4 way header MTA100 6 way header Molex MTA 0 1 header 6 way Molex MTA 0 1 header 8 way Molex MTA 0 1 housing 6 way Molex MTA 0 1 housing 8 way 1 4 sockets Hook up wire 26awg 2 off for LPF and CV DEPTH LP HP to be connected with single core wire F1 F2 Three off PSU Oakley MOTM power supply PWR Synthesizers com power supply UPR for connecting to sockets LWR for connecting to sockets UPR for connecting to sockets LWR for connecting to sockets Eight off mounted either on the Sock8 board or on panel You may well want to use sockets for the ICs I would recommend low profile turned pin types as these are the most reliable You need seven 8 pin DIL and two 8 pin SIL sockets Additional components required if using optional Sock8 board Molex MTA 0 1 header 6 way Molex MTA 0 1 header 8 way Molex MTA 0 1 housing 6 way Molex MTA 0 1 housing 8 way 112APCX Switchcraft 1 4 socket UPR LWR UPR LWR SK1 SK2 SK3 SK4 SK5 SK6 SK7 SK8 If using Molex KK you ll also need at least 28 crimp terminals Suitable lengths of wire to make up the two interconnects and three cable ties Wire Link For this module do not fit the wire link L1 on the Sock8 PCB 11 Mounting the LPF HPF switch Simply connect the switch to the board so that the switch s solder tangs connect to the PCB pad immediately below it This can be
11. aybe thinking why do we need two stages to amplify Surely we could have just used one big amplifier Although it is possible to make a single op amp gain stage of 1000 or more it has one major disadvantage As an amplifier s gain rises we find its bandwidth lowers accordingly The bandwidth of an amplifier is its ability to amplify all input frequencies accordingly A small bandwidth means a restriction in usable frequency response For example it would be not very useful here to have an amplifier with a gain of 1000 but with a maximum frequency of 4kHz By using two lower gain amplifiers in cascade each will have a bandwidth exceeding the audio band so the overall frequency response of both of our amplifiers will not effect the quality of the audible noise Even so I do recommend that you use a good quality fast op amp here The LF412 is an excellent choice and also gives less offset error than a TLO72 To make pink noise we need to create a signal that contains all frequencies as before but this time with equal power per octave Without going into the mathematics of this we need to filter our white noise so that frequencies roll off at a rate of 3dB per octave Now normal one pole RC filters produce roll off not exceeding 6dB octave So to make 3dB octave we must get very clever Fortunately for me other people have done all the difficult work and have published their findings This one I built many years ago from an original design in Wi
12. eck that it doesn t exceed 30mA or so from each rail If you can t check current directly check with your finger that none of the op amps or VCAs are getting warm You will now need a audio VCO and an LFO of some sort Some form of audio monitoring will be needed too so you can hear the output of the Noise Filter module Connect the audio output of the VCO to the IN socket of the low pass filter A sawtooth wave in the bass registers is best Listen to the audio output from the OUT socket Adjusting the LPF FREQ pot you should notice the sound getting duller as you turn the pot anti clockwise This should sound like simple filtering which is exactly what it is At the maximum point the output of the filter should sound more or less the same as the input signal The minimum point should just let through a dull rumble Set up an LFO so that its frequency is at around 1Hz Now attach the LFO s sawtooth output to the CV IN socket Set the LPF FREQ pot to its middle position click the switch to the middle position and rotate the CV DEPTH pot to beyond the central position The cut off frequency should now be being modulated with the LFO Check that the minimum modulation occurs in the middle position of the CV DEPTH pot You should have a dow dow dow sound with the pot in the position and yit yit yit in the position Click the switch into the HPF position and the modulation should stop Now we need to test the hi
13. ffectively regulate the voltage at the emitter with respect to ground since it is behaving as a zener diode However this voltage is not that stable Superimposed on this stable DC voltage is a tiny amount of random noise Noise is a wideband AC signal It contains many different frequencies In fact white noise contains all frequencies within reason at the same average power C17 allows this small AC signal through while blocking the larger DC voltage The next stage is a high gain amplifier with a massive amplification of over a hundred The noise signal is around 2mV or so and this first stage of amplification takes it up to around 200mV It is very possible that DC errors at the input of the op amp U7a will produce output offsets Normally these are very small but any offsets at the input will also be amplified by the op amp C26 removes any DC from the output signal before the next gain stage The second stage of amplification should bring the noise level up to around 5V However due to the variable nature of noise generators in general the gain of this stage is made variable The LEVEL trimmer allows the user to set the output signal to a correct level and account for any differences between noise making devices It is important to set this level correctly otherwise performance of the infra red output will be impaired The white noise output is available from this second stage of amplification via a 1K resistor R62 Now you m
14. filter or SVF for short Unlike the Oakley SVF module this filter is only one pole Only one active part actually controls the cut off frequency and not two like in the SVF module The part that gives us control over the cut off point is an audio voltage controlled amplifier VCA made by the THAT Corporation part number 2180 These parts are normally used in gain control but it is a simple matter to allow them to work in filters too The precise mathematics of the one pole filter is beyond the scope of this User Guide but a more detailed account of its operation can be found in the THAT Design Note number 130 found on their website www thatcorp com Those of you who have built our VC ADSR may recognise the circuitry involved in the one pole filter as I used a similar circuit in the ADSR core To explain how the circuit works we will look at the low pass filter section in more detail Two basic components make up the filter circuitry One is a differential amplifier this is U5b pins 5 6 7 The other is a more complex circuit called an integrator this is based around the VCA and USa pins 1 2 3 An integrator is a charge storage device Think of it like a bucket for electricity It can be filled up and drained at will and the voltage output of the integrator is analogous to the amount of water in our bucket The speed at which the bucket can be drained or filled is controlled by the VCA To be more specific the VCA actually creates a current
15. g low pass filter arrangement This is not a traditional low pass filter as it has two distinct cut off frequencies at 0 2Hz and 0 04Hz Suffice to say its output will respond very slowly to any changes at its input The rapidly changing output of U8a is thus turned into slow sluggish random movements by this lag circuit Furthermore feedback is provided by R47 which feeds back this slow moving output to the comparator This increases the randomness of the signal I found experimenting with the value of R47 produced some unexpected results The ADJ trimmer adjusts the offset voltage applied to the comparator circuit It is provided to keep the overall output centred around 0V Two things seem to affect the amount of offset required One the offset specifications of the op amp I have chosen to use a 1458 as in the original design This does not have the best offset specifications but does have a limited slew rate the speed the op amp s output stages can swing from one voltage to another which will act as a further low pass filter and prevent any fast edges to contaminate our clean analogue design And two the pink noise will contain a certain degree of imbalance which this circuit is sensitive to This seems to be affected not by the DC offsets of the pink noise output amplifier but by the spectra of the noise itself after filtering The output of U8b is typically around 5V to 10V peak The value of which seems to depend on the quality of t
16. gh pass filter Repeat the above section to check whether the high pass filter is working This time as you increase the HPF FREQ pot the output should get fizzier and lose all of its bass end Check also the CV input for the high pass filter Again check that the modulation is at its minimum when the CV DEPTH pot is in the middle position If this is all correct you almost certainly have a working module However we can t check the noise section yet because we haven t inserted the noisy transistor If you have found a fault then the usual course of action is to try and find the wrong component It ll usually be a wrong value resistor like swapping a 100K resistor for a 100R Other things to check are the connections between the socket and main board Make sure too that your headers have been fitted the right way 16 Selecting your transistor and final setting up As soon as you know the filters are working the next thing you need to do once you have built up your module is to fit the noise transistor You should have a few BC547 BC548 or BC549 NPN transistors ready You can try each one in turn into the Q1 position on the PCB and see which one sounds best Connect your power supply up to the module and attach your monitoring amplifier to the white noise socket Keep the volume levels fairly low because as you remove and add the transistors it will thump a bit For the each transistor match the flat side of the device with
17. he chances of mistakes Circuit Description The noise and the two filter sections are essentially separate circuits that only share the same power supply The power supply section is shown on page 2 of the schematics Power is applied to the board through the PSU a 4 way MTA156 Molex 0 156 connector or PWR a special five way MTA header F1 and F2 small axial ferrite beads provide some high frequency resistance and along with C24 and C25 prevent the board from being effected by any noises on the power rails They also help keep any unwanted noises going the other way too Additional decoupling is also provided elsewhere on the board by eight other capacitors shown above right of the power inlet circuitry These capacitors keep the power supply clean of noise and provide a reservoir for the little bursts of current that the circuit takes in normal operation Two grounds are provided one for the circuit itself and one for the earthing of the jack sockets on the front panel The power supplies to each of the op amp ICs are shown separately from the main schematics to avoid cluttering the diagram Still on page 2 we will consider the two filter sections These are actually identical circuits even though one is used as a low pass filter and the other as a high pass filter The only real difference between the two circuits is from where in the circuit we take the output The filters are made from a circuit described as a state variable
18. he noise source After further low pass filtering with R50 and C19 the random output is fed to another op amp to buffer the signal before the final output Issue 4 Noise Filter Parts List 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 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 kilohm R is shorthand for ohm So 22R is 22 ohm 1K5 is 1 500 ohms or 1 5 kilohm 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 Resistors 1 4W 5 or better 22R R63 R48 100R R38 R68 220R R3 R2 330R R67 620R R50 1K R35 R44 R62 R51 R52 R39 R1 R4 R65 1K2 R45 1K5 R59 2K2 R36 R7 3K3 R64 AKT R58 5K K1 R9 R5 R8 R6 6K8 R61 10K R11 R30 R12 R41 R16 14K R26 R17 18K R49 R15 R13 22K R28 36K R10 R37 47K R18 R33 R21 R55 R32 R29 R14 R31 R34 R22 R23 R27 R25 51K R47 100K R46 R43 R40 R24 150K R56 470K R60 R53 R42 1M R54 R57 R66 10M R19 R20 Capacitors 100n axial multilayer ceramic 100p 2 5mm C
19. r issue 4 PCB The issue 4 PCB mounted behind a IU MOTM format Schaeffer panel in natural finish Note the use of the Sock8 board to make easy the complicated socket wiring I have provided space for the three main control pots on the PCB If you use the specified 16mm Alpha pots and matching brackets the PCB can be held firmly to the panel without any additional mounting procedures The pot spacing is 1 625 and is the same as the vertical spacing on the MOTM modular synthesiser and most of our other modules The design requires plus and minus 15V supplies The power supply should be adequately regulated The current consumption is about 30mA for each rail Power is routed onto the PCB by a four way 0 156 MTA156 type connector or the special five way Synthesizers com MTA100 header You could of course wire up the board by soldering on wires directly The four pins that are used on the four way header 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 PCB has four mounting holes for M3 bolts one near each corner These are not required if you are using the two 16mm pot brackets The board size is 101 mm high x 116 mm deep The board has been laid out to accept connection to our Sock8 socket board This small board speeds up the wiring of the four sockets and reduces t
20. reless World It was used in an audio test set and produces a very good quality pink noise Some synthesisers use simpler methods but this one sounds better to my ears so why not use it U9b pins 5 6 7 provides some gain for the losses incurred within the RC network and the pink noise output is available via R52 The infra red output is heavily inspired by the Polyfusion noise module I have made only a few changes to their design since it works so well R43 and C15 provide further low pass filtering of the pink noise and prevent any unwanted switching clicks from the next stage from leaking back into our pink noise output The first half of U8a pins 1 2 3 is configured as a comparator Pin 1 the output will either be at around 14V or 14V depending on the voltages present at its two inputs Without R49 if the or non inverting input is at a higher voltage than the or inverting input the opamp s output will be positive If it is lower then the output is negative But R49 provides hysteresis This means the voltage at the inverting input must be substantially larger then the non inverting input to get the op amp to flip states This prevents the op amp s output from juddering when the two inputs are very close to each other The signal we see at the output of U8a is a rectangular one with random flips from positive to negative We will talk about R41 and the ADJ pot later The next op amp stage is an interestin
21. ter and the unstable chaos of the infra red output circuity The noise generator is Q1 and I will discuss choosing this device in more detail later in this document All devices produce noise of some sort This is caused by the random movement of charge carriers within a semiconductor or metal at temperatures above absolute zero At room temperatures things are really shaking Various devices can be utilised for their noisy behaviour Zener diodes are very noisy particularly around their reverse breakdown voltage but because of their glass enclosures tend to be affected by ambient light levels Transistors can also behave as zeners too when the base emitter junction is connected up the wrong way The voltage in which the zenering affect is seen is called the reverse breakdown potential and we normally try to avoid getting anywhere near it since it produces poor results in amplifiers and other circuits However in this project we lovingly embrace this point of the base emitter junction s V I curve and deliberately force the junction to conduct the wrong way just a little bit The emitter is made more positive than the base via R40 and the resistor thus provides the reverse emitter current The positive end of R40 is a well decoupled filtered version of the 15V supply rail Any extraneous noise on the power supplies is not wanted here and R38 C13 and C16 filter the power supply accordingly The base emitter junction of the transistor will e
22. that the integrator s output will try and follow the input signal but only as fast as the VCA will allow that capacitor to charge and discharge This produces the classic lag circuit This slowness of the integrator s output to react to the input signal gives the desired low pass effect The high pass response from a similar circuit to the low pass filter Indeed the only difference is that we take the signal from the output of the differential amplifier You may want to consider this yourself in more detail However a simple way of looking at it is to consider that this output is formed by the difference in the input signal and low pass output If the low frequencies are taken out of the input signal then you are left with only the high frequencies and thus the high pass response There is one small addition to the high pass filter and that is C14 This low value capacitance in conjunction with R39 acts to stabilise the filter output at very high frequencies It should be noted that a one pole filter creates a roll off of around 6db octave past its cut off frequency So the effect of the filter is less pronounced than ones of higher orders like Moog Ladders and so on It is not possible to make a one pole filter have gain at its cut off point so there is no chance of a resonance control or self oscillation at its cut off frequency Now let us look to the first page of the schematic This covers the noise generator the pink noise fil
Download Pdf Manuals
Related Search