VC-LFO issue 2 User Manual
Contents
1. timing capacitor As an audio VCO the usable range that conforms to 1 V octave to 2cents is over five octaves from approximately 32Hz to 1 2kHz For accurate musical use above this frequency we recommend that you use the Oakley VCO Sync input requires a fast rising gate exceeding 2 5V when active or at least 2V peak pulse or sawtooth not ramp waveform The VC LFO Sync input Synchronisation of oscillators can be quite confusing sometimes Indeed you will find many different methods adopted by designers to get their oscillator cores to synchronise with an incoming clock or other timing waveform Each method produces different sonic effects and although all have their uses no one method probably suits all needs I should clear up one thing first the Oakley VC LFO does not have a clock sync input In my mind a true clock sync input would lock your LFO waveform onto the incoming clock frequency or a multiple of it For example if your clock syne input was 25Hz you may expect the LFO module to produce a perfect triangle wave at a frequency of 25Hz There are cases where a modulating source of this type would be very useful especially when wanting to sync your LFOs to the tempo of a song However the Oakley VC LFO possesses no such cleverness It relies on what I call a hard reset of the core In truth most analogue VCOs and LFOs use this method but each design will use different ways of achieving this Again it is unlikely that an2. tuner or a VST tuner plug in is very useful for setting the V octave trimmer You should use a proper trimmer tool or a fine blade jeweller s screwdriver for adjusting the two multiturn trimmers Vishay and others make trimmer adjusters for less than a pound The three 6mm round trimmers need a small electrician s screwdriver OFF and SHP Plug the sine output into an amplifier and use your ears for this one Set the VC LFO to make a lowish note around 220Hz A below middle C will do Adjust SHP until the sine output sounds pure Its shouldn t be too buzzy or too hollow Then go back and adjust the OFF 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 STEP This one affects the sawtooth and ramp outputs The easiest way to set this is to use an oscilloscope but you can set it by ear if you haven t got one of these If you have a scope then monitor the sawtooth output You ll probably notice a small step or glitch halfway down the ramp part of the wave Set the VC LFO quite high to say about 1kHz Adjust STEP until the little glitch disappears It will reappear when the VC LFO is operating at high frequencies because of the nature of the circuit but this is fine for our purposes If you have no scope then you need to use a lowish frequency
3. Oakley Sound Systems SU Oakley Modular Series VC LFO Low Frequency Oscillator PCB Issue 2 User Manual V2 0 04 Tony Allgood B Eng PGCE Oakley Sound Systems CARLISLE United Kingdom FREQUENCY g OAKLEY VC LFO The suggested panel layout of the issue 2 VC LFO module This is different to the older issue 1 VC LFO panel in that we lose a switch and gain an additional pot Introduction This is the User Manual for the issue 2 VC LFO 5U module from Oakley Sound This document contains an overview of the operation of the unit and all the calibration procedures For the Builder s Guide which contains information on how to construct the module from our PCB and parts kits please visit the main project webpage at http www oakleysound com lfo 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 co Bee i The issue 2 Oakley VC LFO behind a natural finish panel The issue 2 VC LFO Module This is a versatile yet easy to use voltage controlled low frequency oscillator module that can also be used as an audio VCO over a use
4. ful range The unit features sinusoid sawtooth ramp triangle and square outputs All are available simultaneously A sync input is available to reset the waveforms to a fixed point The module features two CV inputs One is fixed at 1V octave The other CV input is controlled by a single pot on the front panel When the pot is turned right of its central position the pot acts as a normal attenuator increasing the sensitivity of the CV input to a maximum of 0 5V octave Left of centre the pot will act in inverting mode Fully anti clockwise the input will respond to 0 5V octave A dual colour LED indicates the output status Power 15V is provided to the board either by our standard Oakley 4 way header or Synthesizers com header Current draw is around 40mA Specifications The VC LFO has an operating range of lower than 0 002Hz one cycle every eight minutes to over 14kHz from the front panel controls The range can be extended with external CV inputs The maximum frequency of the VC LFO is well over 25kHz Range of Frequency control 17 octaves eg When the fine control is central the frequency pot can vary the output from 0 016Hz one cycle per minute to 1 7kHz Range of Fine Frequency control 6 octaves eg When the main frequency pot is central the fine control will vary the rate from 1Hz to 64Hz These frequencies can be altered over a limited range by the onboard trimmer or more significantly by a change in the master
5. in the next picture Again we can see that the waveform always goes negative immediately after the sync pulse has been activated This is true even if the wave was heading upwards just before the sync pulse was received Now with the sawtooth ramp and square outputs the VC LFO s sync function gets a little unusual Remember that the square wave s output is derived from the core and not from waveshaping the triangle wave When the core is reset it resets the squarewave too But not to zero volts it s more complicated than that When a sync pulse is received the square wave output always goes to 5V It stays there for one quarter of the LFO s whole cycle So if you have a frequency of 1Hz the total time for one LFO cycle is one second In this case after the sync pulse is received the square wave output will either go to or stay at its high level state for exactly 0 25 seconds It will then drop down to 5V and continue oscillating at 1Hz So what you get is apparently a 1 4 of a cycle delay in the operation of the synchronisation This is not true though what you are actually getting is the square wave responding to the 4 cycle downward ramp of the triangle wave immediately after the sync pulse is received The sawtooth output behaves again in a slightly unusual but still very predictable manner This is because it is generated from combining a chopped up version of the triangle waveform with the square wave When the sync pulse is
6. out this user manual or have a found a mistake in it then please do let me know 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 March 2010 No part of this document may be copied by whatever means without my permission
7. received the output will be immediately be reset to 2 5V It will then ramp downwards to 5V in the usual way of sawtooths However since it started its descent at 2 5V it will fly upwards to 5V after only one quarter of an LFO cycle The ramp output is simply an inverted version of the sawtooth So when it is synchronised the reset point is 2 5V and it moves upwards at the speed determined by the LFO s own frequency to 5V In audio terms it still sounds like hard synchronisation although it does have different sonic characteristics to our other VCO module However these effects should be considered when using the module as a modulation source The above picture shows the sawtooth output The sync pulse has been delivered just as the LFO s sawtooth output is falling from 5V to 5V The sync pulse resets the sawtooth output to 2 5V and then the sawtooth output falls to its usual 5V lower point and continues to oscillate as if nothing had happened I should add that all these images come from the soft oscilloscope in VAZ Modular VST plug in and are therefore distorted by the sound card s DC filtering analogue to digital converters and anti aliasing filters The real waveforms are considerably sharper and have less ringing Calibration There are five trimmers or presets as we used to call them in the UK on the PCB You do not need any special equipment to set these correctly but an oscilloscope is helpful for the STEP and a digital
8. saw output 0 5Hz or so to modulate a VCO or oscillating VCF You can use the 1 V octave input on the VCO or VCF You should hear a pee ooh sort of sound from the VCO or VCF as its frequency is swept downwards by the VC LFO s output If the STEP trimmer is set incorrectly you ll hear a little jump as the frequency moves downward Adjust the STEP trimmer so that the affect is minimised V OCT Use this to generate a perfect 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 Plug your midi CV convertor or 1 V octave keyboard into the 1V octave input of the VC LFO Play a lowish note on the keyboard then go two octaves higher Adjust V OCT until the interval is as close as you can to 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 main point is that we are setting the musical gap between the notes If you do need to alter the pitch of the VC LFO 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 ac
9. tually 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 FINE FREQ pot on the lower note This will probably require some patience and plenty of twiddling of the front panel controls as well But you will get there It should be remembered that this is a VC LFO and not a audio VCO Thus the circuit has been optimised for its low frequency operation and not keyboard scaling I reckon the design should be able to track over five octaves give or take a few cents At higher frequencies above 1 2kHz or so it will tend to go flat TUNE This trimmer sets the operating frequency of the VC LFO You can set this so that the front panel controls operate over your chosen range There is no right or wrong setting although if you have more than one VC LFO you may wish to make them both behave identically I prefer to set TUNE so that when the FREQUENCY pot is centralised the output frequency is around 1Hz one cycle per second when the FINE FREQ pot is at its minimum Final Comments I hope you enjoy using the Oakley VC LFO 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 have a comment ab
10. y one design will fulfil all expectations The Oakley VC LFO uses a triangle wave oscillator at the core of its circuitry This type of core produces a very good quality square wave for free and we use this to drive the square wave output The key thing to remember is that the square wave output is only positive when the voltage of the triangle wave is falling in value and only negative when the triangle wave is rising in voltage Valid sync pulses to the VC LFO are any fast rising positive voltage of 2 5V or more A typical gate signal that goes from OV to 5V when active is suitable As is the sawtooth or square wave output of your VCO which typically moves from 5V to 5V Ramp waveforms inverted sawtooth waveforms which rise slowly cannot be used When a valid sync pulse is received the triangle waveform is immediately taken to zero volts irrespective of where it was in its cycle A very short time later it is allowed to free run again at its own frequency again The issue 2 VC LFO module triangle output will always ramp downwards from zero volts after a sync pulse has been received as shown in the figure above The sync input frequency has no effect on the frequency of the triangle wave all it does is force the waveform to zero for a brief moment The sine wave is obtained by waveshaping the triangle wave output Thus it too follows the same pattern of resetting to zero and continuing downwards immediately afterwards We can see this
Download Pdf Manuals
Related Search