VCO Function Generator Type B
Square/Sawtooth

The Synthasystem designs and schematics are all
Copyright z 1975, 2010, 2011 Nyle Steiner
They may not be used for profit, sale, or reproduced in any way without the express consent of Nyle Steiner

***New High Frequency***
***Tracking Modification***
HF Correction


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Function

This module produces an oscillating signal whose frequency is based on a voltage input. Typically, this is an audible tone, but this module can oscillate from well below hearing to well above. The control voltage input is typically tuned to a 1 volt/octave scale. Two waveforms are available, Sawtooth, and Square. These two outputs are available at the same time. This module has a very wide useable frequency range from well below audible as a Low Frequency Oscillator to above hearing without re-tuning. Nyle really outdid himself on this module.

Note the square wave will be exactly half the output frequency of the sawtooth.

Use

Inputs/Outputs

This module has two fundamental inputs and two outputs:
  1. VC Inputs - These inputs accept a voltage input, typically between 0 and 10 volts. This can come from any source which provides a voltage output. These are summed and fed to the exponential converter.
  2. Phase Reset - this input accepts a falling edge to reset the phase, or sync, the VCO output frequency to that of the signal at this input. A signal with a steep trailing edge works best, such as a pulse or a "ramp" type sawtooth wave.
  1. Waveform Outputs - The top row of jacks provide the waveform outputs according to the legend on the panel.
Knobs

This module has 5 knobs; 2 for output levels, 1 frequency, 1 fine frequency, and 1 v/Oct trim
  1. Output level - These are attenuating controls to set the relative output levels of the 2 waveforms.
  2. Frequency - This sets the base frequency of the oscillator.
  3. Fine Frequency - This allows a fine adjustment of the base frequency.
  4. V/Oct trim - This front panel trimmer allows more convenient calibration of the V/Oct response. It is only used when re-calibration of this response is required. It may also be located on the PCB.
Switches

There are no switches on this module.

General Connections

Hmmmmm, what good is a VCO in a synthesizer... Wow, hang on. I can think of something...

OK sorry for the bad sarcasm. VCOs are the main module for producing audio. A typical patch would connect a 1 volt per octave controller to one of the VC jacks like a keyboard. If you're Buchla, however, you won't use a keyboard :) Connect an output waveform to a filter, mixer, VCA, then to a speaker or headphones.

However, any varying voltage can be use as an input.
It's only limited by your imagination.

The frequency range of this VCO makes it very useful as an LFO (Low Frequency Oscillator). You can use the output of one VCO to modulate the other.

The Phase Reset is, in more common jargon, a sync input. Use the Saw or Pulse output of one VCO to force a second VCO to oscillate at the same frequency. This is a nice way to combine the outputs of two VCOs and have them stay in tune. However, one of the consequences is the second VCO may reset (probably will reset) before a full cycle of its wave. This will add harmonic content and will (hopefully) make the resulting output more interesting and rich. You can detune the second, third, fourth, etc. sync'd VCO intentionally to emphasize this extra harmonic content

Component Selection

Resistors

This module was originally built with carbon core, 5% resistors with one or two 1% metal film resistors. So, you have a wide range of options here. I recommend using 1% tolerance, metal film resistors everywhere, but the critical resistors are R37 and R38, input summing resistors. These should ideally be hand matched or purchased to 0.1% tolerance to insure consistent response between the inputs.

In the Saw Core, R8, R14, R3, and R1 could be high quality, low temperature coefficient, RN55E series resistors, 0.1% to give the best stability and tracking.

The thermistor in this module should be a standard 1K 3000 to 3500 ppm part. Precision resistor sells them as do other sources like the Bridechamber, Magic Smoke, and others. It just has to span a long way, so make sure your part has sufficient lead length.

On Dave Brown's ModularSynthesis website on this page, about half way down, he shows how he makes a 2k tempco from 2 SMD resistors, Digikey P68CDCT-ND 3300 PPM resistors. You could do something similar with one 1k resistor.

The input summing resistors should be matched, 0.1% if you have the patience or money to buy them at this tolerance.

Capacitors

There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.2 inch on centers. For C2, pick a high quality, low leakage, temperature stable capacitor like a Polystyrene. The tolerance is not as important as the temperature stability. If you can't find a polystyrene, use a polypropylene.

Electrolytics, on power regulation PCB, should be rated at 25 to 35 volts.

Inductors

Who put this here?

ICs

The only ICs really are the LM741 OpAmps. There are many OpAmps with much better specs, but these ones will affect the sound to some degree. If you want a true re-release of a Steiner VCO, use the LM741 or equivalent. If you aren't too worried about it, use a newer OpAmp. Just make sure it is a single OpAmp with the proper pinout. The offset null is not used.

Transistors

The original used 2N5172 NPN and 2N5138 PNP transistors. These are still available, but I could only find a "PN5138" which I think is the same transistor. In any event, you can use any standard NPN or PNP transistors and they should work.

There are three spots compatible with an SSM2210 monolithic NPN transistor pair. The only one which is critical is the exponential converter in the Sawtooth core. The other two can be any small signal NPN transistor matched or not. A matched pair won't hurt which is why I laid it out to accept an SSM2210 or other monolithic pair. If you use the 2N5172s, I'm sorry, you will have to twist them since they don't use the standard EBC pin order. It's a bit of a pain, but can be done. Other small signal NPNs use the common EBC pin order and will fit without a problem.

The exponential pair should really be matched which means the Vbe of two transistors at the same temperature in the same circuit is the same within about 2 millivolts or better.

Either you need to hand match some NPNs, or you need to use a monolithic NPN pair. The SSM2210, or LS318 are good choices, but any small signal monolithic matched pair should work. Analog Devices has a new part, the MAT12 which should work, too.

Q3 is a PNP which is placed physically close to and in thermal contact with the matched NPN pair and it should be in thermal/mechanical contact with the NPN pair and thermistor. As another option (see more detail below) you can use a THAT340 which has a matched NPN pain and an matched PNP pair on the same chip. The PCB pinout will take this chip, too.

So, with that advice and help, in discussions with Nyle, he reminded me they only matched transistors for a short while and in later modules, they weren't matched! So, if you want to be really authentic, don't match any transistors. Just solder up a couple which look like they go together :).

The NPN pair and PNP really, really do need to be in thermal/mechanical contact with each other and the tempco. For mine, I'm going to build at least one with hand matched discrete transistors since that is what I would have done in '75.

The Music From Outer Space website, on this page, has a long discussion on matching transistors with some circuits. There is also a MOOG
(TM) schematic to the right you can use. Note, the PNP matching schematic has an error. There should be a 10K resistor between the transistors like for the NPN.

Bill and Will at Dragonfly Alley, http://dragonflyalley.com/constructionTransistorMatching.htm
also have a great page on matching transistor. Thanks!

I've also put together a PCB for the Moog transistor matching circuit shown on this page and at Dragonfly Alley.

There are two other SSM2210s on the schematic. You can use them or another monolithic matched pair if you want, or even hand matched pairs. Unmatched small signal NPNs will also work fine.

THAT340

As an option, you can use a THAT340 which has an NPN and PNP monolithic pair. This saves the problem of hand matching, or trying to squish 3 transistors and a resistor together and keep them there. If you do this, DO NOT INSTALL Q3. I've warned you. If you do, you will have two transistors hooked up in parallel and I really don't know what that will do. The THAT340 can be purchased at Mouser, P/N 887-340P14-U, THAT P/N 340P14-U.

Make sure whatever configuration you use for the exponential pair and Q3 that these three transistors and the thermistor are in good mechanical and thermal contact. Use a little heatsink grease to help conduct the heat and make up for very small gaps.

Originally, the Steiner modules used a 2N5163 JFET. In later modules, this was replaced with a 2N5246 JFET due to some quality problems with the 2N5163. The 2N5246s are still available and manufactured. Other JFETs will work here, too. I've tried some NTE equivalents for the 2N5163 which seemed to work fine. They are more expensive than the 2N5246s though and the 2N5246 is "vintage" :)

Diodes

Nothing special, 1N4148s are fine.

Pots

Your choice for your panel. If you use the panel I laid out, the holes and spacing will work for the Alpha 12 and 16mm pots. You can probably use nicer BTI, Bourns, etc. 9mm pots with "pot chiclets"

Trimmers

Use good trimmers, please, really on this PCB use good trimmers. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and works well.

The original modules used single turn, carbon trimmers. You can get a Bourns model that will fit the pad pattern if you want to. They cost about as much as the nicer multi-turn pots, though. It's a Bourns 3352H series has the appropriate pin spacing/configuration. The 3329H and 3329W series are sealed versions which will work, too.

For the panel mount trimmers, I made a little PCB chiclet like the ones above to make this easier. I suggest using a Bourns adapter, p/n H-83P. It has plastic 5/16 inch threads, so the hole is 0.3125 inches in diameter. It seems to work with any standard 3.4 inch multi-turn trimmer. Insert the trimmer and push it til it snaps into place.

Another option is a Vishay part p/n 006-1-0 or 006-1-1, or 006-1-2, or 006-1-3. It depends on what type of shaft you want to have. These have a smaller, metal bushing. A 0.22 inch hole works fine. They are a bit smaller than the Bourns and about 3 times as expensive. $5.00 compared to about $1.50. If you use this one, I'd also recommend getting the Vishay trimmer adjustment tool. It's made to fit down the hole if you don't buy the adapter with a shaft. No Vishay part number. Mouser p/n 594-8T000.

The Bourns trimmers which work are the 3006 series. Easy to find.

The Vishay trimmers which work are the model 43P 3/4" 20 turn series. Easy to find.

Jacks

For the panel I laid out, a good 3.5mm or 1/8 inch jack will work. I use the Switchcraft 42A Tini-Jax true 1/8 inch jack. These are switched jacks and they work with 1/8 inch plugs and 3.5 mm plugs.

Switches

No switches on this module.

Build Notes

Soldering

I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.

Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.


Power Supply Regulation/Filtering

This PCB requires the Power/Regulation daughter board. I did it this way to allow this PCB to be parallel to the front panel which allows it to mount in shallow cabinets.

See this page for details on this module.

For the sake of the VCO, you do not need to install the extra filtering capacitors and resistors. the VCO uses only the base +12/-10 volts from the mains.

R2,R6, C4, and C8 are not needed.

Connect only the first three pins between the power PCB and VCO.

Build this PCB first and get the regulators (if you use them) trimmed to +12/-10 volts.

Some additional comments here. These modules are tested to run on +/-12 VDC. The original power supply in the Synthasystem was +12/-10 VDC due to how Nyle designed the -10 volt section of the power supply, not for any magical requirement to have -10 volts.

With that said, the power/regulation PCB has 2 voltage regulators on it which can be set to +12/-10 (or +/-12 volts) depending on your needs. If you are coming from +/-15 volts, you need both regulators and you may as well set one to -10 volts.

If you are coming from +/-12 volts, technically you don't need the regulators, but if you want, install the negative one and set it to -10 volts. When this board is not connected to the main VCO PCB, you have to have a minimal load in order for the regulators to regulate, that's the purpose of the LEDs. you can install these for fun if you want, but are not needed if you don't use the regulators.

Important... if you don't install the regulators, you have to install a jumper between pins 2 and 3 as shown on the Power/Regulation PCB or you won't get power to the VCO.


VCO

Component Placement

The V/Oct and VC3 scale trimmers need some explanation. There are two mounting locations for these two components. ONLY USE ONE LOCATION.

The edge locations are for those who (like Nyle and me) want these trimmers accessible without removing the module and will run a panel mounted trim pot. If you do this, there are a couple of options I've found. First, Vishay/Spectrol makes an adapter to turn a 3/4 inch multi-turn trimmer into a panel mount. The panel hole should be 0.22 inches in diameter.

The Bourns has a 0.3125 inch thread, so the hole should be 0.32 give or take a bit.

If you use my panel, you will get two holes, the V/Oct drilled for the Bourns trimmer adapter 0.32 in, and the VC3 Scale drilled for an Alpha 12mm or 16mm pot, 0.2813 in (7.14 mm).

The shopping cart will give you the option of having this module ship with two trimmer adapter PCBs with this module to make it easier to get wires to the trimmers if you put them on the panel. There is no extra charge for them, I just don't want to waste them.

The exponential pair and Q3 need to be mounted such that all three are in mechanical and thermal contact with the tempco. Be creative here depending on what configuration you decide to build. Use a little heat sink grease to help make sure. I often use small zip ties to hold stuff like this together.

Mounting

This PCB has four holes to allow flexible mounting configurations. The FPE Euro panel is setup to allow this PCB to be mounted parallel to the panel using some 3/4 to 1 inch (typical) standoffs. The mounting holes are connected to ground. The Power Regulation PCB will mount to this PCB using standoffs.

Calibration

There aren't a lot of trimmers for this VCO, so calibration should go fast. You will need a good volt meter and an oscilloscope.

Phase Sync

  1. First, set R29, Frequency Range, close to fully CW (clockwise) (if you don't hear a sound, turn R29 more CW until you hear something). Set R15 or R18, the V/Oct Trimmer (whichever you chose to use) to mid point.
  2. Connect an oscilloscope to the saw output.
  3. Adjust R29 until you hear/see an output. Something in the hundreds of Hertz will be fine for now.
  4. Turn R21, Phase Sync clockwise (towards +12 volts) until the spike on the trailing edge disappears, then just a bit more. (you may also have to adjust R31, sawtooth offset, here to get the waveform into a useable range). An output voltage of about +/-8.5 volts is good.
You should have an output in the +/-8 to +/-11 volt range. If you don't adjust R21 until you do and re-adjust R31 if needed.

Sawtooth Offset

  1. Next, adjust the sawtooth offset to balance the waveform around 0 VDC, R31.

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Schematic

PCB Values

PCB Names

BOM

Power-Regulation Page

Power Reg-Dist BOM
Calibration continued...

V/Oct

First Step: SETUP

  1. You need a control voltage source which will give you a selectable voltage output and a range comparable to that which you use to play the synthesizer.
  2. Turn the HF correction trimmer fully counterclockwise. That is, to ground.
  3. Connect this voltage source to a VC input (not usually VC 3)
  4. Input a known voltage like 0 or 1 volt.
  5. Set the pulse width to something like a 50% duty cycle.
  6. Connect a frequency counter to the pulse output, or if you have a good ear, connect the output to headphones or a speaker.
Second Step: THE TUNING

  1. Press a "low" key, a control voltage around 0 to 1 volt, and tune the Frequency and Fine Frequency knobs to a desired pitch. A good start is around 100 Hz.
  2. Input a voltage twice or four times as great (following a 1 volt per octave scale). I suggest one or two octaves higher.
  3. Adjust R36 (on the PCB if used) or R33 (on the panel if used) to get the proper tracking.  Clockwise on R36 will increase the spread, CCW decreases it. That is, if your low frequency is 200 Hz and your high frequency is 796 Hz, turn R36 CW. Likewise, if your low frequency is 200 Hz and your high frequency is 807 Hz, turn R36 CCW. The oscillator frequency will change when you turn R36. Don't worry about it, this is normal.
  4. Re-input the first voltage. Now, you can either a) reset the Frequency / Fine Frequency to get your original pitch and repeat, or b) just note the new low frequency and multiply it by 2 for one octave, 4 for two octaves, 8 for three octaves, etc. to get the new target pitch for the high end.
  5. It should only take 4 or 5 repetitions or so to get pretty good tracking.
You will note the tracking may be a little flat at higher frequencies. Welcome to the world of analog oscillators! Some VCOs have a high frequency trim. Nyle never added one. You can always tune for better tracking at higher frequencies and live with some low frequency error which is probably less noticeable.

However, there is a High Frequency Tracking adjustment which can be added. (See Here) If you add this modification, here are additional steps for the calibration (Also at the above link):

Third Step: HIGH FREQUENCY TRACKING

  1. Now, hit the highest note on your voltage source and see how close you are to the proper frequency (4 or 5 octaves higher is pretty good). You will almost certainly be low. Turn the HF correction trimmer clockwise. About half way is a good start. This will probably overshoot your target frequency. Go back to step 1 in the "Second Step", then repeat steps 1 thorugh 5 for the V/Oct trimming because adjusting the HF Correction trimmer will affect the V/Oct. When you get good low frequency tracking, 1 or 2 octaves at about 100 to 200 Hz, then re-do the "Third Step" section until you get the desired HF tracking.
Repeat the "Second Step" and the "Third Step" until you get good low and high end tracking.

Doing this, I was able to achieve excellent tracking over 5 octaves at least. I ended up setting the HF trimmer a little more than half way.

Frequency Range

This adjustment was not present in the original module and the original VCOs did not have a V/Oct adjustment. The tracking was set by a knob on the keyboard. This established the tuning for the master VCO. Other VCOs had a trimmer which changed the summing resistor value to match the tracking for the master VCO. Sort of what the VC VAR on  the VC3 input can do.

When building this new release, I knew that with modern controllers, there needed to be a way to tune each oscillator to a standard, not to each other. Nyle helped decide on the V/Oct tuning method and values and I decided there needed to be an adjustment for the frequency range to allow the user to set a useable frequency span. I really hated making these changes, because I didn't want to change a thing on Nyle's designs. But, since Nyle was very involved in testing and tweaking these new releases, I was placated and I'm very happy with the result.

If these modules were only going in my own system, I don't think I would have changed a thing. But since I wanted these to get out to the community and Nyle was very supportive of that idea, it was clear some modifications needed to be made.

On with the last trimmer!

  1. Input the lowest voltage you will use with the VCO. Typically 0 volts but it can be a negative voltage, too.
  2. Connect a frequency counter, scope, or some other indicator (I used my Blacet bar graph)
  3. Turn the Frequency fully counterclockwise.
  4. Turn the Fine Frequency to mid point.
  5. Adjust R52 to output the lowest pitch you want to use.
  6. Input the highest voltage you will use.
  7. Turn the Frequency knob fully clockwise.
  8. Listen to or look at the output pitch and make sure you can output the highest pitch you will want.
Turning R29 CW will increase the lowest and highest frequency you can use and CCW lowers it. You can also play around with the value of R27. Increasing it will shift the adjustable range higher and lowering it will shift it lower.

You should be able to go from fractions of a Hz, to above hearing. Understand you may not have the voltage span on your controller to get this whole range without changing the Frequency control's setting. The utility is that the VCO can be used for LFO applications and/or regular audio settings, or for annoying the neighbor's dog at the really high frequencies :) (C'mon, it's a joke...)

Panel Comments

The panel components installation is straight forward. I would suggest using connectors on the PCB and plugs on the flying wires. The panel I designed is as close to the original as I could, keeping it in a Euro/Frac compatible size. The pot holes are 0.2813 inch diameter. I don't know where the figure came from, but it is perfect for the Alpha 12 and 16 mm pots. The jacks are 0.25 inch. The mounting holes match the PCB so you can mount the PCB parallel to the panel.

Purchasing

See Price List

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NOTE: You need a 10K resistor on the PNP matching circuit comparable to the NPN circuit; between the two transisors.