Super simple V/Hz VCO

nicolas3141 · Posted: Fri Mar 06, 2009 3:56 am Post subject: Super simple V/Hz VCO

The ongoing saga ... here is a VCO to match the ribbon controller http://electro-music.com/forum/topic-32651.html

This VCO does not pretend to be super accurate or be capable of a gazillion octaves of perfection. But it sounds pretty good and is definitely good enough to make music with. The idea is that it is simple enough to easily understand and be easily built in a couple of hours on stripboard using common components. It runs off +/-9V (ie two batteries). It expects a CV in the 0-7V range. The output is about 1V p-p which is suitable to run directly into your effects pedals, DAW, etc.

It is V/Hz which means that if you set it up to give you middle C at 2V, you should get C an octave lower at about 1V and C an octave higher at about 4V. My ribbon controller is similarly scaled so that each 1cm along the ribbon is a semitone higher or lower.

I haven't tested this stripboard layout yet - I would appreciate feedback so I can correct any issues in the drawing. My prototype is slightly different in that the PWM comparator is separate, so if anyone builds this, let me know how you get on.

If you want to try this on +/-15V it might be okay, but you may need to put in an additional protection diode for the transistor. As shown it gets subjected to about 7V of reverse voltage which it seems to handle in practice, but is actually slightly in excess of what the datasheet recommends. With higher power supply voltages the extra reverse voltage might be too much for it.

Cheers,
Nicolas

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synthmonger · Posted: Wed Mar 11, 2009 12:02 pm Post subject:

Did you try adding a fet as a switch across the cap to add sync?

nicolas3141 · Posted: Fri Mar 13, 2009 1:43 am Post subject:

Good idea, but the integrator op-amp inputs are not at virtual ground in this design, so feeding the fet is complicated slightly. I think it could work with the appropriate driver. Might have a play round when I get some time. The idea with this circuit is simplicity - if it starts to get too complicated-with-all-the-bells-and-whistles I would rather use one of the long-proven published circuits. No point in re-inventing the wheel unless there is some point of difference.

Cheers,
Nicolas

synthmonger · Posted: Fri Mar 13, 2009 2:46 am Post subject:

v-un-v · Posted: Fri Mar 13, 2009 4:52 am Post subject:

It's a shame that the comparator used to control pwm couldn't be used as a simple vca instead.

PS- great circuit btw!

nicolas3141 · Posted: Fri Mar 13, 2009 3:40 pm Post subject:

One alternative use for that pwm opamp is to get saw output instead of pwm. As per this diagram.

If you wanted to use the opamp to as a VCA that would be fine too, just require 3 or 4 additional transistors (differential pair with controllable current sink) or alternatively a fet or vactrol.

Cheers,
Nicolas

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v-un-v · Posted: Fri Mar 13, 2009 5:51 pm Post subject:

nicolas3141 · Posted: Fri Mar 13, 2009 7:05 pm Post subject:

Have a look at Rene Schmitz's design:
http://www.uni-bonn.de/~uzs159/vcaneu.png
or a slightly more refined version with more trimmers:
http://www.uni-bonn.de/~uzs159/vca3.png

nicolas3141 · Posted: Tue Apr 07, 2009 1:44 am Post subject:

Its always great to see sdiy being taken out of the back room and into the light. Especially gratifying to see ones own designs getting an outing. Here is my friend Adam Willetts playing his battery powered mini-modular at a recent gig. He is using it for all the sub-textures in this piece while a DSI Evolver provides the midified melody. The video is a bit lo-fi and has a couple of abrupt jump cuts, but you get the general idea:
http://www.youtube.com/watch?v=jNrz8zqQTxs
His mini-modular is entirely built of modules I have designed, most of which are now described in this and other threads. A couple of VCOs, a ribbon controller, two LFOs, ADSR, VCF, etc. The video gives you a wee look at how it sounds and its performability. Obviously being a modular it has a much bigger sound palette available, but switching from one patch to another is not necessarily trivial.

Cheers,
Nicolas

synthmonger · Posted: Tue Apr 07, 2009 10:13 am Post subject:

Did you post your VCF schematic?

nicolas3141 · Posted: Tue Apr 07, 2009 2:07 pm Post subject:

Not yet, but I will do soon. It is a fairly standard state variable type filter based around an LM13600, set up for V/Hz control and the lower voltage standards that I have in my battery powered rig. The resonance control allows it to oscillate reliably so that it can be used as an oscillator as well. I need to tidy up the drawings and then I will post it.

Cheers,
Nicolas

nicolas3141 · Posted: Tue May 19, 2009 4:07 am Post subject:

VCF now up http://electro-music.com/forum/post-245225.html

hway23 · Joined: Jul 23, 2008 Posts: 61 Location: ks

Can I just use another 324 to add saw shaping? Also would I need a 6v supply along with +-9v?

Thanks!

nicolas3141 · Posted: Wed Jun 03, 2009 3:06 am Post subject:

Yes any additional opamp could be used to give you the saw out. If you have another spare opamp you can build that saw shaper circuit again and take the square feed from pin 14 instead of pin 8 and you should get a triangle output that will transition to an octave up as you twist the PWM knob or feed it from an LFO, etc. Fun addition Smile

The +6V is what I use in my synth as a stable reference voltage because my +9V line fluctuates a bit. You can try just using the +9V instead and see how you get on. If you hear crap from other modules bleeding through into your VCO, then sort out a stable 5 or 6V with a 78L05 or 78L06.

Cheers,
Nicolas

nicolas3141 · Posted: Wed Jun 03, 2009 1:53 pm Post subject:

Actually if you used another quad opamp for waveshaping you could use one for saw, one for octave-up and the other two for sine. Now there's an idea.

andrewF · Posted: Wed Jul 08, 2009 9:18 pm Post subject:

Just built 2 of these, all 1 vero-board (with a mixer and headphone amp)
work great - running on +/-12V
tho a little bit of crosstalk.....no biggie Very Happy

going into the
Mega-synth

thanks Nicolas

athleos · Joined: Mar 13, 2010 Posts: 8 Location: Seattle, WA

Hello! Sorry for resurrecting an old thread as my first post! BUT, I just built this lovely little VCO and am running it with a +/- 12V PSU. It works great although, I seem to be getting a pretty nasty 60Hz hum out of the TRI output (though not out of the PWM for some reason...). Is there any way I can get rid of this? Is the diode mentioned in the first post a possible solution? I plan on using this VCO along with a ribbon to make a sort of Tannerin type synth for live use... Any help is much appreciated! Also, just out of curiosity, I've got the ribbon CW terminal connected to +12 and the CCW terminal to -12 with a 100k pot on both in series... Any thoughts on a better way of connecting the ribbon or is this good enough? This is my first DIY analog synth so any pointers are much appreciated!

[edit] mixed up my +/-

nicolas3141 · Posted: Sun Mar 14, 2010 9:16 pm Post subject:

Hi Barry

The hum is most likely a wiring or earthing issue. Is the socket shield connected directly to the power supply earth or the VCO earth or is it not connected at all? Whichever it is, try the other two options to see what happens. Double check your wiring and double check the socket itself. Faulty sockets are common source of bad connections.

But it may be a power supply issue. Are you running it from the mains on an unregulated supply? I run mine on batteries. I specified a couple of 100nF caps to bypass the +V and -V lines, but you may want to try adding 10uF or 100uF electrolytics in parallel with the little caps.

But since the PWM output is okay, I would suspect that its the wiring to the Tri socket or the socket itself that is the problem. The two outputs should not be different except in the way intended.

Oh and about the ribbon - should be 0 to +12V. I don't think we want -V going into the CV of this VCO.

Cheers,
Nicolas

athleos · Joined: Mar 13, 2010 Posts: 8 Location: Seattle, WA

Hey Nicolas, Thanks so much for the reply!! I noticed that the tri output doesn't have any hum during and for a second after a note is sounded but if no voltage is supplied to the ribbon, the tri output has that nasty hum... It is weird that the PWM out doesn't have any hum. Should the CV in be tied high when the ribbon isn't depressed? If not then I may have a ground issue somewhere. I've breadboarded the VCO twice and still the hum. It must be a voltage thing. I'm running the VCO from a regulated +/- 12V PSU that has a very very smooth output on both rails. I checked them with my oscilloscope. So... How would I build your version of the ribbon controller with a 12V bipolar PSU? Maybe I can solve some of my issues using a verified design...

nicolas3141 · Posted: Sun Mar 14, 2010 10:29 pm Post subject:

Oh I see. The problem is quite possibly that the ribbon controller is feeding hum into the CV input. The CV input on the VCO should relax to zero volts when no note is sounding. The fault may lay with the ribbon rather than the VCO. Feed the ribbon circuit from the earth (zero volts) and +12V. It should output a CV that is zero volts when not pressed, and range from about +1V to +6V when pressed. Fix the ribbon circuit first and your VCO hum might go away.

Cheers,
Nicolas

athleos · Joined: Mar 13, 2010 Posts: 8 Location: Seattle, WA

Yeah, the ribbon controller works as it should and is connected to 0V and +12V... Still humming. I wonder if maybe the Transistor is going bad? I'm using the BC557 instead of the 559 and I believe they have the same specs but maybe the 557 is getting to much load? Otherwise, I'll check over my VCO circuit again. Thanks again for your help!

nicolas3141 · Posted: Mon Mar 15, 2010 12:01 am Post subject:

Since the PWM output is okay it is very very unlikely that there is anything wrong with the VCO itself. The things you should be suspicious of are:

1. the CV input, ie the ribbon controller circuit.
- with the ribbon not pressed, connect a guitar amp to the ribbon output and listen for hum, should be zero volts and silent, but a long ribbon may need a bigger value for C3 to quiet the hum.

2. the Tri output socket and associated wiring.
- especially check the earth wiring and watch out for socket faults.

Cheers,
Nicolas

amonk · Joined: May 07, 2010 Posts: 1 Location: olympia

hi there,
I'm building this circuit, along with a bunch of other of nicolas' designs. they sound great!

i want to add fine tuning knobs to the modules.... I know this should be simple, but the couple of approaches i've tried give undesirable results... can anyone help me figure out how to do it?

ben

Sodapep · Joined: Jun 16, 2010 Posts: 1 Location: State College PA

Fantastic thread! I was asking some questions on another forum and was recommended to come and visit your schematics. I am relatively new to to building these synth type circuits, but I've been building stompboxes for a few years now.

I have three questions, if you don't mind me asking.

The first is that on the second schematic, converting the triangle out to saw, you say to tap square from pin 8, but that is not indicated as a square out on the schematic. Could this square out also be used to switch between triangle and square?

'Actually if you used another quad opamp for waveshaping you could use one for saw, one for octave-up and the other two for sine. Now there's an idea.'

Do you have a schematic for this, or could you point me towards one? I wouldn't mind having a sine wave Very Happy

Finally, I'm a bit confused as to how this hooks up to the ribbon controller schematic. I mean, I think the gate comes from the ribbon controller schematic to the ADSR, but I'm not sure if the gate or the CV is the ring, tip, or sheild. I know this has a bit to do with the other two schematics, so hopefully you don't mind me asking in here!

Thanks a lot for organizing all this information. I can't wait to get some of this on a breadboard and learn a bit. Thanks a lot!

-Matthew

anders79 · Joined: Jun 21, 2010 Posts: 21 Location: Sweden

Hello everyone!

I was just wondering if anyone has tried Nicolas V/Hz VCO as an extra VCO in an analog synth? Is it linear enough to be in tune with another VCO in a couple of octaves? Can it be used as an "expander"-VCO?
I want an extra osc in my CSY-1 monosynth, or maybe even two or three...

kind regards, Anders