[sdiy] Help with simple single supply VCA

[Ben Barwise]

Hey ,

Thanks for the book ref Tom, that's a good idea about the bias, I will try it.

Here is the schematic, Its actually all the Boss PC-2 I am doing for
my analogue drum project -
http://clacktronics.co.uk/information/drumlab (I made a syndrum too so
I'll post that soon! annoyingly it uses an ICL 8038 though)
http://www.univertron.com/SAS/manuals/Boss%20%28Roland%29/boss_pc2.gif

trying to make a simple etch yourself PC-2 without the need for the
BA662 or BA6110

I lifted the bias and both input pins of the LM13700 and I could still
hear the noise at the same volume, I can also hear the LFO when HF, so
it is probably a layout problem.

Could it be that the Darlington buffer being lifted by the LED is
causing it? would an op-amp buffer reject noise from the circuit
better?

I have 100N caps across the power supplies of all the op-amps used in
the oscillators and I have tested it by putting thick wire from the
power supply pins of the oscillators to the power supply source to see
if I could reduce the resistance but still no luck. any layout tips?
would a small resistor from the oscillator power pins to the power
supply be a good idea?

I have been sent a real PC-2 and it is completely silent (the BA662
definitely produces less white noise!)

Thanks for the help, the different ways of controlling the bias pin
are interesting



On 14 October 2011 18:10, Tom Wiltshire <tom at electricdruid.net> wrote:
> I know that it's not really recommended, but I'd have thought you can get away with using the resistor as a simple voltage-to-current convertor for the VCA bias input. I've managed ok doing this for 9V single supply VCF applications (envelope wahs) and there's a "Shaped Fuzz" in Penfold's "More Advanced Electronic Music Projects" book (Babani BP174) that uses a single supply VCA like yours with an envelope follower. The envelope output is fed via a 18K resistor direct to the bias input.
>
> Where the offset at the bias input was wrong, I've had success pulling it to a different voltage using a preset resistor across the rails, with a high value resistor (220K? 470K?) to the bias input. The current from this lets you drag the zero point to where you want. It's dirty, but it works.
>
> HTH,
> Tom
>
>
> On 14 Oct 2011, at 13:52, Ben Barwise wrote:
>
>> Hello,
>>
>> I have been trying to design a single supply VCA but I am having lots
>> of problems with the VCO (that shares the same circuit) signal
>> bleeding through. I believe the issue has been caused because OTA's
>> need the envelope to go all the way to -V to turn off? the problem is
>> I use op-amps to make the envelope and they don't swing that low. I
>> fixed it by putting an LED between the negative supply and GND to
>> raise the OTA a bit, and it worked but there is still a bit of signal
>> coming through... veery slightly , but enough to notice. The problem
>> is that I cannot see it on my scope (any idea how I could do this?
>> better probes? )  but I can hear it in an amp.
>>
>> The circuit runs off 9V with a vref supplied by an op-amp.
>>
>> http://img600.imageshack.us/img600/7051/vcasinglesupply.jpg
>>
>> These are the things I have explored
>>
>> *OTA not completely closing - I grounded the control pin .... still there.
>> *through the traces / power supply? I put .1u on the VCO's in the circuit.
>> *the LED is causing the issue? the voltage drop seems to move when I
>> put my multimeter on it
>> *can the signal bleed through if I overdrive the OTA's input? its a
>> bit loud at the moment.
>> *at one point it went away when I connected the output jacks gnd to
>> Vref - but that would short if I plug it into anything else!
>>
>> Any shared wisdom on what I could look for and try would be much appreciated!
>>
>> -Ben
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>
>