[sdiy] Help with simple single supply VCA

[Ben Barwise]

Thanks everyone for your quick replies!

The envelope is 2 LM358 op-amps with a diode and resistor to make it
decay, they definitely can't swing that low!


Harry, I'm fine if it doesn't work below a transistor drop! I'm going
for minimal part design, I have seen designs were there is a BJT
between the input signal and the bias pin with its base connected to
GND is this it?


Oliver , looking at that design trying to understand! what do you mean
its referenced to 0v? you mean the + input is at half voltage? so if I
connect the LM13700 to 0v without a diode and used this circuit , when
my envelope hits 4.5v it will switch off?

can the input being lower than the LM13700's negative rail be
dangerous? I assumed it would be ok ....um ...

A lot of issues in my design could be solved if I could get this damn
envelope to hit 0V is there any techniques to offset voltages with
something small like a BJT?

Brilliant thankyou for your help
Regards
-Ben

On 14 October 2011 14:38, Olivier Gillet <ol.gillet at gmail.com> wrote:
> Not sure if this is related to your problem but the control pin of the
> OTA is at GND + 2 diode drops, and I see two problems with generating
> the Iabc control current from the control voltage through a resistor
> as you do:
> - the "zero" of your envelopes will have to be GND + 2 diode drops
> exactly to shut the VCA, and I don't know how you plan to make sure
> that your envelope exactly rests at this voltage. Not sure how the OTA
> operates when Iabc is in the wrong direction, but this is what happens
> when you ground the input resistor.
> - the output of your envelope generator (especially if it involves
> non-rail-to-rail op-amps powered by 0-9V) might not be able to swing
> that low.
>
> I've recently converted a LM13700 based VCF-VCA to work on a single 9V
> supply, it's using this bit of circuit to generate the control current
> for the OTA:
> http://img42.imageshack.us/img42/2619/screenshot20111014at311.png
> The signal labelled "VCA" is the control signal, referenced at 0V. R1
> can be increased to limit the total current output and add a "knee" in
> the response. C2 can be adjusted to low-pass filter the control
> signal. The output of the op-amp is at "Whatever voltage ensures that
> I_gain matches the current flowing through R3 and R2".
>
> Olivier
>