[sdiy] Help with simple single supply VCA

[Harry Bissell]

Yes, it would be the BJT (PNP) with the base 'grounded'. In your case it should be
held at .6V below your 4V5 reference. That point would need to be pretty low impedance
(high value bias resistors will not work) I did a quick sim and it looks like you could make it
work OK.

H^) harry


----- Original Message -----
From: Ben Barwise <clackjunk at gmail.com>
To: Olivier Gillet <ol.gillet at gmail.com>
Cc: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 14 Oct 2011 09:59:20 -0400 (EDT)
Subject: Re: [sdiy] Help with simple single supply VCA

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
>
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-- 
Harry Bissell & Nora Abdullah 4eva