[sdiy] 1-quadrant multiplier with 2164

cheater cheater cheater00social at gmail.com
Tue Jan 18 09:34:36 CET 2022


The only other option I see is: put the input into the VCA as bipolar, and
then, based on how open the knob is, add the right voltage to the output of
the VCA. So let's say you have 5V on the wiper, the right side of a linear
pot goes to the VCA CV input, and the left side goes to the op amp doing
offset on the output of the VCA. Also, perhaps each end of the
potentiometer should be pulled down to ground via a small resistor.

You save one op amp, and also it'll never be perfectly aligned, and very
likely more temperamental than the offset and de-offset option people
suggest, but who knows, maybe it'll sound cooler.

How do you count how many quadrants a multiplier has? What's the difference
between 1, 2, 3, 4, 5, or 8 quadrant multipliers?

On Tue, Jan 18, 2022 at 2:52 AM David G Dixon via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi Rutger,
>
> It is really easy to do what you want to do.
>
> If I understand you correctly, for an input signal of 10Vpp centred on 0V,
> you basically want the output signal to grow from a baseline of -5V, so
> that when the VCA is off, the output is -5V DC, at 50% gain, the output is
> 5Vpp from -5V to 0V, and at 100% the output is 10Vpp from -5V to +5V.
>
> Let's assume that you have built the standard linearized VCA circuit and
> that you have good 5V and -5V reference voltages available.  Presuming that
> your input resistor into the amplifying 2164 VCA is 30k, and the feedback
> resistor on the I-V converting opamp is also 30k, then all you have to do
> is this:  Sum -5V into the 2164 VCA, in parallel with the input signal,
> through a 30k resistor, and sum +5V into the I-V converter also through a
> 30k resistor.  The +5V into the I-V converter will apply a constant bias of
> -5V to the output signal.  The -5V into the VCA will counteract this bias
> in proportion to the VCA's gain, such that, then the VCA is off, the bias
> is -5V, and when the VCA is at unity gain, the bias is 0V.
>
> Easy peasy, lemon squeezy.  Schematic attached.
>
> Cheers,
> Dave Dixon
> ------------------------------
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Rutger
> Vlek via Synth-diy
> *Sent:* Monday, January 17, 2022 1:04 PM
> *To:* SDIY List
> *Subject:* [sdiy] 1-quadrant multiplier with 2164
>
> [*CAUTION:* Non-UBC Email]
> Dear all,
>
> I think i need a bit of wisdom from the 2164 guru's around here. The case
> is as follows:
> I'm in need of a 1-quadrant multiplier (VCA) that works on a bipolar
> signal. The input would be a 10Vpp triangle wave centered around 0V. The
> output should be the same with the VCA fully open, but... when closing I
> would like to reduce the level with respect to the -5V power bound, so that
> a VCA half-open would result in a triangle of half the amplitude, sitting
> between -5V and 0V and a fully closed VCA would give -5V DC.
>
> Of course I could level-shift both input and output of a 2164 bases VCA,
> but I feel it could be simpler. Would it work to connect a -5V signal to
> the ground pin of a 2164? And would that also require a shifted control
> voltage?
>
> Regards,
>
> Rutger
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