[sdiy] mono/poly mod, electonic basics

[Theo]

Its kinda hard with out a schematic of what you where doing.
But I would say you almost did the right thing.

The opamp is probably hooked up as a summing amplifier,
a difficult way to say mixer :)
The inverting input ( the (-) input) of this opamp works as a virtual ground
point.
So signals brought together at this point have very little influence at each
other.
You can verify this by measuring the (AC) voltage at the negative input,
it should be (almost) zero.

I said you did almost the right thing, cause I think the mod should look
like this:
input > 'top' of potmeter, 'bottom' of pot > gnd.
wiper of pot > mixing resistor > inv. input of opamp.

A log potmeter with a value between  47k to100k should be ok.
For the value of the mixing resistor you should have a look at the other
mixing
resistors (those for the VCOs) for what is approx the right value.
The VCO output may not be at line level,
so you may need a larger or smaller value for the external signal.

There is a feedback resistor between inverting input and the output of the
opamp.
The ratio between the mixing resistors and this feedback resistor set the
amplification factor for each of the signals you are mixing.
A lower value of the mixing resistor gives a larger amplification
and (you never guessed) a higher value mixing resistor lowers the
amplification
factor of the signal.

For added safety it may be wise to add protection diodes to the input.
The 2 diodes should be connected between the input and the positive
and negative powersuply, in a way that the diodes DO NOT CONDUCT.
The trick is that when a harmful high voltage (e.g. a static discharge)
enters the input the diodes guide this voltage away to the power supply.
This way the high voltage does not reach the rest of the circuit and won't
wreck your synth.
Plain 1N4148 or something will be ok for this.

Hope this helps,
Theo



From: Thom Somneevich <thom_s4 at hotmail.com>

> Hello,
>
> I just modded my Mono/Poly to accept an external input to the filter. It
> works, but I was wondering if you guys could tell me exactly why :) and
also
> if I should alter it to protect the synth and/or input.
>
> The filter uses an SSM2044. I used the datasheet, and at first thought of
> adding the input straight to the first pin, w/ a preceding pot to keep the
> signal down if needed. Lava at the Zen Mono/Poly site told me he had
already
> tried this, and it didn't work. Now, I figured it might work to put it at
> the amp which sums the 4 vco signals, and it does. I thought it would
> because I've seen it in so many schematics, but I don't really understand
> it. Why would it work before the amp but not after? At either place the
> signals are being jammed together, right? What makes that op-amp a
'mixer',
> if that is what's happening?
>
> Also, I grounded the jack, but that's it. The mod is jack-->pot-->op-amp.
Is
> there anything I should do there for further protection? It almost seems
too
> simple and good to be true.
>
> Thanks for sharing the knowledge. I'm learning slowly, and I appreciate
all
> the wit and wisdom you guys share.
>
> thom
>
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