[sdiy] mono/poly mod, electonic basics

[Dave Magnuson]

At 10:25 PM 11/19/01 -0600, Thom Somneevich wrote:
>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.


With my SH-101, I added external ins as well.  I did this:  

Input has ~10K resistor to ground.  I then sent the input through a DC
blocking cap to the pot.  Wiper of the pot goes through a series resistor,
to the mixing point.  

The series resistor was found by experimentation... I used a value that
made the external level comparable with the internal signal levels (with a
little extra oomph so I can overdrive it, if needed)

The DC blocking cap might not be needed, but I'm paranoid

Dave

Resonant Frequency:
resfreq at hoohahrecords.com
http://www.hoohahrecords.com/resfreq/index.html