[sdiy] LM13600 tri-VCO

[Henri Kovalainen]

Hello,

I've been aware of the tri VCO design in the datasheets for some time
now but as I'm currently designing an analog drum machine, I finally
prototyped it a couple of days ago.

The thing is that it would be very convenient if I could modify the design
so that the OTA on the right (which works as a comparator) would be a
normal opamp instead. This way I could use the other OTA on the chip as a
VCA.

However my attempts to do so have failed. The VCO does oscillate, but I
lose the voltage control so that the only this that really affects the
frequency is the capacitor. Altering Iabc of the OTA still causes slight
changes of frequency but not much.

Therefore I presume that there's something profoundly wrong in my thinking
and I've decided to give up and ask you guys for help =)

So if anybody knows how it should/could be done, all advice would be very
much appreciated. You can find a schematic of the circuit at:

http://www.kovalainen.fi/~hekovala/lm13600vco.gif

Thank you in advance.

| henri kovalainen   | peltokatu 19 b51   | hekovala at kovalainen.fi
| phone: 0400-683750   90100 oulu finland   hekovala at paju.oulu.fi

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