[sdiy] Help with keyboard control circuit please

[tomg@efm]

What do you need me for? It seems you've done quite well on
your own..;-)

> Now, searching on the net I have found two nice circuits which are
> both very similar and do just this. One is the Two-Note Moog circuit,
> and the other is used in the NFS synth.

The NFS keyboard circut is based on Moog's. I forget which one I
think it was the Source but it was done as early as the Taurus. 

> The circuit feeding the Resistor chain is made up of two op-amps, but
> I am confused as to what exactly is going on - One Op-amp feeds into
> the input of the other via a capacitor. Is it a type of oscillator and
> the circuit is providing an Alternating Current across the chain
> The circuit providing the CV and Gate outputs has two diodes - Is this
> rectifying the AC provided or doing something else? Also, the input to
> this circuit is via a capacitor, leading me again to think that it is
> an AC signal provided.

Good guess...one opamp is a current source for the resistor chain.
The other is a 8KHz oscillator. AC is imposed on the DC. The ota 
sees the low level AC and amplifies it. The diodes change AC to DC 
and use the voltage to drive the gate circut.....or....basically what you
said...;-)

Tom