[sdiy] CV signals and buffering

[jhaible at t-online.de]

kpeek1 at prodigy.net schrieb:
> This is kind of basic but...
> 
> If I want to send a 1 v/oct or gate signal to three
> VCOs or two Env Gen, do I have to "buffer" the signal
> with a distribution amp or can I just "mult" the 
> signal. What is the optimum set-up? 
> 
> If I do need a buffer, what is the best design for 
> stability? I don't mind using OP275 or like op-amps to 
> achieve stability as I want accurate CV signals.
> 

This depends on the "quality" of your 1V/Oct source and
on the capacitance (cable quality and cable length) of
your patch cords.
Ideally, you need a buffer that can drive a high capacitive
load with "zero" output impedance. I have used such a 
buffer in the MOTM VC LAG for instance. If this is your
CV source, you can practically use cables of (almost) unlimited
lenght, and mult it together just as you like.

If you have a simple opamp output in your CV source (no series
resistor), you can do the same, but only with short cables.
It also depends on the type of opamp that is used - the
capacitive load it can drive without oscillation. 
(With a cheap opamp and a cheap - high-capacitive - cable
a few centimeters can be too much.)

I have seen circuits which wanted to avoid oscillation by inserting
a series resistor after the opamp output. This will slightly
attenuate the CV voltage, of course: With 1k series resistor
and 100k input resistance of the VCO, you have 1% error. Now,
this error can be perfectly compensated with a gain of 1.01
in your CV source: You can adjust it to supply a perfect
1V/Oct CV @ 100k load. Now if you try to distribute *this*
CV with a Multiple, the voltage will be too small (0.5% for
two inputs, and so on), and your VCOs will not track properly.
If you have such a CV source, you can build a distribution
amp with a gain of 1.01, and with one 1k output resistor
for *each* CV output. It works, but IMO going for a zero
Ohm / high capacitance driver from the start is more elegant.

JH.