help with isolating audio paths

[DCMagnuson at aol.com]

In a message dated 12/23/00 11:52:23 AM Eastern Standard Time, 
Plutonique9 at symbiocom.com writes:

> Hey people,
>  just joined the list, though I've been on AH for a couple of months 
>  now.
>  
>  I have a question regarding isolating audio signal paths.
>  I know that a common use of diodes is for a "one-way" voltage flow, 
>  yet they don't seem to be working for my application.
>  I have modified my Yamaha CS-01 to have an External audio input to 
>  process external sounds through the synths VCF and VCA.
>  Basically I tapped an input where the CS-01's internal Oscillators 
>  get mixed into the VCF.
>  However, I've also been keen on adding a direct output for the 
>  oscillators. The problem is, I don't want the external signals 
>  getting processed through the synth to come out of the "direct out" 
>  for the oscillators, yet as they are on the same signal path, this is 
>  tricky.
>  I noticed when putting in a diode backwards, that my noise source 
>  still comes through strongly (yucky sounding though), so it seems to 
>  me that a diode may not be too useful for isolating audio signals on 
>  the same audio path, maybe someone could suggest a better method on 
>  how to approach this.
>  Thanks


You're correct, a diode is not good for this purpose.  Personally, I'd 
probably use unity gain opamp buffers.  Run the signal from your oscillators 
into the positive input on the opamp.  Connect the output of the opamp back 
to the negative input.  Anything after the opamp will be isolated from the 
signal before the opamp.  

There are probably more elegant ways of achieving this, but this is really 
simple, and will have an ultra low parts count.

Dave Magnuson