Cmoy volume control mod--seeking advice

If you are using a "shunted linear pot" you dont need the 100K ohm input resistor. The shunt resistor replaces it and does the same thing.

The 100Kohm resistor in the Cmoy is there to make sure that the opamp has a ground reference if the wiper of the pot lifts off of the resistive material. Without this resistor things get REALLY bad really fast if this happens.

You should also note that the input impedance of your amp will now vary WIDELY from 10Kohms to as little as 1.5Kohms. It might not matter but its worth remembering.... Some sources cant drive low impedance inputs. 

Quote:

Originally Posted by Meathands 

Wasn't aware of the impedance issues.  Is that due to using the linear pot?

It sort of is. Its really due to the 1.5Kohm shunt resistor.

When the volume is set to minimum the whole pot is between source and ground, so input impedance is 10K ohms

When the volume is set to 1/2 spin, 1/2 of the pot's impedance + (1/2pot || 1.5K)=~6Kohms

when the volume is set to max you have 10K || 1.5Kohm=~1.5Kohm.

With an "audio" pot its just the nominal impedance of the pot as normally configured

Using a linear pot without a shunt resistor is possible, but I unless you are very careful (or lucky) matching total system gain it wont work well. Even with the shunt resistor a linear pot winds up with less useful range than an analog pot. Just bear it in mind, and dont hesitate to drop the gain in half or even quarter if you cant spin the knob low enough. A non-shunted linear pot only has about 40db (perhaps a hair less) of useful control range (and a little more with the shunt resistor), where audio pots typically have 50-60db of range. Those extra ~15-20db of range make a lot of difference in systems with lots of gain. (too much IMO)