Scratchy Pots?

Your pot is...

• either too old (resistive layer worn off; wiper doesn't connect well anymore...),
• is dirty (has dust particles under the wiper),
• has DC on the wiper,
• or... (let me know if you've experienced other sources)

For a lot of people obvious, for me something I learned recently: If you have DC voltage at the pot in your audio circuit, your pot will sound scratchy as you adjust the level. Once adjusted, no problem, but as you move the dial you hear an annoying scratchy noise.

A typical cause for the DC are defective caps that leak DC into the circuit. In my case, however, it came from an error in the schematic.

Here's the problem area in my tube overdrive circuit (see the earlier RT-1 post):

And here's the fix to the circuit:

To fix it, I added an additional cap after the pot wiper (C17, you may have to scroll to the right to see it). This reduces the effective coupling capacitance by 50%. To remedy this, I roughly doubled the input impedance of the following buffer circuit (R26 & R27).

By the way, see this very useful page about pots in general: http://www.geofex.com/article_folders/potsecrets/potscret.htm