Please forgive me if this has been covered somewhere here and I missed it...
It seems to be common wisdom that changing DC levels through a pot will cause scratchy noises when the pot is turned, and that the typical solution is to block the DC path to ground with a cap at point X in the drawing.
My question is why does changing the DC level cause the noise in the first place?
TIA
It seems to be common wisdom that changing DC levels through a pot will cause scratchy noises when the pot is turned, and that the typical solution is to block the DC path to ground with a cap at point X in the drawing.
My question is why does changing the DC level cause the noise in the first place?
TIA
Attachments
When you move the wiper a change in pressure is applied to the track this allows small DC currents to fluctuate due to any unevenness in the pressure applied by the wiper on the track ergo- "scratchy noise" .
wrong point, the wiper must be DC-blocked, not the track. Thus it can't be the DC bias component...
The problem is the wiper makes intermittent contact with the track as it moves, which in a feedback circuit will slam the opamp output into the rails causing horrible transients.
Add 1µF in line with the wiper and a 47k DC bias resistor to ground on the opamp's input. Select a low impedance pot, like 5k or so - then the 47k resistor doesn't load it down too much.
Last edited:
Not wrong point, quite common in fact. Maybe not your preference, as it will allow any DC offset on the non-inverting input to pass through, but that's assuming there's no-blocking on the input, etc.
I wanted to use the simplest depiction of the situation without bringing in extra assumptions.