[Alex Pogossov]

"DaveC" wrote in message
...
I'd like to add a simple op amp-based tone control circuit to my preamp
for
desktop speakers & sub that I'm modifying from stock.

This is the simplest I found:

http://www.simplecircuitdiagram.com/2010/02/03/single-op-amp-tone-control/

(I have +/- supplies so I will be eliminating all coupling caps and
changing
Vcc/2 to ground.)

What are the positives and negatives to this design?

Better circuit (yet simple)?

Thanks.

This circuit, "mechanically" lifted from the tube era into op-amp
implementation, has a flaw:
If the treble control is set to maximum, then its gain is not limited at
+20dB at 10...20kHz, but keeps rising as far as the gain-bandwidth product
of the op-amp allows. Input impedance goes down accordingly. It might result
in Hf oscillations in the whole audio chain, even to burning the speakers
out if say you leave input unconnected and close to the speaker cable.

To fix the (potential) problem:
- insert 470R in series with the input 4.7uF capacitor or insert 470R
between the wiper of the treble control and the inverting input of the
op-amp;
- throw a 47pF cap from the op-amp output to the op-amp inverting input.