I have this circuit for a tone control with AGC. The problem is the tone doesn't sound good and has little range before distorting. I would like to have a single knob tone in place of the bass/treble control. Any suggestions is much appreciated. So far, everything I've experimented with doesn't work with this op amp configuration.
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As you are feeding the input signal straight to a pair of back-to-back diodes (b-e on the two BJTs) the distortion will be very level-dependent. What is your source impedance? For this circuit to work well you need a highish source impedance (at least a few k), and sufficient gain in the opamp that the input signal is down in the mV region.
Perhaps look at "passive tone controls" and adapt either the bass or treble section to the area you are looking to correct.
The James-Baxandall Passive Tone-Control Network
A quick search yielded this circuit, a One-Knob Wide-Range Tone Control. Looks to be perfect for your needs.
One knob or two, it seems to me your "tone control with AGC" circuit is suitable for a short wave radio receiver and little else. AGC by definition introduces distortions and is a bad idea. Also, it does not help using a circuit with a unipolar power supply (+12V and Gnd). Better to use +/-12V and get rid of DC blocking capacitors in the signal path.
This forum has many ideas for excellent but simple pre-amps, including the Baxandall that Mooly mentions. I use Baxandall's op amp feedback config with the LME49720 op amp and I am very happy. To be honest, now that I have got the bugs out of the overall frequency response, both tone knobs are pretty much unused.
Cheers
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