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.
My main problem is that I really don't want a Bass/Treble control. I would like a decent sounding one knob tone control.
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.
My main problem is that I really don't want a Bass/Treble control. I would like a decent sounding one knob tone control.
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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