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graygem 16th June 2011 03:16 PM

Help needed for tone control circuit
 
1 Attachment(s)
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.

Attachment 227461

DF96 16th June 2011 04:17 PM

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.

graygem 16th June 2011 04:55 PM

1 Attachment(s)
Not sure of impedence.
This is the input channel.

Attachment 227469

DF96 16th June 2011 06:15 PM

OK, the 4.6k resistor at the output will probably give enough source impedance. For this type of simple attenuator you will need to keep signal levels low. Maybe you are overdriving it?

graygem 16th June 2011 06:45 PM

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

Mooly 21st June 2011 07:55 AM

Quote:

Originally Posted by graygem (Post 2608265)
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

sofaspud 21st June 2011 08:58 AM

A quick search yielded this circuit, a One-Knob Wide-Range Tone Control. Looks to be perfect for your needs.

Johno 21st June 2011 09:04 AM

Quote:

Originally Posted by graygem (Post 2608265)
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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