Guitar tone control

[teleman]

Hi Guys,
I have now spend the best part of summer trying to tame this tone circut to no avail!

The original circuit was exhibiting IMHO symptoms of severe saturation & or "pole /phase shift every time I tried to increase the volume control.So taking MJF's advice,I have changed the treble cont.to 100k.lin from 500k.lin,yet the problem is still there,albeit a little less!I then I reduced the cap on the Mid.cont.to 10nF from 22nF & It sounds far better now,but I can still hear some sort of peaking & drastic change in the tone,when I increase the volume half way up! Btw,it's a 10k.log pot.

I even tried replacing the TL072 with an NE5532..but the problem remains!

1.Is this due to impedance loading/mismatch of the tone circuit?

I don't have even a signal generator..& I find "Spice" beyond my ability at the moment!

Can you guys please help me out here? Thanks.

[teleman]

I've read about "Tone Scaling"...perhaps this is what I need to do or can any one spot or give any solutions to this?
Thanks.

[teleman]

Today,I disconnected the entire tone circuit and connected the input buffer to the output.Yep,the amplifier still peaks-mainly the mid freq-beyond a certain point in volume!

[Enzo]

Guitar amps by their nature are anything but flat. And try different speakers, the speaker adds quite a bit of character to the sound.

DUncan has a free downloaded "Tone Stack Calculator" that you might find interesting. It is at the bottom of the page. Very easy to use, and you get real time graphic readout of the circuit response. You can change the component values to see what effect they have.

http://www.duncanamps.com/software.html

[teleman]

Thank you Enzo for your suggestions.I made the following changes to the circuit.
1.Changed the input buffer to an inverting charge amp circuit.
2.Reduced the gain pot of the buffer to 47k lin pot.
3.Then,further fine tuned the buffer gain by reducing the "feedback cap" on the buffer.

Now the circuit is behaving as it should be.

I think I may have found the problem...I am using a Roland AC60 acoustic combo for testing this circuit.Roland has given only the signal level @ -10dBu for the piezo/guitar ch.& nothing else.I suspect that the i/p impedance of this is far lower than the standard 10M for acoustic guitar amps!

To check this I plugged a bare "piezo disc" into this i/p & it responded far better than with my pre.ie; behaved as any piezo would. Being a curious cat,I then plugged the p. into the mic ch, & it gave even even better bass response.

So I think I also have far too much signal on my pre amp...overloading the AC60!
I will definitely try my assumptions on "Duncan" calculator & will let you know if my theory is correct or not.

Oh Man...I should have taken up medicine as a hobby instead I think!!!!

[teleman]

The original circuit has the following gain specs given by the authors;
Bass +/- 15dB @30hz
Mid +/- 19dB @1k
Treble +/- 16dB @10k...this is exactly how they have given these figures!

As you can see from above,IMHO these figures are excessively high for mic/piezo preamps.So am I on the right path if I reduce these values to say +/- 6dB at the most for my purposes!