Tone circuit blues

[sepulchre]

Hi all!

I built this distortion that I found at:

http://www.generalguitargadgets.com/...ion/index.html

It works great except for the tone control which does nothing that I can discern. I thought the value of the capacitor was a bit low for the cutoff to be in a reasonable range so I did the math and tried some other (larger) values, even as high as 0.1 uF but still it doesn't seem to do anything at all. I checked my wiring Several times to make sure it's all correct.

Here's the circuit with the tone control section circled in red:



It looks like it should work to me, but maybe I'm missing something. Any help or info about this would be greatly appreciated.

Thanks,
Ken

[Nigel Goodwin]

It forms a low-pass filter (treble cut) in conjunction with the output impedance of the opamp. As that impedance is going to be pretty low you should try adding a resistor in the wire between the opamp and the top of the tone pot.

[tschrama]

Originally posted by Nigel Goodwin
It forms a low-pass filter (treble cut) in conjunction with the output impedance of the opamp. As that impedance is going to be pretty low you should try adding a resistor in the wire between the opamp and the top of the tone pot.

Indeed, put a 10K resistor between the time POT and the 10uF...

[sepulchre]

Yes, I know it forms a low-pass filter. The problem is - it doesn't do anything. I replaced C6 with a 0.1 uF capacitor, which should produce a cutoff frequency of around 15 Hz but still no effect at all.

I will try the 10k resistor between the 10 uF and the top of the tone pot and see if that has any effect. Even if it doesn't I'll leave it in. The circuit needs the impedance there.

Thanks,
Ken

[sepulchre]

Alright! That did the trick! I added the 10k resistor just after the 10uF cap and put a 0.047uF cap in the low pass section and it works great!

Thanks a million guys!

Ken

[unclejed613]

op amps tend to have very low output impedances, especially with feedback. you might get the impression from a data sheet that a particular op amp has a 5k output impedance. but when negative feedback is added, this figure drops to a few hundred ohms, because the negative feedback causes the op amp to compensate for the internal losses of the op amp. with a wider bandwidth op amp, this effect extends higher in the spectrum. so this circuit might have worked just fine with a RC4558, because the output impedance of a 4558 is high above 1khz. but it might not work properly with a NE5532, because the output impedance above 1khz is much lower.

[sepulchre]

Thanks Uncle Jed!

I hadn't thought about how various opamps may differ. I just thought the author accidentally left a resistor out.
I used a 741, because as the catalog says, "sometimes you don't want fidelity." lol! It works fine with the 10K in place.
I substituted a 0.047uF cap for the little 1nF in the design (at C6) for a bit more effect. Also, C4, at 50nF cuts far too high (about 3.2 kHz) - no guts at all. I put a 1uF cap (15 Hz rolloff) in its place and it sounds much better. Btw, both those values assume the Drive is all the way down. With it all the way up the 50nF rolls off at 31.5 Hz, and the 1 uF rolls off at 1.5 Hz. That's pretty low but I didn't think it would hurt anything. Seems to work okay.

[HankMcSpank]

An old thread - but I have a question related to the overall effect of the treble rolloff of the cct posted at the top of this thread.

I have a 10k resistor fitted as suggested (between the Opamp output & the top of the variable resitor)- I'm using the same capacitor as taken from my guitar's original (passive) treble roll off circuit (unfortunately, I don't have the actual capacitor value to hand)...my issue is the treble rolloff heard when used in the above active cct is minimal in comparison to the amount of treble the same capacitor rolled off in the guitar's passive circuit? (I'm using a 22k variable resistor where R6 is in that cct)


So why is it....

Passive control cct - rolloff is substantial.

Active Opamp output - rolloff is very little

What can I do to get more treble rolloff in the above type of cct?

[Nigel Goodwin]

Originally posted by HankMcSpank
An old thread - but I have a question related to the overall effect of the treble rolloff of the cct posted at the top of this thread.

I have a 10k resistor fitted as suggested (between the Opamp output & the top of the variable resitor)- I'm using the same capacitor as taken from my guitar's original (passive) treble roll off circuit (unfortunately, I don't have the actual capacitor value to hand)...my issue is the treble rolloff heard when used in the above active cct is minimal in comparison to the amount of treble the same capacitor rolled off in the guitar's passive circuit? (I'm using a 22k variable resistor where R6 is in that cct)


So why is it....

Passive control cct - rolloff is substantial.

Active Opamp output - rolloff is very little

What can I do to get more treble rolloff in the above type of cct?

Simply the values are all wrong, with only a 10K in series with the output of the opamp and a 100K pot, most of the range of the pot won't do anything - reduce the value of the pot, and increase the value of the capacitor.

[HankMcSpank]

Many thanks....upping the capacitance helped (it seems there was something line a 4nf cap in there - 47nf has a lot more impact on shunt the treble away!)

I have a 22k *linear* pot in there at the moment (ie where R6 is in the diagram referenced further up this thread), all the rolloff seems to be in the final 3rd, which makes me think I should probably drop a 10K pot in there - question is would log or linear be better (for an audibly smoother tailoff along the whole of the turn - as opposed to just the last part of the turn).

Or would a 22k log probably do me alright?