"MXR distortion iii " Any tips for modding and improving tone

[anotherjim]

Yes, the DC coupling to the clippers means they have to return to Vref. This has the potential to cause signal ripple on vref so C7 is large to send it all down to 0v. If C7 was missing or too small, the signal would get back to pin5 of the opamp and cause positive feedback!

[Vivek]

Does connecting the clipping diodes to VREF sound different from connection g them to GND?

I would have thought that Vref is equivalent to an AC ground

And if there is no DC bias being applied to the diodes, then diode to Vref should be identical in sound to diode to ground

[Rob Strand]

Does connecting the clipping diodes to VREF sound different from connection g them to GND?

I'm thinking of the Rat. I guess the Rat has a cap on either side of the clipping diodes and that is missing here.

The Distortion III is drawn correctly.  You don't need the caps *because* the diodes and R17 (4k7) are connected to Vref.

*if* you wanted to connect the diodes to ground then you would need to add the caps.  If you add the caps then you can
play with the values to make slight changes to the tone.

If people want inspiration for values perhaps checkout the Boss SD-1 schematic.   Notice how the whole tone control section is similar to the MXR Distortion III, and not so much like the TS-9 in terms of values (ie. 10k input and feedback resistors vs 1k).  The SD-1 uses a 10n feedback cap instead of the 22n for the Distortion III.   The input filter on the SD-1 is only 18n,  the equivalent on the Distortion III is between 27n and 39n (approx) - since the Distortion III doesn't have a constant impedance.  IIRC one of the Ibanez pedals was similar as well.

[Vivek]

If people want inspiration for values perhaps checkout the Boss SD-1 schematic.   Notice how the whole tone control section is similar to the MXR Distortion III, and not so much like the TS-9 in terms of values (ie. 10k input and feedback resistors vs 1k).  The SD-1 uses a 10n feedback cap instead of the 22n for the Distortion III.   The input filter on the SD-1 is only 18n,  the equivalent on the Distortion III is between 27n and 39n (approx) - since the Distortion III doesn't have a constant impedance.  IIRC one of the Ibanez pedals was similar as well.

I feel that for tone control circuits, the ratio of components is more important than the value

I mean, if R1.C1 = R2.C2, the circuits behave similar even if R2 is very different than R1

Today I will enter the SD-1 tone control circuit into SPICE and check it. The MRX DISTORTION III seemed to only be a Mid boost rather than a real tone control. It looked like the first target for a mod (if the schematic on the net is correct).

[antonis]

I mean, if R1.C1 = R2.C2, the circuits behave similar even if R2 is very different than R1

What about noise & current limitations..??

[Vivek]

I mean, if R1.C1 = R2.C2, the circuits behave similar even if R2 is very different than R1

What about noise & current limitations..??

Yes, you are right !!!

Noise and current limitations define the boundary of acceptable component values.

Maybe even dissipated power, maximum voltage, minimum voltage, clipping  etc act as design boundaries in some applications.


Still, we have a very large playground within those limits.

[Rob Strand]

I feel that for tone control circuits, the ratio of components is more important than the value

I mean, if R1.C1 = R2.C2, the circuits behave similar even if R2 is very different than R1

Normally yes.  The main difference for the MXR Dist III is the clipping diodes interact with the tone control.
When the diode aren't clipping the input resistor is sort of 4k7+4k7 but when they clip it's more  like 4k7.

If the pot value stays the same and the other parts scale-up or down it affects the way the tone control works.
The MXR uses a W taper pot like the TS9 so it's relatively even to adjust.  However if you use a linear pot the larger
feedback resistors help even out the control.

Today I will enter the SD-1 tone control circuit into SPICE and check it. The MRX DISTORTION III seemed to only be a Mid boost rather than a real tone control. It looked like the first target for a mod (if the schematic on the net is correct).

You should be able to find the original SD-1 schematic.   

The funny thing is there's also SD-1 mods 

[Vivek]

I feel that for tone control circuits, the ratio of components is more important than the value

I mean, if R1.C1 = R2.C2, the circuits behave similar even if R2 is very different than R1

Normally yes.  The main difference for the MXR Dist III is the clipping diodes interact with the tone control.
When the diode aren't clipping the input resistor is sort of 4k7+4k7 but when they clip it's more  like 4k7.

If the pot value stays the same and the other parts scale-up or down it affects the way the tone control works.

Very interesting observation !!!!! Thank you very much !!!!

I have been trying to study :

different Clipping based on input signal frequency

and

Change in output signal harmonic content based on different clipping at different input levels.


All this is supposed to make the output more dynamic and pleasing.


I will now try to study the interaction of the diodes on the tone controls in the Distortion III