Prince of Tone (Pauper) Question

[Wavelength]

Any idea which diodes are used in MXR as replacement of MA856s?

Those look like 1N914 to me. Mixed reviews with those that have both the POT and MXR.

 

[benny_profane]

But why would you want to put the diodes to Vref? That puts dc across the volume control. You could do the same thing and ground then before the blocking cap and make it a hard clipper.
Ok will open mine and see put it on the bench.
Thanks,
Gordon

There’s no dc potential across the volume pot. It’s tied to Vref as well.

It’s an inverting stage. There’s no voltage potential differential at the inputs.

 

[Wavelength]

IMO, clipping elements to VRef make sense because the signal's relative "0" (ground) is VRef. VRef provides the signal's DC center reference for its ±v swings. The power supply ground is different from the signal (AC) ground reference.

Actually the AC ground is the same and remember the opamp output is already centered at Vref.

 

[Wavelength]

There’s no dc potential across the volume pot. It’s tied to Vref as well.

It’s an inverting stage. There’s no voltage potential differential at the inputs.

C14 blocks the DC from the volume control.

 

[benny_profane]

C14 blocks the DC from the volume control.

Ah, the tone and volume controls are both dc isolated. The two blocks are ac coupled and don’t have dc potential across the pots. Neither pot is tied directly to ground.

 

[Cybercow]

Actually the AC ground is the same and remember the opamp output is already centered at Vref.

But for the opamps, VRef is used as the signal (AC) ground. The opamps are being offset so the signal zero-crossing is centered. Setting the hard-clipping elements to ground will present an offset 'zero reference' to the signal, swinging the clipped signal towards ground instead of 'zero reference' (VRef). If examined with a scope, grounded hard-clipping will look different than hard-clipping to VRef.

 

[Robert]

But why would you want to put the diodes to Vref?

Lug 1 of the volume control goes to VREF, not ground, so the potential difference across the pot is near zero.

 

[ntuncer]

Those look like 1N914 to me. Mixed reviews with those that have both the POT and MXR.

Hmm interesting. But I could at least say my POT has ma858s.

 

[Robert]

C14 blocks the DC from the volume control.

C14 doesn't block the path of VREF through D1 / D2, but the volume pot is already at Vref potential and all other paths are AC coupled.

 

[giovanni]

I think the circuit would be theoretically equivalent if all Vref after C14 were ground instead, since that’s the job Vref is doing for the signal path. I wonder if there’s a reason to use Vref at all: could it be to improve the noise floor?

 

[Wavelength]

C14 doesn't block the path of VREF through D1 / D2, but the volume pot is already at Vref potential and all other paths are AC coupled.


View attachment 33664

Thanks, I missed that!
Gordon

 

[Wavelength]

I think the circuit would be theoretically equivalent if all Vref after C14 were ground instead, since that’s the job Vref is doing for the signal path. I wonder if there’s a reason to use Vref at all: could it be to improve the noise floor?

I have to agree with this. One of the reasons I missed the Vref at the pots instead of ground.

Really put the diodes to feedback or ground after C14 and then all the pots to ground. Then loose C8/C9 and go wiper out. Though not a fan of using 100K wiper to an output as that can have an effect on the next pedals response.
Thanks,
Gordon

 

[thesoundofjusticefx]

Ok. I finished tracing my King of Tone and here's what I've found.

1) The High Gain mod is simply changing out the resistor in the feedback loop of the first gain stage that connects to lug 1 of the drive pot. In the low gain it's a 1k and the high gain is 100k.
2) The input cap is 22nF instead of the 10nF in the Prince of Tone.
3) The capacitor in the first gain stage is 750pF .
4) They seem to use Carbon Film resistors in certain portions of the audio path.

Here's the schematic that I built up from tracing my unit. Hope it's helpful to anyone building there own. The carbon comp resistors are noted by CC next to the value. There's only 3 of them.

View attachment 4847

This helps a ton cuz I’m working on a hi gain side right now. But a question I have is it looks like the hi gain mod has the lowest knob setting with 100k of gain. Then with the potentiometer goes to 200k for gain. Looks like the gain starts high then goes higher rather then a low to high gain range. Is that correct? Thank you.