Gain Section Question in Bluesbreaker Pedal

[bushidov]

Hi Guys,

Sorry to bother you folks again with another circuit analysis question, but here it goes!

Schematic Reference


So, this is basically a Bluesbreaker clone. My questions are in regards to the op-amp that is U1B, aka the clipping section.

I believe this is an inverting op amp amplifier where R5 (10K) and R6 (220K) set the gain 220K / 10K of 22. I believe that the C5 (100nF) and R5 (10K) form a bit of a high pass filter into the feedback of the op amp cutting frequencies at around under 159 Hz (1 / (2π ⋅ 10,000 ⋅ 0.0000001) = 159.2 Hz). I get the clipping diodes are there to clip.

What I am not sure about is R7. What is it doing? Does that change the gain equation? Does it change the RC filtering? Am I stupid? (well, that goes without saying, LOL)

Anyways, just trying to get a good understanding of this circuit.

[j_flanders]

Gain of U1B is set by the input resistance, which is not just R5 but also the added series resistance of VR1. VR1 changes the gain of U1A and U1B at the same time. A lot of SS guitar amps use this arrangement.
Check the input section of the Vox Pathfinder 10 for example:


The series resistance to the diodes in U1b softens the clipping. The diodes are in the NFB of an inverting opamp. This kind of clipping is similar to clipping to ground.
If you add some series resistance the clipping characteristics are more similar to diodes in the NFB of a non-inverting opamp.
A random wave form clipping image:

At the left in red is what it would look like when clipping to ground or in the nfb of an inverting opamp.
At the right in green is what it looks like with added series resistance in the nfb of an inverting opamp or in the nfb of a non-inverting opamp (tube screamer for example)

[bushidov]

Hi J,

I was wondering about that gain pot. I just figured it was a "volume" pot going into that next op-amp gain stage. But in regards to my original question about R7, that's just to tame the clipping going into the Diodes? So, if I remove that resistor and just shunt it (essentially a 0 ohm resistor), the clipping will be harsher, but gain and RC filtering will be unaffected? If I increase the resistor value to say 68K (10 times), does it clip even less harshly?

I am just trying to make sure I understand what it is actually doing.

[Mark Hammer]

The 6k8 resistor is there to deliberately "soften" the clipping.  When I came up with the Aefea Drive, I put the principle to use by making that resistance variable, so that degrees of "hardness" (insomuch as it can be achieved with diodes in the feedback path) can be achieved.

The use of a 2+2 diode complement raises the forward voltage and conduction/clipping threshold such that seriously hard clipping isn't going to happen very much anyway.  If you want warmer or less abrasive, just stick a cap in parallel with R6.

The Gain control is clever, adjusting the gain of two op-amp stages simultaneously. Stage 1's gain is adjusted by varying the feedback resistance, while Stage 2's gain is adjusted by varying the input resistance.  Move the Gain pot in either direction and you have an impact on both.  Elegant.  Note that the circuit employs a non-inverting stage, followed by an inverting stage.  This means that the output is phase-reversed, relative to the input.  Not an issue unless you try to combine it with another parallel signal.  But also one of the reasons why splitter/mixer boxes often come with a phase-invert switch for at least one of the paths (see the recent green EHX Tri-Parallel box for an example, or the EQD Swiss Things).

[j_flanders]

So, if I remove that resistor and just shunt it (essentially a 0 ohm resistor), the clipping will be harsher, but gain and RC filtering will be unaffected? If I increase the resistor value to say 68K (10 times), does it clip even less harshly?

Yes, and yes.
I find with (much) higher series resistances, the clipping is a lot less harsh but the clipping or distortion no longer seems to be part of or 'merged' with the audio signal and starts to sound more like a buzz riding on top of your signal. But you may like that.

[Mark Hammer]

If a person wants a "harder" clipping, I'd suggest putting a 2+1 diode trio (2 in one direction and one in the other) in parallel with C7.  U1B has the capability to put out a hot enough signal to be re-clippable by the additional diodes, and C7/R9 will assure that it doesn't get too fizzy.