Why does this sound like ass?

[FingerBlisters]

Nothing particvularly fancy. Gain control into fixed gain stage into hard clipping LEDs, then into a Marshall tone stack with adjustable slope resistor, then into a basic gain recovery stage.

Even bypassing the last stage (so after the 220n to vol 3), and it still sounds very harsh. Very blatty, like a Big Muff with the tone control turned up. The slope resistor thing works very well but turning the treble control is basically nothing but this very hard and unpleasant high end.

I did think it might be the phase of the opamps into the recovery stage, but it looks ok to my dumb eyes.

Does anything immediately jump out as being dum dum? Impedances might be an issue now that I look but that's  beyond my pay grade.

All help gratefully received.

[Rob Strand]

Does anything immediately jump out as being dum dum? Impedances might be an issue now that I look but that's  beyond my pay grade

Perhaps add a resistor in series with the cap between the output of IC1B and the clipping diodes.  Could also tweak the cap value.

Have a look at the Marshall Guvnor schematic.

As an experiment drop the level after the tone control / before the recovery stage, or, reduce the gain of the recover stage.  If the recovery stage overloads maybe that is what sounds bad as the tone control cannot shape the tone of a clipped last stage.

[marcelomd]

This kind of circuit, which looks a lot like the Marshall Guvnor, tends to be shrill, in my experience.

Suggestion: Put 2 high cut filters, 10k in series + 2n2 to gnd, twice, somewhere after the tonestack. Between tonestack and recovery stage, or between recovery and volume. Lots of Marshall-in-a-box circuits do this.

Also, Rob is right. Check if you are overdriving the recovery stage. The Guvnor is hot enough not to need one.

Last thing, you mentioned phase. This is not an issue in "linear" circuits like this. Only when you are combining parallel (parts of) circuits.

[aron]

Marshal Guv'nor:
https://www.electrosmash.com/marshall-guvnor-analysis

[FingerBlisters]

Thank you, gentlemen.

Since posting I made the second opamp stage a 1k/2k in input/feedback for a gain of 2, and adjusted the capacitors in both feedback loops to roll off around 2khz. This was an attempt to roll off all the treble I thought was at fault and hopefully stop any opamp clipping occuring, which never sounds good.

Still terrible.

Then I added a 1k/47nf RC filter to ground after the 1u and before the LEDs to shave off some high end there too. Still terrible.

I didn't consider clipping of the recovery stage but that's a good shout.

Someone mentioned to "check if the recovery stage is overloading". I'm not sure how to do that or how to find that out.

However, dropping signal after the tonestack is something I can handle, presumably a resistor in series with treble lug 2 will do the job.

Then, if that works, the recovery stage can do it's thing more happily.

As an aside, I did have this pedal hooked up without the recovery stage, so wiper of the treble went through a 1uf into volume lug 3. It was ok but only above unity when the gain was almost dimed, so lower gain stuff wasn't really an option.

[merlinb]

Do what Rob said. Stop driving the LEDs directly from the opamp output!

"add a resistor in series with the cap between the output of IC1B and the clipping diodes."

[FingerBlisters]

Do what Rob said. Stop driving the LEDs directly from the opamp output!

"add a resistor in series with the cap between the output of IC1B and the clipping diodes."

Is that not what I did?

"Then I added a 1k/47nf RC filter to ground after the 1u and before the LEDs"

1u then a 1k in series, with a cap off to ground (therefore in parallel with the leds). Or are you saying a bigger resistor is required?

[fryingpan]

Do what Rob said. Stop driving the LEDs directly from the opamp output!

"add a resistor in series with the cap between the output of IC1B and the clipping diodes."

Is that not what I did?

"Then I added a 1k/47nf RC filter to ground after the 1u and before the LEDs"

1u then a 1k in series, with a cap off to ground (therefore in parallel with the leds). Or are you saying a bigger resistor is required?

You could try with 2.2k and upwards. Basically, when "hard clipping" (I prefer to say "when you clip to ground"), the resistor value determines the hardness of the clipping. Also, keep in mind that most "cheap" opamps (RC4558 and the sort) really like a medium impedance after it, if you look at the specifications for the opamp they will state a maximum output current (for low-end opamps, it might be around +/-10mA) and they will provide figures for maximum output swing at a couple of loads (eg. RL=3k and RL=600), maybe skew towards the higher load. That said, I don't think it's the opamp "suffering" (although they will "suffer" if you just clip to ground with no series resistor).

[FingerBlisters]

Roger that. Thanks for the tip. I'll up the value tomorrow and see if that changes anything. Cheers.

[Mark Hammer]

My own advice would echo Rob's.  Clipping diodes, regardless of diode category, generally require some form of current-limiting resistor "in front" to behave in the desirable manner.

As well, remember that clipping adds harmonics of whatever one has fed the clipping circuit.  The 100k/330p pair in the feedback loop of IC1b rolls off starting just under 5khz.  You may want to up that 330p to 470p or 560p to nudge that rolloff downward and rein in some fizz.

Finally, consider changing the dual op-amp used.  I'll put in a vote for the humble LM1458.  Not normally considered a "desirable" chip, and certainly not any sort of "magic" chip, its bandwidth limitations at higher gains can be advantageous sometimes.  The two cascaded gain stages will easily exceed the headroom of the chip when gain is turned up, meaning that some of the clipping you are hearing is coming from the chip itself, rather than the diode pair.  Indeed, if you don't already have a socket in there for the chip, I'd recommend one, because the circuit provides an excellent platform for auditioning different dual op-amps, based on what they do to the signal.  You might be surprised.

[marcelomd]

I missed the missing resistor before the LEDs.

Another suggestion: Copy the Guvnor and use your tonestack. It's a known circuit that behaves well. Then you change one thing at a time until you get to your design.

1- Add the recovery stage, if you think it's needed. This should change volume only. Note the extra volume may overload whatever is next (amp, interface, etc.);
2- Change the gain of the second stage to your values;
3- Change the capacitor of the second stage;
3- etc.

It's totally possible that the circuit is behaving correctly, and the values you chose don't sound like you want :shrug: