Tube Breaker? Blues Sound Fuzz?

[Bill Mountain]

I don't know what to call it but I whipped this up on the breadboard last night and it sounds pretty alright...

I have been wanting to build with CMOS Inverters again but my batch of 4049's must be bad because the overdrive sound is...I hate to say it...digital sounding?  Does that make sense?  It's like super ultra glitchy like it was misbiased on purpose.

So anyways...not wanting to give up on Mr. Anderton's beautifully simple design I decided to use some opamp stages and clipping diodes instead.

I have always wanted to revisit the Marshall Blues Breaker circuit and I saw this as a perfect opportunity to mate the two and see what their offspring sounded like.

It is definitely open to lots of experimentation.  I just chose some standard parts/values as a proof of concept.  EQ options are endless as well.

[Mark Hammer]

I was thinking of something very similar the other day: several cascaded stages of softer clipping.

The only suggestion I'd make, looking atwhat you've drawn here, is to make the feedback cap in the first stage a little larger value, maybe 82-100pf.  While you do have a 3khz rolloff in stage 2 (thank you!), keep in mind that it is a shallow rolloff, that the diodes are adding more harmonic content, and anything less than max gain in stage 1 will have a higher (and similarly shallow) rolloff, feeding stage 2 with harmonic content that stage 2 will exaggerate.

My own preference is to rein in the top end in earlier stages, and spool it out as you progress through subsequent gain stages.  And, since:
- the two stages together will only produce a combined gain of 110x,
- the 2+2 diode combos will up the clipping threshold, and
- the 6k8 "softening resistors" will reduce the apparent clipping,

I'm going to suggest not only making the feedback cap a little higher in stage 1, but also dropping the 100k resistors in that stage to 33k.  That will get you a max gain of 31x in stage 1, and a total max gain of 310x in the overall circuit.  A 31x gain in stage 1 will get you some clipping, though not a lot.  However the modest clipping, and low treble rolloff will yield a more pleasing clip from stage 2.

I suppose if you wanted to get an even thicker overdrive tone, consider changing two of the diodes in stage 1 for either germanium or shottky, so that their total forward voltage is higher than a 1+1 pair, but less than a 2+2 silicon combo.  You probably want to be able to get some predictable clippping in stage 1, and not have to depend on diming it to do so.

[Quackzed]

another thing to try is to adjust the respective 'softness' resistors of each stage , raise the first stage resistor to say22k and leave the second at 6k8, for a more dramatic soft/dynamic to smooshed/compressed variation when playing soft to hard.... but note that a bigger resistor in series with the diodes allows more signal to go beyond the threshold of diodes and can more easily hit the rails of the opamp... you could use this to your advantage as a sort of hard clipping limit, as the softness allows your signal to get 'past' the feedback diodes limit... but i like it , 2 stages of soft clipping, i did something similar with my 'siggy' overdrive only used bjt's instead of op amps...

[Keppy]

I have been wanting to build with CMOS Inverters again but my batch of 4049's must be bad because the overdrive sound is...I hate to say it...digital sounding?  Does that make sense?  It's like super ultra glitchy like it was misbiased on purpose.

You got the 4049UBE, right? This is one of those places where the suffix matters.

http://www.diystompboxes.com/smfforum/index.php?topic=4687.msg26920#msg26920
http://www.diystompboxes.com/smfforum/index.php?topic=46791.msg344487#msg344487

[Mark Hammer]

another thing to try is to adjust the respective 'softness' resistors of each stage , raise the first stage resistor to say22k and leave the second at 6k8, for a more dramatic soft/dynamic to smooshed/compressed variation when playing soft to hard.... but note that a bigger resistor in series with the diodes allows more signal to go beyond the threshold of diodes and can more easily hit the rails of the opamp... you could use this to your advantage as a sort of hard clipping limit, as the softness allows your signal to get 'past' the feedback diodes limit... but i like it , 2 stages of soft clipping, i did something similar with my 'siggy' overdrive only used bjt's instead of op amps...

Good point.  We tend to forget that at least some of what we hear in such circuits has little to do with the diode action, and is simply what it sounds like when a chip, powered by +9v, runs out of headroom.

[Bill Mountain]

Yes UBE.

I plan to try a 3.5k low pass filter (12 or 24 dB per octave) after the distortion stage.  I find if I kill too may highs in the distortion circuit it sounds muffled.

I experimented really quickly with a 470pf in the second stage and that gave is a darker but still usable tone.

There are some great ideas in here.  Keep in mind I posted this to spark a discussion (which it has).  Nothing is set in stone.

I also want to try some series capacitance on the clipping stage to let through some lows (like a BMP).  I could also just build a BMP with double diodes and a series resistor in place of the regular anti-parallel pair.

I could put some lower voltage diodes in series with a capacitor to let through the lows through and then keep the soft clipping diodes in the circuit to limit the maximum low end.

As for EQ, this is giving me a tube overdrive kind of vibe so I think I'll experiment with a FMV stack.

Good point about the headroom.  When I plugged it in I played for a few minutes and was surprised how open and clear it sounded.  Then I looked at my bench-top power supply.  It was set at 30 volts!

I know it shouldn't make a huge difference when the diodes are involved but it did get muddier when I set it back down to 9V.  I lowered the input cap and that seemed to help clean it up a little.