Bluesbreaker overdrive... is there any hard clipping?

[Steben]

HI guys,

Never really plunged into the BB.
Looking at the schematic once more, I see soft clipping in the second stage, but a lot of hard clipping in the first stage unless the 27/33k resistors are used.
Or am I wrong here?

[GGBB]

If by "hard-clipping" you mean op-amp saturation (hitting the rails) then yes there would probably be some of that in the upper range of the gain knob. Note that with the smaller resistors a *lot* of low frequency response is removed and that is where the signal peaks typically live, so there might not be as much clipping as you might think.

[GibsonGM]

It's a little weak for my taste...maybe on a few (blues!) numbers.  Just sayin'...it doesn't feel like a 'hard clipper'...

[iainpunk]

the coolest thing about the blues breaker is how the clipping knee changes with the gain.
the clipping diodes have that 6k8 in series which makes it soft clip, instead of hard, but the level after the threshold is determined by that resistance and the input resistance. at very low gain settings, that 6k8 is not much compared to the 110k -ish resistance, giving quite hard clipping, but when you turn down that resistance, the 6k8 becomes comparatively more and more, making it clip softer.

in experimenting with the circuit, i found that the first opamp doesn't clip much with most guitars, only on the attack of the note, just a tid-bit, even on max gain, due to its low end roll off, the second gain stage has that variable hardness of clipping going on.

i like the circuit with the gain control at 4 o'clock (using a linear pot on my breadboard setup)

cheers

[MG]

It's a little weak for my taste...maybe on a few (blues!) numbers.  Just sayin'...it doesn't feel like a 'hard clipper'...

I have one. I often use a Marshall Shredmaster (badly misnamed, it's not really a shred pedal, and I'm not a shredder).
So...I grabbed the Bluesbreaker pedal instead of a Shredmaster for a jam, and it is indeed way lower gain.  But it made me work harder to get sustaining blues sounds, which is a good thing.  Enforced clarity.  But not tonally versatile like the Shredmaster.

You could get the first stage to clip a bit if you really cranked the gain, but ultimately, it's more of a 'push the front end of the amp' kinda pedal.

[Mark Hammer]

I take the stance the "hard" vs "soft" clipping is not well understood.  Much of what we attribute to "hard" clipping is really double-clipping.

Consider that when an op-amp is asked to provide guitar-signal gains of >50, using a 9V supply, with an op-amp that generally can't swing more than 3.5V above and below Vref, you're running up against, and exceeding, the headroom limits of the chip, and the chip's output is then being clipped again by diodes to ground.  So the diodes are clipping a signal that has already been clipped.  If it was the case that the op-amp was being powered by a +/-15V supply than I imagine what you'd hear would be strictly a matter of the diodes to ground, but that is rarely the case.

Contrast this with what gets called "soft" clipping, as in the case of a Tube Screamer, or a Bluesbreaker.  When the output of the op-amp reaches or exceeds the forward voltage of the diodes in the feedback loop (generally around 550-600mv), the diodes conduct, and the gain is momentarily reduced.  You can set the gain of the op-amp as high as you want, and that gain will be applied to tiny signals (e.g., a gain of 250x, applied to the 2mv "tail" of a decaying note will bring it up to 500mv), but much of the signal can't be raised any higher than the +/-600mv limit of the diodes.  So it never hits the headroom limits of the chip, and all the clipping is done via the diodes.  The series resistor alters how the diodes conduct, but still doesn't hit the headroom limits of the chip.

[Steben]

I take the stance the "hard" vs "soft" clipping is not well understood.  Much of what we attribute to "hard" clipping is really double-clipping.

Consider that when an op-amp is asked to provide guitar-signal gains of >50, using a 9V supply, with an op-amp that generally can't swing more than 3.5V above and below Vref, you're running up against, and exceeding, the headroom limits of the chip, and the chip's output is then being clipped again by diodes to ground.  So the diodes are clipping a signal that has already been clipped.  If it was the case that the op-amp was being powered by a +/-15V supply than I imagine what you'd hear would be strictly a matter of the diodes to ground, but that is rarely the case.

Contrast this with what gets called "soft" clipping, as in the case of a Tube Screamer, or a Bluesbreaker.  When the output of the op-amp reaches or exceeds the forward voltage of the diodes in the feedback loop (generally around 550-600mv), the diodes conduct, and the gain is momentarily reduced.  You can set the gain of the op-amp as high as you want, and that gain will be applied to tiny signals (e.g., a gain of 250x, applied to the 2mv "tail" of a decaying note will bring it up to 500mv), but much of the signal can't be raised any higher than the +/-600mv limit of the diodes.  So it never hits the headroom limits of the chip, and all the clipping is done via the diodes.  The series resistor alters how the diodes conduct, but still doesn't hit the headroom limits of the chip.

Hi Mark, I was looking at the first stage of the BB. Given the soft clipping character of stage 2 (soft clipping is often quoted as "some clean signal mixed in") any hitting of the rails by the first stage would be strongly present in the upper "soft clipped" portion in stage 2. At least compared to a diode to ground clipper.
First stage surely hits the rails if the lower region of impedances are used in the tail to ground section (3k3 and 4k7) instead of the 27k and 33k.

[Steben]

the coolest thing about the blues breaker is how the clipping knee changes with the gain.
the clipping diodes have that 6k8 in series which makes it soft clip, instead of hard, but the level after the threshold is determined by that resistance and the input resistance. at very low gain settings, that 6k8 is not much compared to the 110k -ish resistance, giving quite hard clipping, but when you turn down that resistance, the 6k8 becomes comparatively more and more, making it clip softer.

in experimenting with the circuit, i found that the first opamp doesn't clip much with most guitars, only on the attack of the note, just a tid-bit, even on max gain, due to its low end roll off, the second gain stage has that variable hardness of clipping going on.

i like the circuit with the gain control at 4 o'clock (using a linear pot on my breadboard setup)

cheers

Yes, that is what I mean.
One of the crucial drawbacks of soft clippers in the TS style is the fact they are designed for typical guitar signal levels. If the signal in front is too big, the "clean portion" coming through is much bigger than the treshold which leads to rapid opamp clipping again. The result is not pleasant (IMHO).
In other words: TS style stuff is made to be in the front of the chain. Any use of the circuit topology as "amp like" drive is bound to have extra limiting involved such as clipping diodes in front which reduce the max input of the TS stage, avoiding opamp clipping.

The BB on the other hand will cut off the signal in the first stage going to the second stage if the input is too big.
The versions with low gain in stage 1 will hardly get to that point. But the high gain versions will.
Just think of the use of a treble booster in front of the BB.

[iainpunk]

EDIT: wow, i just now find out that there are ''original'' blues breakers with 4k7 and 3k3 in the feedback loop of the first gain stage, i always thought the first gain stage had 27k and 33k.
[the following response was written not knowing this fact]

One of the crucial drawbacks of soft clippers in the TS style is the fact they are designed for typical guitar signal levels. If the signal in front is too big, the "clean portion" coming through is much bigger than the treshold which leads to rapid opamp clipping again. The result is not pleasant (IMHO).
In other words: TS style stuff is made to be in the front of the chain. Any use of the circuit topology as "amp like" drive is bound to have extra limiting involved such as clipping diodes in front which reduce the max input of the TS stage, avoiding opamp clipping.

The BB on the other hand will cut off the signal in the first stage going to the second stage if the input is too big.
The versions with low gain in stage 1 will hardly get to that point. But the high gain versions will.
Just think of the use of a treble booster in front of the BB.

yeah, i have guitars that passively make red LED's clip, so when using a TS style clipping circuit with that guitar, it sounds like $hit, almost as if a bad chip with cross over distortion was used. (chip crossover sounds different from diode or transistor crossover distortion, its more of a wiggle in the zero cross than the typical flat spot you find on push-pull transistors or series diodes)
when you have the gain of a BB very high it does soft clip, and if the input signal is large like that guitar mentioned before, it will possibly have the same 'problem' as the TS, but when you turn down the gain slightly, like at 4 o'clock, it clips harder and doesn't have that problem.

cheers

[GGBB]

EDIT: wow, i just now find out that there are ''original'' blues breakers with 4k7 and 3k3 in the feedback loop of the first gain stage, i always thought the first gain stage had 27k and 33k.

I thought it was the other way around - 27/33 is the first version - 4.7/3.3 was the revision.

[iainpunk]

EDIT: wow, i just now find out that there are ''original'' blues breakers with 4k7 and 3k3 in the feedback loop of the first gain stage, i always thought the first gain stage had 27k and 33k.

I thought it was the other way around - 27/33 is the first version - 4.7/3.3 was the revision.

i didn't mean that its the first, but that its a real marshall blues breaker, not a modded version or clone

cheers

[Steben]

yeah, i have guitars that passively make red LED's clip, so when using a TS style clipping circuit with that guitar, it sounds like $hit, almost as if a bad chip with cross over distortion was used. (chip crossover sounds different from diode or transistor crossover distortion, its more of a wiggle in the zero cross than the typical flat spot you find on push-pull transistors or series diodes)
when you have the gain of a BB very high it does soft clip, and if the input signal is large like that guitar mentioned before, it will possibly have the same 'problem' as the TS, but when you turn down the gain slightly, like at 4 o'clock, it clips harder and doesn't have that problem.

cheers

Yet in that case the first stage will likely clip

[Steben]

EDIT: wow, i just now find out that there are ''original'' blues breakers with 4k7 and 3k3 in the feedback loop of the first gain stage, i always thought the first gain stage had 27k and 33k.

I thought it was the other way around - 27/33 is the first version - 4.7/3.3 was the revision.

i didn't mean that its the first, but that its a real marshall blues breaker, not a modded version or clone

cheers

The small impedance version mainly boosts high frequencies IIRC.
So the conclusion is the (hard) clipping the first stage happens in the "pleasant" zone?
But this means slightly boosting the front might not be that bad....
Haven't built one yet... So can't experiment.