Another silly dual-stage overdrive

[Mark Hammer]

I made a Bluesbreaker clone some time back and just found it too shrill and finicky.  But I was intrigued by the nature of the gain control, as well as the use of a "softened" clipping in the second stage.  I mused about it and thought that I might be able to get a nice thick sound if I made better use of the first op-amp stage, and did some tone-shaping.  I'm a fan of double clipping anyway.

I'm really pleased with the outcome.  Just a nice thick-sounding drive, with decent sustain even at lower volumes.  I called it the "Wattbreaker", since it reminded me very much of an amp I used some 40 years ago.  I had sat in at a jam in the campus pub, and the band hosting let me plug into a lovely Hi-Watt 50W.  I was impressed with the stoutness and sturdyness of the guitar's sound through that amp.  It just seemed so muscular, even when not diming the drive.  This circuit kind of reminded me of that, although heaven knows how accurate my recollection is after 40+ years.

Two knobs deliver what it has to offer.  Given the filtering distributed throughout, I can't see the need for a Tone control, but I leave that to your tastes.  I've shown where the various rolloffs are throughout the circuit so you can see the logic of it.  There is some extra push for the mids in stage 1, and the additional gain and clipping diodes in stage 1 adds more harmonic content, but any frizz is seriously tamed by both feedback caps and the lowpass on the output.

Just a nice sensible drive that likes both neck and bridge pickups.  I experimented with sticking a back-to-back pair of schottky diodes in series, just ahead of the volume control for some crossover distortion, but it didn't really add anything useful, so I'm not showing that.  Let's keep it as a utilitarian 2-knobber that gives "stout" meat-and-potatoes drive.  Nice bottom end, low center-of-gravity.

Happy New Year.

[Vivek]

Thank you sir !

I learn a lot from your posts

[Fancy Lime]

And a happy new Year to you, as well!

Funny, I have been toying with the "Marshall pedal topology" as well recently. So I have no difficulty believing that the Wattbreaker sound the way you describe it. Really great topology for ampish sounding overdrives! I ended up somewhere quite similar yet quite different. More of an Orange kind of deal. It really is a remarkably versatile and good sounding topology. Surprised that it seems to see relatively little use in DIY circles compared to other "topological families" despite the success of the KoT and some other family members.

I was planning to release mine once I finally had the layout done but maybe I should take your post as a sign and publish the schematic right away.

Cheers,
Andy

[iainpunk]

that dual clipping idea seems interesting, how does it fair with guitars with insanely high output? i have had some disappointing results with soft clippers in combination with that guitar.
one thing tho, the first gain stage's NFB terminates at the Vb, shouldn't it terminate at ground? i feel like the low resistances would bootstrap and influence the gain via the Vb

Andy, cant wait to see your idea

cheers and Happy New Year, Iain

[Mark Hammer]

that dual clipping idea seems interesting, how does it fair with guitars with insanely high output? i have had some disappointing results with soft clippers in combination with that guitar.
one thing tho, the first gain stage's NFB terminates at the Vb, shouldn't it terminate at ground? i feel like the low resistances would bootstrap and influence the gain via the Vb

Andy, cant wait to see your idea

cheers and Happy New Year, Iain

That's what I thought, as well.  But it didn't work.  Had to tie those ground legs to Vref to get anything.  I honestly don't know what the relative output of my various guitars is, though I have an old Vantage with some double-creme Schaller pickups that might fit the bill.  I'll try it out and report back.

As for how to make the circuit handle hot pickups, I suspect the answer is to be found by reducing the value of that 20k feedback resistor, and maybe letting the 20k and 12k resistors trade places.  As shown stage 1 has a max gain of around 55x and stage 2 has a max gain of 18x, yielding a combined gain of just over 1000x (  ), so more aggressive than a Bluesbreaker but not quite into Rat territory.  Set to minimum, stage 1 has a gain of 10x and stage 2 a gain of just under 2x, yielding a combined gain of 20x.

If we switch around the 20k and 12k resistors, we get a combined max gain of just over 570x and a min gain of just under 12x.  So that ought to handle hot pickups a little better.

The basic idea of the circuit is that stage 1 provide a pre-clipped signal with suitable treble cut, that is then pushed a bit harder in stage 2, and re-clipped, again with suitable treble cut.  The stock Bluesbreaker strikes me as providing insufficient bandwidth-management until one gets to the tone control.  Maybe that's why it is so strident to my ears.  Much like a TS-9, the gain in stage 1 also loses top end as gain is increased, such that things like two-pickup "cluck" are still quite audible at low gains, but the tone thickens up when gain is cranked and bandwidth of stage 1 gets decreased.

Again, I have to give credit to the genius of the gain control in the Bluesbreaker.

Andy, do whatever makes you happy.  Me, I have no urge to protect I.P. and get my kicks from simply having something work out as intended.

[ElectricDruid]

Funny, I have been toying with the "Marshall pedal topology" as well recently.

There must be something in the water! I've also been playing with bluesbreaker/KoT-alikes. I've been working on giving a pedal programmability using MCP42xxx digital pots, so I decided to start with a nice simple four-knob drive pedal, and I needed something that only uses either 10K, 50K and 100K pots since that's all you can get in the digipot range! The bluesbreaker/KoT fit the bill pretty nicely, so that's what I've been working with. Mark, I think your version looks quite a bit better developed than mine, although in my defence I was mostly interested in the programmability and the overdrive was really only a proof-of-concept. That's my excuse for why it sounds a bit hopeless, anyway Thanks for the inspiration though - I think I might try a few tactical modifications to mine!

One side effect of the digipots was that the thing has to run at 5V (there *are* higher voltage digipots, but these aren't them) so I swapped the op-amps for MCP6002s and the whole thing is a 5V overdrive. The lack of headroom is a bit of a nightmare in a gain pedal as you can imagine, but the worst problems come with the active tone control I stuck after it. At least for the gain stage, the clipping can limit things to a reasonable level. It's *not* easy to design audio circuits for only 5V is what I'm finding...the programmability was the easy part in some ways!

[jubal81]

Funny, I have been toying with the "Marshall pedal topology" as well recently.

There must be something in the water! I've also been playing with bluesbreaker/KoT-alikes. I've been working on giving a pedal programmability using MCP42xxx digital pots, so I decided to start with a nice simple four-knob drive pedal, and I needed something that only uses either 10K, 50K and 100K pots since that's all you can get in the digipot range! The bluesbreaker/KoT fit the bill pretty nicely, so that's what I've been working with. Mark, I think your version looks quite a bit better developed than mine, although in my defence I was mostly interested in the programmability and the overdrive was really only a proof-of-concept. That's my excuse for why it sounds a bit hopeless, anyway Thanks for the inspiration though - I think I might try a few tactical modifications to mine!

One side effect of the digipots was that the thing has to run at 5V (there *are* higher voltage digipots, but these aren't them) so I swapped the op-amps for MCP6002s and the whole thing is a 5V overdrive. The lack of headroom is a bit of a nightmare in a gain pedal as you can imagine, but the worst problems come with the active tone control I stuck after it. At least for the gain stage, the clipping can limit things to a reasonable level. It's *not* easy to design audio circuits for only 5V is what I'm finding...the programmability was the easy part in some ways!

Hate to veer of on Hammer's thread (very cool design), but as far as using programmable digipots, what about putting them in a 10+ band EQ to make a programmable analog cab sim?

[iainpunk]

That's what I thought, as well.  But it didn't work.  Had to tie those ground legs to Vref to get anything.  I honestly don't know what the relative output of my various guitars is, though I have an old Vantage with some double-creme Schaller pickups that might fit the bill.  I'll try it out and report back

the problem i run in to is the ratio between the distorted signal and the clean signal that gets through after the clipping threshold, the portion of clean is so large that the distorted middle of the wave sounds more like crossover distortion than overdrive...
it has enough output to clip LED's on the initial attack and 4148's in the sustain, without needing any boost. i think leaving out that 6k8  or replacing it with a 10k trimmer, would do better with this particular guitar. (it also overdrives a phase 90 on the initial attack)

the gain control topology is genius indeed, i might steal that and put it in the wave folder fuzz i'm building.

cheers, Iain

[Fancy Lime]

....
Andy, cant wait to see your idea
...

There you go: https://www.diystompboxes.com/smfforum/index.php?topic=126139.0

...
Andy, do whatever makes you happy.  Me, I have no urge to protect I.P. and get my kicks from simply having something work out as intended.

Oh, me neither. I do this all just for fun anyway. The idea was to present it all as a package so people could go make one if they want to, without the tedium of having to make their own layout. I'm just so darn slow with those. I approach DIY stompboxology as a scientist, not an entrepreneur: knowledge shared is knowledge gained. I generally consider my own "intellectual property" to be public domain.

...
There must be something in the water! I've also been playing with bluesbreaker/KoT-alikes.
...

Hehe, nice! Strange place, the universe. Sometimes some ideas just pop up all over the place. Maybe 2021 is the year for this particular idea to be in the spotlight again.

Cheers,
Andy

[Mark Hammer]

Funny, I have been toying with the "Marshall pedal topology" as well recently.

There must be something in the water! I've also been playing with bluesbreaker/KoT-alikes. I've been working on giving a pedal programmability using MCP42xxx digital pots, so I decided to start with a nice simple four-knob drive pedal, and I needed something that only uses either 10K, 50K and 100K pots since that's all you can get in the digipot range! The bluesbreaker/KoT fit the bill pretty nicely, so that's what I've been working with. Mark, I think your version looks quite a bit better developed than mine, although in my defence I was mostly interested in the programmability and the overdrive was really only a proof-of-concept. That's my excuse for why it sounds a bit hopeless, anyway Thanks for the inspiration though - I think I might try a few tactical modifications to mine!

One side effect of the digipots was that the thing has to run at 5V (there *are* higher voltage digipots, but these aren't them) so I swapped the op-amps for MCP6002s and the whole thing is a 5V overdrive. The lack of headroom is a bit of a nightmare in a gain pedal as you can imagine, but the worst problems come with the active tone control I stuck after it. At least for the gain stage, the clipping can limit things to a reasonable level. It's *not* easy to design audio circuits for only 5V is what I'm finding...the programmability was the easy part in some ways!

Could be worse.  Could be 3.3V.

[MikeA]

that dual clipping idea seems interesting, how does it fair with guitars with insanely high output? i have had some disappointing results with soft clippers in combination with that guitar.

When I run this in LTSpice, I notice two things that are unusual for overdrives (granted that simulations are not reality!)
1. The output wave shape doesn't vary much with the input level, and is only mildly sensitive to the Gain knob setting.  Pretty much the same waveform out with 100 mV to 5V in, so I would predict that this layout sounds about the same with hot pickups.  It's a softly rounded symmetrical clip that reminds me of the Ibanez MT10.  Clip increases just a bit at higher Gain settings, but not much.
2. The frequency response doesn't vary with the Gain setting.  All that changes is the output level.  This, along with the voicing, might contribute to the ability to work without a tone control.
Nicely done, Mark. 

[Mark Hammer]

that dual clipping idea seems interesting, how does it fair with guitars with insanely high output? i have had some disappointing results with soft clippers in combination with that guitar.

When I run this in LTSpice, I notice two things that are unusual for overdrives (granted that simulations are not reality!)
1. The output wave shape doesn't vary much with the input level, and is only mildly sensitive to the Gain knob setting.  Pretty much the same waveform out with 100 mV to 5V in, so I would predict that this layout sounds about the same with hot pickups.  It's a softly rounded symmetrical clip that reminds me of the Ibanez MT10.  Clip increases just a bit at higher Gain settings, but not much.
2. The frequency response doesn't vary with the Gain setting.  All that changes is the output level.  This, along with the voicing, might contribute to the ability to work without a tone control.
Nicely done, Mark.

It does and it doesn't.  Certainly one expects more harmonic content to be generated as gain and clipping is increased.  However the 680pf cap in stage 1 "pulls back" on that harmonic content as the gain is increased, yielding a fairly consistent amount of harmonic content across gain settings, and the subsequent filtering supports that mission.  At least that was part of the plan.  I imagine that if you measured bandwidth at the output of stage 1, there probably would be some variation in frequency response with gain changes.  Not gobs of it, but some.

Thanks for the appreciation, but a bigger thanks for taking the extra steps to sim the circuit.  Much obliged.

I like the reference to the MT10.  I received my shipment of five CA3260 chips from China a few weeks ago and put one into an MT10 clone I made.  Dare I try one out here?

[Vivek]

When I run this in LTSpice,

I was thinking of entering this into LTSPICE too

But seeing that you already did,

Could you share your file please ?

[11-90-an]

Happy new year (which has since aged around 5 days, but I digress...)

Thanks for sharing this ckt, MH. Did it on breadboard and sounds lovely. Probably will make a quick perf layout and build it soon...

A quick mod, as I find myself looking for more treble (my taste), so replacing the 10k resistor on the output with a 4k7 in series with a 10k pot gives a tad bit of treble control without too much hiss, etc. This moves the LPF corner frequency from around 1kHz to 3.8kHz

As I said before, great circuit and great sound, thanks for sharing! 

[bluebunny]

We've just been locked down again, so what better than an interesting new overdrive to build?  Thanks Mark.  HNY.

[Mark Hammer]

Seeing as it is fundamentally an adaptation/perversion of the basic Bluesbreaker circuit, I'm curious as to how may other Bluesbreaker-derivatives there are out there.  Certainly not as many as there are Tube Screamer derivatives, but between Analogman (KoT) and JHS (Morning Glory) that can't be the entire spectrum.

[MikeA]

I like the reference to the MT10.  I received my shipment of five CA3260 chips from China a few weeks ago and put one into an MT10 clone I made.  Dare I try one out here?

I think you can just drop one in, the bias should be adequate and everything else looks like it will work.

I was thinking of entering this into LTSPICE too

But seeing that you already did,

Could you share your file please ?

  Sure, I'm thinking I can just copy and paste the contents of the .asc file as text into a message?  Or is there a more elegant way to share an LTSpice file here?   

[PRR]

...I'm thinking I can just copy and paste the contents of the .asc file as text into a message?

Yes, the ASC file seems to be 99% sufficient (unlike my other simmer which needs multiple files).

Use a CODE {#} box here --- it keeps it neat and easy to Select and copy to clipboard.

Code: [Select]

Version 4
SHEET 1 880 680
WIRE 128 -48 -48 -48
WIRE 128 80 128 -48
WIRE -48 96 -48 -48
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WIRE -48 272 -48 176
WIRE 32 272 32 128
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FLAG 96 320 0
SYMBOL Misc\\battery -48 80 R0
WINDOW 123 0 0 Left 2
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SYMATTR InstName V1
SYMATTR Value 200
SYMATTR SpiceLine Rser=0.01
SYMBOL Misc\\triode 128 128 R0
SYMATTR InstName U1
TEXT -82 232 Left 2 !.op

It is possible/likely that some non-generic parts will need their bits copied-in from a library. Above I have a "misc\\triode" which may not be stock on your system.

[MikeA]

Thanks Paul.  Mark's Wattbreaker V1 (LTSpice .asc file in text form), does not include detail on power section from schematic.

Code: [Select]

Version 4
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[MikeA]

Seeing as it is fundamentally an adaptation/perversion of the basic Bluesbreaker circuit, I'm curious as to how may other Bluesbreaker-derivatives there are out there.  Certainly not as many as there are Tube Screamer derivatives, but between Analogman (KoT) and JHS (Morning Glory) that can't be the entire spectrum.

In addition to these, I'm aware of the Wampler Pantheon and the Heavy Hand side (left half) of King Tone's The Duellist as derivative of Bluesbreakers.  Kevin had a nice Bluesbreaker analysis on the Aion website, but it's not there on his new site.