# HM2 / Promethium distortion mod - bypass distortion?



## mnemonic

Hi everyone,

I have just got a Promethium distortion pcb in the mail, I've waited long enough for Boss to make a Waza HM2, so I'll just make one myself.

I was thinking of also making a tagboard HM2 EQ only for use with other distortions, or just for some 'HM2 flavor' without the clipping, but then I thought, shouldn't it be possible to just bypass  the distortion section of the pedal with a SPST switch? I've (poorly) drawn over the schematic from the build docs in mspaint, just jumping from the input buffer to after all the clipping, would this work?  Or do I need to use a larger switch and lift connections in the distortion circuit also?

Thanks


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## BurntFingers

You'd need to put the switch earlier, between the transistor and the 47n cap, then have one lug to the circuit and one lug as bypass to where you have it.

I'd be interested to see if there were noise etc poking through because all the circuit is still on in the enclosure, just not engaged, but it's still getting power and doing it's thing, but rudimentarily I think your plan would work.

You could make it a pot too, for some blend action.


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## Danbieranowski

If you don’t want to have to roll your own for the EQ part, I know Kurt Ballou makes a pedal of just the EQ section. You can pick up the build kit for it at Small Bear and the PCB through God City Instruments.


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## mnemonic

BurntFingers said:


> You'd need to put the switch earlier, between the transistor and the 47n cap, then have one lug to the circuit and one lug as bypass to where you have it.
> 
> I'd be interested to see if there were noise etc poking through because all the circuit is still on in the enclosure, just not engaged, but it's still getting power and doing it's thing, but rudimentarily I think your plan would work.
> 
> You could make it a pot too, for some blend action.



so I should ‘intercept’ the circuit before the Q1 transistor (I assume input buffer)?

good point about using an spst switch so I lift the connection going into the distortion part of the circuit.


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## BurntFingers

You could probably splice it before c1 and after c12.


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## mnemonic

BurntFingers said:


> You could probably splice it before c1 and after c12.



cool, I will give that a try, may start this one this weekend.


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## mnemonic

I got all my parts ready, and I realised I forgot to order two of the transistors needed for this one (2N5457 and 2N3906) 

I have a spare J201 that I believe would work in place of the 2N5457 (a google search shows some people saying that in that position any n-channel jfet should work), but I have no other PNP transistors to sub for the 2N3906.  I ordered some so completion will need to wait until those arrive.

in the meantime i have wired everything else up, and put sockets for the missing transistors.

I have done it like this, with a DPDT switch, hopefully it works!





unfortunately MS paint is the only software I have to edit pictures. 

The color codes are the color of wire I used for each connection, since I put the switch under the board, between the knobs (a decision i may come to regret).


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## Devoureddeth

Wouldn't you eliminate noise by grounding the distortion portion using a switch?


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## benny_profane

If you’re using a DPDT, definitely ground the input/output of the distortion block. Also, I’d put the switch _after _C1 rather than before so you maintain a DC-blocking capacitor in either arrangement (i.e., with and without distortion). The positioning after C12 is fine as-is.


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## mnemonic

benny_profane said:


> If you’re using a DPDT, definitely ground the input/output of the distortion block. Also, I’d put the switch _after _C1 rather than before so you maintain a DC-blocking capacitor in either arrangement (i.e., with and without distortion). The positioning after C12 is fine as-is.


This is a good call I found out the hard way.

i noticed my missing transistors are in the distortion stage so I finished wiring in the footswitch and gave it a try, no noise in ‘normal’ mode which is expected, but in EQ-only mode, very sputtery. I added a cap after the 10k resistor on the input, (an odd value, like 82nf, just what I had since I ran out of 47nf’s), now it works great.

Very fun before a tight modern distortion sound. Like a more modern HM2, since it’s got the same EQ curve, just a different distortion character.

Cant wait for my correct transistors to arrive now.


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## mnemonic

I’ve never been that good with wire management. At least it’s not veroboard





as we all know, knowing what the knob does is not necessary with an HM2.


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## benny_profane

Looking good! Glad it worked out for you. Did you end up grounding the distortion circuit at all (assuming you've received the transistors for that circuit block)?


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## mnemonic

benny_profane said:


> Looking good! Glad it worked out for you. Did you end up grounding the distortion circuit at all (assuming you've received the transistors for that circuit block)?



I did receive the transistors yesterday and it’s now fully up and running.

I thought I may need to ground the distortion circuit when bypassed but I don’t notice any additional noise when it is bypassed so I just left it floating. I would need to pull everything out of the enclosure to get to the switch so really I’m just being lazy.

despite how high gain the distortion side is, I guess there isn’t much bleed-over there.


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## Devoureddeth

Would anybody be interested in some deluxe promethium mods? I will have to trace them out but it might be worth sharing.


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## benny_profane

Devoureddeth said:


> Would anybody be interested in some deluxe promethium mods? I will have to trace them out but it might be worth sharing.


Sure. What are they?


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## Devoureddeth

It is a deluxe version of a boutique HM-2 I will have to get it from a friend.


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## Nostradoomus

Is it a lone wolf audio THE FIRST AND BEST AND ONLY BOOTEEK HM2 CLONE? I hope so, fuck that guy haha


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## BurntFingers

Nostradoomus said:


> Is it a lone wolf audio THE FIRST AND BEST AND ONLY BOOTEEK HM2 CLONE? I hope so, fuck that guy haha



I'd never heard of them before today but man do they sound pretentious.


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## daeg

What a strange design. I see asymmetrical soft clipping, then crossover distortion, then symmetrical hard clipping. A PNP BJT amplifier (Q3) into a non-inverting OpAmp (IC2) for what reason?

Never have seen a distortion control like that either. Looks like it cuts input signal to the left and increases gain to the right, to provide a wider sweep than you'd get from a standard control.


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## benny_profane

I think the crossover distortion is just a consequence of implementing the rudimentary noise gate with the diodes. With everything else destroying the sound wave, that’s not going to be the most present distortion artifact.

Also, this circuit has gain stages all over the place. Most likely trying to keep noise low (evidenced by the gate block too) while keeping the op amp stable. At least that’s my guess.


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## daeg

benny_profane said:


> I think the crossover distortion is just a consequence of implementing the rudimentary noise gate with the diodes. With everything else destroying the sound wave, that’s not going to be the most present distortion artifact.
> 
> Also, this circuit has gain stages all over the place. Most likely trying to keep noise low (evidenced by the gate block too) while keeping the op amp stable. At least that’s my guess.



Smart man. A problem with that design is crossover distortion will increase and become more apparent as you turn the gain now (amplitude is lower, so more sine gets chopped off by the diode forward voltage + less harmonics mess to cover it up).


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## daeg

daeg said:


> Smart man. A problem with that design is crossover distortion will increase and become more apparent as you turn the gain now (amplitude is lower, so more sine gets chopped off by the diode forward voltage + less harmonics mess to cover it up).



This is perhaps one of the many reasons the pedal is only used in 'full-speed-ahead' mode with the gain control dimed.


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## benny_profane

daeg said:


> This is perhaps one of the many reasons the pedal is only used in 'full-speed-ahead' mode with the gain control dimed.


Oh that’s interesting, I hadn’t thought of that implication. Very good point. With the soft and hard clipping, how present is the crossover distortion at lower gain levels? The gate will cut anything lower than ~300mV and the threshold for hard clipping will be twice that. The soft clipping is a part of that crazy gain stage with the BJT in the feedback path...without doing the math I can’t easily see how that’s coming into play.


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## Nostradoomus

Listen to On The Turning Away by Pink Floyd, that would be an HM-2 on the solo...they sound awesome haha


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## benny_profane

Nostradoomus said:


> Listen to On The Turning Away by Pink Floyd, that would be an HM-2 on the solo...they sound awesome haha


Oh I’m familiar with the pedal...just curious about how all the stages come together and what each adds.


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## daeg

benny_profane said:


> The soft clipping is a part of that crazy gain stage with the BJT in the feedback path...without doing the math I can’t easily see how that’s coming into play.



It only has the _appearance_ of a feedback path. If you look closer, you'll notice that the wiper of Dist pot is grounded. No Feedback.

Funny that this came up. PRR over at DIYSB corrected some of us in his typical terse way when we were discussing the 'Nature' pot of the in the BJF Honey Bee. When I first looked at that HM-2 schematic, I was thinking Q3 was to invert the signal and create NFB for the Dist pot, which is connected to the Non-Inverting OpAmp. I further speculated that maybe they used a Non-Inverting OpAmp because when it clips (meaning the diodes are turned on) the Gain doesn't go lower than 1 as it would with a Non-Inverting stage. That narrative went completely out the window once I realized there was no NFB.


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## Devoureddeth

Nostradoomus said:


> Listen to On The Turning Away by Pink Floyd, that would be an HM-2 on the solo...they sound awesome haha


Such a bassy solo, probably the best sounding HM-2.


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## Nostradoomus

Devoureddeth said:


> Such a bassy solo, probably the best sounding HM-2.



It’s such a great lead sound...love playing it with a harmonic energizer too


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## yazooligan

Devoureddeth said:


> Would anybody be interested in some deluxe promethium mods? I will have to trace them out but it might be worth sharing.


Yes please.


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## yazooligan

mnemonic said:


> I’ve never been that good with wire management. At least it’s not veroboard


This is such a great idea. It makes it a much more versatile pedal if you ask me. I'm about to do a Promethium build myself and I'd love to get your insight before attempting this same mod. Thanks!


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## blueimpala

Devoureddeth said:


> Would anybody be interested in some deluxe promethium mods? I will have to trace them out but it might be worth sharing.


Double yes please. That would be awesome!


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## mnemonic

yazooligan said:


> This is such a great idea. It makes it a much more versatile pedal if you ask me. I'm about to do a Promethium build myself and I'd love to get your insight before attempting this same mod. Thanks!



not to much to it really, if you want to do this same mod,I would just follow the schematic changes in post #7, but do it after C1 rather than before. Or if you do it before C1 like in the picture, just add a 47nf off the switch (Where it’s marked orange).

mine is technically a ‘bodge’ since I added a cap off R1 since I didn’t want to pull the board again to do it right. So technically I have two caps in series when in normal mode.

you can see in the pics where I had only soldered on one leg of C1 and c12 so I could add in the switch.


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## yazooligan

Nostradoomus said:


> Is it a lone wolf audio THE FIRST AND BEST AND ONLY BOOTEEK HM2 CLONE? I hope so, fuck that guy haha



For the prices he’s asking I’d expect a little more visual flair to the enclosures. They’re kind of uninspired if you ask me. There’s a video of him at 2017 summer NAMM showing off a bunch of his pedals, but no actual playing, and it’s like, uh, you do want to sell those right? Might not hurt for people to hear them. Maybe he’s a pair of camo cargo shorts come to life...


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## Nostradoomus

Definitely. For a 400 dollar HM-2 clone you should at least drill the holes straight too, it’s baffling.


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## benny_profane

Is that the HM-2 derivative with all the weird add-ons that oscillate and are unusable? I remember seeing one of them that just made me think, “wow, this couldn’t be more poorly designed.”


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## ANGRY_SWEDE

Devoureddeth said:


> Would anybody be interested in some deluxe promethium mods? I will have to trace them out but it might be worth sharing.


Yes, I would greatly appreciate this!


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