# HM-2 style circuit questions



## Stelloh42 (Aug 10, 2020)

Planning on building a promethium PCB up into an HM-2 style distortion. And I'm looking for some advice on how to replicate my fav HM-2 sound. I have an older Behringer HM300 Heavy Metal that just sounds uglier, more ragged and almost fuzz-like on the lower end than anything I have built so far.

Of the couple of HM-2 style circuits that I have built so far, they use 2n5457 jfets, and a couple of different opamps (usually JRC4558 or TL072) and while they have that characteristic 1k honk that the HM-2 is known for, they never get quite as ugly as this one Behringer that I have. I've tried other HM300's and they don't have the same quality.

the majority of the board on this one particular pedal that I have is smd but, from what I can tell, the through-hole jfet is a 2SK184.

I know that descriptions like "ugly" and "fuzzy" are subjective but any insight that anyone has into the HM-2 design would be greatly appreciated. Thanks!


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## twebb6778 (Aug 11, 2020)

2SK184 and 2N5457 appear to have the same pinout. I'd socket them and see how it affects the overall sound.


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## Stelloh42 (Aug 11, 2020)

i took a look at the smd side of the board and it looks like there quite a few diodes in this Behringer pedal. Like the promethium has a total of 8 including the rectifier, where as my HM300 has the rectifier and one pair of diodes thru hole, then another 10-11 of them on the surface mount side.

starting to think there is more than just a component value difference that is causing the noticeable audio disparity.

still - I do have some sockets so I should at the very least try out the Jfets I have.
Thanks!

-Chris


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## Chuck D. Bones (Aug 11, 2020)

It's not the JFET.  It's wired as a source-follower and as long as it's biased correctly, they will all be pretty transparent.  IC1.2 _may _get driven to saturation when DIST is dimed.  If so, then the choice of opamp for IC1 will affect the tone. IC2 & IC3 will not be driven to saturation, so any of the usual opamps will sound the same.  I suspect that most of the tonal qualities are in Q2, Q3 and the germanium diodes.  

BUT...
Without knowing what's in your old HM300, it's going to be pretty hard to guess how to get that particular sound.  Maybe you feel like tracing the schematic?


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## Stelloh42 (Aug 11, 2020)

going to do some looking around and see if I can figure out what op amps and transistors Behringer put in this thing. The majority of the bits in this thing are tiny surface mount components.


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## Chuck D. Bones (Aug 11, 2020)

Knowing which parts are inside is only half of the story.  Without knowing how they're connected, we're still in the dark.


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## Nostradoomus (Aug 11, 2020)

It probably has more to do with the values and tolerances of the parts around the gyrator eq than anything. If Behringer straight up cloned the HM-2 there’s going to be input and output buffers which would account for the extra parts.


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## Stelloh42 (Aug 11, 2020)

At this point - I might be a little too much of a noob to trace out a circuit, let alone one that is over 2/3 smd parts crammed into a boss-size enclosure. I am going to try and get to that point, tho. This really is the best sounding heavy distortion pedal I've used and I'd like to know what makes it tick.


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## Nostradoomus (Aug 11, 2020)

It’s the gyrator EQ that gives it that special chainsaw sound. Using a calculator like this http://www.muzique.com/lab/gyrator.htm you can see that the values of the gyrator connected to the Low knob give you a boost around 88hz and the High knob does 2 bands at once...one around 860hz and one at around 1300...this is where you’ll want to tinker on the promethium.


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## Stelloh42 (Aug 11, 2020)

Awesome - I think I'm going to check the resistor and cap values that control the lows on the HM300 at that point in the circuit too. I'll let you know what I find. As always - you've been super helpful.


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## Stelloh42 (Aug 23, 2020)

Nostradoomus said:


> It’s the gyrator EQ that gives it that special chainsaw sound. Using a calculator like this http://www.muzique.com/lab/gyrator.htm you can see that the values of the gyrator connected to the Low knob give you a boost around 88hz and the High knob does 2 bands at once...one around 860hz and one at around 1300...this is where you’ll want to tinker on the promethium.
> 
> View attachment 5826


So - I built up the promethium with a couple changes. 1st - put a higher gain transistor in q3 - I think I used a 2n5087. 2nd lowered R25 to 47 ohms. Third, raised C13 to 150n. And I used JRC4558s instead of TL072s cause it was what I had 3 of.

And - holy crap what a difference. It totally surpassed the Behringer in terms of both high and low end ugliness. I actually had to dial the EQ knobs back just a touch. All 4 levels dimed is the Swedish "death metal" tone, but I usually back the highs off to about 3 o clock. I'm having to lower the bass knob to about 4 o clock now too.

total success - thanks for the help!


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## Nostradoomus (Aug 23, 2020)

Awesome! It’s a fun circuit to play around with, so many variables happening with just a component swap or two.


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## yazooligan (Aug 24, 2020)

Not to hijack the thread but I’m about to build one of these myself, and I’m wondering how close the sound of the stock build is to the HM-2?


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## Chuck D. Bones (Aug 24, 2020)

Stelloh42 said:


> So - I built up the promethium with a couple changes. 1st - put a higher gain transistor in q3 - I think I used a 2n5087. 2nd lowered R25 to 47 ohms. Third, raised C13 to 150n. And I used JRC4558s instead of TL072s cause it was what I had 3 of.
> 
> And - holy crap what a difference. It totally surpassed the Behringer in terms of both high and low end ugliness. I actually had to dial the EQ knobs back just a touch. All 4 levels dimed is the Swedish "death metal" tone, but I usually back the highs off to about 3 o clock. I'm having to lower the bass knob to about 4 o clock now too.
> 
> total success - thanks for the help!



Cool!  Always great to hear about successful mods.

Upping the HFE on Q2 or Q3 won't make much difference because the resistors around Q3 set the gain.  Still, low noise transistors like 2N5087 are always a good idea in a high-gain dirt pedal.

TL072 is quieter than JRC4558.  IC1.2 can be driven into saturation, so different opamps will sound different when you dime the DIST control.  Whatever sounds good to you, that's the one to use.

Be cautious about playing with the resistor ratio in a gyrator.  The intended purpose of a gyrator is to be an impedance transformer.  Make synthetic inductors, multiply capacitance, that sort of thing.  The price we pay is that gyrators are noise amplifiers.  Whatever noise is in IC2.2, it gets multiplied by R21/R25.  I ran into this problem bigtime trying to replace the inductor in a Wah-Wah pedal with a gyrator.  The good news is that the gyrators are after all of the gain stages, so more noise from IC2.2 may not be noticeable.


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## ANGRY_SWEDE (Jan 27, 2021)

yazooligan said:


> Not to hijack the thread but I’m about to build one of these myself, and I’m wondering how close the sound of the stock build is to the HM-2?


If you stick to the BOM it is 99% identical. You always must take into consideration parts tolerances we are using are much better than the original Boss units from the 80's.


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