Boss hm2 questions

[JebemMajke]

For a long time, I have been searching for a nice od/dist/fuzz for bass. A buddy of mine and I are making a death metal band. One guitar only, so I figured bass has to be dirty and heavy.

I tried all sorts of things. Darkglass stuff, pharaoh fuzz, supercollider, black Russian muff, all those bass overdrives. Boss od3b.

And it wasn't what i wanted.

I was kind of going after that Jo Bench bass sound ( Boltthrower ). That stuff sounds like a bloody tank.

And yesterday I had a chance to try boss hm2. It is the bomb!

Like, it sounds like death itself <3. So @#$%ing heavy!

I  L O V E  I T!

Ps my bass is Yamaha rbx 550, made in Japan. PJ stock pickups ( with musicman active electronics inside ) and it is fretless ( i made it fretless ). It still has the mwah of a fretless. Pedal reacts nicely to the volume pot of a bass. Great stuff!

So that is it, i found the sound i was after, and now it is building time.

I have all the parts, even those strange transistors ( 2sc2240 and 2sa970 ).

I was wondering, should i bias them. Like use trimpots and bias that fuzz face looking circuit

BTW here is the schematic



Has anyone here built the thing?

Are there any suggestions?

Ps

I am also looking into adding a fet preamp to run in parallel to HM2 so that i can put in a blend pot, and to blend dirty and clean signals.

[Mark Hammer]

I have one.  The key to the HM-2 is the use of both clipping and crossover distortion.  Secondary to that is the resonant filtering of the "High" and "Low" controls.  The pair of germanium diodes in series with the signal essentially clip the "sides" of the waveform, rather than the tops.  It also serves as a crude noise gate by blocking any signal that does not reach the forward voltage of the diodes.

Crossover distortion is willing to be "just friends" with neck pickups but loves bridge pickups.  Before you spend time building an HM-2, if you have another clipping/distortion pedal, try sticking a back-to-back pair of diodes in series with the signal at some point in the circuit.  They can be either germanium or Schottky.  Note that they will subtract whatever their forward voltage is from the signal amplitude.

[Fancy Lime]

Hi Miša,

depends on what you mean by "suggestions". First let me crowbar in some shameless advertising. If you don't have an HM-2 and would need to build one yourself, I would suggest you try this thing first:

https://www.diystompboxes.com/smfforum/index.php?topic=122928.msg1160772#msg1160772

It sounds fairly similar to the HM-2 but is a *lot* easier and quicker to build. But it is not an HM-2. It's maybe a bit rawer and gnarlier, less nice than the HM-2 but I have a feeling you might like that.

If you *want* to build an HM-2, I think there are PCB's out there, I just don't know the names. If you do that I would suggest experimenting with the crossover clipping diodes. I find BAT41's or BAT42's to sound best in this role, much better than Ge. But that is a matter of personal taste, so make sure to experiment at this point. If you build on vero or perf from scratch, I would redesign the whole input gain stage (the original one is unnecessarily complicated and sports a very inefficient gain control) and integrate a clean path with a mixing stage at the end (for bass use).

Another possibility (if you do not fancy extensive mods to the original circuit) is to just by a Behringer HM300, which is a cheap copy of the HM-2 in a flimsy pink plastic housing. These often go for about 20$ on the used market. You can take out the guts and put them into a more sturdy enclosure.

Hope that helps,
Andy

[thetragichero]

the magic is in the tone stack section. only dirt pedal on my bass board is basically the "allies fuzz" with the knobs maxed (aka left out the controls) going into a pot to blend it with the clean signal (handy with bass) going into the tone stack from the hm-2
not sure if i ever drew iii the full schematic. I'll check when i get home

[thetragichero]

here you go. i quickly drew it so not as pretty as i usually do. as somebody who HATES the crossover diodes in an hm-2 (one of the first things i do is jumper them so i can get back my sustain) this works a lot better than me, and it's a lot simpler to build on perf than 85000 ICs