Question about a schematic(Boss HM2)

[BraindeadAudio]

I made a board in EAGLE to see how this schematic would sound vs the 3 HM2 pedals I have here, and I am having touble with the sound going dead after 2sk30 transistor. I cant find any info about a replacement for the 2SC2240 and the 2SA970 pinouts...I was thinking I could use BC550 or 2n3904/2n3906 matched? I have a small ammount of 2sk30s here for Q1....just wondering if this schematic looks like it even makes any sense and would work? http://www.freeinfosociety.com/electronics/schematics/audio/pictures/bosshm2.gif


As opposed to this one, which is the original Japanese HM2 schematic? http://www.hobby-hour.com/electronics/s/hm2-heavy-metal.php

[BraindeadAudio]

Ok, just for reference, I wired it up, using a BC549C in Q1, a BC550 with the legs bent to work in Q2 and a 2N3906 in Q3, the pedal sounds quiet with minimal disortion and the tone stack sounds like it doesnt work?

[BraindeadAudio]

Anyone?

[the3secondrule]

I can't help, but I would totally buy a board for a verified HM2 project...

[oldhousescott]

Q1 is a source follower, so any N-channel jfet (or indeed NPN silicon transistor) should work in that position. The pinouts for Q6 and Q7 are both ECB, so your 2N3906 will need to swap pins 1 and 2.

EDIT: whoops! make that pins 2 and 3. 2N3904/06 are EBC.

[BraindeadAudio]

Yes, but is the simplified schematic correct vs the bigger one?

[oldhousescott]

Yes, mostly the same. The simplified version doesn't have the final emitter-follower buffer, but that's OK. Also, the caps from collector to base across both Q6/7, and in the feedback loop of the following opamp stage, have been reduced from 100p to 10p. This will extend the high frequency response in those stages. Likewise, there is a 470p cap in the opamp stage of the tone control in the original that is not present in the simplified version. This will again extend the high frequency response in that stage. Based on these changes, I would expect the simplified version to be brighter/clearer, perhaps less nasally, but also maybe excessively bright.

Curiously, there is what looks like a typo in the original that was corrected in the simplified version. The original shows 9v being fed through a 68k resistor to the positive terminal of IC1b. I would expect that to be 4.5v as the simplified version shows, and as is connected in both cases to the positive terminal of IC1a.

[BraindeadAudio]

And the rest of the schematics transistors, that arent in the simplified schematic are all in the power section? It doesnt need those to achieve the sound? I sorta want it to sound exactly the same as my Japanese ones.

[oldhousescott]

All of the other transistors (excepting the follower-buffer as stated above) have to do with the soft-switching. Well, actually there is another buffer for the bypassed path, but it is also not necessary.

[BraindeadAudio]

And those caps on the gyrators are Tantalum correct?

Sorry for all the questions just trying to figure this thing out.

[oldhousescott]

Yes, best I can tell looking at some images, the 1.5, .15 and .1uF (C35, C28, and C29) are tantalums.

[BraindeadAudio]

Now, if I was to take this...and Add the HM2 tone stack into it, how do you think it would sound?

Up until the tone stack they look absolutely Identical...with some minor parts value changes

http://4.bp.blogspot.com/-mMD0Cdhzk0E/T-_GwsLouII/AAAAAAAABas/u3jeCR9emZk/s1600/DOD%2BFX59.gif