Boss HM-3 mods - recommendations?

[Jorge_S]

I recently bought a Boss HM-3 for cheap (and I mean REALLY cheap). It sounds too 80's for me (too "nasal"), I would like more bass and overall more headroom.

Anyone has some info on which parts to replace?

I've searched on google but just found some info on turning it into a feedback pedal:
http://www.guitargeek.com/chat/stompboxes-7/boss-hm-3-mods-71047/

and a guy saying removing the germanium diodes reduces compression and pedal sounds more "open":
http://www.guitargeek.com/chat/stompboxes-7/boss-hm-3-a-96590/

and here is the schematic:
http://www.freeinfosociety.com/electronics/schemview.php?id=137

Any help or suggestions will be very appreciated!

[sault]

The simplest mod would be to raise the input capacitor. 10k || 47n = HP at 330-ish hz, so right there that's going to limit your incoming chug. To 1 uF would probably be too much (HP at 20-ish hZ), so I would think anywhere in the 100-220 pF region  (for a highpass of 160 to 220 hZ, respectively) might be a place to start. Admitting more bass does mean less headroom, though, so keep that in mind.

Let's see... there are three places where diode clipping is taking place - the first is in the op-amp loop driving the gain knob's negative feedback, the second is with the serial germanium diodes, the third with the shunt diodes. Changing any of these will change the degree of clipping. Personally, I'd take out the germanium diodes. I mean, the Metal Zone only has two shunt diodes for its diode clipping... you could also change the number of or types of diodes in that op-amp loop. A higher diode drop (either from more diodes or an LED, say) means more overhead, as well, again at the expense of less clipping.

Not sure about the feedback idea, don't think it would really be useful in this situation.

[Jorge_S]

The simplest mod would be to raise the input capacitor. 10k || 47n = HP at 330-ish hz, so right there that's going to limit your incoming chug. To 1 uF would probably be too much (HP at 20-ish hZ), so I would think anywhere in the 100-220 pF region  (for a highpass of 160 to 220 hZ, respectively) might be a place to start. Admitting more bass does mean less headroom, though, so keep that in mind.

I will do that! I'll take my chances with a 220pF. Would you also recommend changing the output capacitor 1uf electrolitic for a 1uf poly film?

Also, would you recommend changing those op-amps for hi-fi modern ones? like TL072 or maybe burr-browns? I don't know if pins will match with a few cables though...

Not sure about the feedback idea, don't think it would really be useful in this situation.

I was just mentioning the mods I found on google, so when someone searches this forum for HM-3 would have those links at hand 

[DavenPaget]

The simplest mod would be to raise the input capacitor. 10k || 47n = HP at 330-ish hz, so right there that's going to limit your incoming chug. To 1 uF would probably be too much (HP at 20-ish hZ), so I would think anywhere in the 100-220 pF region  (for a highpass of 160 to 220 hZ, respectively) might be a place to start. Admitting more bass does mean less headroom, though, so keep that in mind.

I will do that! I'll take my chances with a 220pF. Would you also recommend changing the output capacitor 1uf electrolitic for a 1uf poly film?

Also, would you recommend changing those op-amps for hi-fi modern ones? like TL072 or maybe burr-browns? I don't know if pins will match with a few cables though...

Not sure about the feedback idea, don't think it would really be useful in this situation.

I was just mentioning the mods I found on google, so when someone searches this forum for HM-3 would have those links at hand  

I don't think 220pF is going to admit any bass at all ._.
All op-amp's pinout are the same , except for single , dual , quad , octo (?) of course
If you have a single now , swap with a 071 , dually ? 072 .

[sault]

Could you explain that, Dave? Perhaps we're talking about a different cap.

Here are some basic mod suggestions. Lots of good tips here, Aron really knows his stuff.

http://www.diystompboxes.com/cnews/mods.html

Aron suggests swapping caps in the tone circuits at the very least to try and get a smoother sound... I have a feeling that for a high-gain pedal you'll want 'em in around your gain, too.

Totally swap out the IC. I like the TL-series, but of course any time you start talking about Jfets (in op amps or out) my heart beats a little faster.

[Henry89789]

This mod comes from the Brian Wampler book but it is for the HM-2.   I don't know if it is the same circuit. I did this mod to a HM-2 and really liked the sound after the changes.
A pic of the pcb is copied below so you can see if it is the same as yours

Boss HM-2 mod
Location Stock
value
Mod value What it effects
C30 .068uf   change to .022uf Lower value for more bottom end
C6 .047uf  change to  1uf Adds bottom end
D3 1n4148   change to 1n4001
D5 1n4148    change to led
D8 1n4148   change to Led
D9 1n4148    change to led
D6 Germaniu  remove and Jumper
D7 Germaniu  remove and Jumper
These changes will remove as much of the
fuzziness as possible, making it as rich and
full of a distortion as the circuit will allow.
C15 1uf elec   change to 1 uf (film cap) Will quiet some of the background noise.
C12 1 uf elec   change  to  .033 uf Will lower the bass before clipping, this will
help eliminate some fuzz as well.
*any size/shape/color led will work the same when used as clipping diodes as they are
with this pedal.

[DavenPaget]

Could you explain that, Dave? Perhaps we're talking about a different cap.

Here are some basic mod suggestions. Lots of good tips here, Aron really knows his stuff.

http://www.diystompboxes.com/cnews/mods.html

Aron suggests swapping caps in the tone circuits at the very least to try and get a smoother sound... I have a feeling that for a high-gain pedal you'll want 'em in around your gain, too.

Totally swap out the IC. I like the TL-series, but of course any time you start talking about Jfets (in op amps or out) my heart beats a little faster.

Yes ! I love TL07X for guitar applications , for input caps and output caps you mean µf don't you ?