Boss HM-3 Hyper Metal with HM-2 EQ?

mynameisjonas · Post by **mynameisjonas** » 13 Apr 2022, 06:03

Hi,
I got an HM-3 dirt cheap a few years ago to use as a complement to my HM-2, but I always found it really hard to dial in. It's just got way too much top end sizzle regardless of where i set the EQ knobs, and there's not enough mids. So the other day I decided to try and turn the EQ section into HM-2 specs, and although it does sound a lot better now, I'm not quite there.
I need to state up front that I have no idea how to properly translate the values of the opamp EQ of the HM-2 to the transistor based EQ of the HM-3. But here's what I did:

C29: changed from 0.039uF to 0.15uF
C30: changed from 8.2nF to 6.8nF
C31: changed from 0.022uF to 0.1uF
C32: changed from 1.2nF to 4,7nF
R42: changed from 1k to 330R

HM-3 schematic
HM-2 schematic

It sounds good, but I'm not quite getting the chainsaw sound of the HM-2, plus it's still got that piercing top end sizzle (primarily noticeable when you turn down the HIGH control). Can anyone give me any pointers?

phatt · Post by **phatt** » 13 Apr 2022, 06:16

Never owned one but,,,
Try removing C11, lift one end to test.
If too much then use a lower value Cap for C11.
Phil.

mynameisjonas · Post by **mynameisjonas** » 14 Apr 2022, 16:03

Thanks for the suggestion! Removing C11 did indeed remove some of the fizz.
I looked a little closer at the components around C11 and realized it was part of a mid scoop filter together with R14, R15 and C12. I tried swapping the values of R14 and R15 (so R14 was halved and R15 was doubled), to lessen the effect of the filter, and that did the trick. I don't have my HM-2 here to compare, but the HM-3 does a really mean chainsaw sound now. It's still a bit fizzy when the HIGH control is turned down below 12 o'clock, but I doubt I'll ever find myself wanting to do that.