Unity gain mixer project -- anyone taken apart a Boss KM2?

[niftyprose]

Hi guys, I'm designing an unusual micromixer. I'll spare you all the details but the basic idea is to use a very simple unity gain mixer circuit like the Boss KM2 of yore as one link in a chain. The completed box will enable me to chain several such circuits together. I know the theory for a unity mixer is prettty easy and I'm sure I could work out a circuit from first principles. However, if anyone has hands-on with a KM2 or a decent schematic, I'd be delighted to see how Boss tackled this one. Best, NP.

[PRR]

https://desitol.exblog.jp/24192797/

[niftyprose]

Wow, thanks, that's even simpler than I thought. Got the circuit? Best, NP.

[PRR]

FWIW: it's not "unity gain", unless you turn the gain-knobs to the {UNITY} mark.

By the simplicity, it is probably a single transistor inverting amplifier. For more than a few signal paths, sanity recommends non-inverting.

The inverting mixer with over-unity internal gain is susceptible to hiss troubles.

I think in this day of 19 cent dual opamps, you can do better from a clean sheet. The woods (well, innernet) is full of mixer pages. Jack AMZ Orman, R.G. Geofex Keene, Rod ESP Elliott are some no-BS sources.

[niftyprose]

Thx guys, definitely food for thought.... NP

[2SC732GR]

the trannies are 3 2SC732GR's and 1 2SC970. google boss km2 pictures and there's a detailed facebook post
by a japanese guy that show the pcb gut shot front and backside. the first transistor pair are the 732 and 970,
second are 2 732's.

i built it on a perfboard and it turned out to be a great distortion for synths and guitars.
a bit noisy if you dime it but lovely never the less.

[2SC732GR]

2SA970, sorry, not 2SC970

[hair force one]

I build one too because it's used by Hank Shermann from MERCYFUL FATE! (best 80's heavy metal band ever imho) -> https://hankshermann.com/wp-content/uploads/2015/03/savage-studio-1bw.jpg

This is in fact a discrete op-amp that may predate boss OD-2, same topology but without any fets, full bjts here. Lot's of gain and grit to push any amp, love it. I tried a germanium for the PNP to tame this fuzzy beast a bit, put a gain pot in the feedback loop, some switchable back to back diodes too.

Like an empty box of mods waiting to be filled with yours.   

[t1redhands]

I build one too because it's used by Hank Shermann from MERCYFUL FATE! (best 80's heavy metal band ever imho) -> https://hankshermann.com/wp-content/uploads/2015/03/savage-studio-1bw.jpg

This is in fact a discrete op-amp that may predate boss OD-2, same topology but without any fets, full bjts here. Lot's of gain and grit to push any amp, love it. I tried a germanium for the PNP to tame this fuzzy beast a bit, put a gain pot in the feedback loop, some switchable back to back diodes too.

Like an empty box of mods waiting to be filled with yours.

that cinches it, i'm making one of these.

[hair force one]

Boxed the km-2 but without the second input, nice overdrive-gain till distorsionsmith pedal. Also had a hard time trying to screenprint it, didn't succeed at all and turned out utterly ugly.

Made some tiny diode modules to test the various options!

[Krystal]

I think it looks lovely. I love a slightly offset screen print, it gives it a lovely texture.