Just bought this one on ebay. It looks like really nice preamps with tranformers on the input and one tranformer on the output.
Not bad transformers, either, as I recall.
What i'm thinking to do (after reading the schematics):
- swap quad opamps on the signal path with modern and faster ones like LME49740 or LT1359 (will add 0,1uf psu bypasses as well);
Probably a good idea, but watch out for oscillations -- I guess you're already cautious about that, since you're planning to use psu bypasses.
What opamps are in the box now?
- upgrade psu smoothing caps to 2200uf and add a parallel 0,1uf bypass;
Can't hurt, probably will help -- and besides, you'll almost certainly be replacing the supply electrolytics anyway.
- upgrade all EL caps on the signal path to at least 47uf/25v Panasonic FM's;
- change the output voltage on the LM317 regulator from 30v to 34v, or 36v if i use the LT's instead of the LME's (more juice to the opamps and phantom power);
Probably okay, but check the bias and temperature of the output transistors to make sure they're not cooking.
- adding unbalanced outs from each preamp.
Where? As I recall, the back panels of these mixers are pretty short on free space. Also, of course, you'll need electrolytic coupling caps, since the opamp outputs are biased at half the supply voltage. Space may be an issue with them too.
What you guys think of the circuit and the mods i'm planning to do?
As you gather from the above, I think your plans are eminently sensible (at least if you can find a place for the channel output jacks). And my experience with this mixer is that it's not half bad sounding, certainly a HUGE improvement on the M67. I made some nice recordings with it back in my TV days.