Shure M68RM to 4 Channel Mic Pre Build

[Hibernate Audio]

A buddy of mine introduced me to these mixers a couple years ago, and I picked up the reverb unit just to see. I was expecting a gritty lo-fi box that I could hack into and play with occasionally - but this one sounded surprisingly hi-fi. I liked it enough to start down the rabbit hole of racking it up and building a whole preamp box around it...






Chassis and I/O
I kept the original chassis mostly intact, and mounted it in a 2U redco chassis. The faceplate is a Front Panel Express panel with UV printing and standoffs for the "input cards." The input cards let you select mic/line, phantom, instrument input, and pad settings. XLR and TRS inputs and TS unbalanced outputs on the back, TS instrument inputs on the front. Unbalanced TS input and output for the spring reverb channel.

Circuit Design
Circuit-wise, I clipped channels 1 through 3 out of the summing gain stage and wired them to each of their outputs. Channel 4 I left connected to the summing gain stage to keep one "high gain" channel.
I also clipped the reverb driver and post-amplifier out of the summing gain stage and wired them to their own I/O. I added a fuse to the auxiliary DC power output of the Shure power supply and wired it to each input card as 24V phantom power. Maybe someday I'll put a DCDC converter in there, but my more modern condensers are fine with the lower voltage standard. Lastly, I added an IEC connector and fuse and wired the Shure power supply into it.

End of Project Thoughts
Sound-wise, I'm pretty happy!

Pros:
- It was still cheaper than most four channel transformer-input mic pres.
- It sounds great, silky smooth with a really nice low end glue. It's amazing as a rhodes DI. I plan to normal my drum kit mics to it, we'll see how that goes.
- Whisper quiet, very little audible noise. Haven't done any measurements yet, but planning to take it into work soon.
- The reverb channel is super fun and sounds great
- Has my logo on it. Nothing beats that!

Cons:
- So much wiring.
- Channels 1 through 3 are pretty low gain, but it works really well as the first gain stage before going to an audio interface that has more gain on tap.
- It clips easy, and has no level indicators or gain control

Lessons learned:
- A 60s sheet metal chassis won't be square, CAD isn't real life. I think I would do the front panel cutout differently if I were to do it again.
- Next time I'm just going to build my own preamp instead of frankenstein-ing around a mixer...
- New rule for me: single channel only

Planning to get some audio samples later this week and start putting it through its paces.

Happy to share schematics, etc for this one if anyone would like. Took a lot of cues from ruffrecords, hairball, hamptone, etc for my input card design - thank you all.

Any comments/feedback/suggestions are welcome - it was a big project with a lot of compromises, so no ego here as far as things I could've done differently.

 

[TwentyTrees]

Great job, I can well imagine how much work all this was! Fantastic result though. I've also got a soft spot for the M67 & M68 preamps, simple as they are, so your idea to make them more fit for the modern studio is a very good one in my view.

Are the input cards your own design?

 

[Hibernate Audio]

Great job, I can well imagine how much work all this was! Fantastic result though. I've also got a soft spot for the M67 & M68 preamps, simple as they are, so your idea to make them more fit for the modern studio is a very good one in my view.

Are the input cards your own design?

Thank you! Really appreciate it. Making it fit into the modern studio is exactly what I was going for.

Yes, the input cards are my own design:

 

[TwentyTrees]

Nice! An elegant design, great job.

 

[Bo Deadly]

This circuit should have quite a bit of grit to it. What little feedback there is is isolated to each gain stage. But if you want a little grit, that would be a good way to get it because clipping with feedback is nasty. This might overdrive more gracefully.

Note: Definitely take the output from the mic out w/ low-impedance setting and then amplify with a good low noise / high gain mic stage (your average pro-sumer grade mixer like xenyx or onyx or whatever would be great for this purpose).

 

[TwentyTrees]

Note: Definitely take the output from the mic out w/ low-impedance setting and then amplify with a good low noise / high gain mic stage (your average pro-sumer grade mixer like xenyx or onyx or whatever would be great for this purpose).

Agreed! Bob Ohlsson (ex-Motown) has mentioned that he used to do the same with a Helios preamp (M67 rather than M68, but they're very similar).

 

[pvision]

Throwing the box away at the start of the project was a great move. They're too cramped to work in

 

[Hibernate Audio]

Agreed! Bob Ohlsson (ex-Motown) has mentioned that he used to do the same with a Helios preamp (M67 rather than M68, but they're very similar).

Ah yeah, I remember reading that thread way back at the beginning of my project - that and the tape op direct out mod thread.

I always planned to add an output gain stage and transformer to each output, but for the money and time I'm not sure if I'll go down that route. It's working pretty well just pulling the output directly off of each volume pot and going unbalanced into my interface. Basically a transformer "mojo" box, and I'm cool with that