A little history
The Shure M67 is a great vintage mic preamp that has an interesting history—presumably used to record the Woodstock festival, and used at countless churches and VFW halls from the late 60s through the 80s. It also has a rich history in the hardware-mod space.
There are a lot of mods out there, from things as simple as adding direct outs to as complex as modifying the gain structure to handle high-gain line-in, and even phantom power. But in short, you get four mic inputs, with four pretty great mu-metal input transformers. For similar transformers today, these would cost quite a bit—usually much more than what one of these M67s go for on eBay or Reverb.com—around $25-$50.
The back gives you the four mic inputs, and a single mic mix output. It also has quite an interesting variation of plug types—RCA, banana plugs, blade connectors, wire-nuts, XLR, 1⁄4”. Somewhat annoying by today’s standards, but it’s okay. We’re going to mod this up pretty heavily to make it more conducive to a modern studio setup.
The inside looks pretty standard for 1960s-vintage gear. Lots of point-to-point wiring, twisted signal wires, old carbon comp resistors, large transformers, and a rugged design. Feels a little bit like a tube guitar amp inside (minus the tubes of course!)
The printed circuit board (PCB) is just beginning to make its debut in this era of equipment, as you can see in the brown board in the middle.
Onto the Mods!
Lemurland has an excellent compiled list of approaches to modding this device. I think since Ron Childress’s site went offline (one of the original M67 modders), Lemurland has the archives and can be considered the de-facto resource.
They propose a fairly flexible modification of having two high-gain inputs, one line-level input, and having two channels mix into the mic mix output. Here’s their schematic:
I wanted to make my mods a little more flexible for both mic preamps as well as re-amping existing tracks from the DAW (because, those mu-metal transformers… sooo nice!)
Here is my mod list:
Remove oscillator circuit
Four separate inputs, with high-gain stage capability on all four channels (aka, an input gain pot pre-input-transformer) using dual-concentric pots
Knobs for the dual-concentric pots
Every channel has direct outs–just great for adding a bit of color
Make unbalanced direct outs balanced, following the advice in this fantastic white paper by Jensen:
Every channel maintains a low-cut switch
Low-cut capacitor value choice reconsidered in case the default -3dB @ 200Hz is too aggressive a low-cut
Any channel still able to be mixed into the master bus via a “direct/mix” switch (considered keeping both but it will likely load down the output too much)
The master bus (and its output transformer/additional gain/600Ω output) output moved from banana plugs to the “mic out” XLR jack
Mix line out/mic out switchable from the front (the old vu range switch used here)
Headphone amp removed to make room for other mods
Retwist all wires and consider a ferrite bead/choke to reduce remaining noise if heard