Modding a Shure M67 for use as a recording pre

[Derringer]

I need a project, been looking at pres and this guy seems like fun
https://pubs.shure.com/guide/M67/en-US


if that link doesn't work, just search shure m67 schematic and shure has a pdf of what I assume is the original owner's manual with schematic.

My goal is just to make this into something that sounds different from the regular inputs of my tascam interface.

Reading around, folks say thata it's got too much gainn as is for any loud source, like drums or guitar. So one mod is to move the position of each channel's gain pot from after the darlington pair to between the input transformer and the darlington pair, to become a pre-gain control. Makes sense to me. Futhermore, folks have then made the unit have four individual outputs by sending the signal directly from the coupling caps off the outputs of the darlington pairs.

I'd like to try this, perhaps keep one channel stock, going through the summing amp and out through the microphone output and make the other three into individual channels.

The outputs of the three individuals would be unbalanced, which hasn't been a problem in my experience with my interface.
However, my interface has line inputs that have a 10K input impedance.
Do you think I'd benefit from adding a bjt buffer right after that darlington pair? It looks like the voltage there is already about 1/2 V, so it seems rather straight forward if I do.


thanks

[PRR]

I think the '67 is a piece of history, an audio icon, and you could leave it alone.

And if you think you need ALL those changes, maybe it isn't what you want at all?

[Derringer]

ooooooophff  ... fair enough

if small signal transformers were more affordable, I wouldn't have chosen this route

but I want to see what all the hubub about them is about.

[tubegeek]

Well,  personally I have busted them open to get the transformers, so there's that.

Also, it is Audio Lore (there are pictures!) that Woodstock was mixed on a stack of Shures ganged together for more channels.

Bill Hanley at Woodstock:

[garcho]

https://sound-au.com/projects.htm

^ If you need projects, that's a good place for inspiration. He has solid designs for mic pres and many other useful devices, as well as helpful information and history. A bit like an internet version of Doug Self.

[Derringer]

Well,  personally I have busted them open to get the transformers, so there's that.

Also, it is Audio Lore (there are pictures!) that Woodstock was mixed on a stack of Shures ganged together for more channels.

Bill Hanley at Woodstock:

Do you know if the transformers in the M68 are good for anything in particular?
In the few things I read, everyone seems to prefer the M67 transformers.

[tubegeek]

Do you know if the transformers in the M68 are good for anything in particular?
In the few things I read, everyone seems to prefer the M67 transformers.

Not off the top of my head - are they a different ratio or something?

[wavley]

If I remember correctly, the 67 and 68 are the same, one has a meter.

I use these sometimes for color, haven't bothered modding mine, but you can and I've done it for a couple folks.  There's a huge thread about it over at the Tape Op message board.

The easiest thing to do is just take an out off the low cut switch, clip the cap and solder a jack to it.  Low cut off, everything is normal, low cut on, switched out of the buss and is a direct out.  Leave ch 1 alone, that way you still get low cut and the output section/transformer.  A lot of the no good headroom is the tiny input transformer, get or make an XLR plug in pad and it's fixed.

I use M267s all the time, they live in my compressor rack, there's always a plug in pad on the output so I can push them hard.  Really fattens a snare or room mic.  My m67s live in my mic pre rack, honestly, they're so cheap I recapped the power supplies and use them as single mic pres (sub mixing I do on an old Yamaha that sounds lovely)

[Derringer]

Do you know if the transformers in the M68 are good for anything in particular?
In the few things I read, everyone seems to prefer the M67 transformers.

Not off the top of my head - are they a different ratio or something?

I couldn't find anything definite whether they use the same xformers or not. The manual for the 67 makes it a point to to call the transformers "mu-metal" shielded or something. The m68 manual doesn't point anything out about its transformers. The 68s are less expensive than the 67s, I'm just thinking about getting decent transformers on the cheap. If these are infact decent transformers, I have zero experience running a mic through an iron input, hence why I'm pursuing this project. So far though, I like how they sound. I just need to figure out how to use them best.

[Derringer]

If I remember correctly, the 67 and 68 are the same, one has a meter.

I use these sometimes for color, haven't bothered modding mine, but you can and I've done it for a couple folks.  There's a huge thread about it over at the Tape Op message board.

The easiest thing to do is just take an out off the low cut switch, clip the cap and solder a jack to it.  Low cut off, everything is normal, low cut on, switched out of the buss and is a direct out.  Leave ch 1 alone, that way you still get low cut and the output section/transformer.  A lot of the no good headroom is the tiny input transformer, get or make an XLR plug in pad and it's fixed.

I use M267s all the time, they live in my compressor rack, there's always a plug in pad on the output so I can push them hard.  Really fattens a snare or room mic.  My m67s live in my mic pre rack, honestly, they're so cheap I recapped the power supplies and use them as single mic pres (sub mixing I do on an old Yamaha that sounds lovely)

Yup! That article is the one that got me going on this mixer.

68 and 67 have different gain stages in the schemes I looked at

I modded the first two channels of my 67, left 3 and 4 alone for now.
I basically did what you said there except that on channel 1, I moved the gain pot infront of the gain stage. In channel two, I tried a dual concentric pot so that I have a volume control fore and aft the gain stage. Both work great. I repurposed the filter switch to switch the signal onto/away from the bus to individual unbalanced outs, like you stated, as well. The way I did channel two seems to be the way to go. I can dial in a little clipping without blowing out my interface with too hot of a signal.

When you recapped, there are two caps in the power supply that are soldered to terminal strips. The strips are riveted, hell just about everything in this is riveted, in place and are tightly packed behind the power transformer and snugged next to the VU meter. To get at those caps to replace them, did you have to disassemble the whole power section/VU meter housing first? Or is there an easier way that I haven't noticed?

Didn't know there was such a thing as an XLR pad. Thanks for the tip and thanks for chiming in!

[tubegeek]

http://www.uneeda-audio.com/pads/ All About Pads by Rick Chinn

[Derringer]

http://www.uneeda-audio.com/pads/ All About Pads by Rick Chinn

What I mean was that I didn't know that something like this existed.
https://www.shure.com/en-US/products/accessories/a15as

but, i didn't know there were all those different ways to pad either, so thanks TGeek!

[tubegeek]

Lotta pad options for sure. Rick Chinn's no dummy, and he's done some great summarizing and organizing there.

If you want EVEN MORE pad info, Howard Tremaine "Audio Cyclopedia" has EVEN MORE.