untune

Shure M67; Restoring and Ideas
« on: March 11, 2021, 06:09:45 AM »
Hi all,

A year or two back I picked up a job lot of Shure M-series mixers for a reasonable amount.  I only wanted one particular piece of gear and the rest were a bonus that have sat untouched ever since; specifically I have an M67, M68 and M688 sat doing nothing.  I was getting the 'lockdown-fix-something' itch again, so I decided to open the M67 up, if only to get it to a working state.  It came to me without a plug, but I fitted one and it powered up fine.

I started toying with the idea of making some kind of 'colour box' since, despite lots of conflicting information about how good the transformers are/aren't, I can't see much point in trying to fashion a high-end pre when there are better options.  I put together a pseudo-balanced cable terminating the XLR side with a pair of 33k/100ohm resistors using the same method that the M67 uses in channel 4 to step line level down to play nicely with the transformer input. Running a synth through it, I actually quite like the sound - doesn't quite overdrive it, but maxing the input pot and keeping the output on 2 gives a decent level out (after moving the line output from the screw terminals across to the heaphone TRS jack and putting a 600 ohm resistor across hot/cold.)

What I've noticed is that there is a bit of a noise floor with all inputs/outputs turned completely down, even before intoducing the hiss from upping the output level.  It dances around the -60dB mark in my DAW—not the best way of measuring I'm sure, but it's practical for my uses.  Noise seems to be highest somewhere between 10 and 20Hz, and there's a spike (albeit a lower one) around 150Hz which I assume is power supply-related.  I know these units are considered inherently noisy but I wondered if there may be any way to reduce it.

So far I've:

Power supply:
1. Replaced C28 (1000uF)
2. Replaced C29 (400uF but with a 470uF as that was all I had, figured it wouldn't hurt in this application)
3. Replaced C27 (250uF but with 220+33uF in parallel as I had them to hand)


Main board:
4. Replaced C31 (5uf but with 4.7uF)
5. Replaced C21 (a green mylar with a polyester film; the green one had darkened considerably compared to the others and was covered in a 'frosty' layer of shiny particles which I assumed meant something had gone wrong)


I have yet to change C25 (250uF) which is the only large electrolytic left untouched; it runs ahead of the output transformer and most of the VU circuit is behind it; I did see a video of a guy who had a leaky cap in this position which was causing odd VU behaviour but mine seems fine in this regard.  Not sure if it would have any effect on the noise level. C24 has not been touched either, another 5uF near transistor Q13.

Of course, it may just be that this level of noise is inherent in the design and it's still certainly useable as it is, just want to explore the options before I start doing anything else.  I assume the transistors could be contributing but I don't want to change too much.

I also have another (US voltage) M67 from years back that was heavily modded when I obtained it and would be better as a parts donor these days.  I'm wondering if there's anything I can do with those transformers?  I've been wondering if I can't make a passive DI or some kind of simple colour/distortion device with those - could they be used in reverse for anything?

So the main questions are:

1. Are there any other ways to reduce noise or is it just a side effect of the topology?
2. EDIT: Is there any need for the pair of inductors that go to the hot/cold line out?
3. Any ideas what I could do with the 4x input transformers and 1x output transformer from the donor?


Many thanks for any input; schematic attached

Cheers!
« Last Edit: March 11, 2021, 06:17:55 AM by untune »


TwentyTrees

Re: Shure M67; Restoring and Ideas
« Reply #1 on: March 11, 2021, 11:45:58 AM »
I've had a couple of M67s & M68s over the years. I agree about the noise, though I do quite like the overall sound in a way. In a pinch they make a semi-decent way of getting some extra inputs into a pre (Bob Ohlson has posted some tips for getting the best result when used in this way), but generally I've gutted them and used the bits for something else. Built a couple of fully passive EQs in M68 chassis, that sort of thing - having a box with xlrs and holes pre-drilled makes for a good playground.

The mic input transformers are small and quite coloured, don't take a huge amount of level, but can be serviceable especially for prototyping - try them on FET or tube inputs (or even something like a Helios!), and they make fun "colour" DIs when reversed.

mjrippe

Re: Shure M67; Restoring and Ideas
« Reply #2 on: March 11, 2021, 02:59:27 PM »
A TwentyTrees said, I racked up four inputs in reverse for easy keyboard DIs in the SSL room at Studio G.  Just don't use them into a pre with phantom power turned on!

untune

Re: Shure M67; Restoring and Ideas
« Reply #3 on: March 11, 2021, 06:32:05 PM »
I've had a couple of M67s & M68s over the years. I agree about the noise, though I do quite like the overall sound in a way. In a pinch they make a semi-decent way of getting some extra inputs into a pre (Bob Ohlson has posted some tips for getting the best result when used in this way), but generally I've gutted them and used the bits for something else. Built a couple of fully passive EQs in M68 chassis, that sort of thing - having a box with xlrs and holes pre-drilled makes for a good playground.

The mic input transformers are small and quite coloured, don't take a huge amount of level, but can be serviceable especially for prototyping - try them on FET or tube inputs (or even something like a Helios!), and they make fun "colour" DIs when reversed.

Agreed re the sound! I have replaced a couple of other smaller electrolytics to no avail, but tonight I ran another quick test putting my Prophet 6 through it and then running the same sequence directly into the line in on my interface.   There's a very clear difference—the Shure does something really nice with the mids and upper harmonics and I can see it being useful for helping certain sounds sit in a mix.  It's just that hump in the sub bass that won't go away!

I noticed a slight discrepancy in that C25 (250uf/40VDC on the schematic) is different in the two models I have: in the UK voltage version it's a 250/25V and in the US one it's a 400uF/25V.  Wondered if there might be any reason for that.

I do have a pair of valve preamps from an Akai M8 that I intended to make into *something* eventually, and they'd make decent mic input transformers for that.  But the last Akai pre project I did took a lot of time and effort and I feel like it's a way off yet ;D

A TwentyTrees said, I racked up four inputs in reverse for easy keyboard DIs in the SSL room at Studio G.  Just don't use them into a pre with phantom power turned on!

Having four of them racked in reverse as DI's sounds like a nice easy project actually, I might start with that and go from there. Thanks for the input from both of you :)

untune

Re: Shure M67; Restoring and Ideas
« Reply #4 on: March 14, 2021, 09:59:35 AM »
Did a bit more experimentation... picked up a couple of plugs and got the Shure M68 working, sounds fine, no modifications at all.  Noise is lower than the other but then it doesn't appear to have such a drastic gain stage after mixing.

I used the unbalanced-line to balanced-mic attenuator cable to test it with the output of my interface and back in, found that other than additional noise and a slight roll off at 20Hz there's very little if any difference between what goes out and what comes back in.

I copied the attenuator (a u-pad I believe) from channel 4 of the M67, which accepts a line input into the mic transformer by means of a switch (set to bridging and ignoring the 600 termination.) I figured that Shure will have known best how to pad the input down.  It uses 33K series resistors and 100R shunts that meet and go to ground.  The input TX is different between the M68 and M67, but most info is a mystery.  People have speculated the M67 to be 1:14 and M68 to be 1:10 or 1:12 but I can't say for sure.  I have attached an image of the cable I made along with info for each model, and have a couple of questions.

1. Reading about u-pads, the shunt resistor usually appears to be a single resistor that goes from one side to the other, here it comprises of two resistors that meet in the middle and then go to ground.  What's the purpose of this, if any?
2. Could there be a more suiatable combination of resistors to pad the M68? It's very clean, I'd like to try a few different levels of attenuation to boost the level but I'm unsure which numbers to pluck from the info I have in order to correctly calculate the pad values.  I have read the great info on uneeda audio but I'm still not sure if the shunt values should be different, and whether the answer to my last question will inform that.

Cheers!

Scodiddly

Re: Shure M67; Restoring and Ideas
« Reply #5 on: March 15, 2021, 09:58:07 PM »
First guess is that the ground connection in the middle of the pad is for unbalanced inputs.


 

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