untune
Well-known member
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!
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!