Shure M67 13.4v increase

[jmeger]

Hi!

I'm working on modding a Shure M67 microphone mixer and had a quick question up potentially upping the voltage in the first stage of the unit. Schematic here: https://pubs.shure.com/guide/M67/en-US.

I'm doing the mod that has been pretty well documented (https://messageboard.tapeop.com/view...&highlight=m67) where you take some or all of the channels out of the mix bus and run them to direct outs before the summing stage.

My question is : could increasing the voltage of that first stage improve headroom, THD, noise and possibly squeeze out a few extra db of gain without consequences? I've been able to try it a little with a JLM T Rex power supply. It seems to work well if I remove the line bringing 34v volt to the board. If I leave that in there is feedback in the channel that is still in the mix bus. Is this because of the section in the summing stage that is 13.4v being increased and messing with the next stage? Or something else? I've been hesitant to increase it past 20v since there are some 25v caps in those sections and wanted to ask about this here first before swapping those and pursuing this any further.

Thanks in advance!

 

[abbey road d enfer]

I'm doing the mod that has been pretty well documented (https://messageboard.tapeop.com/view...&highlight=m67) where you take some or all of the channels out of the mix bus and run them to direct outs before the summing stage.

this link does not work.

  could increasing the voltage of that first stage improve headroom, THD,

Yes.

  noise and possibly squeeze out a few extra db of gain without consequences?

Noise may possibly increase marginally, gain should not vary because iit's governed by the ratio of resistors.

  If I leave that in there is feedback in the channel that is still in the mix bus. Is this because of the section in the summing stage that is 13.4v being increased and messing with the next stage? Or something else?

Hard to tell from a distance. Changing the voltage should not matter significantly. But apparently it does...  ??? Maybe marginal design...

 

[jmeger]

This link should work for the mods : https://messageboard.tapeop.com/viewtopic.php?t=32687

 

[abbey road d enfer]

This link should work for the mods : https://messageboard.tapeop.com/viewtopic.php?t=32687

Is this " It seems to work well if I remove the line bringing 34v volt to the board. If I leave that in there is feedback in the channel that is still in the mix bus." your only issue?
It is not clear to me. Are you certain this "feedback" is in the channel or in the mix/master circuit? Do the channel/master pots act on this?
One thing I didn't get from the link is how it's supposed to be used. Are these direct outputs meant to be connected to mic or line inputs?

 

[jmeger]

Thanks for the replies here!

One thing I didn't get from the link is how it's supposed to be used. Are these direct outputs meant to be connected to mic or line inputs?

My intention is to connect them to line inputs. In my case 1 - 3 have been removed from the mix bus, the 4th is still in there.

Are you certain this "feedback" is in the channel or in the mix/master circuit? Do the channel/master pots act on this?

The feedback was only happening in the master channel and would swell up with both the 4th channel pot and master pot completely off.

I've tracked a couple of other issues since the original post though. Now I get a really high feedback on channel 4 with the channel pot all the way up and the master anywhere from 8 or higher. So I started testing caps to see if I fried anything with all my power supply tests and it seems like C32 might be dead. So maybe that's my issue?

Another quick beginner's question here. Is anyone able to explain what's going on with the mod here: https://images.reverb.com/image/upload/s--7-sMyTwH--/f_auto,t_supersize/v1565242202/j8kptw5bbcp0uwvxmbtn.jpg ?  Specifically, it looks like a small film cap has been placed in parallel to R5,13,20,32 and a 475k100v cap has maybe replaced either R8,11,18,30 or R6,14,21,33 (gain). In the second case if it is replacing R6 I guess that's just to decrease the gain and noise?

 

[abbey road d enfer]

it seems like C32 might be dead. So maybe that's my issue?

It is possible, since it decouples the voltage rail taht powers a whole section.

Specifically, it looks like a small film cap has been placed in parallel to R5,13,20,32 and a 475k100v cap has maybe replaced either R8,11,18,30 or R6,14,21,33 (gain). In the second case if it is replacing R6 I guess that's just to decrease the gain and noise?

For the life of me I couldn't guess. I've never seen something like this coming from a respected company. Looks like it's been assembled with a shotgun.

 

[rackmonkey]

For the life of me I couldn't guess. I've never seen something like this coming from a respected company. Looks like it's been assembled with a shotgun.

It’s obviously been heavily modified. The originals I’ve messed with are reasonably neatly assembled. At least you can look at them and tell what’s going on.