Help Identifying PCB

[DSpenceATL]

Hello all,

I recently acquired a bunch of gear in a trade, and amongst the pile was a U47 body which the other guy claimed was a “solid state U47 style mic”. Cool, I thought, but it wasn’t really my top priority, just some cool studio decor since it has a Neumann badge glued on it.

I finally got around to opening this thing up and see what the deal is, and I don’t really recognize the PCB, it certainly doesn’t appear to be anything U47-ish to my eyes. As far as I can tell, it’s a Transound TSB-25AX glued onto the deck of a somewhat decent Chinese U47 body, with this unknown PCB.

Testing yields a pretty loud ground hum over the signal, but I’m not sure I want to even put in the time trying to track it down if this PCB just belongs in the trash.

I certainly don’t expect this to be anything revolutionary, but if it’s even a half decent utility mic, it might be fun to play with.

Thanks all!

 

[Khron]

What in the hell is THAT electronic abomination?!???

That ground hum you're getting is likely because the phantom power supply collapses to next to nothing, with those three(!) dual opamps. Just the idle current of those adds up to at least 12mA (phantom power can supply up to ~14mA into a dead short, but you get 0v that way).

 

[DSpenceATL]

What in the hell is THAT electronic abomination?!???

That ground hum you're getting is likely because the phantom power supply collapses to next to nothing, with those three(!) dual opamps. Just the idle current of those adds up to at least 12mA (phantom power can supply up to ~14mA into a dead short, but you get 0v that way).

Yeah I have no earthly idea. Hoping somebody will recognize it and shed some light. My curiosity is driving me crazy on this one.

 

[Khron]

Well, you could consider reverse engineering the circuit - doesn't seem to be too-too complicated of a thing.

 

[Ike Zimbel]

Well, mics are not really my thing but...a few things jump out at me:
1) I question whether those extra long leads from the capsule are adding to the noise susceptibility?
2) The circuit board, which from its shape must have been intended for a u87 type body, is not grounded to the mic body/frame. Probably should be.
3) The soldering on the two 10uf/100v caps at the top is definitely suspect...no flow-through at all on one lead each, and on the solder-side pic, you can see the outline of one lead, which usually means it's not really soldered.
4) I can't get my head around the need for a 5 watt resistor in a mic...

 

[DSpenceATL]

Well, mics are not really my thing but...a few things jump out at me:
1) I question whether those extra long leads from the capsule are adding to the noise susceptibility?
2) The circuit board, which from its shape must have been intended for a u87 type body, is not grounded to the mic body/frame. Probably should be.
3) The soldering on the two 10uf/100v caps at the top is definitely suspect...no flow-through at all on one lead each, and on the solder-side pic, you can see the outline of one lead, which usually means it's not really soldered.
4) I can't get my head around the need for a 5 watt resistor in a mic...

Yeah, everything about this thing seems insane. I was thinking maybe this PCB came from some aliexpress U87 type mic based on the taper of that board, but couldn’t find any lookalikes. It’s pretty tempting to just rip all of this out, use the Transound capsule for something else, and try to build some kind of 47ish thing with the body, but even the design of this Chinese body is kinda questionable. With Only 2 posts on the side, I don’t think I can fit any Poctop boards or anything in here easily.

 

[rock soderstrom]

It’s pretty tempting to just rip all of this out, use the Transound capsule for something else, and try to build some kind of 47ish thing with the body, but even the design of this Chinese body is kinda questionable. With Only 2 posts on the side, I don’t think I can fit any Poctop boards or anything in here easily.

Good idea! With a little elbow grease you can make your own holders for the PCB

 

[DSpenceATL]

Good idea! With a little elbow grease you can make your own holders for the PCB

I always liked the idea of the rounded Fet 47 poctop boards, but I suppose I could rig up the square ones to fit a little easier, and then I would just need to find a way to keep the transformer locked in place..

 

[Voyager10]

What the actual ...?

I'm laughing at the way the board is tie-wrapped into place. It looks like there may not be a reliable connection between XLR pin 1 / circuit ground and the metal case. What's supposed to happen here is those exposed tinned traces make contact with the frame because it's bolted in place.

(Also ... a five watt 4.7 ohm resistor? That's, like, an amp of current. Where's that coming from, then?)