Demeter vtmp-2b tube balanced outs with no Xformers?

[Derrils]

Hey all,

I recently noticed some strange things regarding a used Demeter VTMP-2b I bought, and one in particular I wanted to ask the internet about.

I used the 1/4" output at first then realized that it is unbalanced, to keep the signal path completely tube (aside from the input transformers). The "b" version, which this preamp is, also has balanced xlr outs. Since my curiosity was piqued, I opened it up to see what's inside.

Am I missing something here? I don't see any output transformers or IC's. How can the xlr be balanced in this case?

I apparently have an early model, as the input transformers are Jensen, which were old stock and no longer in production. Is it possible that at this point in Demeter's history that they added an unbalanced xlr?

Bonus question, since I've only a hobbyist's knowledge of Electrical Engineering:

Would this unit benefit from re-capping? There are a decent amount of electrolytics for such a simple circuit, but I've heard mixed things about the value of recap jobs.

My jpg seems to be failing a security check, so the pics are here: https://imgur.com/a/PUJN5QJ

 

[emrr]

impedance balanced output maybe. 

 

[ruffrecords]

I am curious about that odd component immediately above the output jacks. Is that a 3.5mm jack?

As far as I can see from the wiring, the XLR output is not balanced. Unless there is a resistor on that switch PCB in which case it could be impedance balanced as EmRR said.

Cheers

Ian

 

[pucho812]

there was a hand drawn schematic that was floating around here.  maybe it's on the tech documents section?

 

[Derrils]

I am curious about that odd component immediately above the output jacks. Is that a 3.5mm jack?

It's a little bigger than a 3.5mm jack.  I'm assuming it's TT for connecting straight to a patch bay, but again, not sure. 

I believe the attached are the schematics that were on this board, but they're too small for me to make sense of.

 

[Derrils]

Got around to testing it out with different situations.

I have to conclude that it is impedence balanced - though I admit I don't have the technical knowledge to say so by just looking.

I tried using the 1/4" outs as well as the xlr outs and got different results. I used a handheld dynamic mic very close up to speak into. The 1/4" didn't seem to have the boomy proximity-effect low end and was certainly brighter, but at about 15 dB lower level.

The 1/4" out gave unpredictable results being patched into other balanced gear while the  xlr was consistent and sounded as expected.

There is no visible resistor on the output switch as ruffrecords suggested, but maybe there is on the main pcb prior to the wires connecting the switch. The wire connections run under the pcb and I don't have it far enough apart to see.

Thanks for your help everyone.  I'd still love to hear opinions on whether it's worth recapping.

 

[fazer]

A friend has one and I repaired it one time.  It’s unbalanced to XLR out.  Look at mastering labs mic pre puncho put up.  It’s the same way.  Back in late days of analog tape, there were many unbalanced lose  the transformer to get rid of the transformer colorations.  I new a tech that would pull the transformers on ampex and MCI tape machines and run unbalanced for faster rise time.  Today most people don’t like fast rise time and digital sound.  They want fat / at times muddy glue.  Usually that means less rise time with phase shift.

 

[pucho812]

It's a little bigger than a 3.5mm jack.  I'm assuming it's TT for connecting straight to a patch bay, but again, not sure. 

I believe the attached are the schematics that were on this board, but they're too small for me to make sense of.

yes it is a TT jack for going directly into a TT patchbay.

 

[PRR]

Your pictures show a switch "Pin 2 hot" "Pin 3 hot". Could be a clue?

 

[pucho812]

Your pictures show a switch "Pin 2 hot" "Pin 3 hot". Could be a clue?

Not sure who's pictures you reference, but the mastering lab tube mic pre has that and is unbalanced out.

 

[ruffrecords]

The blurry schematic looks rather reminiscent of the Pultec MB1 tube mic pre without the output transformer and  with modified gain switching using Vactrols.

https://medias.audiofanzine.com/files/pultec-mb-1-471599.pdf

Cheers

Ian

 

[Derrils]

That's interesting.  If it is actually unbalanced, I wonder what would account for the different performance I get when using TRS vs XLR outputs.

 

[gyraf]

Output seem to be a simple cascode 12BH7..

/Jakob E.

 

[abbey road d enfer]

Not sure who's pictures you reference, but the mastering lab tube mic pre has that and is unbalanced out.

Yet there are 3 wires coming to the switch, which suggests the impedance balanced components are located on teh main board, close to the active leg. that would be just right.

 

[PRR]

Output seem to be a simple cascode 12BH7..

White cathode follower.

 

[gyraf]

.. yep..