Neumann U47FET Clone: D-U47 FET Microphone Project PCB Build thread

[Spencerleehorton]

T3 looks like you have a problem as more than 0.6 between E and B.

 

[duantro]

:

T3 looks like you have a problem as more than 0.6 between E and B.

That's right!, I'll switch it out and see if it helps.
Edit: Swapped T3 out and same measurements.  Checked all connections, and component values. Would a fake 2n3819 cause this? I've read about people getting fake ones? Not sure what is up here...

 

[Spencerleehorton]

I would change T1 and remeasure

 

[duantro]

I changed T1 and same voltages everywhere.  I get get a pretty quiet signal on the mic, which I'm assuming from the low polarization voltage. What the hell is going on here?!

 

[duantro]

Still banging my head against the wall on this one. Do the outer screw holes have to be grounded for the circuit to work? The mouser cart has 4/40 screws that are way to big for these holes, ...without drilling. I'm using Spencer's bodies that have flat rails and don't work with the mounting hardware, without some  fabrication. I'm noticing that the holes on the hiZ side don't have continuity to the opposite boards mounting holes. Would this have any affect on it working?

 

[duantro]

Since I had concern with Pad "A" being shorted out and damaging something, I swapped out the pcb's for the 2nd set of boards that I've populated the same,  and still really low output. So frustrating.  What now....The telefunken transformer has a bit higher resistance than the others have posted here 2k2: 120r. Would that affect the circuit adversely? Trying to think outside of the box here.

 

[Spencerleehorton]

I’ve got 4 of these circuits on the bench, when I get some time I’ll help, anyway you could just have capsule in headbasket and test circuit out of body?

Also continuity test all connections as much as you can, I’m sure you’ve probably done this!

I’ve got a working version so will get some voltages for you,

 

[duantro]

Thanks Spencer.
I put it down for a minute and built my second pair  km84’s  that just showed up. I finished them and will calibrate tomorrow. I used the same 2n3819’s and figured I’d at least see if the batch I received were suspects in the fet47’s.

 

[duantro]

Ok, so the Km84s work like a charm (3 hour build with zero head scratching!) with 2n3819's from the same batch, so that doesn't seem like the issue. In comparison, the idle room noise with the gain cranked up on my Neve pre is -10db on the km84's, and -60 on the fet47. When checking polarization voltage on r8 starts out at around 38vdc and drops to 15 vdc when meter is applied. The only thing I can guess is something is dragging polarization voltage down.
edit:
Changed T1 yet again, changed capsule wires, and checked component values again.  Same results. Maybe this would sound good on a 100db kick drum?

 

[Spencerleehorton]

Dodgy capsule maybe?

What is you phantom volts on pins 1/2?

Do you have a frequency generator and a scope?

 

[poctop]

See Page 1    4th picture allthe requirement ,
also ake sure you have the backplate connected on the right side of the connector there is only one side of this connector that goes to gnd ,

Backplate is ground in this case , just to make sure ,
Best,
Dan,

 

[duantro]

Unfortunately my tek 465 just died, so I'm kind of shooting from the hip here, until I can get another one.  I tossed it on a kick drum, and think it may be working, but possibly a transformer issue. The signal is clean, just real quiet. I'm going to check the wires on it.

edit: Thanks guys, Back plate is correctly connected. Getting phantom power as T7 collector is showing 44.74. What are you getting for voltage at r8? Is this transformer's impedance an issue? Could the polarity of the transformer cause a drop in level if not connected correctly? This transformer has all clear coated wires. Output phasing is correct with my beta 52.
If I boost the clip gain by 15db in pro tools, the signal is great. 

 

[Spencerleehorton]

If it sounds right and it’s just your level, it could be your transformer, try it the other way round?
Quick fix to try.

 

[Spencerleehorton]

do you have another transformer you can try? i have some OEP 6.5:1 ratios ones that i've used to test before, ideally a 9:1, 9.5:1 or 10:1 will work nicely.

 

[duantro]

Yeah, I’ve switched the primaries and secondaries around the other direction, and it gets quieter.  Can anyone verify voltage at r8?

 

[Spencerleehorton]

I’ll have a look in the morning, I’m knackered now after 2 full van loads of crap from the garden, monster dog walk then football practice!!!

 

[duantro]

I’ll have a look in the morning, I’m knackered now after 2 full van loads of crap from the garden, monster dog walk then football practice!!!

That sounds like a full day! Let me know what you find out when you get a minute to check. I plan on saving up for a pair of capsules and transformers that match at some point, so if my voltages are on with yours here, maybe that's my next step.

 

[Spencerleehorton]

Could you email me some photos or post up some?
Do you have any other transformers you could try?
Most of the time it’s not about finding the fault but eliminating the possible problems.

 

[duantro]

I posted photos of the build  a page back. The only other mic transformer I have is a 2:1 mxl, and a 3:1 lomo 19a9, so nothing that I think would work.
I agree about finding a possible problem, that’s why I was asking about polarization voltages  at r8
Edit: I forgot, I do have a t14 transformer that I can try. It has about the same impedance as the telefinken.

 

[Spencerleehorton]

T14 is 12:1 so would work and if level is better would then prove your txf is faulty.