Neumann Vintage U87 Clone : Build Thread.

[JimSalabim]

OK, so now that I have also built a U87ai from the current Mic & Mod kit, I have now put its K87 capsule (with isolated backplates) in the U87i (the one we’re talking about here) just for trying.
Now everything is working, BUT I have a very high noise floor in figure-8 mode now. No hum, rather hiss and crackling noise, and only in figure-8.
Any idea how I can solve this issue?
If I’m successful then I would probably just order another K87 capsule so that both mics work properly.

EDIT: Cleaning the PCBs helped removing the noise completely. With the K87 capsule everything is working PERFECTLY.

 

[Horizoneer]

I've ordered everything I need to make a pair of these, except I'm having trouble finding 68Mohm resistors in the UK. Would anyone mind pointing me in the right direction? I found a few 1w ones from Taiwan but that seems overkill...

 

[Ricardus]

I looked around and there aren't a lot of good choices. 60 megohm will work fine there also, FYI.

 

[Horizoneer]

I'm struggling with those as well. Maybe I could line the inside of the metal body with 60 1m resistors in series, they could help dampen resonances too.

Are carbon comps an option? I know they tend to be noisier...

 

[abbey road d enfer]

Are carbon comps an option? I know they tend to be noisier...

What difference do you expect with carbon comp resistors in this position?

 

[Horizoneer]

What difference do you expect with carbon comp resistors in this position?

I don't know, it's why I'm asking. In guitar amps they can add noise in certain positions but those are much higher voltage and current situations, I just don't know in this circuit.

 

[abbey road d enfer]

Carbon comp resistors are very imperfect elements, in particular they have a high voltage coefficient (the resitance varies with the voltage applied), which results in rather significant distortion. In tube guitar amps, it results in 2nd-order harmonics, which are supposed to be euphonic on single notes (but bad on double/triple stops) and chords. They are also quite noisy when submitted to voltage.
In a mic head amp, the voltage across is very small, so none of these effects is really significant. However, being more expensive and less reliable than metal film, there is no good reason for using them.

 

[Horizoneer]

Carbon comp resistors are very imperfect elements, in particular they have a high voltage coefficient (the resistance varies with the voltage applied), which results in rather significant distortion. In tube guitar amps, it results in 2nd-order harmonics, which are supposed to be euphonic on single notes (but bad on double/triple stops) and chords. They are also quite noisy when submitted to voltage.
In a mic head amp, the voltage across is very small, so none of these effects is really significant. However, being more expensive and less reliable than metal film, there is no good reason for using them.

Yeah, I know about the voltage coefficient in guitar amp applications - I was just wondering because the low currents and very high resistances involved in my question, which is about this particular circuit, are well beyond my own direct experience (which started when I spent sessions swapping components in a Vox ac30 trying to discern what and where component choices matter)

Is there still no good reason for using them if you literally can't find any source for non carbon comp choices?

 

[EarthTone]

I believe I found 68Mohm resistors in stock at Newark which is a UK company

 

[Horizoneer]

I believe I found 68Mohm resistors in stock at Newark which is a UK company

Perfect, got them! Much obliged, now I just need to wait for the postman to do his thing, and I've got a project to keep me occupied on evenings after my kid's gone to sleep

 

[egroys]

I am realizing my 87s are out of phase.

Can someone please confirm, since there appears to be conflicting data, which is the correct wiring for the Cinemag 13113?

is it like on the guide on Dany's site:
red = p+
brown = p-
yellow = s+
orange =s-

or:
P+ = Brown
P- = Red
S- = Yellow
S+ = Orange

 

[abbey road d enfer]

I am realizing my 87s are out of phase.

Can someone please confirm, since there appears to be conflicting data, which is the correct wiring for the Cinemag 13113?

is it like on the guide on Dany's site:
red = p+
brown = p-
yellow = s+
orange =s-

or:
P+ = Brown
P- = Red
S- = Yellow
S+ = Orange

Both are equivalent. One has polarity reversed on both primary and secondary, which makes it also correct.

 

[vinylwall]

True that either way is fine. However, this is from Cinemag's website:

RT = P+ = Brown
SW = P- = Red
WS = S+ = Orange
BL = S- = Yellow

 

[egroys]

So I have mine wired to Cinemag’s specs and my polarity is in fact reversed.
Is the best/correct/easiest way to mitigate this by inverting pins 2&3 or rewiring the transformer?

 

[terry setter]

Which ever change is easier to make is the best way. You accomplish the same thing.

As for Cinemag, think of the rainbow, or colors of resistors (even better) Brown, R, O, Y, G, B... (I left out black). Cinemag seems to "from brown, up". So, Brown will be primary positive. Red will be primary negative. Orange will be secondary positive, and yellow will be secondary negative. Of course, if you flip both pairs (as per the earlier comment) you get the same functionality, just different wire colors. So, if you think red should always be the hot lead and brown seems like a good ground color, go with the flipped version.

 

[Ricardus]

So I have mine wired to Cinemag’s specs and my polarity is in fact reversed.
Is the best/correct/easiest way to mitigate this by inverting pins 2&3 or rewiring the transformer?

But either wiring scheme will be in phase. So what's going on?

 

[egroys]

But either wiring scheme will be in phase. So what's going on?

My understanding (and how I came about noticing this) is the signal is inverted compared to say my SM57 or whatever...

 

[Ricardus]

My understanding (and how I came about noticing this) is the signal is inverted compared to say my SM57 or whatever...

from the way you're writing that, it doesn't sound like you're completely convinced.

 

[egroys]

from the way you're writing that, it doesn't sound like you're completely convinced.

What I’m not clear on is what you mean by out of phase. The microphone sounds excellent, but the polarity is (was) inverted from other microphones.

I just flipped pins 2&3 last night so everything is consistent and copacetic

 

[Ricardus]

What I’m not clear on is what you mean by out of phase. The microphone sounds excellent, but the polarity is (was) inverted from other microphones.

I just flipped pins 2&3 last night so everything is consistent and copacetic

But the polarity should have been correct if you wired the transformer in either way, so I'm still confused. Maybe the other mics are wrong?