Neumann U67 Clone : D-U67 Tube Microphone Build Thread.

[Moby]

You seem to know a thing or two about transformers
Is it possible though that the slope in the low cut with ioaudio transformer is more broad? That would explain the dark character despite the seemingly correct calibration

Huh... No. Low end is the result ot the LC filter, actually it's the -3db point. I can't say what exactly "cuts" the high end in this case, but probably the wrong ratio of the feedback windings.

 

[Purplenoise]

Huh... No. Low end is the result ot the LC filter, actually it's the -3db point. I can't say what exactly "cuts" the high end in this case, but probably the wrong ratio of the feedback windings.

Sorry I meant to say low pass ( high cut)…

 

[dmp]

I measured the windings on a different mic transformer BV11 last night with no problem. Voltages made sense, signals in phase, n=10 as it was supposed to be.
Tried to measure the BV12s again - same deal, voltages don't make sense, signals out of phase like the windings are open. I get the same thing on Ioaudio and Moby. No idea what I'm doing wrong. I'm following the color code on the schematic to pick the wires and the wires do show ohms with a multimeter (not open).

 

[JerryPbury]

Tried this?

 

[dmp]

Yes, that's how I did it

 

[JerryPbury]

I changed the drawing because the ground lead was originally incorrect. Make sure it’s in the right spot.

 

[dmp]

I didn't hook up the black 'ground' wires when I connected the sig gen / scope. The black grounds are for the cores I believe. Shouldn't matter. Or maybe that's what I'm doing wrong? I have the different board style. But when I connect the transformers up to the pcb, the mic works with them. I'm just trying to figure out the feedback winding.

 

[JerryPbury]

That’s how I hooked mine up (2 mics) and they work perfect. I tend not to investigate the components unless I am troubleshooting. All voltages were to spec. Great mics!

 

[dmp]

That’s how I hooked mine up (2 mics) and they work perfect. I tend not to investigate the components unless I am troubleshooting.

Was the amplifier frequency response correct?

 

[Studio Mollan]

There's a big difference between 'sounds dark' and statements about the amplifier frequency response being wrong with the ioaudio. I have a build with ioaudio transformer, c17=100pf, and the freq response measures correct (-10db @ 15khz). Without c17 installed the mic will be MUCH brighter. Not sure how the calibration could be right without c17 since that is the negative feedback path.
Be nice to figure out with some measurements what is going on. I've been curious about this for years.

That's interesting. Maybe just some of the IOaudio transformers are bad then?
I built two identical microphones with IOaudio transformers. The pair sounded identical, and both were off with ANY cap installed @C17. Only if I removed it I got the correct HF response, -10dB @15k.
First build was on Dans, poctop, boards, and then on a PCB set that IOaudio made with all new components except capsule and transformer. Both versions sounded EXACTLY the same. -> the transformers or capsule were bad, and equally so between the pair.
Jim50Herts got the same results and bought a genuine Neumann capsule to compare with. beesneez k6, and still same dull response. -> it is the transformers. I'm fairly confident a better replica transformer would transform mine to sound closer to a u67.

They are still prey useable as they are now though, with the dull transformer and no C17 but a slightly different animal. More "wild" sounding, and not as controlled as a genuine u67.

 

[dmp]

That's interesting. Maybe just some of the IOaudio transformers are bad then?
I built two identical microphones with IOaudio transformers. The pair sounded identical, and both were off with ANY cap installed @C17. Only if I removed it I got the correct HF response, -10dB @15k.
First build was on Dans, poctop, boards, and then on a PCB set that IOaudio made with all new components except capsule and transformer. Both versions sounded EXACTLY the same. -> the transformers or capsule were bad, and equally so between the pair.
Jim50Herts got the same results and bought a genuine Neumann capsule to compare with. beesneez k6, and still same dull response. -> it is the transformers. I'm fairly confident a better replica transformer would transform mine to sound closer to a u67.

They are still prey useable as they are now though, with the dull transformer and no C17 but a slightly different animal. More "wild" sounding, and not as controlled as a genuine u67.

Very interesting, thanks for sharing. It has been hard to figure this out because some people have them and think they are correct, others don't. Maybe there is some variation between the ioaudio transformers. I think a lot of people have put in Moby's transformer to get closer to the real thing.
I have a M269 and U67 built with ioaudio transformers and they share the same character (like you're describing). They are good mics, but distinct that way.
Maybe some of the transformers had a high freq rolloff and a problem with the tertiary winding? Without C17 I think the transformer must have been severely rolling off the highs

 

[Jim50hertz]

I'm still testing but I reckon with Moby's transformers I'm going to hit 250-270pf in C17. With around 210pf in there I'm around -7db or so at 15kHz. For this value at c17 the mic on test exhibits a lot more presence than with the previous transformer. When I get it to spec I'll have more of an idea of the difference.

 

[dmp]

So you something similar to what I showed with the 'typical' neumann ~100pf feedback cap? (-4.5 dB @ 15kHz)
A larger cap needed to increase the negative feedback signal.
Does that imply the inductance is quite a bit different than the Neumann transformer?

 

[Purplenoise]

I'm still testing but I reckon with Moby's transformers I'm going to hit 250-270pf in C17. With around 210pf in there I'm around -7db or so at 15kHz. For this value at c17 the mic on test exhibits a lot more presence than with the previous transformer. When I get it to spec I'll have more of an idea of the difference.

Those are the values I ended up with in both my 67s. One is with Moby but the other one is with haufe ( same as Neumann reissue) so I don’t think Mobys transformer is bright. Maybe something is different in the circuitry compared to Neumann?

 

[Jim50hertz]

Those are the values I ended up with in both my 67s. One is with Moby but the other one is with haufe ( same as Neumann reissue) so I don’t think Mobys transformer is bright. Maybe something is different in the circuitry compared to Neumann?

You’ve mentioned comparing yours to vintage 67s?

 

[Purplenoise]

You’ve mentioned comparing yours to vintage 67s?

Yes vintage and current reissue

 

[Jim50hertz]

Yes vintage and current reissue

And favourably so, did you measure the 15k response with your 250pf C17? Or tuned by ear?

 

[Purplenoise]

And favourably so, did you measure the 15k response with your 250pf C17? Or tuned by ear?

By ear, tried to match the original. I have c17 socketed for easy swap. On both I have original k67s so with other capsules it might be different

 

[JerryPbury]

Was the amplifier frequency response correct?

I have not measured it. What was your test procedure?

 

[Jim50hertz]

250pf seems about right from what I’m measuring so far, I don’t have an original or reissue to compare. I’ve a K67 and a K6, testing with the K67.