[Winston O'Boogie]

Thanks for the explanation Ruud, makes sense to me.
Do you think that Neumann taking the feedback for de-emphasis from a tertiary winding was mostly to straighten out  non-linearities in the transformer they used? 

[RuudNL]

Yes, I think so.
The problem is that a good transformer with a tertiary winding is hard to find and if you can find them, they are expensive.
A transformer is nothing else than a 'translator' between electricity and magnetism. (And back of course.)
That means that the core properties also reflect to a certain extend to the primary winding. (Lenz's law.)
I have chosen to use HF feedback from the primary winding in order not to overcomplicate the circuit and keep the output transformer (relatively) low cost.
And judging by the people who have replicated the design, everybody is happy with the result.

[Winston O'Boogie]

 Thanks again Ruud. 
Yes I like not being tied to the available transformers with a tertiary, and I like that your HF feedback is not over complicated. 
I was going to be using a Neumann U87ai capsule & headbasket along with some spare Neumann internal rails and bottom bell so I don't know that the kit  boards will physically work there but, I'd like to try your circuit so I'll do a wee pcb layout myself soon.

Cheers 
 

[RuudNL]

Great! I would love to hear about your results.
Some experimentation with the de-emphasis capacitor may be needed for the best result.
Because it depends upon the microphone capsule you are using.

Success!

[Gus]

I waited a few days to see if anyone noticed what side of the output cap C9 picks up the signal

[dmp]

I compared mine with a 'real' U67, but I didn't hear any difference!

You don't hear a difference between the difference in cathode bias to a U67?

[RuudNL]

You don't hear a difference between the difference in cathode bias to a U67?

No. There may be a small difference, but I didn't hear it.
Anyway: it is good enough for me as it is.

[abbey road d enfer]

You don't hear a difference between the difference in cathode bias to a U67?

The difference between cathode bias and fixed bias is that LF resposne of teh former decreases starting at about 10Hz, falling by 10dB at 1Hz. Does it really matter?
I know that a certain microphone historian claims it is audible because of some elusive phase artifact, but I don't subscribe.

[Winston O'Boogie]

I know that a certain microphone historian claims it is audible because of some elusive phase artifact, but I don't subscribe.

I'm aware who you're referring to and I have to say, I've wondered sometimes about the extent of his technical expertise.  I don't wish to speak badly of anyone and  I know he's respected for what he does but...  Hmm?
I'd be curious if there's  a noise difference between both bias schemes due to the impedance at the cathode though.


Regarding Gus's comment over the location of cap C9 in this schematic:  my thinking was more curiosity about the following shunt resistors'  loading effect on the J-Fet (*Edit* : of course, I meant valve)  signal vs the implementation in the U67 where these are shunted across a low impedance tertiary winding.    But I haven't built  or analysed the circuit here so?




 

[abbey road d enfer]

I'd be curious if there's  a noise difference between both bias schemes due to the impedance at the cathode though.

Regarding Gus's comment over the location of cap C9 in this schematic:  my thinking was more curiosity about the following shunt resistors'  loading effect on the J-Fet signal vs the implementation in the U67 where these are shunted across a low impedance tertiary winding.    But I haven't built  or analysed the circuit here so?

My sim shows no visible difference.
The roll-off seems quite intense, though. -3dB @3kHz, -13 @10kHz.Much more than a real U67.
As far as noise is concerned, I would think the major difference is in the pentode connection, with associated partition noise, as opposed to Neumann's triode connection.

[Winston O'Boogie]

OK, earlier I wrote "J-Fet  signal" which was clearly  an idiotic mistake  .

Regarding  the partition noise you mentioned -  the schematic here shows a triode connection so I'm completely lost now! 
 

[abbey road d enfer]

Regarding  the partition noise you mentioned -  the schematic here shows a triode connection so I'm completely lost now! 

You're absolutely right! I don't know what I was thinking...

[Winston O'Boogie]

You're absolutely right! I don't know what I was thinking...

You said Pentode, I said J-Fet, let's call the whole thing off!   

[RuudNL]

The roll-off seems quite intense, though. -3dB @3kHz, -13 @10kHz.Much more than a real U67.

I think here is a difference between the 'real thing' and a simulation.
With a signal injected in the 'test input', I measured about -5 dB @ 15 KHz. (Re. 1 KHz.) That is with the capsule in place.
-13 dB @10 KHz would result in a very dull sounding microphone!
With an original Neumann capsule, I even had to increase the value of C9 a bit, because with 120 pF is was a bit too bright, compared to a U87ai.

[abbey road d enfer]

I think here is a difference between the 'real thing' and a simulation.

Don't I know! 

With a signal injected in the 'test input', I measured about -5 dB @ 15 KHz. (Re. 1 KHz.)
[/quote] OK; I have to reconcile that with the simulation.

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