Microphone preamp impedance and "phantom" resistors

abbey road d enfer said:

DC magnetization is caused by different turn number, not DC resistance differences. It is perfectly normal for windings to have slightly different DCR even when the turn number is the same. Different average diameter.
However, it may result in some differential DC voltage, which the xfmr in the mic may not like.

Click to expand...

?????
Sorry Abbey, maybe I wasn't clear.
I'll try again...

Suppose the HQ UTC transformer has a turn number exactly equal in both halves.
Both halves have a different DC resistance. By phantom over the center tap the DC current drawn by the microphone will not be the same on both sides.

Nor the DC loss over the winding, nor the current drawn is equal on both sides. Cancellation will not be perfect. This is causing magnetization. (Very very very little as proven by my measures.)


'However, it may result in some differential DC voltage'
Different DC resistance causing differential DC voltage= DC over the primary=magnetization.
Different DC resistance in both windings is causing (very little) magnetization.

some differential DC voltage, which the xfmr in the mic may not like.
= because also this xfmr will suffer (very very little) from magnetization...

Of course, this is all sooooooooooo ridiculously minimal, as shown in my previous post...

Paul

[Edit:]
Sorry again Abbey, now I see...
You probably meant to say:
"Values are so low that magnetization caused by DC resistance differences is neglectable, quasi non-existent..."

I didn't get it!

Thanks for looking at that in detail!

aazaa said:

Suppose the HQ UTC transformer has a turn number exactly equal in both halves.
Both halves have a different DC resistance. By phantom over the center tap the DC current drawn by the microphone will not be the same on both sides. ....
...Different DC resistance causing differential DC voltage= DC over the primary=magnetization.

Click to expand...

Think it over.
Hypothetically, the microphone could be a perfectly balanced current sink, as a perfect Schoeps circuit could be.
Each primary half would deliver the same current so there would be no magnetization. However the voltage at each end would be slightly different due to the difference in DCR.

In practice, interaction with the microphone is equally important as the input xfmr balance.
It's a Wheatstone bridge where current must be nulled, not voltage.

Interesting measurements. Thanks.

abbey road d enfer said:

Think it over.
Hypothetically, the microphone could be a perfectly balanced current sink, as a perfect Schoeps circuit could be.
Each primary half would deliver the same current so there would be no magnetization. However the voltage at each end would be slightly different due to the difference in DCR.

In practice, interaction with the microphone is equally important as the input xfmr balance.
It's a Wheatstone bridge where current must be nulled, not voltage.

Click to expand...

Indeed, interaction with the microphone is important as shown in my measurements. Different microphones are giving different results.

Ok Abbey, about the rare case of a Schoeps microphone having the ability to compensate for DC differences.
Most microphones do not have this ability.
In most cases, for example with Neumann U87, if there is different DC at both ends, the microphone will draw different current on both sides - causing magnetization. Different DC at both ends will also cause transformer magnetization inside this microphone.

But we are hopeless, aren't we ???, discussing this irrelevant, extremely low, near zero magnetization ...

Paul