Another interesting mic preamp

An interesting (I think ) design for a mic preamp optimized for the stereo miking techniques.
I have chosen the 12BZ7 for it's lower Ri, which paralleled reaches around 16K .
The interstage transformer has multitapped connections on both sec's, to enable accurate attenuation from the transformer ratio instead of resistors or pots.
Stages are fixed biased, as you can see.

Please post your opinion/comments/suggestions. :roll:

Respect,

Val.

:idea:

what do i think? i think you should look at the ssl ethic thread in the brewery. LOL

Interesting design. Two possible weak points:

The transformers are unterminated, and will probably ring. On the other hand, this is the kind of load classic ribbon microphones were designed to see.

The stepped gain controls, with no trim, leave you with no way to compensate for unequal tube gains or unequal mic sensitivities. Not an issue with individual microphones, but an issue for stereo pairs.

Peace,
Paul

Nice!

I am wondering if when the innerstage, when usuing taps, will load
down the tube grid too much.

If you link to that, we can lose the side bar.

[quote author="solder_city"]what do i think? i think you should look at the ssl ethic thread in the brewery. LOL[/quote]

He's at it again....groan

analag

C1 C3 are drawn with wrong polarity.

You need a way to check bias. I suppose that measuring plate voltage will work, by measuring current in R7 R9.

The cost of the many-many-tap transformer is A Problem.

[quote author="pstamler"]The stepped gain controls, with no trim, leave you with no way to compensate for unequal tube gains or unequal mic sensitivities. Not an issue with individual microphones, but an issue for stereo pairs.
[/quote]

This design involves the careful selection of the tubes pairs and also matching the internal triodes.
Do not think it's a big issue for selected stereo pair mics.

[quote author="PRR"]C1 C3 are drawn with wrong polarity.[/quote]
Updated the schem.

[quote author="CJ"]I am wondering if when the innerstage, when usuing taps, will load down the tube grid too much.[/quote]

Unavoidable in this configuration.

[quote author="pstamler"]The transformers are unterminated, and will probably ring[/quote]
When I'm building the prototype, if there is ringing, I will AOT the termination resistors, and add them accordingly.

RASpect,

Val.