C again

[Gus]

just did a fast calc. I was guessing the Id for the J210 (IDSS .2ma to 1 ma)

I am guessing <= .4ma Id because it gives....

.4ma X 2.7K = 1.08V source to ground

.4ma X 19.1K drain R = 7.64V voltage drop across the Drain R

.4ma X 30K =12V

20.72 total giving about 3VDC to 4VDC drain to source.

This would kind of work out if the microphone uses a 24V zener, also the capsule voltage is affected by the current draw. I have not found the current draw of the phantom in specs. If we know this the 10K and zener voltage will let us figure out the capsule voltage

It seems this design balances the voltage drops against the capsule voltage giving a lower voltage at the capsule than most "standard" designs(45V to 60V) it also has what looks like a higher gain active stage because of the SF I guess for the possable lower ratio output transformer.
Could also be the capsule is designed for lower voltage.

bcarso anyone any thoughts?

 

[Rossi]

Gus, your were looking for current draw. The German website says 4mA.

Self noise of the Phantom C is spec'ed at 8mA. So the circuit can't be that noisy.

It's kind of unusual that there are two fets in the circuit. Do you know any other mic circuit that's similar? As far as I know the BC246 is an obsolete part. There must be a reason for using a part that's not easy to source. Also, the J201 isn't a very common part in Germany. I would guess that Dirk Brauner used it for a reason. Maybe he tried different fets and decided for the J201 based on listening tests.

 

[Gus]

BC246 is NS process 50 TYPE(different man different process step #) part(from my databook) speced for IDSS of 7 to 12ma a bf something should be the same or close at the same IDSS. 3 or so ma would be a good spot for the source follower fet with it rating of 7 to 12 ma.

I don't understand not using a more predictable BJT as an EF, unless it is for some harmonics set by both bias points of the fets for a certain gain stage "sound" at high levels.

The J201 has "tight" IDSS spec looking at the numbers of .2ma to 1ma

Some china microphones have a fet and a pnp and a 2:1 transformer.

4ma in the phantom resistors would be about a 13.6v drop for about 35v at the capsule.