Help with diode ID?

[abbey road d enfer]

Quite sure I copied it down accurately.

On the handwritten schemo, you have one pin floating and the on ethat carries signal shorted to ground.
Now with the printed schemo, I can see how it works.

Look Ma, no source resistor.

R202+R116

 

[k brown]

Ah, I see - I misdrew the ground connection on the T3BM.

R202 is just part of an RF filter, along with R201 on the other input pin (with C201 and C202).

I just meant no source resistor to ground, as commonly done.

 

[abbey road d enfer]

R202 is just part of an RF filter

It's also the galvanic connection to the less positive source.
Somehow, it goes to ground, albeit in a tortuous way.

I just meant no source resistor to ground, as commonly done.

Ther are many ways to skin a cat, and many ways to power an element. Ground is just a convention.

 

[FIX]

Their were a whole series of diodes that had a drop of about a diode and a half, that were very stable and used in many discrete op-amps for current source biasing. They aren't available any more, if you come across one, a regular diode in series with a Schottky diode comes close.

 

[k brown]

Interesting idea.

Anyone care to speculate on any advantage this biasing method may have compared to a simple source resistor?

 

[abbey road d enfer]

Anyone care to speculate on any advantage this biasing method may have compared to a simple source resistor?

Compared to the simple resistor, this regulates the capsule voltage over a wide range of phantom voltage.
I belive it is somewhat related to AT's conformity to the 9-52V standard.

 

[k brown]

That would make sense, since there's no zener in this.

 

[abbey road d enfer]

That would make sense, since there's no zener in this.

There's a lot of confusion here, since googling ISS226 results in three different diodes, only one being a zener.
There acid test is to measure the voltage across it. According to measurements on the schemo, I suspect it's a low-voltage (about 2V) zener.

 

[analogguru]

There's a lot of confusion here, since googling ISS226 results in three different diodes, only one being a zener.
There acid test is to measure the voltage across it. According to measurements on the schemo, I suspect it's a low-voltage (about 2V) zener.

It is not an "I" it is a (one), 1ss226 and this is two normal diodes in series, that is 2x 0,65V = 1,3V. 2x 1N4148 (in series) or a BAV99 should do the trick.

 

[abbey road d enfer]

OK, so that means the FET in the capsule operates with this very low voltage. I'm not sure what is the advantage...?

 

[analogguru]

OK, so that means the FET in the capsule operates with this very low voltage. I'm not sure what is the advantage...?

1,2V (1,5V) is a "normal" voltage for the FET's used in electret-microphones. The "advantage" is that it can be operated with a single 1,5V battery (too). The current source shown is probably a 2mA current source. A selected FET with shorted G and S should do the trick.

 

[k brown]

FWIW, the modules in question do not have a battery option - they are phantom only. All the AT modules I've seen the schems for that have a battery option use a simple source-to-gnd resistor (AT8531, etc.).

I copied the schems carefully (despite my drawing error on the input pins) - the AT schems all show that diode with the standard diode symbol; not as a zener.

It'd be a bit surprising if the circuit is operating the FET at that low voltage; the specs for mics using this powering circuit state 138dB max. SPL

 

[k brown]

Just re-confirmed the voltage I noted on my drawn schem.

On the center tap of the xfrmr: 40v (what my mixer generates).
Between each pin the FET connects to, and ground (so between drain/source and gnd): 3.6 - 4v (with capsule/FET connected).

 

[abbey road d enfer]

Just re-confirmed the voltage I noted on my drawn schem.

On the center tap of the xfrmr: 40v (what my mixer generates).
Between each pin the FET connects to, and ground (so between drain/source and gnd): 3.6 - 4v.

I don't get it. Can you complete the attached dwg?

 

[analogguru]

I don't get it. Can you complete the attached dwg?

"what is this" is the mentioned current-source-("diode", made of a FET), delivering probably 2mA.

 

[abbey road d enfer]

"what is this" is the mentioned current-source-("diode", made of a FET), delivering probably 2mA.

There's been a lot of confusion and speculation between this current-regulating diode and the other that's used to bias the capsule.
I'm out.

 

[k brown]

Yeah, that circle-drawn diode aways puzzled me; never seen it before - that's the way it's shown on AT's schems..

Voltage at the output of the 2SA1174 is 2.2v.
Voltage on the source is 3.6v

 

[k brown]

My original question only ever concerned the diode biasing the FET.

 

[abbey road d enfer]

Yeah, that circle-drawn diode aways puzzled me; never seen it before

Well that's a pretty standard symbol, but TBH I thought it was something like a connector, the way it was drawn.

- that's the way it's shown on AT's schems. It's a glass diode on both the SMD and through-hole boards, not a FET.

Current regulating diodes are actually FET's with only two pins.

 

[k brown]

Hmm - even the glass ones? - didn't know that.

I know this is frustrating, but as I said, AT seems to have paid someone to toally scrub the internet of all schematics. Years ago every AT mic info sheet contained a schematic (though sans parts values).

I did find a scan of the AT8532 PM, but it's so low res I didn't think it would be of any use.