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.
R202+R116
Help with diode ID?
- Thread starter k brown
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Now with the printed schemo, I can see how it works.
k brown
Well-known member
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.
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.
It's also the galvanic connection to the less positive source.
Somehow, it goes to ground, albeit in a tortuous way.
Ther are many ways to skin a cat, and many ways to power an element. Ground is just a convention.
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
Well-known member
Interesting idea.
Anyone care to speculate on any advantage this biasing method may have compared to a simple source resistor?
Anyone care to speculate on any advantage this biasing method may have compared to a simple source resistor?
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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
Well-known member
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
Well-known member
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.
OK, so that means the FET in the capsule operates with this very low voltage. I'm not sure what is the advantage...?
analogguru
Well-known member
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
Well-known member
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
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
Well-known member
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).
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).
I don't get it. Can you complete the attached dwg?
Attachments
analogguru
Well-known member
"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
Well-known member
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
Voltage at the output of the 2SA1174 is 2.2v.
Voltage on the source is 3.6v
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k brown
Well-known member
My original question only ever concerned the diode biasing the FET.
Well that's a pretty standard symbol, but TBH I thought it was something like a connector, the way it was drawn.
Current regulating diodes are actually FET's with only two pins.
k brown
Well-known member
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.
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.
Attachments
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