Any drawbacks/compromises with this simple capsule polarization?

[k brown]

Just a center-tapped output trafo feeding phantom to the capsule (plus, I suppose a 1M/small cap ripple/noise filter before the 1 Gig resistor).

This would obviously be for capsules that are happy at 40-45v polarization.

 

[Khron]

Not at all, as far as i'm concerned. That 40-45v's assuming the circuitry would draw next to no current though (you still have the 2x 6.8k inside the preamp)

 

[k brown]

Not at all, as far as i'm concerned. That 40-45v's assuming the circuitry would draw next to no current though (you still have the 2x 6.8k inside the preamp)

Yeah, the circuit I first wanted to try this with has only a single bipolar transistor in it.

 

[ccaudle]

the circuit I first wanted to try this with has only a single bipolar transistor in it.

Bipolar transistors don't have high enough input impedance to work effectively as a condenser capsule buffer. There is a reason why every mic circuit you have seen starts with a jfet.

 

[k brown]

Bipolar transistors don't have high enough input impedance to work effectively as a condenser capsule buffer. There is a reason why every mic circuit you have seen starts with a jfet.

That's a FET on the schem between the capsule and the circuit block (labeled 'FET bias / output circuit') that has the single bipolar buffer Q in it.

To quote Eliza Doolittle: "Whaddya tyke me fo'?".

 

[k brown]

Now, the next question is, what would be the effect (if any) of changing the output circuit to impedance-balanced, and disconnecting the primary of the trafo from the circuit and only using it to tap off the phantom voltage. Affect the output Z or CMRR?

 

[ccaudle]

That's a FET on the schem

Yes, I saw the schematic, but you said the circuit only has a single bipolar transistor.
So by "single bipolar transistor" you mean "single bipolar transistor and also a JFET?" Or by "single bipolar transistor" you mean "single JFET?"

What is the bias current of the JFET and the bipolar?

 

[k brown]

'The circuit' I referred to as having a single bipolar Q in it is the block labeled "FET bias/output circuit".

The FET is 2SK660, and the bipolar is 2SA1174 (collector current 0.05 A).

 

[doqmemory]

Is your intention to put the capsule bias from the center tap directly on the FET gate? I would think a 1000 pF capacitor would be needed to couple the energized capsule to the FET gate.

 

[k brown]

Is your intention to put the capsule bias from the center tap directly on the FET gate? I would think a 1000 pF capacitor would be needed to couple to the FET gate.

Good catch - no, I just failed to draw it (focussed on the output arrangement).

 

[doqmemory]

As long as the capsule works with the 40-45V polarization, you should be fine. I have a U47-type microphone built with a Phaedrus SPIRIT Supertube that gets the capsule polarization from the center tap of the Bv8. The Thiersch M7 "red line" capsule sounds so great with that polarization that my brother won't return it to me .

The U87 and KM84 just bias the FET and go straight from the FET drain to the coupling capacitor before the transformer, with no bipolar buffer or other circuitry in between and "do just fine". Any other circuitry in between will contribute whatever drawbacks/compromises they introduce.

 

[ccaudle]

The FET is 2SK660

Bias current?

the bipolar is 2SA1174 (collector current 0.05 A)

50 mA is the maximum. What is the bias current?

Bias current of the JFET and the bipolar stage has to come from the phantom power.
Both together have to be around 2mA if you do not want to go below 40V at the cartridge.

 

[k brown]

Bias current?



50 mA is the maximum. What is the bias current?

Bias current of the JFET and the bipolar stage has to come from the phantom power.
Both together have to be around 2mA if you do not want to go below 40V at the cartridge.

I'm not an EE; just a mic hobbyist, so I don't know how to calc bias current.

This would be a mod of the Audio Technica AT8533x power module, which (when powering an electret containing the 2SK660 FET) AT says draws 2mA phantom power.

Here's my reverse engineering of the module (may contain errors). I don't know what the diodes are. The K660 in the mic head connects directly to the TB3M input connector. As you can see, it has a rather unusual way of biasing the FET; only thing similar I've seen is the Shure SM90/91 PMs.

 

[ccaudle]

an electret containing the 2SK660 FET

The 2SK660 is made specifically for buffering electret capsules; it contains the gate bias resistor (or equivalent, possibly made using the small leakage current of a PN junction) and a JFET made to be able to run with 0V gate-source.
The capsule head with that JFET is basically like the small electret capsules you can get with built in buffer that just need a resistor to the drain from a voltage source, and you capacitively couple the signal from the drain/resistor junction.

I don't know what the diodes are

Probably a zener diode operating as a basic regulator to drop the phantom power voltage down to 9V or so. You can't put 40V directly on a 2SK660.

disconnecting the primary of the trafo from the circuit and only using it to tap off the phantom voltage

If you are going to disconnect the transformer just take it out. To tap the phantom power you just need two equal value resistors each connected to pins 2 and 3. Join them together and that is your center tap. 2K each would drop another couple of volts from the phantom power; is that why you are trying to use the transformer instead of resistors?

 

[k brown]

The 2SK660 is made specifically for buffering electret capsules; it contains the gate bias resistor (or equivalent, possibly made using the small leakage current of a PN junction) and a JFET made to be able to run with 0V gate-source.
The capsule head with that JFET is basically like the small electret capsules you can get with built in buffer that just need a resistor to the drain from a voltage source, and you capacitively couple the signal from the drain/resistor junction.



Probably a zener diode operating as a basic regulator to drop the phantom power voltage down to 9V or so. You can't put 40V directly on a 2SK660.



If you are going to disconnect the transformer just take it out. To tap the phantom power you just need two equal value resistors each connected to pins 2 and 3. Join them together and that is your center tap. 2K each would drop another couple of volts from the phantom power; is that why you are trying to use the transformer instead of resistors?

I don't think it's a Zener; I drew it just as AT did (back when they published schems [sans component values]) - a circle with a line to the left (never seen that symbol on other schems); others have speculated it's a current regulating diode.

I did wonder if the trafo would drop phantom less than a resistor pair (plus it's already in there!).