Alctron SDC Tube Microphone

[k brown]

If the switch only affects to the vents pattern (not sure) it could be easily done this way but guess it would need some further experiments

One can achieve a reasonably useful subcardioid by blocking half the vents (will probably need some EQ correction), but blocking the external vents completely to get to omni, will in most cases result in a very poor frequency response.

The Schoeps switchable capsule is a very sophisticated design, and even then Schoeps says it performs best at either full card or full omni settings.

 

[kingkorg]

If the switch only affects to the vents pattern (not sure) it could be easily done this way but guess it would need some further experiments

The problem with such solutions is that it creates a resonant cavity behind the capsule and meses up FR. Think microphone cupping. Rear openings should be closed at the capsule. Shouldn't be too difficult to make an additional ring on top of rear backplate that can be rotated in order to close the holes.

 

[GeorgeToledo]

For similar reasons, one shouldn’t put plastic omni ball accessories on most cardioid SDC with the intent of converting them to omni.

 

[Icantthinkofaname]

The problem with such solutions is that it creates a resonant cavity behind the capsule and meses up FR. Think microphone cupping. Rear openings should be closed at the capsule. Shouldn't be too difficult to make an additional ring on top of rear backplate that can be rotated in order to close the holes.

No, just not really something easily DIYable. I mean I'm sure somebody could disassemble a capsule and stick a switch in one of the vents on a cardioid capsule and add some sort of shutter/ring.

 

[k brown]

No, just not really something easily DIYable. I mean I'm sure somebody could disassemble a capsule and stick a switch in one of the vents on a cardioid capsule and add some sort of shutter/ring.

A LOT more complicated than that - look at the Schoeps cutaway again.

 

[Gus]

I could not find any information with a quick search
Does the inside look something like in the picture?
http://recordinghacks.com/microphones/groovetubes/GT-44It might be a hybrid cascode.

 

[LevinGuitar]

It is indeed

 

[Voyager10]

Interesting question whether the tube in this circuit is making much contribution to the sound (in the sense of "transfer function from the input to the transformer primary").

The anode current and cathode current are essentially identical, so the anode voltage (dropped across R4) is fixed by the FET's drain current Id. The only place the tube characteristics come into play is changing the FET's Vds (as its Vg varies with anode current and voltage).

The 2SK117 data sheet shows Id being largely independent of Vds for Vds above 1V or so, and here it's a good 6V.

So I wonder what the reasoning is behind going to the trouble of using a tube is, but not in a conventional common-cathode amplifier?

 

[kingkorg]

Interesting question whether the tube in this circuit is making much contribution to the sound (in the sense of "transfer function from the input to the transformer primary").

The anode current and cathode current are essentially identical, so the anode voltage (dropped across R4) is fixed by the FET's drain current Id. The only place the tube characteristics come into play is changing the FET's Vds (as its Vg varies with anode current and voltage).

The 2SK117 data sheet shows Id being largely independent of Vds for Vds above 1V or so, and here it's a good 6V.

So I wonder what the reasoning is behind going to the trouble of using a tube is, but not in a conventional common-cathode amplifier?

https://www.tubecad.com/2007/09/blog0119.htm
Also some clues in the description of Josephson mic.

https://www.rspeaudio.com/josephson-engineering-c725-dual-diaphragm-hybrid-condenser-microphone
And this
https://www.labjc.com/?p=1179

 

[Migs 31]

I could not find any information with a quick search
Does the inside look something like in the picture?
http://recordinghacks.com/microphones/groovetubes/GT-44It might be a hybrid cascode.

@LevinGuitar. thanks for the Schemo.

If it was me, I would change this into an AKG C28 circuit with a 5840 valve (smaller than 6072).
Which is the "spirit" (marketing bluff) these things tried to convey... without the amazing CK28.
My 2 cents.

M

 

[Marik]

@LevinGuitar. thanks for the Schemo.

If it was me, I would change this into an AKG C28 circuit with a 5840 valve (smaller than 6072).
Which is the "spirit" (marketing bluff) these things tried to convey... without the amazing CK28.
My 2 cents.

M

Why, if I may ask? This is a perfectly fine (in fact, very fine) and very clever schematic, which makes lots of sense and perfectly takes care of all deficiencies of the tubes, such as noise and input impedance and still has the best tube advantages and sonics. What is the point of ruining a good circuit for sake of yet another copy-paste, which doesn't make any sense? In any case, any system is only as good as it's weakest point, which in this case is a capsule...

Best, M