Some questions about the Sony C37a - mics with cathode follower

[k brown]

FWIW, here's Dave Royer on the 5840 CF's ability to drive cables without an output transformer:

"I would not recommend a length longer than fifty feet UNLESS you use something like the Mogami cable that is intended for tube microphones— otherwise, the voltage drop in the conductor for the tube heater (which carries 6 volts at 150 mA) can become excessive. Aside from that, the cable length can be up to a few hundred feet; the cathode follower has a low enough impedance (approximately 450 ohms) that high frequency losses are a nonissue— the problem is that excessive capacitive loading will lead to loss of headroom and increased distortion at high frequencies, however, with up to a few hundred feet of cable, this effect will not be significant unless the microphone is being subjected to sound pressure levels approaching 130 dB at frequencies of 8 kHz and above. . . Quite unlikely in practice."

https://mogamicable.com/category/bulk/microphone/tube/
The C-37A's factory-supplied cable between the mic and the power supply was 10 meters long.

 

[rock soderstrom]

Thanks for info. I have also read this elsewhere on the WWW where a studio owner swears by this type of signal routing.

I'm also running my first experimental setup right now without a transformer. No problems so far.

 

[rock soderstrom]

A question for the owners of original C37a's and their matching power supplies.

How noisy are the original PSUs? Problems with hum compared to other tube mics?

 

[Tubetec]

I was able to scan the docs I have relating to the B&K CF circuits , but I have a tecnical issue preventing me from posting ,

15 feet of cable off the B&K Cf circuit still gives 200khz bandwidth into hi-z , its only the very small 1/8 inch capsules that get anywhere near that , still goes to show the circuit is beyond reproach regarding fidelity .

You can always arrange the following preamp circuit to add colour as required .

 

[Marik]

I always found it strange that anyone would wish for this sort of sound - weakest point here being the capsule IMO, but the CF dosen't add anything nice either. The selling point of this mic was apparently it's ability to drive looong mic cables...

Jakob,

Re: CF sort of sound... from memory the Radiotron 4th Ed, as well as John Brosky both insist the CF still need to be loaded by the same as an AF transformer ratio, regardless of notion the CF has lower output impedance. The 10K:600 is a ratio of about 4:1, which is a bit too low, if believe those. So perhaps it is not about the media, but implementation.

Best, M

 

[Tubetec]

Im a firm believer in CF tube mics , Im glad peoples interest here has opened up the discussion .
The B&K CF preamp doesnt run a transformer directly off the output , in conjunction with the 2801PSU , a transistor is used before the step down transformer , I tend to use the direct CF output feeding a simple one tube preamp , it just works , the issue is on higher level sources is trying to get rid of gain without having to attenuate . The CF itself is capable of putting out around 20 v into high z before distortion becomes objectionable .

Ive listened to Dave Royers talks about his CF mics but I didnt find the bit that K Brown quoted above , interesting stuff .

 

[k brown]

Im a firm believer in CF tube mics , Im glad peoples interest here has opened up the discussion .
The B&K CF preamp doesnt run a transformer directly off the output , in conjunction with the 2801PSU , a transistor is used before the step down transformer , I tend to use the direct CF output feeding a simple one tube preamp , it just works , the issue is on higher level sources is trying to get rid of gain without having to attenuate . The CF itself is capable of putting out around 20 v into high z before distortion becomes objectionable .

Ive listened to Dave Royers talks about his CF mics but I didnt find the bit that K Brown quoted above , interesting stuff .

The Royer quote was from a response to an inquiry I'd made through the Mojave website regarding how much extension could be put between an MA-100 and it's PSU. The response I got was from D.R. himself.


You don't see C-37A used as a live mic very often, much less hand-held! (though these were ID'd online as taken at the Sands, Bob Ohlsson has said that the 'Sinatra at the Sands' album was done by Wally Heider with Frank using an SM57; which one can see on the album cover - actually looks like one of the '57's predecessors, 545S https://coutant.org/shure545s/shure545s.pdf):

 

[Tubetec]

Daniel Lanois is a strong proponent of the C37a , if you look at him recording he kinda sings across the mic , in very very close , the horse shoe mount helps in this respect .
I also have a bunch of the old Shures , I got rid of the stand mounts and switches as there not needed for my usage . They sound a little different to the modern 57 , not hugely different but just enough that they add a slightly different colour to what ever it is you use them for , these old mics are always good to have on standby .

 

[rock soderstrom]

Since the name John Broskie was mentioned here, how about using the Aikido Cathode Follower as an impedance converter in a microphone? That way you could put the second triode system to good use, improving the PSRR is a welcome bonus in a microphone.

12AU7, 6CG7, 6SN7, E88CC or 6n1p would be the usual suspects.

 

[rock soderstrom]

Another microphone with a cathode follower is the Audio Technica AT4060. Strictly speaking, a E88CC based white cathode follower with an output transformer. Very interesting, what the hell are all those diodes doing in the input section? Capsule polarization and grid resistor substitute?

Edit: The designer liked diodes, the cathode biasing is also done with diodes.

 

[limo]

I rescued two sets of C37A's and one microphone from parts that were about to be thrown away from the warehouse of a friend who retired due to illness.
That was last week.
The two microphones were the same, but the PSUs were CP-3 and CP-3B.




The difference between the two is the presence or absence of a board.
The rectifier diode "3" is SONY 1T201, and "3B" is NEC 1N121 and 1N125.
The audio transformer was changed from the large Fuji Electric to the small Sony. However, it looks the same as Fuji Electric's inductor.

The AC cables were cut, which is a common practice for companies when they remove equipment from their ledgers and dispose of it.
I hurriedly installed a new AC cable and replaced the rotten XLR, Telefunken, and NEC vacuum tubes with new Toshiba tubes, and they both sounded fine.
Neither of them have been repaired, but they have exactly the same sound.

The voltage are AC28.5V , AC280V for transformer out.
DC16.3V, DC282V for no connect microphone
DC6.3V, DC226V for connected microphone

 

[Tubetec]

Is that a paper in oil capacitor on the far right in the second picture ?

It looks like it might be the first filter cap after the diodes ,

Do you have the schematic ?

 

[k brown]

Notsurpr

I rescued two sets of C37A's and one microphone from parts that were about to be thrown away from the warehouse of a friend who retired due to illness.
That was last week.
The two microphones were the same, but the PSUs were CP-3 and CP-3B.
View attachment 126362
View attachment 126363
View attachment 126364
View attachment 126365View attachment 126366
The difference between the two is the presence or absence of a board.
The rectifier diode "3" is SONY 1T201, and "3B" is NEC 1N121 and 1N125.
The audio transformer was changed from the large Fuji Electric to the small Sony. However, it looks the same as Fuji Electric's inductor.

The AC cables were cut, which is a common practice for companies when they remove equipment from their ledgers and dispose of it.
I hurriedly installed a new AC cable and replaced the rotten XLR, Telefunken, and NEC vacuum tubes with new Toshiba tubes, and they both sounded fine.
Neither of them have been repaired, but they have exactly the same sound.

The voltage are AC28.5V , AC280V for transformer out.
DC16.3V, DC282V for no connect microphone
DC6.3V, DC226V for connected microphone

View attachment 126369

Not surpising they sound the same - CF mics tend to be rather tolerant of PSU variations.

 

[limo]

Is that a paper in oil capacitor on the far right in the second picture ?

It looks like it might be the first filter cap after the diodes ,

Do you have the schematic ?

This is audio line coupling.
Nii-Chikudenki = Nippon Chemi-con