Some questions about the Sony C37a - mics with cathode follower

[rock soderstrom]

I am currently studying tube microphone circuits with cathode followers, so I have some questions about this classic.



1. does anyone here have a C37a in use? What do you prefer to use it for? Pros and Cons?

2. how high is B+?

3. what is the polarization voltage of the C-3 capsule and how is it supplied?

4. what is the transfer ratio of the output transformer?

Here you can find a lot of info and photos about the C37a.

https://www.coutant.org/sonyc37a/index.html
Cheers

 

[Tubetec]

My experience with cathode followers is there nearly impossible to distort , you get ultra high input z ( a multiple of the grid resistance value ),the grid volts are where you want them at for direct polarisation of the capsule ,
for higher level spl sources the CF ticks all the boxes for me , for vocals up close and personal anode load wins out unless you have a were wolf howling at the mic .

 

[gyraf]

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...

Polarization looks like being at cathode-bias-level

Transformers and thus voltages are hinted at annotated image below. I love the hand-drawn company logo

/Jakob E.

 

[Tubetec]

The 100M grid resistance appears much larger due to NFB , that means the capsules output voltage is higher at the grid , that helps offset the low gain of the CF to some degree .
Ive tried a few different capsule with a CF , your typical LDC works fairly well ,with smaller capsules the output drops considderably and you end up with extra noise on lower level sources . B&K used 200v for capsule polarisation on their CF mics , thats not feasible with standard capsules as the membrane will collapse and become stuck to the back plate .
The CF isnt so popular here I see ,but Im a firm believer if its used correctly it can give a very high quality results . Its a fair point to say the CF doesnt do anything nice to the sound in the way an anode loaded stage might when it distorts . Another benefit of the CF is any chance of microphonics becoming an issue is greatly reduced , especially in conjunction with sub mini tubes which themselves already have a very high resistance to vibration .

 

[gyraf]

I agree that the CF has the potential to give you a good deal more precision - but it's simply not what I would want or expect from a mic. I understand using early B&K hv-mics for classical and mechanical/telemetry - but really not much for things I'd like to "sound" good. I always attributed this to the CF topology, but I'm not at all 100% sure about this.

I think perhaps the CF with its precision does a poor job of masking impurities in capsule performance?

/Jakob E.

 

[Tim Campbell]

I did make a simplified version of this capsule for one of my clients and I have repaired many over the years. This was Sony's attempt at a very flat frequency response but I just think they sound a bit strange and the mechanical pattern mechanism does very little.

 

[Tubetec]

The MKL-2500 is another CF based mic I use and very much like the sound of , its a lot less flattering in the top end that most modern condensers , Im not certain if thats due to either capsule or CF or both .

 

[soapfoot]

Altec M11 system is also a cathode-follower system from way back.

 

[rock soderstrom]

Thank you all for your opinions, info and data!

It's interesting how differently CF mics are perceived. For example, some online reviews compare the Sony C37a with other classics from Neumann and AKG. They use the C37a as their main microphone for vocals and important instrument recordings and praise the intimate "bigger than life" sound of the mic. Others see more use in an environment where the CF characteristics are advantageous, such as low distortion and precision.

What speaks for CF mics is the low component count. They are very easy to build and you can also use small enclosures, because the low impedance output of the CF allows the transformer and large coupling capacitors to be separated from the mic body into the PSU enclosure.

The capsule of the Sony C37a is a bit special for me, I didn't know the concept of mechanical switching of the polar pattern before. Here you can see the switch, directly on the grille.


Is the capsule, apart from the mechanical switching, to be regarded as something of its own in terms of its characteristics or is it based on German or austrian models? How does the somewhat "strange" sound of the original capsule manifest itself? Is the capsule from the C800 identical or very similar to the C-3 capsule from the C37a?

If one were to build a C37a today, which capsules would be more suitable? Flat types like K47 or rather K67/87? I think the former, as there is no circuit compensation for the treble of the latter.

 

[rock soderstrom]

Sony C37a in action:

 

[rock soderstrom]

I love the hand-drawn company logo

Me too

Thanks for the circuit info!

 

[rock soderstrom]

A growing list of tube microphones with only a cathode follower as impedance converter:

historic:

Sony C37a, C800
Altec M11 "Coke Bottle"
Altec 165a "Lipstick"
Oktava MKL-2500
AKG C12a, C60, C61
Bruel&Kjaer
RFT MV101
Radiometer "Kunstigt øre" (artificial ear)

modern:
Audio Technica AT4060
Royer Labs MA-37
Royer Labs R-122v
Royer Labs SF-24V
Royer SDC mod
Mojave Audio MA-100
Mojave Audio MA-202ST
McHugh AMM-68
"The Country Boy's Capacitor Microphone"

If you know of any, please post them.

Edit: added some schematics for the mentioned microphones.





Keywords: Cathode follower mics microphones Mikrofone Kathodenfolger, schematic, Schaltplan

 

[Tim Campbell]

Well the capsule isn't like any other and is not similar in sound to any european capsule. Sony's original intent was to create something like the U89 but a tube mic. I believe Josephson and I are the only 2 companies producing versions of this capsule. The sound is boring to say the least but they make a U89 sound exciting. The first time I heard one of these mics I thought it was broken.

The real shame is that it is easy to make this capsule sound similar to a CK12 if it is set up right but then it wouldn't be as Sony intended.

If you want to build a copy and can't get the right capsule you could probably use the K89.

 

[inkster]

I love mine

 

[k brown]

That gubbin on the back of the head basket is not a switch; it's simply the access hole for the mechanical 'vane' on the back of the capsule that seals off the rear vents when set to omni. A flat head screwdriver goes in there. Scares the hell out of a lot people to poke a screwdriver into the back of a condenser mic - especially a vintage one.

Always loved the Kingston Trio cover with three '47s on vox and three C-37As on the instruments. Says it all - '47s for color, and C-37As for neutrality. [Great album, too]

 

[kidb]

Great mic, these days one could sound different from another c37, simple circuit high headroom, small for a tube condenser, since they moved the output transformer and hpf to the psu. B+ measures 247v on mine. 6au6 tubes are not expensive but it does require a quiet one, so going nos you’ll want a few to listen to. They did change the psu over the years as well as the output transformer I believe, the cp-2 psu has a 10k:600 opt ratio 4.08:1

 

[andYz00m]

@SparkleBear and I have been scheming on making a simple CF LDC tube mic in the C37a style. The goal being a mic that can take high SPL and provide a nice smooth tube condenser response. The capsule is surely the big swing. To Tim's point, you can either target the original sound (differing opinions on this) or try something else! For me, I'm less interested in the acoustic labyrinth, and polar pattern options at all, than I am in the high SPL.

Watching some interviews with David Royer he also mentions the careful consideration of the output transformer. The CF can go straight through a 1:1 output trafo but he opted for a 2:1 Lundahl to make sure that even preamps with low input impedance would still see the high SPL handling benefits of the circuit design.

I love my Altec 165As and want more tube mics that can handle high SPL (I record primarily loud rock music).

Well the capsule isn't like any other and is not similar in sound to any european capsule. Sony's original intent was to create something like the U89 but a tube mic. I believe Josephson and I are the only 2 companies producing versions of this capsule. The sound is boring to say the least but they make a U89 sound exciting. The first time I heard one of these mics I thought it was broken.

The real shame is that it is easy to make this capsule sound similar to a CK12 if it is set up right but then it wouldn't be as Sony intended.

If you want to build a copy and can't get the right capsule you could probably use the K89.

What about if youre going for a "new" sound? Im considering building a CF mic body and trying out a bunch of different capsules to find something... interesting! Maybe start with a CK12 and one of queens flat47s.

 

[k brown]

Can also do without a transformer, and just go impedance-balanced. CF can drive cables on it's own.

For years I used my Royer-style CF mics (6205s in Oktava MK-012) without transformers, single-ended into transformerless tube pres placed right by the mics; never picked up a bit of noise with such short cables.

 

[Tubetec]

Simplicity and low component count ,
I did a version of the B&K cf circuit with only the tube , capsule and grid resistor contained in the mic body , everything else was back at the psu end ,
As I said on a post previously I tend to take the cf output directly to the grid of the following preamp tube , 1M ohm input z , and plenty of headroom .

 

[Tim Campbell]

Years ago I built a mic very similar to the C37 using a russian 6J4P and a CK12 capsule. It sounded brilliant.

JJ Blair, who has a great mic locker, only seemed to find 1 use for his C37 as a tom mic but he swears it is the best tom mic.