tube CF microphones

[Gus]

I don't understand why anyone would want a TUBE cathode follower (CF)microphone.

Anyone have a good reason for a CF tube microphone?

I understand wanting the capsule and transformer.
I don’t understand why you would want a tube and not a solid state follower.
You could design a higher voltage solid state circuit that is direct coupled and charges the capsule. I drew something up some time ago maybe I should build it.

I would need to think about the math(charge voltage, capsule, and input resistance) but I don't think( I could be wrong here) the CF will have an input greater than 2V peak to peak.

 

[Tubetec]

One benefit of CF tube mic is the input impedence the capsule sees can be made very large ,a multiple of the actual grid resistor value ,  as an example of very good design Id recomend you have a look at the Bruel & Kjaer cathode follower microphone circuit ,
the CF itself has a bandwidth of 200khz and different capsules to cover the entire range , the mics will put out 20 volts into a high z before distortion becomes objectionable . The other CF mic I have direct experience of is the Oktava MKL2500 , thats a great all rounder , Ive used it on acoustic guitars and voice  to good effect.

 

[ln76d]

I don't understand why anyone would want a solid state follower microphone.

All depends, there are some pretty nice cathode followers and there are some junk cathode followers.
From my perspective benefits are low noise and low sensitivity. And still we have tube not silicon.
All the chinese and rebranded chinese mics bring up on the mic market that tendency of high sensitivity circuits.
Usual sensitivity is lowered in the circuits with follower output (FET or Tube CCDA) which also makes a "stamp" on overall "response".  Ok for vocal tracking, yeah sure, if you have weak and noisy preamp then cool. But for drums or loud guitars? Not really?!?! Double PAD - one in mic (lowering polarisation voltage), second in preamp? Another "stamps" on chain repsonse etc.
Lets look on Sony C37a (6AU6) or CU2 (AC701). High main voltage around 280VDC-300VDC (if memory serves me well), polarisation voltage around 140-150VDC (here of course transducer is prepared to work with), output transformer 4:1.
Sensitivity is oscilating around 3mV/Pa. It have pretty good headroom, takes nicely high SPL. Incredible on guitar or drums tracks. Nice tube coloration which many people loves especially on drums. Nothing which you could get with solidstate mic and all the 0110 noise emulators.

 

[Recording Engineer]

I’d certainly be curious in any conclusion here.

You’re right about the capsule and transformer. Transformer choice here really seems to be the difference between good and cool. Same as the capsule choice. I still haven’t heard it, but Chris at Barbaric really likes it with the Maiku edge-terminated (hasn’t tried true CK12 style with it yet), but admits there’s really something really special every time he hears my pair with Sony C38B capsules. I also have a pair with EV RE200 capsules that have an amazing vibe on acoustic guitar.

One thing we haven’t experimented what-so-ever has been different tubes or otherwise. We’ve always used 5840w tubes there.

 

[Tubetec]

Sub mini wire ended tubes are killer in tube mics 5840 5899 especially , they were made to withstand escape velocity in a rocket , a far cry from typical studio conditions ,save the odd crash landing  :-[
Overheads on a seriously loud drumkit , CF with a directional one inch diaphragm and your usual 60 volts polarising voltage , close the feedback loop at the far end of the line , and match it into a hi-z low gain tube line stage , you may think you do better with more complexity on paper , but it probably wont be as kind on the ear when the meters are in the red .

 

[mjrippe]

The Altec M11 "Coke Bottle", one of my all time favorites, uses a 6AU6 in a cathode follower configuration.  Great for high SPL and the low end goes down so far there is a 20cps* high pass setting!

*cps = cycles per second.  It's what they used to call it before Hertz was honored for his work...

 

[ln76d]

5840 is pretty ok, for non sub mini similar effect 5654 as direct replacement is ultra nice. I prefer 6AU6 most for many different circuits CF and PF. Especially it can handle much bigger voltages than many other small tubes (even for plate follower).
Sony transducers are amazing, overall i'm not a huge fan of bright capsules, but this and "brass" CK12 are exceptions.
I reworked C37a to CU2, so before little adjustemnts of PSU i could compare difference between 6AU6 and AC701 in the same circuit. Yes, different, even for CF. Both pleasant, but the main difference was in the upper corner of frequency response. Maybe not so drastic like 6S6B and AC701 in M49B topology, but still pretty audible.

 

[lampas]

Maybe not so drastic like 6S6B and AC701 in M49B topology, but still pretty audible.

Is there  a big difference ? 

 

[Winston OBoogie]

I'm not a mic design guy so a pinch of salt is required with this but, if I were free from the constraints of using P48, I don't see any reason why a solid-state follower would not equal and (if required) better any conventional valve cathode follower in a head amp/impedance converter. 

 

[ln76d]

Is there  a big difference ?

More noticable with KK47/KK49 (which is brighter) than with M7. Compared with fixed HPF and without. Never compared with fixed LPF, because never used it for that circuit. With 6s6b bias set fot AC701 and with custom bias for 6s6b, both options 4V (underhating) and 6V as standard heating (range 5.7-6.7V if i remeber correctly). AC701 of course only 4V for heaters.

I'm not a mic design guy so a pinch of salt is required with this but, if I were free from the constraints of using P48, I don't see any reason why a solid-state follower would not equal and (if required) better any conventional valve cathode follower in a head amp/impedance converter. 

It would end probably like standard discussion about  FET vs. Tube mics. Not sure it have any sense here. Some people prefer one thing  some people other thing, some both etc.  If you are reffering to my "I don't understand why anyone would want a solid state follower microphone", then this was just a joke Have nothing completely to solid state follower idea, have no experience except output emitter followers popular in mic circuits (here i'm not a huge fan) and this could be something nice if well designed, so keep fingers crossed for Gus. I have long list of different ideas for microphones, so someday  maybe try to design some solidstate follower, but today is not the day. For low sensitivity FET mics for loud SPL, now working on single FET Impedance converters, maybe someday will be the day for followers here.

 

[Winston OBoogie]

If you are reffering to my "I don't understand why anyone would want a solid state follower microphone", then this was just a joke  

My dear fellow  Gallifreyan 
No my post was just a general response to Gus' original one and not in reference to anyone or anything in particular.
I'm not trying to open up a valve vs fet debate, I like both and I've used both.

I would think the distortions from the capsule and transformer would contribute more to a mic signature than those from a well designed valve follower.  I also believe, from experience, that any non linearities from that follower working at these levels could be  emulated with discrete SS and, likewise, could be eliminated all together for all intent and purpose.
I would advocate using a dedicated external supply for this rather than be restricted by P48, but that's me.
Cheers,

 

[Recording Engineer]

Now that I’m thinking of it, Phaedrus now has a “SS tube” for CF.

 

[Winston OBoogie]

As far as valve cathode followers, I'm curious if anyone has played around with converting a U47/U48 to a follower circuit.   
Rudy Van Gelder supposedly had Rein Narma convert his U47 head amps so they better suited a close mixing technique.  Compared to other contemporary recordings, I certainly can't fault Rudy Van Gelder's results. 

 

[mjrippe]

I would advocate using a dedicated external supply for this rather than be restricted by P48, but that's me.
Cheers,

I have worked with a designer using discrete SS circuits with higher voltage/current supplies for mic head amps and they can be very good sounding.  Of course these were coupled with custom capsules and transformers so it is hard to say what is what.  A handful (less than 10) made it into circulation but I don't know where most of them went.  If the current round goes into production I'll let you know.

 

[Tubetec]

Someone with a loud voice in close  can easily distort a U47, loud guitar cabs will distort it also  but depending on how distorted the guitar is to begin with you may not even notice the extra distortion added by the mic . I also saw U47's used as OH on loud drum kits , even though your probably getting quite high distortion on the peaks its not that apparent to the ear . 
Those old Neumans fetch a high price and that price is set to rise , doing something like gutting it and rebuilding into a CF ,no matter how well engineered is going to leave you with something much less valuable in the end . 

Ive done one or two CF mics based loosely around the B&K circuit , the first one I just allowed the grid voltage on the CF to polarise the capsule  and another thats designed with its own seperate polarising voltage and cap coupled to the grid , CF directly drives the unbalanced output cable and it gets matched into a hi-z tube preamp with lowish gain , the cathode resistor and coupler cap are placed in a small hammond box at the far end of the cable ,said little box plugs directly into the preamp.
The responce of the CF and preamp is excellent out to around 100khz ,but of course with the usual 1 inch side address capsule responce falls away sharply around 20khz either way .

 

[Winston OBoogie]

I have worked with a designer using discrete SS circuits with higher voltage/current supplies for mic head amps and they can be very good sounding.  Of course these were coupled with custom capsules and transformers so it is hard to say what is what.  A handful (less than 10) made it into circulation but I don't know where most of them went.  If the current round goes into production I'll let you know.

That's cool, I hope it can happen for your designer guy and yourself.
As I said, I'm not a mic guy but, having messed around with valve and solid state amps,  it makes sense to me.   

 

[Winston OBoogie]

 
Those old Neumans fetch a high price and that price is set to rise , doing something like gutting it and rebuilding into a CF ,no matter how well engineered is going to leave you with something much less valuable in the end . 

Haha yeah  . I wouldn't suggest rearranging your $15K microphone.  But using one of the better K47/49 or M7 capsule copies (or a new real Neumann K49) with one of the best transformers, I'd like to see how it performed.
Get a high gm  valve in there and configure it as a  follower, however you felt was best.  Lots of possibilities.

 

[kingkorg]

I find it amusing no one ever comes with any kind of measurements in posts like this. Even though they are fairly simple to do. It just adds to already existing confusion surrounding mics.

Even plate follower modded Apex460 with stock transformer, decent tube and bias adjusted can take 140db of spl, with maybe 1.5-2% thd. So i cant help but wonder what kind of SPL are you are talking about? I challenge btw anyone to notice 2% thd on a recording.

Mic distortion caused by guitar amp is very noticeable since it's not filtered by guitar speaker. It just sounds horrible.

My two cents would be any type of topology will work great if desingned and built properly for what the mic is intended. I've heard AND MEASURED some amazing mics with ICs in them, and seen baffled listeners in double blind shootouts against tubes and transformers.

I regularly see people swapping parts for worse performing ones, and removing CF while using mics in conditions where it would not make any difference if the mic is PF, CF, or even solid state for that matter.

 

[Winston OBoogie]

...I challenge btw anyone to notice 2% thd on a recording...

It really depends on the spectrum of this 2% distortion.    2% of mostly H2 & H4, probably not.  Even 2% ofH2 followed by H3 and so on...  But 2% of mostly higher partials would be very easily heard by most people I think.
In my *very* humble opinion 

 

[kingkorg]

Thd i did write. I believe most harmonics would be second and third order, not so much others. But even then test can be easily done, and i would gladly post examples with any kind of harmonic content on any kind of material. But, not many would take part in listening test i'm sure.