S76 - Tube Microphone (6AU6, K67, deemphasis, 9.5:1-10:1)

[ln76d]

Starting new project.
This time with deemphasis circuit for K67 capsule!
Will be updated from time to time
Every new project topic motivates me to finish all previous

Circuit changed to use 6UA6

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All microphones projects:
https://groupdiy.com/index.php?topic=62423.0
https://groupdiy.com/index.php?topic=62441.0
https://groupdiy.com/index.php?topic=64687.0
https://groupdiy.com/index.php?topic=62601.0
https://groupdiy.com/index.php?topic=63436.0
https://groupdiy.com/index.php?topic=63682.0
https://groupdiy.com/index.php?topic=62262.0
https://groupdiy.com/index.php?topic=62682.0

 

[ln76d]

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[ln76d]

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[ln76d]

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[ln76d]

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[tardishead]

Hi there
Has anyone tried this circuit?
Any comments?

 

[RuudNL]

Although it looks a bit 'experimental' (some components have no value), I don't see any reason why this shouldn't work.
Personally  I don't like the 220 V HT. Especially because there is a significant voltage drop over the 30 K. resistor.
The deemphasis network looks like a copy of a famous German microphone manufacturer...  ;D
But maybe the topic starter can tell us more about his creation, almost a year later...

 

[ln76d]

Ok, i can tell something more
It was a really long year
I build this mike, i had problem to set proper values for deemphasis network - grid resistor is a huge problem here.
Anyway i had to reuse donor body for different mike, so i didn't finish this project. Someday i will reuse board and finish it, but currently have no time for this.
I finished yesterday similar project but with different tube - 6112, running in parallel.
Works like a charm
For 6AU6 - why not 220V? It works really good under high voltage (see C800G for example).
I chose high voltage to make that project easy for DIY with use of chinese PSU. Most is running on 200V DC with huge drop from 300VAC. 20V was reserve for extra 33K drop and i don't see Rudolph why the drop should be problematic?
Main idea was to get a liittle higher impedance with 133k. Rest depends on cathode setting. More current more juice Anyway currently i'm not using,  in any project,  extra filtering cap at the plate - to get better phase relation. But am using often higher plate resistors than typical 100k (sometimes lower - depends on the circuit).
For deemphasis network - of course it's a copy, show some which isn't Show me some tube mike circuit which also isn't a copy of any other (i don't have in mind any hybrid weirdos).
I experimented with other option of feedback and filtering, but for now i didn't found anything exciting.
For the values - grid resistor need to be 1G to get nice filtering curve.
HF fitering cap 220pF, resistor which goes to ground 8.2k (better curve shape - in both ends -  than with 6.3k in U87).

 

[RuudNL]

why not 220V? It works really good under high voltage ...

220 V gives bigger sparks than say 105 V. 

 

[ln76d]

And ca. 200V drop on a three resistors can cause quite good fireplace

 

[rmburrow]

Although it looks a bit 'experimental' (some components have no value), I don't see any reason why this shouldn't work.
Personally  I don't like the 220 V HT. Especially because there is a significant voltage drop over the 30 K. resistor.
The deemphasis network looks like a copy of a famous German microphone manufacturer...  ;D
But maybe the topic starter can tell us more about his creation, almost a year later...

Ruud, don't you get 220 volts out of your wall?  (Not suggesting power transformerless operation..)

I don't have a problem with either the 220 volt HT or the 6AU6 valve.  If I recall, Dr. Schoeps used the 6AU6 in his early product, before the IRT made the German mic makers switch over to the AC701.

The Neumann NU67 sends 210 volts up to the U67 mic using the EF86 (6267) valve.

The 10 meg dropping resistors to the K67 capsule seem low to me...those should be in the 100  meg range.  Need to be careful about the overall time constant of resistors connected to the capsule falling in the audio range.  Also, there should be NO more than 60 volts dc between any section of the capsule and the fixed back plate.  (like 0-60 volts on one side and 60-120 volts on the other side, back plate at fixed 60 volts for variable pattern)... Drop the 390k to 330 k, maybe even 270 k.  Measure the voltage before connecting the capsule.  Excess voltage across the capsule can collapse a membrane.  What about the unconnected membrane?

Just my $0.02 worth...

 

[ln76d]

10M is sufficient, you shouldn't notice any difference.
U87 use in this position 330k for first half of backplate. schoeps cmc dc converter have mostly 1M, sometimes 10M, few other mikes use also 1M.
Anyway i don't see problem to use 100M for example, i have it in my last build.
I wish to be easier  adjust proper curve for 100M at the grid, but it isn't
Without drop at the plate there should be ca. 61.5V at the backplate.
Most 34mm capsules can be running up to 70V without any problem (safe up to 65V), 32mm to 100V.
For multipattern depends on the capsule.
There's no any norm of voltage polarisation values. It depends on a capsule design, diaphragm tension and it manufacturer.
Akg in later versions of "teflon" capsules run it from a 90V, edge terminated capsules in Violet Desing Dolly over 80V, some Chinese tube mikes 32mm k67 100V, Rode NT1a 75V (but this wasn't walid for their version with k67 - it was cause of fault, with edge terminated works better but best with 65V like in current NT1) etc.
Each capsule type also have its sweet spot
Best option is just to set proper value for the capsule you have. It also affect response and noise spec as also headroom. Choose whatever works best for you
Here's other circuit with variable polarisation voltage:
https://groupdiy.com/index.php?topic=64687.0
I tested it with 35mm and 32mm k67. There's significant audible difference in response between 3 setups - first, middle, last.
For 32mm range can be much higher.

 

[RuudNL]

Ruud, don't you get 220 volts out of your wall?

No, the mains voltage is 230 V here! 

Of course it is a personal vision, but I don't see any advantage in using a very high HT voltage.
Say the maximum output of a microphone is in the order of 800 mV.
That would mean 8 volts AC at the anode when using a 10:1 output transformer.
This voltage swing can easily be reached with a much lower +HT voltage.
(Neumann even uses +72 V in their M149 microphones.)
IMHO a higher +HT voltage means more dissipation, but doesn't contribute necessarily to a higher quality.
(Please convince me if I'm wrong...)

 

[ln76d]

Of course it is a personal vision, but I don't see any advantage in using a very high HT voltage.

To make life easier
As i wrote before, main advantage was for non problematic setup B+ from typical chinese PSU.
It's project for diy-iers, some have different level of knowledge, some just completely don't understand circuits and just want to build something. When you have chinese donor you can easily make from its PSU low noise supply running it at high voltage, you can even make it passive and reduce zener regulators. So why not?
6au6 is really great pentode for microphone circuits and can be used with higher plate voltages without any problem.
If max for triode connection is 275V, then 200V - 220V is really decent value.
If there will be need to use really low  plate voltages, then 6AK5 would be great candidate here.
In a very simplified way there is also a little bit more headroom with higher voltage, also i like bigger cathode resistors than you can find in most commercial designs and with higher plate voltage is often easier to get sweetspot.
I changed schematic to looks like more how it would look if i will start this project today

 

[RuudNL]

No cathode decoupling capacitor?
(Higher output but more distortion.)

 

[Khron]

How else are you supposed to get that "choob' warmth"? ;D

No cathode decoupling capacitor?
(Higher output but more distortion.)

 

[ln76d]

No cathode decoupling capacitor?
(Higher output but more distortion.)

In my builds output is that high as i need or just response is better, i would consider use decoupling cap from different perspective.
I like both options anyway as also different kind of biasing like m49b or even u47.
All depends on the circuit, tube, capsule and headbasket.
Not all distortions are bad as we can find in todays recording world  - people looks much more for distortions now, since digital sterilisation is boring and lifeless.

Mentioned by you in other topic -  m147 - shows exactly how ends fight with virtual noise and distortions

In some circuits difference is neglible, in some is huge.
One of my experiments was switchable cathode biasing between resistor and resistor with cap (both calculated to get same level).  In one circuit was big difference and really positive for the single resistor, in other results were truly similar.
In case circuit like in this topic cathode cap would add another phase shift, so this is one of the reasons, other is that 6au6 sounds really good with that kind of biasing and another one is that it's good to get a little more coloration for k67 capsules.

 

[moamps]

IMHO a higher +HT voltage means more dissipation, but doesn't contribute necessarily to a higher quality.
(Please convince me if I'm wrong...)

The higher +B means higher value for anode resistor for the same tube's DC working point (bias), so  it means lower load and lower distortion for AC. 

 

[ln76d]

The higher +B means higher value for anode resistor for the same tube's DC working point (bias), so  it means lower load and lower distortion for AC.

Good point

 

[JessJackson]

Use 6J4P, (long body 6au6a) what Sony originally used, sounds way better imo.