FET847 Microphone Project Circuit By Jonathan Burtner PCB Layout By Poctop

[Deepdark]

Svm900 will fit

 

[poctop]

What is the function of C1 ("Tuned to taste")?
How do we "tune" this?

C1 as in the 4pF NFB cap? That cap certainly provides some NFB back into the gate portion which in my understanding stabilizes the amp, reduces gain, and I believe it also reduces output impedance... I'll read up in my RDH4 on NFB maths...
For tuning the specific value try some different values, sweep the DUT / graph the results for quantitative measurements, listen in context for qualitative measurements...
I don't know of another procedure used by the OEM but would like to hear about it if anyone knows or any additional discussion on the topic for that matter.

Cheers!
-jb

our Best Friend Oliver Used the Mic,  a Room and,  Acoustic Piano to tune Those.
Best,
Dan,

 

[Deepdark]

For your information, Advanced Audio carries some headbasket replacement.

 

[Gus]

Schematic looks like something Flatpicker posted years ago. 

It is all in the details

 

[rockinrob86]

Is it best to order the transformer direct from cinemag?  I can't find it anywhere...

 

[Deepdark]

You have to ask David from cinemag directly, the transformer isn't listed on their website

 

[geebeeVIE]

Sorry for the dumb question,  but what polarization voltage does it supply to the capsule?

If it does supply, like the KM84, only "less then 48V" - it will not be suitable for any of the mentioned capsules as they ask for 60V polarization voltage to work to their specifications....

 

[0dbfs]

Sorry for the dumb question,  but what polarization voltage does it supply to the capsule?

If it does supply, like the KM84, only "less then 48V" - it will not be suitable for any of the mentioned capsules as they ask for 60V polarization voltage to work to their specifications....

That's a great observation and yes the polarization voltage supplied to the capsule is in the area of "slightly less than 48V" like the KM84 or vintage U87 circuits which both use different capsules biased at that voltage. It is my understanding that the 60V (or perhaps even 70V) polarization spec is a max value before the diaphragm becomes sucked to the backplate due to electrostatic attraction.

The slightly lower polarization voltage does at least two things - It creates slightly less tension on the diaphragm and it slightly reduces the signal level from the capsule into the FET. In my tests the mic is very quiet and has more signal output than a vintage U87.

In reference to capsule specifications and polarization voltages I would like to point out that in many tube mic designs with variable patterns (for instance) that the rear diaphragm will see 0-120V depending on the selected pattern, the backplate will see a fixed 60V, and the front diaphragm will see 0V so if you have one of these tube mic's that uses that type of pattern adjustment and you would like to perform some polarization / sonic tests you could ground the front capsule (instead of connecting it to the grid), and use only the rear diaphragm with the variable bias / rear-diaphragm-voltage.

Cheers!
-jonathan

 

[bruno2000]

I have a collection of U64 mics (which I love), and consequently have a stash of NOS 7586 tubes.  Thought about doing something like this with a 47 or 49 capsule, but using the 7586 tube.  Thoughts?
Thanks!
Best,
Bruno2000

 

[0dbfs]

I have a collection of U64 mics (which I love), and consequently have a stash of NOS 7586 tubes.  Thought about doing something like this with a 47 or 49 capsule, but using the 7586 tube.  Thoughts?
Thanks!
Best,
Bruno2000

I think that would turn out similar to the original with the 7586 paired to the BV107 / CM5722 from Cinemag in what looks like a fairly standard plate-loaded self-bias circuit (close-to-stock chinese PSU would work well since it's not fixed bias). You could change the polarization scheme slightly such that you polarize the backplate and take the signal directly from the diaphragm while losing the capsule-to-grid blocking cap. Looks like the U64 uses a 51V capsule polarization divided down from 120V so you could also tune that as desired You could probably P2P it pretty easily too in a small body like this one for tight placement and a fixed omni/card pattern too.

Cheers!
-jb

 

[poctop]

Only  10 Left , 
Dan,

 

[Deepdark]

Be patient, I'll come with at least 4 of them soon

 

[poctop]

only 6 Left ,
Best,
Dan,

 

[poctop]

2 Left , and this is it ,
Best,
Dan,

 

[poctop]

KDOKELeeDO,  all gone ,  :'(
thanks To All and Enjoy your FET847 Project ,
Best,
Dan,

 

[Deepdark]

KDOKELeeDO,  all gone ,  :'(
thanks To All and Enjoy your FET847 Project ,
Best,
Dan,

Now we have to get enought interest to start another batch  ;D

 

[Spencerleehorton]

might be a silly question but why is there 22uf tant in the BOM? C9 optional tantalum?

regards

Spence.

 

[poctop]

80-T350D226K006AT T350D226K006AT Kemet Tantalum Capacitors - Solid Leaded 6.3volts
22uF 10%

C2 (more bass option) 1

Best,
Dan,

 

[Spencerleehorton]

Ah ok,

So could that be put to a switch and tuned to taste for bass?

Spence.

 

[Deepdark]

Ah ok,

So could that be put to a switch and tuned to taste for bass?

Spence.

You could put some socket (kinda dip socket) and simply swap different caps value to find out which one you prefer. Croc clip could be used to try them out, too