Gefell / RFT m94 in Royer SDC mod

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vitopower

Well-known member
Joined
Jul 23, 2009
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Location
St. Louis, MO
Would anyone be willing to help me understand how I might use an RFT m94 capsule with the Royer SDC mod (schematic posted below)? 

I've successfully got it working with an oktava mc012 but not sure how to proceed with this capsule.

There is a resistor inside the m94, which I understand is to bring the standard MV body 60v down to 40v.

I can’t find info on whether the backplate or diaphragm for m94 should be polarized, also whether the capsule pin is the backplate or diaphragm.  Also not sure if either are attached to body,  ehh.. :-\ and how this would affect the circuit.

THANK YOU!

 

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vitopower said:
Would anyone be willing to help me understand how I might use an RFT m94 capsule with the Royer SDC mod (schematic posted below)? 
It looks like this works by providing bias via grid-leak; since the cathode is at about 40V, the grid is very close to that (a little less though).

There is a resistor inside the m94, which I understand is to bring the standard MV body 60v down to 40v.
Grid-leak bias cannot supply any noticeable current (femtoamps at best); even with gigaohm resistors, that would not provide any useful bias voltage. This resistor must go!

I can’t find info on whether the backplate or diaphragm for m94 should be polarized,
Any electrode in a condenser capsule can be polarized (there are cases where both the backplate and the diaphragm are polarized).

also whether the capsule pin is the backplate or diaphragm. 
You may try; the only difference would be the polarity of signal.

Also not sure if either are attached to body
If there's only one pin, I'm 99% sure it's the diaphragm, and the body is connected to the backplate.
 
The body of the Gefell mics is grounded, the capsule element  is isolated from the housing (like Schoeps CMT-/CMC capsules).

Backplate -> central pin -> signal
Diaphragm -> contact ring ->  Vpol
Housing -> GND
 
Thank you Abbey and Bremusound! ;D

I understand the resistor is connecting the capsule to the housing.  Should I be concerned about damaging the capsule by removing the housing? 

If I don't want to risk damaging the capsule by taking it apart, could I just ground the diaphragm or would the resistor still be a problem? 

I'm using the octava body and the adapter has capsule grounded and Vpol at pin, so this would make it easier mechanically.
 
This mic amplifier is virtually doing the same thing as done with the old Altec M-30 mic which also used a 5840 tube.  The cathode voltage is developed across the 39 k resistor to ground; approximately 1 ma of current flows through the tube; the grid bias is grid leak, and the grid leak resistance is essentially that of the tube...which may be a few hundred megohms on a good day...
 
abbey road d enfer said:
You may try; the only difference would be the polarity of signal.
No, it doesn't make any difference, the polarity will stay the same.
With an increase in pressure, the diaphragm is moved towards the backplate.
The capacity will increase. In this case it doesn't matter if the polarisation voltage is on the membrane or the backplate.
After all: the only thing that counts is the change in capacity.
And that is the same from membrane to backplate, as from backplate to membrane!
 
RuudNL said:
No, it doesn't make any difference, the polarity will stay the same.
With an increase in pressure, the diaphragm is moved towards the backplate.
The capacity will increase. In this case it doesn't matter if the polarisation voltage is on the membrane or the backplate.
After all: the only thing that counts is the change in capacity.
And that is the same from membrane to backplate, as from backplate to membrane!
Doh!
 
vitopower said:
I understand the resistor is connecting the capsule to the housing. 
Which side of the capsule? The diaphragm or the back plate?
A schemo of the RFT mic would help. Is it the same as MV692?
If it's the same, I would say the resistor is about 10 megohm. But, If I understand the schemo, the polarization voltage is applied to the  electrode that is not connected directly to the FET's gate. If it's really the case, you should be able to connect directly the electrode that is connected to the body via the 10 Meg resistor to the body (ground).

Should I be concerned about damaging the capsule by removing the housing? 
I would be concerned; that's a very tedious (and sometimes impossible) task.
 
vitopower said:
I've successfully got it working with an oktava mc012 but not sure how to proceed with this capsule.

I can’t find info on whether the backplate or diaphragm for m94 should be polarized, also whether the capsule pin is the backplate or diaphragm. 

Curious what it takes to get one of those capsules on an Oktava MK-012 body. 

Polarization:  look at all the ways the U67/U77/U87/U87AI capsules are polarized, that's a good basis for observed learning. 
 
abbey road d enfer said:
Which side of the capsule? The diaphragm or the back plate?
A schemo of the RFT mic would help. Is it the same as MV692?
If it's the same, I would say the resistor is about 10 megohm. But, If I understand the schemo, the polarization voltage is applied to the  electrode that is not connected directly to the FET's gate. If it's really the case, you should be able to connect directly the electrode that is connected to the body via the 10 Meg resistor to the body (ground).
I would be concerned; that's a very tedious (and sometimes impossible) task.

Thanks Abbey,
I have attached a MV692 schem (p48 ver). 

This is from user Sredna, who seems to be very familiar with these mics:

Sredna said:
The PM750 is (as you know) meant for singing but more versatile witout the HP and pad,

so that's a good and easy mod.


The capsule is indeed mechanically like an M94 but not electrically, the real M94 has a 10M resistor between the membrane side and capsule housing (=GND). The reason is that the M94 can take

just 40V polarizing voltage but the other capsules in the SMS70 series use 60V.


The PM750 capsule lacks this resistor because this is already taken care of by R2 in the microphone amp. (see schematic)


If you remove R2 you could use the other capsules with full potentional.

(can't use the old capsule anymore though...)
 

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Here is pic of the capsule from the other forum:
http://repforums.prosoundweb.com/index.php?action=dlattach;topic=36984.0;attach=2544;image
I would consider removing 10M resistor, but!
Other option, if you truly want to use Royer cathode follower  - if the 10M is connected between diaphragm and ground, then why not just connect diaphragm direct to ground (short 10M)? This is connection which you need for Royer  cathode follower.
 
EmRR said:
Curious what it takes to get one of those capsules on an Oktava MK-012 body. 

Polarization:  look at all the ways the U67/U77/U87/U87AI capsules are polarized, that's a good basis for observed learning.

Thanks for the pointer EMRR, still new to mics, about to build a u67, will check this out.    I used a hollowed out
oktava 10db pad, it screws onto the body and the pin goes into a loop formed by the 5840 signal wire.  Some rubber grommets fit the shell of the Gefell body tightly to the oktava, the copper spring in the pic is temporary and was from before I knew the shell is isolated from the backplate/capsule.  Now it looks like I will still need a solid connection from the body to ground.
 

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ln76d said:
Here is pic of the capsule from the other forum:
http://repforums.prosoundweb.com/index.php?action=dlattach;topic=36984.0;attach=2544;image
I would consider removing 10M resistor, but!
Other option, if you truly want to use Royer cathode follower  - if the 10M is connected between diaphragm and ground, then why not just connect diaphragm direct to ground (short 10M)? This is connection which you need for Royer  cathode follower.
Thanks, ln76D!

From the help from RuudNL, Abbey and EMRR, whose posts I’ve been reading  for years, I was realizing this and mentioning it in my reply to EMRR while you were posting.

I’m using the Royer circuit because its here and working and one less thing I have to buy a transformer for or build.  Would you recommend a more preferable tube circuit for the m94?
 
vitopower said:
  I used a hollowed out oktava 10db pad, it screws onto the body and the pin goes into a loop formed by the 5840 signal wire.  Some rubber grommets fit the shell of the Gefell body tightly to the oktava, the copper spring in the pic is temporary and was from before I knew the shell is isolated from the backplate/capsule.  Now it looks like I will still need a solid connection from the body to ground.

Hmm, I'm still not totally sure what I'm looking at there.  Does the m94 match the oktava threads and diameter? 

Here's what I did once, it got lost in the shuffle along the way and never finished up. 

https://groupdiy.com/index.php?topic=24058.0
 
EmRR said:
Hmm, I'm still not totally sure what I'm looking at there.  Does the m94 match the oktava threads and diameter? 

Here's what I did once, it got lost in the shuffle along the way and never finished up. 

https://groupdiy.com/index.php?topic=24058.0

haha ;D, no the gefell is bigger, but the o-rings keep the oktava centered and stable and the pin receptacle of the pad has a enough room in it's indentation to makeup for a few mm if it they don't line up exactly.  The pin receptacle is also spring loaded and goes down a few mm when the capsule is screwed on.  The area surrounding the pin receptacle is isolated by plastic, is metal and connected to the shell, so it can connect the m94 diaphragm contact ring via the spring. 

I think I have a spare Mil Spec Raytheon/Philips ECG NOS 5840W if you decide to pick up your tubed oktava project.  Mine is dead quiet after some ferrite and RFI cap work.
 
vitopower said:
I’m using the Royer circuit because its here and working and one less thing I have to buy a transformer for or build.  Would you recommend a more preferable tube circuit for the m94?

Personally i prefer plate follower than cathode follower. Especially when you are able make true cardioid connection for the capsule after removing 10M resistor. This isn't common in SDC capsules so if there's  a chance, then why not.
Most circuits which fit LDC, will fit SDC - maybe you will have to slightly reduce low end.
I truly like Schoeps CV60 topology (of course without input capacitor,  true cardioid connection).You should be able to fit 6au6 in gefell type body without any problem.  Here you will find schematic:
http://www.sdiy.org/oid/mics.html
For sub mini tubes, just simple follower is enough. Like M49C without all the "broadcast filters". Anyway i would keep feedback cap frome plate to grid.
Here's truly simple circuit i posted. This was for grounded diaphragm SDC capsule, so the difference os only to remove C2, R3 to diaphragm, R2 to backplate and case to ground. It can be more simplified.
https://groupdiy.com/index.php?topic=63436.msg803459#msg803459
Forum member Vac11 is currently building similar circuit, but with 6S6B-V, maybe he will post here his experience with this circuit.
For transformers - ultra small, good and not so expensive - Beyerdynamic, Sennheiser, Neutrik. Little bit bigger and not expensive 3UAudio, Pikatron. There's lot of options.
 
ln76d said:
Personally i prefer plate follower than cathode follower. Especially when you are able make true cardioid connection for the capsule after removing 10M resistor. This isn't common in SDC capsules so if there's  a chance, then why not.
Most circuits which fit LDC, will fit SDC - maybe you will have to slightly reduce low end.
I truly like Schoeps CV60 topology (of course without input capacitor,  true cardioid connection).You should be able to fit 6au6 in gefell type body without any problem.  Here you will find schematic:
http://www.sdiy.org/oid/mics.html
For sub mini tubes, just simple follower is enough. Like M49C without all the "broadcast filters". Anyway i would keep feedback cap frome plate to grid.
Here's truly simple circuit i posted. This was for grounded diaphragm SDC capsule, so the difference os only to remove C2, R3 to diaphragm, R2 to backplate and case to ground. It can be more simplified.
https://groupdiy.com/index.php?topic=63436.msg803459#msg803459
Forum member Vac11 is currently building similar circuit, but with 6S6B-V, maybe he will post here his experience with this circuit.
For transformers - ultra small, good and not so expensive - Beyerdynamic, Sennheiser, Neutrik. Little bit bigger and not expensive 3UAudio, Pikatron. There's lot of options.

This is so cool, 8) and I have these tubes here.  Thanks again,  ln76d!
 

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