Gefell MV 691 Circuit description and Tolerance
« on: July 26, 2020, 04:35:28 AM »
Schematic is Attached .  I'm looking at refurbishing this microphone and hopefully improving it because it seems Gefell used some cheaper low tolerance parts.  I'm specifically looking at R9 and R10 (4.7 Mohm) feeding the polarisation voltage to the capsule I believe.  It looks like there's symetry there for some reason and it seems reasonable to me that those two resistors would benefit by being a closely matched low noise pair.  How critical is the value here?  I assume there isn't a lot of current flowing here so there must not be much voltage drop.  Could I go smaller to 3.8 M ohms without causing problems.  Vishay makes precision resistors in this value down to .05% and I can only find 4.7 Mohm in 1%.  Going to 3.8 Mom should reduce thermal noise in this application as well, is that correct?  Anyway, this is all speculation so it would be great to hear from someone who knows what's actually going on here.



Gus

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #1 on: July 26, 2020, 06:14:24 AM »
The two resistor work with C8 and C9 as a RC filter for the capsules charge voltages.
So IMO you picked the two resistors that have the smallest effect if any at all unless they are broken.

In your microphone what are C1 and C2?  If they are Al electrolytics they might have dried out with age if they are tantalums I would leave them alone unless they are bad.

What kind of cap is C3 in your microphone?

Are the gain stage resistors metal film?

NOTE the nice table in the upper right for the resistor values for the JFET used.



gyraf

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #2 on: July 26, 2020, 06:43:15 AM »
..and please don't underestimate the good people of Gefell - I've found that they know EXACTLY what they are doing for a surprising large part of their business :-)

They may have had temporary shortages of certain high-grade parts back in their east-german days, but that sure is a long time ago. And even that they countered very successfully by creative use of available materials.

/Jakob E.
..note to self: don't let Harman run your company..

Gus

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #3 on: July 26, 2020, 08:26:09 AM »
To be clear I would only change things that are out of spec/bad like dried out caps.
It is a well designed circuit the gain is controlled the transformers are fine.

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #4 on: July 26, 2020, 02:47:10 PM »
The two resistor work with C8 and C9 as a RC filter for the capsules charge voltages.
So IMO you picked the two resistors that have the smallest effect if any at all unless they are broken.

In your microphone what are C1 and C2?  If they are Al electrolytics they might have dried out with age if they are tantalums I would leave them alone unless they are bad.

What kind of cap is C3 in your microphone?

Are the gain stage resistors metal film?

NOTE the nice table in the upper right for the resistor values for the JFET used.

Thank you gentleman for lending your knowledge here.  That does make sense to me that it is the filter for the polarisation voltage.  Hoever, why would we not want to use high quality, low noise, and closely matched parts?  If the resistors have higher noise here, would that not still contribute to the overall noise.  Also why would they use 22 nF on one line and 10 nF on the other?   When I looked at the next generation MV 692 schematic, they used 22 nF on both lines.  I was planning on upgrading those old Eastern ceramic caps to new COG 1% or 5% parts.

C1 and C2 are polarised aluminum electrolytics.  Could someone please explain their function in this circuit?  I'm having a hard time understanding C2 and it's positive polarity facing toward the transformer primary.  I was thinking about replacing this with a Nichicon Muse Bipolar of the same value.  Similarly, I'm not sure what's going on this C2 either with it's Positive end on the Drain of the Fet.  Is this coupling the signal from the drain?  I was also thinking about replacing this with a Muse bipolar of the same value. 

C4 and C5 are also polarised aluminum electrolytics.  They are 1 uf "Tesla" brand.  I was planning on replacing them with new 4.7 uF Vishay 2021.  I think that the increase in capacitance could be beneficial here in what seems to be a ripple suppression role.

I'm not sure what C3 is exactly but it looks similar to other small polystyrene types I've seen with a clear coating over a silver body.  All the pF value caps in my mic are the same type. 

I'm not sure if the gain stage resistors are metal film or not as they look quite different from modern western resistors.  Based on pictures I could find, it looks like they could be either old soviet metal film or carbon film resistors. 

It sounds like the old soviet FET is probably the biggest contributor to noise in this circuit and I would really love to reduce the noise of this mic and improve the clarity of the sound.  The noise isn't unbearably loud but it is noticeably louder than modern condensor mic's and with compression, it can certainly be distracting.  I could look into finding a replacement FET with Ugs in the range of the table and replace the old biasing resistors for the FET with new low noise metal films. 

Let me know what you think!


Gus

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #5 on: July 27, 2020, 08:32:27 AM »
The 692 I had did not seem to have any more noise that other SD microphones that used similar charge voltages.
Have you checked them against other SDs or tried a different known good capsule?

moamps

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #6 on: July 27, 2020, 05:12:14 PM »
C1 and C2 are polarised aluminum electrolytics.  Could someone please explain their function in this circuit?

C1 and C2 are coupling caps. Their quality and value are critical. They are polarized that way because transformer's common point is +Vcc, not usually used 0V.

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #7 on: July 27, 2020, 11:25:13 PM »
The 692 I had did not seem to have any more noise that other SD microphones that used similar charge voltages.
Have you checked them against other SDs or tried a different known good capsule?

The 691 and 692 are different circuits and use different FET's (although the circuits are similar.  The 691 is older and discrete and mine has an old soviet FET and old soviet caps and resistors.  I believe you can still buy the 692 today so very different compenets in one of those examples vs my mic.

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #8 on: July 27, 2020, 11:27:45 PM »
C1 and C2 are coupling caps. Their quality and value are critical. They are polarized that way because transformer's common point is +Vcc, not usually used 0V.

Thank you; That was my suspicion, but the topology looked a little different than what I was used to.  It shouldn't be a problem to use bipolar electros there, correct?  I'm going with Nichicon muse there in the same values as the schematic.  Do you have any other ideas for improving the components in the mic; any other critical parts I should look at?

Also, does anybody now the size and thread of the screw that fixes the amplifier to the body tube.   It's tiny and I dropped it and lost it in the carpet.  :-\

thank you.

moamps

Re: Gefell MV 691 Circuit description and Tolerance
« Reply #9 on: July 28, 2020, 08:33:56 AM »
Do you use mod for +48V phantom  powering?
C4, C5 good electrolytics, can have higher capacitance,
C6,7,8 and 9 good low leakage polypropylene, can have higher capacitance,
C3 polystyrene  or styroflex, the same value
Check out also parts in capsule head, there should be one capacitor and few resistors IIRC.


Re: Gefell MV 691 Circuit description and Tolerance
« Reply #10 on: July 29, 2020, 01:28:47 AM »
Do you use mod for +48V phantom  powering?
C4, C5 good electrolytics, can have higher capacitance,
C6,7,8 and 9 good low leakage polypropylene, can have higher capacitance,
C3 polystyrene  or styroflex, the same value
Check out also parts in capsule head, there should be one capacitor and few resistors IIRC.

Thanks Moamps!  Your responses have been very helpful and generous :).  Yes, this mic has a mod for +48 power with a small circuit board with an IC and other passive SMD components,  From what I can tell, this is one of the better types of solutions for using one of these mics with +48VDC.

C4. and C5.  Yes!  I was able to figure out these are reservoir/smoothing capacitors and I've upgraded these to Vishay 021 4.7 uf parts!

For C8 and C9, I've purchased to high tolerance 22 nF c0g ceramic capacitors.  My research (not knowledge) tells me that ceramic capacitors in this application are a superior choice for shunting radio frequencies to ground. 
 
For C6 and C7 these appear to me to already be some type of film cap so I was considering leaving them stock.  The have some type of clear epoxy coating over a silver body;  They look very similar to polystyrene capacitors I have seen but, really I have no idea because I'm not totally familiar with all of the different Soviet parts out there.

C3 is also the same type of the suspected film caps mentioned above so I thought I would keep this stock also

The capsule I have is the M70 and I don't see any safe way to gain access to components that may be inside.

I'm excited to get started on these mods when the parts arrive!


 

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