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[hasbeen]

I recently bought a Gefell MV692/UM70 which I was told had been wired to accept regular phantom power. It is a Tuchel to XLR cable. The mic is noisy and sounds like crap. No problem with a return and full refund but the seller has offered a solution in the form of a Gefell N692 preamp/power supply and cable.

My question is: since the body has been modified for phantom power, will the preamp improve or even work with the mic using the older 7 pin cable? Or in general, will any of these old 691-692 bodies still work with the old cable and power supply once they have been modified to accept phantom power through an XLR?  Thanks

[klaus]

I don't get the "modified to phantom power" part of your post: according to my original MG manuals, the MV692 is powered according to P48 phantom supply- the original and still widely used DIN 45596 standard. Therefore, no special preamp is needed, but any mic pre with built-in phantom power will do (which is pretty much any mic pre sold for decades.)

Bottom line: You do not need a specialized supply or preamp when powering a stock UM70/MV692, and I would not buy a supposedly "phantom powered" mic that does not readily work with the universal, standard phantom power as per DIN 45596/IEC 61938.

Read more about that standard here: http://en.wikipedia.org/wiki/Phantom_power

[hasbeen]

If you go back far enough the MV 690, MV 691 and MV 692 models had no phantom power whatsoever and required a power supply to operate them. Phantom powered versions didn't come along until the MV 692 P48V version, although earlier versions could be converted. I have a coverted one.

[panman]

If you go back far enough the MV 690, MV 691 and MV 692 models had no phantom power whatsoever and required a power supply to operate them. Phantom powered versions didn't come along until the MV 692 P48V version, although earlier versions could be converted. I have a coverted one.

Yes, that is correct. However, you have not given enough information about the conversion to make any conclusions, but you seem to talk about a simple rewiring using a Tuchel to XLR cable and not a "real" conversion inside the mic-amp? Both are possible, but the rewiring leaves the capsule voltages too high, like 52V(my memory only) so, that it is not safe to use M94, that should have 40V max. The other SMS-capsules do not mind.
Anyway, you cannot use a N692 with a P48-converted mic.

[klaus]

To avoid further confusion:

At this point, it seems most logical to adapt your P48-converted mic and XLR-3 connector-equipped cable (with proper pinout) so that it can work with any modern phantom-power equipped mic  pre/mixer, rather than taking a step back and adding extra cost and hassle getting an old MG power supply.

[hasbeen]

Great info, thanks. It is a UM70 capsule but I am willing to bet the seller just did the rewiring and not the electronics.

[hasbeen]

To avoid further confusion:

At this point, it seems most logical to adapt your P48-converted mic and XLR-3 connector-equipped cable (with proper pinout) so that it can work with any modern phantom-power equipped mic  pre/mixer, rather than taking a step back and adding extra cost and hassle getting an old MG power supply.

Although the seller has offered the power supply for free I may just take this advice. I need to find out if there was ever any real electronic work done inside the mic body and not just a rewired cable. Any recommendations for the conversion work?

[Sredna]

Any recommendations for the conversion work?

I have the schematic for the 48V->12V circuit in the MV692P48 and a circuit for an adapter but it seems that I cant upload them due to "Your attachment has failed security checks and cannot be uploaded"

They are in JPG format and about 150kB in size, what's wrong?

To avoid capacitors in the signal path it's better to do the inside mic conversion.

Best,

Anders

[Sredna]

Just to clear some things up about the (Microtech) Gefell MV692:

Before about 1990 the name of the company was "RFT VEB Mikrofontechnik Gefell" an the model was named just MV692. It had a 7-pin connector and needed about 12 Volts (12.6V). This voltage could be given either as Phantom Voltage (P12) on pins 1&2 or directly at pin 4, pin 3 is ground. The dedicated power supplies was N692 (w. preamp), N691 and N690.

In the begnning of the 1990s the company was renamed into "Microtech Gefell GMBH" (MTG).
The first attemts by MTG to convert the mic to 48V was to just fit an XLR contact and use a resistor or pot (47k) to limit the current draw, the mic worked with 48V though. Later a small PCB was contructed to fit in the mic body with a simple circuit that converts 48V->12V. The model name was then MV692 P48.

...but the rewiring leaves the capsule voltages too high, like 52V(my memory only) so, that it is not safe to use M94, that should have 40V max. The other SMS-capsules do not mind.
Anyway, you cannot use a N692 with a P48-converted mic.

It depends how the mic was converted. The capsule element of M94 uses max 40 Volts but in the capsule housing there's a resistor to get the voltage from the MV692 (~60V) down. So the M94 can be used in the same way as the other SMS70-series capsules (M70, M71, UM70, M93, M73)

Best,

Anders

[Ernie Black]

I have the schematic for the 48V->12V circuit in the MV692P48 and a circuit for an adapter but it seems that I cant upload them due to "Your attachment has failed security checks and cannot be uploaded"

They are in JPG format and about 150kB in size, what's wrong?

To avoid capacitors in the signal path it's better to do the inside mic conversion.

Best,

Anders

This has cropped up from time to time in the PSW forums (and elsewhere). There was a discussion last December on the PSW SR/LAB forum about this: http://forums.prosoundweb.com/index.php?topic=135238.0 (actually shows up high in a Google search for the error message you got). Assuming the same issue, here's my reply from that thread:

A trigger for this appears to be when people try to attach Photoshopped images which have xml with links to other websites in the metadata. But then we can't expect people to edit their xml data.

Or can we? If you use Save for Web & Devices, the xml data is stripped down to just a few lines, which may well explain why in the above referenced thread I was able to post a jpg after saving for web & devices that I could not post before. Something in the xml data is triggering the security check fail. So try Saving for Web and reposting.

There is a way to turn off image checking security altogether, but doing so could expose the site and server to possible exploitation which is not a good option. If you are unable to process the photo as suggested, you can send it to me ([email protected]) and I'll give it a go.

[Sredna]

Ernie, many thanks, now it works!

Best,

Anders

[hasbeen]

Thanks for the great info! Unfortunately I am unable to do the mod myself due to lack of skills. Anyone know a good tech who does the in body mod?

Also, I do have the Tuchel to XLR connector and there may be a chance that the wiring is wrong. How could I make sure the wiring from the 7 pin connector on the mic is attached correctly to the XLR end? If there is a diagram available that would be great. Thanks again.

[panman]

I posted a drawing about the XLR to Gefell 7-pin wiring some years ago on the old forum, but was unable to find it. I have the drawing, but for some reason it does not let me attach it here. Anyway, it is the same wiring as in the second pic, that Anders was posting, but without the resistors, capasitors and the zener, XLR pin 2 going straight to Gefell pin 1 and XLR pin 3 to Gefell pin 2, XLR pin 1 to Gefell pin 3. Gefell pin 4 as well all the rest remain unconnected.
And yes, Anders is right, that M94 can also be used like other SMS-capsules, because of that resistor inside. But what I actually had in mind and was mixing up with, was the PM 750 from the same Gefell-family, that also has M94 capsule, but without that resistor. For this mic you better not use this "straight conversion".

[Sredna]

Esa, please read the answer from Ernie Black. I followed his tip and used "Save for Web & Devices" in Photoshop, worked...

Regarding PM750: it can be converted like MV692 ,its actually is a (kind of) MV692 without the pad and hi pass switches. The capsule in PM750 does indeed miss the internal resistor but the voltage drop is taken care of in the circuit (R2) as well as 10 dB pad (C1) and hi pass (C6). See attached schematic.

[klaus]

Thank you all for excellent documentations! The forum at its best!

KH