Gefell PM860 and PM750 48V phantom

[sdelsolray]

You need to either (a) get a 48v to 12v converter cable/unit or (b) convert the mics to 48v with 3 pin XLR connectors and whatever else you have to do to get there.

[Nobilmente]

Originally Posted by vanakaru

Hi.
I got recently pair of PM750 with M94 capsule and a pair of PM860 with capsule no markings. I have seen these mics with the same capsule before (kind of open design with a spring in the center). The guy who sold them said that I could power these strait from modern mixer, but as far as I know these work off 12V by the design. The preamps are M691 and M692 as far I can tell.
I have been suspicious to try them with 48V without rebuilding the preamps. I have not got a good deal original power supply yet, but still looking.
So, what about the 48V phantom. These are 7pin stecker ones.

Have a look on ebay for a young man called Leo Schaal. I spoke to him about designing a battery free converter, so he made one for me powered by 48v. He's selling those on ebay now, along with a battery version. They're very convenient. He'll also supply you with an adapter lead if needed too. I have both versions here, but finally had my microphones converted to 48v powering. There are several ex-employees of Gefell around who will do the work, or Gefell themselves. Gefell may want to replace parts that don't actually need replacing, such as mountings and non-wearing items, so ask them if that is really necessary or can the existing parts be used without any detriment, it will cost more than needs be otherwise. I think it's easier for them to replace whole units rather than just the parts that need replacing.

I'd say mention my name to Leo, but I'm using Nobilmente on the forum, and he won't know that one!

All the best

[vanakaru]

Thanks!
I tried the 48V mod by installing 24k resistor(not the pot) on a transformer circuit. It reduces the voltage successfully to 12V. I got the original PSU now as well and find the converted one more noisy than the original setup wth N691.

[jensen1]

I hope this thread is still of interest to people who want to convert their Gefell stuff.

I wouldn't recommend using a resistor to get 48V down to 12V for Gefell mics. This is unreliable because it only works if the phantom power is always at 48V precisely. I'd recommend using a regulated converter design.

I have built external adaptors for Gefell mics, as well as converter units which go directly inside the mic. At least an external converter is pretty easy to build. You need a transistor, a zener diode, some caps, some resistors. That's it. This will also keep noise low, so that you don't hear any difference between the origninal PSU and a converter.

Installing a converter directly inside of the mic actually makes the circuit even simpler, because you don't need big blocking capacitors for decoupling. This also gives the best audio quality. The DC is directly taken off the center tap of the transformer. The only problem is that you can only make this converter using SMD parts, otherwise there is not enough space for a "traditional" design.

[jensen1]

Here's a picture of the unit after installation in the mic.

[vanakaru]

Thanks for that! I am still working on the best solution.
I have been experimenting with different possibilities. This resistor mod was supposedly only for use with specific sound card. And I know that phantom power has no standard what so ever (15V-58V). It is my thought as well you need to use zener in any way. The only concern was its noise that may be added to the line. What is your experience with zener noise and what kind you use? Do you care for resistor type?
It would be nice if you share a schematics for your mod as well.
I built the external adapter according to this layout (on the very bottom of the page)
ELEKTROAKUSTIKA® RFT MV692 - condenser microphone
But since the old 7pin connectors are a crap I have been thinking more and more to mod the 692 circuit itself and replace the connector.
BTW there is a picture of XLR version of 692 in the UM71 somebody was selling on ebay. You can see the same add on probably with zener.

[jensen1]

Noise from a zener is no issue, if it is bypassed by a cap. Also it is always better to use a regulated design with a transistor. The schematic from your link is only useable for testing the mic or very low-fi applications, but not for serious recordings. Electrolytic caps in the audio path are not without problems, do add some more concerns. I have done my adaptor with 22µF MKT (they are a bit pricey and also very big). But you don't need this stuff, when you put the circuit in the mic.

There are some versions of MV692 (PM860 is the same circuit, only a different housing) which were equipped with a regulator directly at the center tap from the transformer. I have attached a copy for you. These parts replace D1/2 and C12 from the original circuit. This works very good, no noise issues. The only problem is, as I said, that you need to use smd parts, at least for some parts. I used a cheap 12V SMD zener. Nothing special.

I have some mics, which were converted to 48V directly by Gefell (I didn't want to mess up the mics at that time....). When I compare them to the ones which I converted, I can't hear any difference in noise.

As for the 7-pin Tuchel connector: These are still pretty easy to get here in Germany (with some patience) as NOS. If the connectors are in good condition, there's no problem with the contact. So I just left my mics with the Tuchel connector and made adaptor cables, Tuchel to XLR. No problems with that.

[vanakaru]

Thank you! What transistor it is? I shall try this tomorrow.

[jensen1]

I think this is not very critical. Any transistor which can handle the voltage and current can be used, as long as it is an npn type. Make sure to check the circuit before you install it.

[vanakaru]

I did not have much success. The mic has less volume and much more noise.
I used BC550C 0.5W transistor and BZX55C12 0.4W zener and ceramic resistors.

[jensen1]

How much V did you measure at the output of the circuit? Did you use it internally or externally? The mic may suffer from other problems, which don't have to do anything with the powering modification.

[vanakaru]

The mic was in excellent working condition while using N691 power supply. I used the internal build. And I get 16V on emitter(R6 C9) and pin1(shield). Any thoughts?

[Omega Arts]

Very interesting that I found this thread, I was about to post a thread about the PM750 mic and M691 power supply.

I picked two PM750's and the dual M691 PSU in Muldova several years ago and have never used them because the PSU is 220/50.

Can someone tell me what I have here, something about the mics and what they would be used for. I really would like to have them active but I'm reluctant to have them modded to 48v unless I have to.
Is there a step up tranny I could use here in the states to use the M691 PSU.

I'm always looking for STUFF when I travel and someone gave me these two mics and PSU. They are in almost mint condition with just a few minor scratches. The capsule doesn't have a number I can find. Where would I look for the capsule number?

Thanks guys for any info on these mics,
Larry

[jensen1]

Originally Posted by vanakaru

The mic was in excellent working condition while using N691 power supply. I used the internal build. And I get 16V on emitter(R6 C9) and pin1(shield). Any thoughts?

In that case your regulator is not working properly. The internal working voltage of the mic is something like 11.2 Volts, since the original diode D1/2 or D 1/1 (depending where you apply the powering) is dropping the 12 V power by some 0.7 V (give or take). You should get something like 11 Volts on junction R6/C9 (no matter if you use the regulator or N691). You should test the regulator without connecting it to the mic. It is always a good idea to install a variable resistor behind the circuit in order to not over power the mic when testing. For instance take a 50k potentiometer and put it between the output of the regulator and junction R6/C9. Adjust the resistor to max and turn on power. Measure and adjust the resistor, keep measuring until you find that the resistor is on min. and the regulator works. If you have too much power coming from the regulator it simply doesn't work as intented. Check the circuit for errors (zener soldered the wrong way round.....).

@Omega Arts:

The PM 750 is a smal diaphragm condenser mic. It is basically identical to the Gefell MV 692 (same circuit) with M94 capsule. The main difference is that in PM 750 the capsule has something like a "shock mount" which makes this mic suitable for using on stage. Thus the bigger housing compared to MV 692.

You can certainly use a step up transformer for powering your N691. This is easy to get. You don't need to convert the mics to 48V. It just makes them more easy to use, especially when you have a bunch of them, like I do.

[vanakaru]

Originally Posted by Omega Arts

Very interesting that I found this thread, I was about to post a thread about the PM750 mic and M691 power supply.

I picked two PM750's and the dual M691 PSU in Muldova several years ago and have never used them because the PSU is 220/50.

Can someone tell me what I have here, something about the mics and what they would be used for. I really would like to have them active but I'm reluctant to have them modded to 48v unless I have to.
Is there a step up tranny I could use here in the states to use the M691 PSU.

I'm always looking for STUFF when I travel and someone gave me these two mics and PSU. They are in almost mint condition with just a few minor scratches. The capsule doesn't have a number I can find. Where would I look for the capsule number?

Thanks guys for any info on these mics,
Larry

It would be rather easy to replace the transformer in the N691. There is plenty of room inside. Just get well made one that puts out 23-24V - that what I measure on my transformer output in N691.

[vanakaru]

I got the voltages right - 11.6V on R6/C9. But comparing to unconverted mic with N691 there is significant noise/hiss increase. I wonder how much the quality or materials of the parts contribute to the noise.
I would try the external adapter too. Could you share the schematics for that as well.

[jensen1]

Are you sure you bypassed the zener with an electrolytic cap of 22µF? You may have to check the quality of your phantom power source. If you can, check the mic with another mixer (or mic preamp). You may have some crappy phantom power with a lot of noise, to begin with.

I added the schematics for the external adaptor (sorry for the poor quality). It's pretty much the same like the internal circuit. Make sure to use a 13V zener. This gives s.t. like 12.4 V output. This unit should work with noisy phantom power as well, since there's some caps for buffering. The circuit powers the mic with power on pin 4 of the mic-plug, but it shouldn't be too difficult to modify it for phantom powering. Instead of the 22µF caps you can use electrolytics as well, since MKT's of this size are a bit hard to get and pricy. But I'd bypass the caps with smaller sized MKT's in that case to improve the sound quality.

[alistair]

Thank you so much to Jensen for this great little circuit. I've successfully installed it in a pair of MV691's and I'm very happy with it.

Has anyone found a suitable XLR jack to use with the existing Gefell mount? Or is a matter of machining something to replace the Gefell mount?
My initial searching has led me to the Binder connector company, which seems to make a lot of connectors that would drop in to the existing mount, but their only 3 pin units are DIN, not XLR. I also know an XLR jack is a little bit narrower than the 7 pin Tuchel.

[tinn]

Hi Jensen,

your regulated circuit will produce a bit of noise.
Maybe the supply in the mic amp will be halfway clean.
But the transistor sees a very low Z at it's base and this
way you get a lot of noise current in the collector
which might be audible, if your input is not perfectly balanced.

The unregulated version with the resistor should be fine.
The phantom power has to be exactly 48 volts in this case.
I converted my microphones with the circuit below.
The circuit looks a bit weird. The reason is, that it
fits exactly onto the old board like that.

Cheers, Tinn

[verbos]

I had the original power supply for my PM750 that I bought when I lived in Germany 10 years ago. I hated using it because it was such a pain in the ass to plug in the supply, power converter, blah blah.

I modded it a couple years ago by putting in a 47k resistor in place of D1/2 and C9. It worked but then shortly after that lost all its gain. I could get sound out of it, but only if I cranked the preamp up to MAX and then the noise was unbearable so I shelfed it.

Yesterday I dug it out again, and found this thread. I switched the resistor to a 2N3904 and 12v zener, etc and it didn't help the lack of gain. I did a bit more research and decided I had to un-pot the 29-25. I replaced the SMT PNP in there with an MMBT3906 and the gain came back! There was still some popcorn in the background. So, I took apart the capsule portion and replaced the FET with a J201 and it's now back to 100%.

So, to sum it up.... I now have it working from phantom power, using a cable with Tuchel on one end and XLR on the other. All electronics are inside the body and the sound is clean and ..... well it's still a noisy mic, but not the shit-show it was. It's back to how it was when I got it, and still had the external power supply.

Mark