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[AusTex64]

I've taken on applying Oliver's UM92 mods for a friend's mic. Here's are the "before and after" schematics from the Tab Funkenwerk web site http://www.tab-funkenwerk.com/id71.html

A few observations and questions.

This mod isn't nearly as easy as it looks. There are significant mechanical changes to how the capsule mates to the impedance converter. It's also pretty crowded inside the upper deck of the mic. Turns out modifying mics is a lot harder (for me) than building them from scratch. I'm guessing Klaus can relate.... :-P

The PSU pinout on both of these drawings don't match up to the supplied Gefell PSU. From reading some posts where Oliver says the Gefell PSU is "crummy" and the pinout info, I'm figuring he assumed the use of a custom PSU, or one of his M49 style PSU's.

Being a glutton for punishment, I'm trying to make it work with the Gefell PSU. So far, so good. Heater voltage (5.8V) and B+ (120V) from the Gefell supply look OK. Rear diaphragm voltages for pattern change concern me though, I'm getting 150V in fig 8, 75V in cardioid, and 0 in omni. This is with 68V on the backplate. The mic is working, passing audio and sounds pretty good with these voltages. However, in fig 8 the front side of the capsule isn'tas lively and sexy sounding as the back side. In cardiod, front side sounds about the same as fig 8. In omni, both sides are not as loud as fig 8 or cardioid, but both side sound very similar. Is this a result of the high voltages?

I have an AMI M49 style PSU here, would have change the Gefell's Tuchel to 7 pin XLR and remove the regulator in the mic to make it work. Would like to be sure that's 100% necessary before resorting to that though.

Your thoughts?

Thank you,

Robert Mokry
Austin, TX

[klaus]

Before adding any more confusion, or trying some answers, can you explain how you get 150VDC from a power supply that only puts out 120VDC B+?
And how did you measure 68VDC at the backplate? Do you have a high impedance probe?

[AusTex64]

Thanks for your response Klaus.

I'm getting 68V on the tube plate, not the capsule back plate, my bad.

However, on the red wire (pattern change going to rear diaphragm), I'm getting 150V in fig 8, 75V in cardioid and 0V in omni. This is measured at the Tuchel coming into the mic. I am not sure how the PSU is supplying those voltages, but it is. There are several semiconductors in that PSU, and I don't have a schematic, so can only guess there is a voltage multiplier in there somewhere? I am measuring about 1/10 of those voltages (15V fig 8 and 7.5V cardioid) after the 100M resistor and 1uf cap, before the 1G resistor going to the rear diaphragm.

My main question is how much can back diaphragm pattern change voltages affect frequency response on a capsule like the M7 in this mic? The patterns are working as I would expect, there's just difference frequency responses in different patterns. At least to my ears.

[klaus]

You are asking two unrelated questions, which require two answers:

1. You have 150VDC polarization voltage on the rear diaphragm, because the power supply does not put out  a B+ of 120VDC, as you wrote, but 150VDC.

2. The M7 diaphragms can handle a max of about 75VDC (which is also the same electrostatic pull of the rear side at 150VDC).

If you notice stark differences in sound between front and rear in figure eight (after reversing the audio polarity of your headphone signal when listening to the back!) it would be due to different diaphragm pre-tensions, different wear, different aging of the two sides.
Besides that, multi-pattern, dual-diaphragm capsules sound very different in the three prevalent patterns, cardioid, omni, and figure eight, so don't ever judge front and rear uniformity of sound with anything but figure eight.
 

[AusTex64]

Thanks for your patience with my questions. This is a terrific resource. FWIW, I'm doing this mod for Stephen Bright. I've learned several things from your reply. Especially about reversing polarity of the headphones and how the polar patterns sound different.

To be more clear, Here are the voltages from the PSU:

9.6V for heater (regulated down to 5.6V in the mic)
120V for tube plate and voltage divider powering the capsule backplate
0 to 150V for pattern change

I own a high level electronic repair shop, so I'm having our power supply expert look at the PSU to determine where the 150V is coming from. I emailed Michael at Microtech Gefell last night requesting a schematic for the PSU, but I'm not holding my breath for that.

[radardoug]

The idea behind the variable voltage is that the capsule backplate is held at 1/2 the maximum rear capsule voltage, so swinging the rear capsule voltage from 120 to 0 will effectively reverse thevoltage on the rear capsule. So the backplate should be at exactly 1/2 the variable voltage. If it is not, you will not get accurate polar patterns. In the cardioid position the voltage on both sides of the capsule must be equal.
If the backplate is at 60v, then for omni both capsules outer voltage should be zero. 

[klaus]

(...) So the backplate should be at exactly 1/2 the variable voltage. If it is not, you will not get accurate polar patterns.

That is only true for figure-of-eight.

[AusTex64]

This article explains polar pattern methods quite well, it helped me a lot http://www.foxaudioresearch.ca/Polarpatterns.htm

[klaus]

Hello Robert, we need to revisit your first post.

First off, it is unclear what mic or power supply you actually are working on, because the model you mention in your header-UM92.1- does not exist. MG issued two mics with EC92 tube: the UM92 and the UM92s. These came with UN920 power supply (127VDC B+). Then MG changed the tube to an EF86 and relabeled the mic "UM92.1S". It came with the UM920.1 power supply (122VDC B+).

In the supplies for both tube models the rear diaphragm polarisation voltage for figure of eight is exactly double the center backplate voltage: 127VDC vs. 63.5VDC. and 122VDC vs 61VDC, respectively. You should aim for these voltages, depending which of the two tubes you have in your mic.

Best,
KH

[AusTex64]

So, the UM92S story ends like this...

My bad, the mic I worked on IS a Gefell UM92S. Uses the EC92 tube. I should have been more precise in my first post. I've corrected the title of the string to reflect this inaccuracy.

After searching and studying posts from Oliver Archut about his mods to the UM92S, he made it clear that the Gefell PSU is a POS and to replace it with a better PSU. So that what I ended up doing, using an AMI M49 style PSU and rewiring the connector to match. Never figured out why the Gefell PSU was outputting such strange voltages. With the AMI PSU all voltages now match the AMI mod schematic for UM92S, and it has all the M49 patterns too. Replaced the Sylvania tube that came in the mic with a NOS Telefunken EC92. It's halfway into the KH approved 72 hour burn in phase, and so far sounds quiet and delightful. Also added a 5K mini trimpot to dial in the 1.6VDC cathode bias, a la Lucas CS-1. Real handy trick Oliver turned me onto.

So, my Reader's Digest condensed version of the AMI UM92S mod is:

- count on replacing the Gefell PSU. FYI, Gefell doesn't use color coded wire wire for the mic cable, all 7 conductors are white or blue... :-(
- the AMI schematics aren't entirely correct, especially the pinout on the stock UM92 drawing.
- be prepared for physical mods regarding how the capsule mates to the impedance converter. You will be running wires down to the top deck.
- It's worth it. This mic sounds pretty damn good now.

Thanks again for your patience. A big thank you to Oliver (RIP) and KH (still kicking) for sharing their knowledge with the world. Hope my observations help anyone else that dares to take on this mod.

Happy New Year,

Robert Mokry
Austin, TX

[klaus]

Thanks for sharing, and let' also not forget that some of the sound quality derives form a superb M7 PVC capsule MG made at that time.
Best,
KH (still kicking, though you never know how much longer, until the kicking motions come to a standstill!)