Neumann um57 PSU

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renx

Well-known member
Joined
Jan 3, 2007
Messages
59
Location
Zagreb, Croatia
Hello everyone,
I got this beautiful neumann um57 mic without a psu. I built my own according to the following specs:

um57:
pin1 = GND
pin2 = signal +
pin3 = anode voltage = +160v
pin4 = heater= +5.8v
pin5 = signal -
pin6 = pattern (from 0v to 160v)





I used Gyrafs g7 psu schematic almost entirely except for the lm317t resistor (I needed 5.8v instead of 6.3v). I also connected ...





now, everything works fine except when I put the pattern switch on cardiod (+80v) I get hum-noise.
I put a 1000uF capacitor in parallel with the bottom 100k resistor and it killed the noise totally, but when I switch between the patterns sparks fly out of the switch and eventually it stops working!

Any ideas would be very helpful.

Regards!
 
[quote author="renx"]
I put a 1000uF capacitor in parallel with the bottom 100k resistor and it killed the noise totally, but when I switch between the patterns sparks fly out of the switch and eventually it stops working![/quote]

sounds like you use a make-before-break (shorting) switch. it shorts out the capacitor ---- 1000nF? or µF ? :shock:
better put a 10k resistor on pin6 inside the microphone in series and put a cap (470nF) after the resistor to ground. that way you filter the polarisation directly at the mic and avoid shorting the filter cap with the 10k resistor.

-max
 
max thanks a lot,
I did that on the psu because I didn't want to alter the mic. It worked!!

Now I hear huuuummm around 160Hz. Could this be the tube?? It is Philips EC92. It sounds to me like a grounding problem, but I just can't solve it! I tried grounding different parts of the mic, but it didn't work! On one occasion I accidently touched an unisolated wire coming out of the central part of the membrane and it stopped humming just for a second...

Renx.
 
solved the hum...

The transformers I bought were really shitty, a lot of magnetic flux leaking out. Closed them in a metal box along with graetzes.

Regards.
 
Funnily enough, I just went through exactly the same process, trying to hook up a UM57 to a G7 PSU.

It looks like the cause of the problem is that the UM57 power supply has a 500K resitor and 1uF filter/stabilisation cap following the pattern selector inside the psu. For the G7 there is a resistor (33M) and cap inside the mic.

It only seems to affect the mid-voltage position as the B+ is well filtered anyway.

Hope that helps someone with the same!
 
Yes, the problem is that the G7 has polarisation-voltage-filtering inside the mic, where Gefell put it in the PSU.

Simplest fix would probably be to have a 470K-1M resistor in series with the pol-voltage in the PSU, and then a 100n-1uF/160V to ground.

This would also keep the mic and PSU compatible with other Gefell's that may show up over time (they tend to do that).

Jakob E.
 
I simply put a 2 meg resistor and a 47nF film cap in the cable at the mic end. There's plenty of room in the big connector, and it keeps the filtering as close to the mic as possible, without modifying the nice old Neumann.

But Jakob's method is nice because you won't have to modify the cable each time.
 
renx said:
UM57:
pin1 = GND
pin2 = signal +
pin3 = anode voltage = +160v
pin4 = heater= +5.8v
pin5 = signal -
pin6 = pattern (from 0v to 160v)

I think the pattern voltages are officially meant to be 120v, rather than 160v, though?

As follows:

UM57:
pin1 = GND
pin2 = signal +
pin3 = anode voltage = +120v
pin4 = heater= +5.8v
pin5 = signal -
pin6 = pattern (from 0v to 120v)

(that's what checks out on my mic's here, with original supplies)
 
Yes, older Gefell mics use +120V for anode voltage and thus +60V polarization voltage for the front diaphragm via voltage divider.

Later Gefell mics use +80V for capsule polarization - that's what the S in the model number stands for.

UM92S -> 80V,
UM57 -> 60V.

Some aftermarket PSUs use 160V/80V for older mics, too.
 
zebra50 said:
Wow! Old thread!  8)

Ha.  Indeed.  Hi Stewart.
Yes.. it is old.. I came to this thread trying to figure out for myself what are the correct voltages for this mic / PSU ..

I'm trying to go over a couple of old UM57's the past couple of days, and I think I have things pretty well figured out, now, but the polarisation voltages are not correct yet on my UN61 supplies.

On one power supply (with mic disconnected) I have 0, 30, and 80v.
(Omni, Cardioid, Figure Eight)

On the other I get 0, 0, and 80v.

Which also reflects just what I hear with each of the mic's plugged in.

Can anyone suggest which components to check to resolved this?
I haven't updated or checked any cap's on the supplies.

Thanks!
Evan.
 
>On one power supply (with mic disconnected) I have 0, 30, and 80v. (Omni, Cardioid, Figure Eight)

>On the other I get 0, 0, and 80v.

Well, on the first I would check first that the two 500K resistors that make up the voltage divider are still in spec. On the second I would do that and also check the switch for a short.
:)
 
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