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Author Topic: U87 low level background noise  (Read 301 times)

LLL

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U87 low level background noise
« on: June 13, 2024, 09:53:25 AM »

Hello Klaus / all,

I have recently been asked to have a look at re-building a friend of a friends' U87. The U87 was found in parts, in a box marked Noisy !....while recently clearing out his sadly deceased techs' home workshop.
Various resistors and FET missing, but I have replaced these according to schematic 102419012.7 (a copy was in the box, although, on the output transformer, it states 930-20 ? I have cross referenced the values against another U87 of same model ), selected a TI 2N3819 (Idss - 3.37mA, trim pot R=4.17K - used 4K), and biased as per Klaus' method for highest output, lowest distortion ( sig gen / scope) I have also replaced the tantalums for new.
I have powered it up, with a pair of 56pF SM caps to simulate the capsule assembly and placed in a screening can.

However, there is a very slight, but really annoying low level intermittent noise in the background, and I am having difficulty tracking it down.
Would any of you have any insight / experience of similar issue ?

I have also tried the mic with a donor capsule assembly - the mic sounds nice..... apart from that noise !

I have noticed, that the +48V supply, drops to 41.6V, - voltages at GR1 is correct at 33V, and node between R12/R14 is bang on at 29V. Next step is to try and isolate why the polarising voltage is being pulled down ?

A question regarding voltages at capsules - how much voltage drop due to loading, would be expected while measuring with a Fluke 289 ? I appreciate too, that some of these voltages are way off !

Measured at capsule socket with 56pF's fitted -
Pin 1 - 0.31V - drawing states 47V
Pin 2 - 35.4V - drawing states 47V
Pin 3 - 5.2V - drawing states 47V
Pin 4 - 0V - FET I/P
Pin 5 - 5.2V - drawing states 47V
Pin 6 - 38V - drawing states 47V
Pin 7 - 0V

No difference in measurements with capsule head fitted.

FET Voltages
Drain - 16V
Source 1.8V

Now, feeding a signal into the calibration inputs, and listening to the output on headphones, this background noise is not present !

Has anyone had an issue with possibly a noisy resistor / cap within the 'cup' section of the capsule head socket ?
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klaus

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Re: U87 low level background noise
« Reply #1 on: June 13, 2024, 11:22:07 AM »

Again, an audio sample of the noise (capsule connected) would help.

The reduced polarization voltage you measured is most likely due to the meter's internal resistance pulling down the 10,000,000,000Ω impedance of the capsule. So measuring with a meter is notoriously unreliable unless you have a genuine high-impedance probe in between meter and mic
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

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Re: U87 low level background noise
« Reply #2 on: June 13, 2024, 12:40:31 PM »

Try cleaning the high impedance part (around the FET) with a mix of isopropyl alcohol and distilled water, using a little brush.

Use a vac to suck away the solvent before it dries.
This way no residue stays.
Repeat several times with a fresh drip of solvent.


By no means let any fluid come near the transformer, hold the mic accordingly.


If this doesn’t help, the FET is No.1 suspect.
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