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R/E/P => Klaus Heyne's Mic Lab => Topic started by: SDVIG on November 12, 2017, 07:56:51 am

Title: Neumann U67 C9 capacitor.
Post by: SDVIG on November 12, 2017, 07:56:51 am
Dear Claus.

I am very grateful to you for your help.
Your advices help me a lot.

I want to ask you a question about the problem with my microphone U67. I'm familiar with a skilled craftsman. He is known as one of the best masters in musical audio equipment. For example, it can parse and compile SSL 4000. But he is not a specialist in microphones. Now to the point. I feel my microphone is losing the low end. The master tells me that the capacitor C9 is responsible for this. When we extracted it from the microphone, the capacity was normal(Capacitance is OK). But the master does not have a stand in order to test this capacitor. If we did not have a capacitor of a suitable size, we put two capacitors with a superior capacity of 4 times. These capacitors have a voltage of 200 volts. Master said that this is dangerous for the microphone. With the audio test, the microphone received a low end, but the sound turned out to be harsh. Please advise which film capacitor should be placed in the microphone?
At the second edition of U67, Neumann used film capacitors.

I see such suggestions on Ebay:

MKC 1860 250V 0.47uF 474 10% HV Non-polar Film Capacitor:

 CAP, FILM, PET, 470NF, 250V, AXIAL MKT1813447255G By VISHAY

1. Could you advise me how to act?
2. Why is the U67 Schematic talking about 250 volts, but in reality I saw in several microphones 350 volts?
3. Maybe it's not a condenser?

PS: Any advice is welcome.

Second edition of U67: & My capacitor:
Title: Re: Neumann U67 C9 capacitor.
Post by: klaus on November 12, 2017, 12:23:14 pm
I am dubious that the original electrolytic 0.5mfd cap is your problem. It never has, during all my years of attending to U67. But there could of course always be a first.

Did you measure it outside the circuit? Every half-decent meter these days has a capacitance tester built in. I want you to conform that it measures low before you move on.

As to voltages in a replacement: You can safely install a film & foil polypropylene capacitor with 0.5mfd capacitance and 200VDC rating, as long as it fits. I would not use inferior material in this position as it's decoupling DC from audio going into the transformer.
Title: Re: Neumann U67 C9 capacitor.
Post by: Kai on November 13, 2017, 04:11:42 pm
Klaus, are you sure 200V rating is enough?
On start, when the tube is not already active and conducting, the voltage is a bit higher.
I see 225V B+ with NU67 PSU at nominal lines voltage.
I'd go for at least 250V for this cap, although PPs usually have some safety margin.
For an electrolytic I'd stay with 350V.

Testing the electrolytic with a normal multimeter cap tester doesn't tell the whole story, because DC leakage is not tested, which may lead to a bad low frequency response due to biasing the transformer.
Title: Re: Neumann U67 C9 capacitor.
Post by: klaus on November 14, 2017, 12:11:40 am
Klaus, are you sure 200V rating is enough?
Yes, plenty, and for the reason you mention: safety margin of f & f caps.

I have never had a failure in all the years I have installed them. Besides, there is too little space in that mic for a larger physical size with a higher voltage rating without sacrificing quality.