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R/E/P => Klaus Heyne's Mic Lab => Topic started by: Milec on May 08, 2018, 04:57:17 am

Title: Neumann M49b Making High Frequency Noise
Post by: Milec on May 08, 2018, 04:57:17 am
Hello Klaus,
I do not find on this forum a post corresponding to my problem:

One of my Neumann M49b generates a significant noise at approximately 10kHz, regardless of the audio signal at the input. I changed all capacitors and finally all the resistors! But without any improvement.

When I change polar patterns, the frequency goes down for 2 or 3 seconds and then goes back up to a stable sand loud 10kHz.

I exclude a power supply problem because my other M49b are working perfectly with the same PSU.
I think now to incriminate the AC701k tube but before to go further, I would like to know your opinion about this failure. Thank you in advance.
Title: Re: M49b making high frequency noise
Post by: klaus on May 08, 2018, 05:42:12 am
Hello  Mike,
Even before I finished reading your story, I was already thinking along the same line as you: a vacuum leak of the AC701 tube, or a nasty case of filament microphonics may be at the root cause of the high frequency squeal.

You may be able to change the squeal's resonant frequency or its intensity by gently knocking the tube with the handle of a small screw driver while the mic is on. If the squeal indeed changes behavior, this would confirm the tube as the culprit.

And should this be an issue of an oscillating filament inside the tube, there is a sledgehammer cure that sometimes works: give the tube a really hard knock with the screwdriver handle (remember: the next step would be replacement anyway!) Sometimes that knock will reorient the oscillating filament and quiet it down. This cure of course will not work if a vacuum leak is the origin of the squeal.

A side note: I have observed that in many cases where there is an AC 01 with a high level of microphonics, including run-away microphonics, the heater voltage was often set too high.

So make sure to start your testing procedure by correctly calibrating your power supply's tube heater voltage to no more than 4.00VDC, as measured directly at the heater of the tube (not inside the power supply!)
I set the heater voltage for all AC701-powered mics at no more than 3.95VDC, with a minimum of 3.90VDC.
Title: Re: Neumann M49b Making High Frequency Noise
Post by: Milec on May 10, 2018, 04:28:38 am
I thank you very much for your fast reply.
I will test firstly the DC heating voltage on the AC701 tube itself. I always mesured this voltage on the output of the PSU...
I keep you in touch!
Title: Re: Neumann M49b Making High Frequency Noise
Post by: klaus on May 10, 2018, 04:57:55 pm
I should have mentioned why one should measure the heater voltage of any tube mic at the tube itself, rather than at the power supply:

Depending on the length of the cable, there is a voltage drop due to wire resistance that cannot be generalized. Measuring heater voltage at the tube itself eliminates guessing the voltage drop.
Title: Re: Neumann M49b Making High Frequency Noise
Post by: Milec on May 25, 2018, 04:43:08 pm
I finally found  time for measuring the heater voltage on the AC701 itself. There is indeed a difference of -0.2V compared to the measurement at the output of the power supply. I calibrated the power supply to get 3.90V at the heater of the tube.
Unfortunately, this does not change the behavior of the microphone. The high frequency remains.
I tried to give a few light knocks on the tube. This causes a very audible "microphonic effect" louder and louder as you get closer to the top of the tube. But that did not change the high resonance frequency value. Next step: make a test with the AC701 of one of my other microphones that works fine ... hoping not to damage it during the (un)soldering steps.
Sorry for my basic knowledge of English!
Best regards
Title: Re: Neumann M49b Making High Frequency Noise
Post by: klaus on May 25, 2018, 05:12:53 pm
Can you upload a sound file of the noise to the forum or email it to me, before you undertake tube change-out?
Oftentimes, a good listen can reign in possible causes.
Title: Re: Neumann M49b Making High Frequency Noise
Post by: Milec on July 12, 2018, 04:44:06 am
Dear Klaus,
Your diagnosis was the right one! The AC701 is well the culprit. I finally decided to replace the tube and everything is back to normal. In addition, since I had previously replaced all capacitors and some resistors that could be the cause of this instability, the combination of the new tube and these new components give an excellent signal/noise ratio. Thank you, thank you very much!