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91
Klaus Heyne's Mic Lab / Which Transformer T14/1 for a DIY ELA M251 ?
« Last post by Avgatzeblouz on May 09, 2018, 09:05:42 am »
Hi everybody ! As I'm scouting the internet to find people experiences with current offer on T14/1 transformers, I find myself confused, as I found lots of posts all saying different things.

So I would like to have the advice from experienced people on this forum: what is to your opinion the closest replica one can find that matches the T14/1 found in the vintage ELAM 251 ? And is the current Haufe offering actually the real deal, and no need to look any further ?

Thanks a lot !
92
Klaus Heyne's Mic Lab / Re: Does This Work To Self-Diagnose Mic Noise?
« Last post by Kai on May 09, 2018, 08:45:22 am »
The methods you describe perfectly works, preamp self noise should be of no concern in this case because the microphone's self noise is way higher than any quality preamp's.
Analyzing the noise by ear is OK too, because what you hear is what is annoying in the noise.
The method could be simplified if you have a stereo preamp (I assume that both channels are of the same quality):
Both microphones could be recorded on separate tracks AT THE SAME TIME, mics located as close as possible side by side, with any kind of signal (except pure tones) as reference, e.g. speach or white or pink noise out of a smartphones speaker.
After level matching the signal in the DAW the noise level can be compared either by ear or with any kind of meters.
The only problem that might show up is: you need a very very quiet room, especially if you want to measure the noise with a meter.
When I do noise measurements on microphones I put them inside a damped box in my very very quiet recording room.
So record the reference signal, then close the box while continue recording the selfnoise, without changing any setting inbetween, would be the way to go.
93
Klaus Heyne's Mic Lab / Re: How to (Re-)Mount Badges on Microphone Bodies
« Last post by vinylwall on May 08, 2018, 05:58:54 pm »
These instructions are spot on.  I'd like to thank Klaus for taking my phone call and explaining this process to me.
94
Klaus Heyne's Mic Lab / Re: Does This Work To Self-Diagnose Mic Noise?
« Last post by klaus on May 08, 2018, 03:04:35 pm »
The intention here was to self-help someone who does not have access to sophisticated and expensive test equipment. So, Jim, will the procedure I recommended work in this specific case?
95
Klaus Heyne's Mic Lab / Re: Does This Work To Self-Diagnose Mic Noise?
« Last post by Jim Williams on May 08, 2018, 12:41:42 pm »
The scientific way without subjective guess work is to use a graphical analyzer to "view" the noise vs frequency plots from 20~20k hz. That will show whether the noise is hi frequency hiss, mid range grunge or lower frequency hum components, or all three.

Before that is done the mic preamp needs to be tested in the same way. Some designs like Audio Precision allow one to extrapolate the mic noise from the measured preamp noise to eliminate that preamp noise contribution as all noise sources are accumulative.

Then one would have a proven, scientific base line of the specs of those models in which to compare to others. AP will allow you to nest those sweeps into a group to show the baseline and any variants. Then you can create a bell curve to show all the permutations as well as the base line at the top of the bell.

This sort of advanced testing does require an initial investment in rather expensive test equipment which is not commonly found in the smaller pro audio companies.
96
Klaus Heyne's Mic Lab / Re: M49b making high frequency noise
« Last 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.
97
Klaus Heyne's Mic Lab / Neumann M49b Making High Frequency Noise
« Last 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.
Mike
98
Klaus Heyne's Mic Lab / AKG C451 PAD Pin issue
« Last post by MikeCheck on May 07, 2018, 08:23:18 pm »
Hello All!
I have a AKG C451 E with a -20db PAD. The pin is too short on the PAD and does not reach the amp.

I've attached a photo. Obviously the PAD did not come with this mic based on the finish.

I'm curious if anyone has dealt with this before and if there is a resolution. A replacement pin maybe?

Thanks!
99
Klaus Heyne's Mic Lab / Re: Does This Work To Self-Diagnose Mic Noise?
« Last post by RuudNL on May 07, 2018, 01:05:23 pm »
IMHO this is a very reliable and objective method to check the self noise of a microphone.
Anyway, this is the method I also use myself.
An other way of checking for noise (if you have a second microphone of the same type), is to listen to the two microphones with headphones.
One in the left channel and the other one in the right channel.
Carefully adjust for the same level when speaking in both microphones close together and after that listen to the noise in a quiet room. If there is any significant difference in self noise, you will hear the noise 'out of center'.
100
Klaus Heyne's Mic Lab / Does This Work To Self-Diagnose Mic Noise?
« Last post by klaus on May 07, 2018, 12:59:44 am »
So I get this email from a gent who suspects that his KM84 Neumann is hissy noisy "when I increase gain".

Here is what I wrote him back:

... before you go to the next step and involve professional help, I recommend a simple self-diagnostic.

First, you need to put the hissing you perceive into perspective by defining a baseline that allows you to compare the noise of your mic to that of another. That will determine:

*is this just the normal noise floor of a KM84 (they are not the quietest) you are hearing, or
*does your KM84 objectively have too much noise, compared to its specs? 

As you may not have a second KM84 to compare noise types and floors with, I recommend to record with your KM84 a sine wave or, preferably white noise at a reasonable distance and level from the speaker that has been fed the test sound. Then turn the tone/noise generator off, but continue to record for a few more seconds (i.e. quite room).

Now, on a second channel, record with a different professional condenser mic-hopefully a healthy one-the same white noise, after adjusting the level on the recording meter to exactly the same as for the KM84 recording. Continue recording a bit more without sound input (quiet room).

After you have equalized playback levels between channels during playback of the test tones, crank up the listening level and listen for the self-noise level during the quiet section at the end. THAT will give you the information whether your KM84 is unusually noisy, or still within a 3dB range* of the other mic's noise floor, therefore "within spec".


I have used the testing method I described for many years, and never had anyone tell me that the results were not at least roughly in sync with either their hearing or more sophisticated methods of noise analysis.

But i am open to modify or abandon how I comparatively test noise, if any of you deep thinkers see a fatal flaw in my methodology. To emphasize, for this kind of fault finding I am not chasing after the last dB. That's the realm of fine tuning at a different level for a different purpose.

*Yes, I find that the hissy kind of noise level of professional mics still varies by as much as 3dB from one specimen to the next.


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