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Author Topic: AKG C414EB - elevated noise floor  (Read 415 times)

Matt Ogaz

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AKG C414EB - elevated noise floor
« on: September 06, 2021, 11:40:20 PM »

I have two Silver AKG C414EB's with Teflon capsules.  One of them has a noise floor 4 or 5db higher than the other one.  The level of noise is not affected by the low cut or pad switches.   They sound the same aside from the elevated noise floor in one mic, output levels are within 1 or 2 db of each other.  The noisy mic appears to be a different revision than the other, some of the cable routing to the capsule and switches is different, among other things.

I noticed that when I switch from cardioid or fig-8 to omni on the quieter mic, I get a  slight increase in noise that quickly drops back to normal level.  When I do this with the noisy mic, the noise level instead drops much lower for several seconds  before coming back up.  During this time when the noise goes away the mic is outputting audio and sounds normal.

I have read elsewhere that noise like this can be caused by a noisy jFet or bipolar transistor in the amplifier and I should replace those.  Does this sound likely?  Thanks for any insight you can offer.

The offending noisy mic is on the right in the photos.
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klaus

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Re: AKG C414EB - elevated noise floor
« Reply #1 on: September 07, 2021, 02:35:49 AM »

Snip the wire bridge "Br" - http://recordinghacks.com/pdf/akg/C414EB-service-doc.pdf) to activate phantom power on both mics and report back.

That bridge needs to be cut to reduce current consumption in these mics, down to a level that can be handled by phantom supplies.
The wire bridge, visible on the first set of your pictures, connects the upper (red wire) and center stand-off columns on the far right side of the circuit board (lower column has a black wire connection).
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Klaus Heyne
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Kai

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Re: AKG C414EB - elevated noise floor
« Reply #2 on: September 08, 2021, 09:39:11 AM »

Snip the wire bridge "Br" - http://recordinghacks.com/pdf/akg/C414EB-service-doc.pdf) to activate phantom power on both mics and report back.

That bridge needs to be cut to reduce current consumption in these mics, down to a level that can be handled by phantom supplies.
The wire bridge, visible on the first set of your pictures, connects the upper (red wire) and center stand-off columns on the far right side of the circuit board (lower column has a black wire connection).
In the right pictured mic the red wire is soldered wrong.
It should go to the upper post, not the middle.

This doesn’t matter as long as the wirebridge Br is in place.
Once you cut the bridge this mic won’t work as intended any more.


If the noise doesn‘t go away, my major suspects are the bigger sized Tantalum Electrolytic capacitors, The bigger blue and green ones in the right picture.
These have a significant tendency to fail.
Very carefully handle the polarity when replacing.
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klaus

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Re: AKG C414EB - elevated noise floor
« Reply #3 on: September 08, 2021, 11:57:37 AM »

Good eye! Indeed, someone had rewired connection. Snipping the bridge it without re-soldering the red wire to the top post would render the mic inoperable.
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Klaus Heyne
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Matt Ogaz

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Re: AKG C414EB - elevated noise floor
« Reply #4 on: September 13, 2021, 03:43:27 AM »

Thank you both for the information! 

I cut the wire bridge on both mics and moved the termination of the red wire to the upper post. (I replaced the wire actually as it was too short to reach.)  After this, the formerly noisy mic is now a tiny bit quieter than the other one.  I am going to compare both mics to a pair in frequent use at a studio I work in. 

The pattern switches on both mics are somewhat noisy when switching.  I just ordered some deoxit gold to clean and lubricate them (in a needle applicator bottle, based on a recommendation found in another post on this forum.)

Thanks again for your help.

-Matt



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klaus

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Re: AKG C414EB - elevated noise floor
« Reply #5 on: September 13, 2021, 11:40:33 AM »

Good news.
Please report back after your comparison test.
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Klaus Heyne
German Masterworks®
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David Satz

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Re: AKG C414EB - elevated noise floor
« Reply #6 on: September 17, 2021, 05:40:12 PM »

Matt, the switch's contacts should certainly be kept clean, but I don't think a totally silent changeover between patterns should be expected with this type of microphone. I don't know of any "vintage" microphone that was designed specifically so that the pattern could be switched while the microphone was in the middle of picking up production sound. That just wasn't an expectation that the designers of that time sought to fulfill.

Each time you change switch positions, the DC converter disconnects one circuit branch from the rear membrane and connects a different one in its place. The switch physically breaks and makes contacts; current flows into the new branch and charges a capacitor via a resistor over a certain amount of time, and the back half of the capsule shifts its charge according to the new voltage being supplied to it via a high-value resistor.

It's straightforward, and by today's standards you might call it unsophisticated, since there's no filtering to remove switching transients or smooth the transitions. But I think that the recording engineer was expected to choose the microphone's pattern before starting a take, or between takes if an adjustment was necessary, and not during a take.

--best regards
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