R/E/P > Klaus Heyne's Mic Lab

AKG C414EB Noise

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One of my c414eb (original brass ring ck12 capsule) has developed a problem.  I use the mics frequently, and during a session this one began exhibiting intermittent "pop" sounds every few seconds.  (pop.......pop..pop.......pop..........)

I believe I have isolated the problem somewhat.  The noise was happening in all patterns, so in cardioid, I swapped the leads from the front and back diaphragms (effectively putting the back diaphragm in cardiod), and the noise persisted, so I'm assuming (possibly incorrectly) that the capsule is not the source of the noise.

The "popping" sound is also attenuated by the pad, and filtered by the rolloff.  So it seems unlikely to be one of the tantalum caps on the amplifier board as they are downstream from the pad and filters.. In fact, C1 is the only capacitor before the filter and pad switches, so I feel like I should replace it?  There is also a tantalum (C19) on the DC to DC converter board.  I intend to replace that too.

Do you feel like my diagnosis and trouble shooting are valid and that I should proceed with capacitor replacement?  If so, could you recommend values and types etc for C1 and C19?  And will I want to replace those same caps in the other c414eb to maintain sonic similarity?  Or will the effect be minimal?

If not, could you recommend any other steps I should take to isolate the problem?

Thanks in advance,

Below is a prudent procedure to isolate where the problem might have originated.

First, try to determine whether this in a capsule or a mic processor problem. Usually that's pretty easy to do:
In your case, you put on a pair of headphones, and breathe onto the capsule (with the mic in cardioid, to limit variables at this stage of troubleshooting). If the volume or nature of the discharge noise can be influenced by your breath, you need look no further.

For example, if the frequency of the popping suddenly increases with the added humidity you introduce to the capsule, the culprit is most likely capsule contamination.

If the discharge sounds remain steady, regardless of your moist breath, then it's likely a component on the mic amp that caused the noise. I have found that the filter capacitors, especially C19 on the DC board (brown Tantalum in the picture), to be a consistent suspect with the C414EB: if you replace it, you should increase the capacitance, and, as you can see on the picture, you can even solder a huge electrolytic capacitor between (+) and (-) of the supply voltage stand-off posts. Replacing C19 usually gets rid of low frequency motor-boating, but that may not necessarily be your issue.

But before you solder around on the boards, and wildly start substituting components, first determine whether it's a capsule or amp issue.

Thank you Klaus. What capacitance and voltage rating is that big electrolytic?  And what would you recommend increasing c19 to?

Thanks again,

I am not sure, because I got the picture from the internet.
Double C19 and try a 47mfd as the big capacitor between the stand-offs. Voltage rating should be at least 50V.

But let us know first what your troubleshooting revealed!

Thanks again.  And do I need to replace C19 with a rant, or can that be electrolytic too?



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