The modifications to the output should be the prime suspect for your RF
problems....
A single layer of screen should be sufficient as long as it still has a low
resistance connection to the rest of the case. The RF wavelength of the
holes should be small enough to get the job done.

Regards
Eric

"David Satz" wrote in message
m...
Michael,

I only have a few data points for your collection, plus some advice:
Incremental improvements in sound quality are great, but let's take care
to test everything that might be relevant after making a change.

Here's my story: Earlier this year I bought two used AKG C 414 B-ULS
microphones, had them checked out by AKG in Nashville, then sent them to
be modified by a well respected (including by me) person who specializes
in upgrading studio equipment. The modifications included removing one
layer of metal screening from around the capsules, and the microphones'
output transformers were also removed.

With that person's knowledge--he is notably the opposite of an egomaniac--
I then sent one of the modified microphones to some friends in Germany to
test. My friends know their way around condenser microphones and have a
test setup that I trust. Also, a stock (unmodified) C 414-B ULS was on
hand there for comparison--these people do, after all, pay attention to
what's on the market as part of their work.

After making the standard frequency and polar response measurements, the
nice people in Germany decided to check out the modified microphone's
immunity to RFI. Unfortunately the modified microphone was found to be
highly susceptible to interference. Large amounts of noise resulted from
using a mobile phone anywhere in the same room. No such problem was found
with the stock microphone.

So while the removal of the output transformer could also be implicated
in this problem, let's please consider that grounded metal mesh screens
may have multiple functions, and let's test accordingly, OK?

Also, I've seen one instance in which a more "open" design for a screened
capsule head caused a greater increase in that capsule's _low frequency_
response than in its high frequency response. It would be very nice to
understand the physical reason for that, but at present I sure don't.
Thus I suspect that as non-experts, our common-sense expectations about
the effects of screens on the sound of a capsule may be misplaced, too
simple, or perhaps both--and I suggest proceeding cautiously before we
turn any unproven assumptions into dogma.

--Since some people here can probably guess who did the above-mentioned
mods and who did the aforementioned testing, let me add that in other
respects the modified C 414 B-ULS was found to be improved: 2 dB lower
noise, output impedance reduced by half, and improved (i.e. increased)
high frequency response--though the degree of that was considered rather
slight as it was told to me.

None of the above should be construed as an endorsement by anyone of
anything (nor the contrary, if you please); it's just data points. Let's
add 'em to the pile and keep on having open minds, at least until someone
who knows more about this than we do comes along to explain it to us.

--best regards