[Truth]

On Sep 13, 7:48 am, "Arny Krueger" wrote:
"Truth" wrote in message

ups.com...

I have a ribbon microphone that is picking up AM radio.

I don't think so. What you have is a mic that is picking up RF and a mic
preamp or something else that is demodulating the RF and presenting the
modulation of the RF as audio.

I had no idea
what it was until today. It's usually a squeal and I don't hear the
voice or music. So I was guessing and trying different things all to
no avail. I was turning off equipment, moving it far away from the
mics, removing any fluorescent lights, etc. Problem remained. I even
got some special power cords for the mic power supplies. Still, the
squeal is there.

The squeal might be a hetrodyne between two different RF sources. Hetrodynes
are due to nonlinearity. The nonlinearity is probably due to some active
component being driven outside its linear range.

But today, in addition to the squeal like you're getting a bad radio
signal, I could hear the radio content as well which made it clear
what the problem was. I turned on the radio downstairs and tuned it
to AM. I found the station the mic was picking up in my recording and
it was the same type of squeal, so it seems this is the problem.

Seems consistent with the other things that you've said so far.

The problem is not constant, it comes and goes.

Moving mics and cables around can cause these sorts of changes.

I've tried the mic with
different preamps and I get the same problem with both.

Either the two preamps share the same sensitivity to RF, or the problem is
elsewhere.

I've also
tried different microphones. Only the ribbon mic picks up this RMI
squeal.

One major variable is that ribbon mics usually demand that a lot more gain
be used to amplify them.

Now I'm just wondering what is picking up the interference, is it the
mic itself, the mic cables, or the main power?

A corroded connection in the mic itself could be providing the necessary
nonlinearity.

Or, your mic preamps might be doing just fine all by themselves, but are
outputting too much RF for some downstream component to handle linearly.

Is this problem due to
noisy power lines and will getting a power conditioner eliminate it do
you think?

No. I suspect that your mic cables are acting like antennas.

I had my equipment in the same room as the mic. I moved
it out of the room and into the hall to test and the problem remains.
Replaced all fluorescent bulbs with regular incandescent ones. There
is a dimmer in the house, but it's located down the hall and far away
and is turned off.

Dimmer noise is generally overlaid with the power line frequency. So, its
more like a buzz than a squeal or a radio station program.

Nothing is powered on in the same room as the mic
now and the squeal is still there. I've turned off all radios in the
house as well. Unplugged a wireless router. None of this helps,
which makes me think it's the power.

The RF is most likely in the space in and around your house.

Something as simple as a low value ceramic capacitor across the mics
terminals could make a difference.

I've found that mic preamps vary tremendously in how well-equipped they are
to reject RF. Some have multiple-section LC filters (good) and some have
nothing in particular or maybe just a capacitor across the input terminals
(not so good).

I think that the best mic preamp I have in terms of RF rejection is my SX
202. Frankly, for whatever reason, I've mostly used the SX 202 in my remote
recording sessions, some of which have been in the high-RF parts of my city.
I don't know how many problems I've never experienced because of my choice
of equipment.

I don't use ribbon mics at all, so I don't know what would happen with a
more difficult microphone than the common kinds of condensor mics that I
prefer.

These are two brand new Royer 122V Tube Ribbon Microphones. I sent
the first two I received back to Royer and they sent me two brand new
replacements. They told me to keep them away from dimmers, motors,
fluorescent lights, etc. They also mentioned that the mics can act
like antennas. If that is the case, then what can be done about it?
Could the problem originate in the power source and made worse by long
cable runs? I have to get in contact with them again and see what
they say as well. Perhaps getting shorter ribbon mic cables will
help, since you say the problem could be the ribbon mic cables acting
as antennas.