I was test driving my newly installed patchbay during a session
yesterday and had some strange behavior with one of my preamps. The
problem was brought about by operator error, but I'm wondering if it's
evidence that I might have some deeper problem with the way I've wired
things. Here's the senario:



There is a mic snake running to the back of the preamps. The two
preamps involved are a Phoenix DRS-2 (fed by channels 3&4) and a Sytek
(fed by channels 5-8).

I plugged in two mics into what I thought was channels 3&4 of the snake
and patched the outputs of the DRS-2 into the inputs for tracks 3&4 on
the tape deck (Otari 5050).

The meters on the DRS-2 weren't showing any input so I kept turning the
gain up (the mics are fairly low output and the source was fairly
quiet). Nothing until the gain was up at 70dB and then all of a suddden
it's peaking the DRS-2 and the Otari meters. Whatever signal was
causing the peaking must have been very high or very low frequency,
because I couldn't hear it anything coming from the headphone out for
channels 3&4 on the Otari. (The Otari has an individual headphone out
for each pair of channels).

If I disconnected the inputs to the Otari, the peaking stopped on both
the Otari and the DRS-2, but if I reconnected EITHER of the patch
cables to the Otari, the DRS-2 would peak on BOTH channels.

Then I discovered I had plugged the mics into channels 5&6 of the snake
(feeding the Sytek) instead of 3&4 (feeding the DRS-2). I dropped the
gain on the DRS-2, swapped to 3&4 and things seemed to work fine.

I was in the middle of tracking, so I didn't have a chance to test
further. Before I go back in to look at it more today, I wondering how
disturbed I should be by this behavior. My shot in the dark
explaination would be that there is some kind of capacitive coupling
between the DRS-2 ins and outs when the Otari is connected and that's
causing some high frequency oscillation when the gain is cranked. But I
don't know, (A) if that's correct (B) if I should worry about it (C) If
I SHOULD worry about it, where I should start looking for the problem.

Any advice would be much appreciated.

Thanks, Andy

wrote:
My shot in the dark
explaination would be that there is some kind of capacitive coupling
between the DRS-2 ins and outs when the Otari is connected and that's
causing some high frequency oscillation when the gain is cranked. But I
don't know, (A) if that's correct (B) if I should worry about it (C) If
I SHOULD worry about it, where I should start looking for the problem.

If there's any unshielded wires on the inputs and outputs running near
each other, capacitive feedback is a possible explanation. Even so, this
should not really happen with balanced inputs.

It probably won't ever happen when you have a mic plugged in - if your
preamp has a high input impedance it can pick up far more capacitive
feedback on open circuit than it will when a mic is connected to it. The
mic's low impedance will shunt the feedback path when connected.

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