Re: Installing internal microphones without feedback.

On 2009-08-13, Viktor <viktor@xxxxxx> wrote:
Hi!

 What would make this easier to trace
would be to have a stereo output (it could be a three-circuit 1/4"
(6.35mm) phone plug which uses the sleeve as a common ground, tip as the
left channel and ring as the right channel.  Then you could individually
control the volumes at the sound system's mixer, fully turning down the
treble (at the mixer) and seeing whether you could create feedback using
just the bass microphones.  I suspect that you can't, and that the
feedback (not "feed" BTW) is coming from the treble side.

Don, I don't understand the difference between your suggestion, and
what I can do right now. Probably I have not explained exactly what
happens. Maybe you did understand me, and I don't get it! :-)

Just to clarify: I have two volume knobs on the accordion, one for
bass-side and one for treble-side, sending the sound out together in
mono.

Yes -- I assumed that. This probably means that you have
something like the following (view this with a fixed pitch font like
Courier -- proportional pitch fonts will cause things to squish to the
left by differing amounts and things will no longer line up properly,
leading to confusion.

|)-|>--+
Z TREB
Z<------o--+-----o-----v || Mono jack
Z | o---------
Z | |
| | ---
___ | -
- |
|
|)-|>--+ |
Z |
Z<------o--+
Z BASS
Z
|
___
-

Now -- the "|>" symbols are microphone preamps, and the "|)"
symbols are microphones. The 'Z' columns are a resistance element of a
volume control.

Assume that the bass volume control is turned all the way up,
and the output impedance of the preamps are rather low compared with the
resistance of the volume controls. This means that the output point
sees the very low output impedance of the preamp, because the wiper of
the potentiometer is all the way up, resulting in very little resistance
between the preamp output and the output line from the volume control.

This means that the treble (which is not all the way up) has to
go through the resistance of the upper section of its volume control and
can't drive the outout of the bass preamp very well. (This is the
"loading".

If, instead, there is a resistance from the output of each of the
volume controls to the point where the two are joined to go to the jack,
then you don't have this loading effect.

However -- what I was suggesting is what is below -- using a
stereo phone cable (Tip-Ring-Sleeve) instead of a two conductor mono
one, which is only tip and sleeve). This allows each side to go into a
different input preamp in the mixer, and the design of any good mixer
will prevent any such loading effect. This way, you can see what is
sensed by each microphone set, and control whether it gets to the
speaker without affecting the other.

|)-|>--+
Z TREB
Z<------o--+-----o-----v || Stereo jack
Z +-----o------^||
Z | o-------||
| | |
___ | ---
- | -
|
|)-|>--+ |
Z |
Z<------o--+
Z BASS
Z
|
___
-

When I turn down bass (0) and turn up treble (full) I have some
feedback, when I turn up bass (full) and turn down treble (0) I have
no feedback.

Which says that the feedback is coming through the treble side
only.

When I turn up both (3/4) I have some feedback.
But the thing is when I turn bass up full and keep treble on 3/4 I get
less feedback(!) than both on 3/4.

Yes -- because the loading from the bass preamp through a fully
up volume control is reducing the amount of the treble which actually
gets out to the amplifier.

It seems to me to be a phase-problem of some sort.

I don't think so, because the phase will vary with how far the
bellows is extended, and you don't mention that as one of the things
which affect your feedback.

Logically I don't
get it, since the bass-side (after felting) is very well acoustically
isolated and gives very little feedback, suggesting that the bass-side
microphone should not phase interfere with the treble-side.

It should not -- but the bass side preamp can load down the
output of the treble side with the volume controls set as you have
described.

Therefore I was wondering if maybe it was a wiring-problem of some
sort.

Only in that there is no isolation following the volume
controls. Ideally, there would be a tiny mixer there with the two
volume controls feeding through different resistors into a summing
amplifier.

Maybe that is also what you suggest when you write:

The electrical loading of the treble side's output by the bass
side seems to make sense to me.

I don't understand how the treble sides output can be electronically
loaded by the bass-side. If this is the problem, what I can do?

It is quite likely, if the wiring is as I drew it in the first
drawing above, and I hope that my explanation made it make more sense.

You can eliminate it by either re-wiring as a stereo output as I
drew in the second drawing, assuming that your mixer can treat the sides
of a stereo jack as separate (look for a balance pot on the channel of
the mixer into which you are plugging it. Some may not have stereo
inputs at all.

Or -- you can put an active mixer circuit between the volume
controls and the output jack. You can probably find a lot of drawings
showing such using operational amplifiers. The schematic gets a bit
more complicated and I don't want to try drawing it using ASCII graphics
as I did above.

 Out of curiosity -- is your current amplifier and speaker an
open backed arrangement?  And is it standing in front of you on the
stage?

Actually I am trying different options out right now. I must say that
the feedback problem is not so big anymore. I WOULD have liked some
more headroom, but I think that is impossible when you think of how
loud my band is playing (crazy).

I hope that you are wearing hearing protection while you play
there.

Its actually not so much a live
problem, but more when in rehearsal-studio. In a live situation I have
the amp behind me as a guitar player would have. When I turn my body
towards the amp, I turn counter-clock-wise to avoid the treble-side
being exposed in front of the speaker.

That helps -- though it would make more sense to have the
speakers in front of you, with closed backs, so they minimize the
exposure of the instrument's microphones to the speaker's sound.
However, you probably need the current placement of the speakers to hear
what the others in the band are playing -- if you don't have separate
stage monitors and house mix -- with someone running a mix board to do
this properly.

Best of luck,
DoN.

--
Email: <dnichols@xxxxxxxxxxx> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
.

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