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xy
 
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Default question about "feedback"

hi guys,

i was checking out the gain-before-feedback levels of a mixer/mic
combo. i got it to where it was about to howl, and then backed the mic
input level down just a bit so it was in the "safe" zone.

then i brought up the power amp level. i thought it would tip it back
over the edge and start to get the "about to howl" situation. but it
didn't.

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?

i thought once you were at the edge of feedback, any increase in gain
anywhere in the signal chain would tip it over the edge. but i seem
to have gotten away with some extra "gain without penalty" from the
power amp.

why?


my next related question is this: if you are trying to get a PA
situation set up, should you turn the power amps way up and then keep
the gain controls on the mixer comparatively low to get a better
gain-before-feedback ratio?
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George
 
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In article ,
(xy) wrote:

hi guys,

i was checking out the gain-before-feedback levels of a mixer/mic
combo. i got it to where it was about to howl, and then backed the mic
input level down just a bit so it was in the "safe" zone.

then i brought up the power amp level. i thought it would tip it back
over the edge and start to get the "about to howl" situation. but it
didn't.

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?

i thought once you were at the edge of feedback, any increase in gain
anywhere in the signal chain would tip it over the edge. but i seem
to have gotten away with some extra "gain without penalty" from the
power amp.

why?


power amps are fixed gain devices the volume control effects the amount
of voltage required to achieve rated output at the fixed gain(often
about 29 to 32 dB) but the gain of the amp does not change as you turn
the volume control
preamp often have 60 to70 dB of gain
a small turn on a pre amp is much more gain than a huge turn at the
power amp
of course I might be really confused but I think this is a explaination
for what you experianced


my next related question is this: if you are trying to get a PA
situation set up, should you turn the power amps way up and then keep
the gain controls on the mixer comparatively low to get a better
gain-before-feedback ratio?


run all you gear at the levels proper as indicated on the meters then
turn your amps up to the required level
the amp always can put out the same wattage what the volume control does
is effect the voltage going INTO the amp
George
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Logan Shaw
 
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xy wrote:
i was checking out the gain-before-feedback levels of a mixer/mic
combo. i got it to where it was about to howl, and then backed the mic
input level down just a bit so it was in the "safe" zone.

then i brought up the power amp level. i thought it would tip it back
over the edge and start to get the "about to howl" situation. but it
didn't.

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?


Perhaps the gain knob on the mixer's channel is more sensitive
(more decibels per degree of rotation) than the one on the power
amp, so it's sort of an illusion.

Or perhaps you have a bi-amped or tri-amped system and you turned
the knob of the wrong amp. :-)

Either way, it shouldn't work the way you're describing.

I guess one other possible explanation is this: in a *totally*
silent room, you can crank the gain as high as you want, and
there will be no feedback. In other words, there has to be a
trigger for the feedback to start. Only once it has started is
it self-sustaining. So if you are trying to detect feedback,
being totally silent and then turning knobs isn't the most
reliable way to do it.

Or another possible explanation: you had the mic closer to one
speaker than the other, but you were adjusting the level on the
channel of the amp corresponding to the one the mic is further
away from.

- Logan
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Scott Dorsey
 
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xy wrote:

i was checking out the gain-before-feedback levels of a mixer/mic
combo. i got it to where it was about to howl, and then backed the mic
input level down just a bit so it was in the "safe" zone.


Mixer/mic/speaker/room combo. The latter two are extremely important.

then i brought up the power amp level. i thought it would tip it back
over the edge and start to get the "about to howl" situation. but it
didn't.

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?


I bet a nickel it's because a tiny tweak of the trim control is a huge
increase in gain, and a large turn of the power amp control is a very
small increase in gain.

i thought once you were at the edge of feedback, any increase in gain
anywhere in the signal chain would tip it over the edge. but i seem
to have gotten away with some extra "gain without penalty" from the
power amp.


In a case when the system is linear, that is the case. If something is
starting to clip or get badly nonlinear, this might not be the case.
But I bet a nickel that what you are noticing is the difference in
control sensitivity and that the actual system gain that causes the
system to feedback is the same no matter where that gain is in the chain.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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David Grant
 
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Default


I guess one other possible explanation is this: in a *totally*
silent room, you can crank the gain as high as you want, and
there will be no feedback.


This would require the electronics to be completely silent as well, of
course.






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Justin Ulysses Morse
 
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xy wrote:

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?


Aside from the good answers you've already heard, another possibility
is that you had been overdriving the preamp in the mixer until you
backed off the gain control. When you overdrive a circuit, you produce
a whole lot of harmonic distortion. This results in new signal being
added at various frequencies that weren't there before. If those
frequencies are the wrong ones, they can trigger feedback in the room.
You'll notice this with guitar amps a lot: It's easier to get a
distorted guitar sound to feedback than a clean sound. With a clean
sound, you generally need more volume.

So it's possible that you turned up the channel trim to the point that
it was distorting , and you heard feedback, so you turned it down a
bit. Then when you turned up the power amp, you were increasing the
volume level without overloading any circuits.

I still agree though that the most likely answer is that the gain knob
on the mixer is more sensitive than the attenuator on the power amp.

ulysses
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Justin Ulysses Morse
 
Posts: n/a
Default

xy wrote:

how come i could bring the power amp levels up more without feedback,
but if i barely increased the gain on the mic channel, it would
proceed toward feedback?


Aside from the good answers you've already heard, another possibility
is that you had been overdriving the preamp in the mixer until you
backed off the gain control. When you overdrive a circuit, you produce
a whole lot of harmonic distortion. This results in new signal being
added at various frequencies that weren't there before. If those
frequencies are the wrong ones, they can trigger feedback in the room.
You'll notice this with guitar amps a lot: It's easier to get a
distorted guitar sound to feedback than a clean sound. With a clean
sound, you generally need more volume.

So it's possible that you turned up the channel trim to the point that
it was distorting , and you heard feedback, so you turned it down a
bit. Then when you turned up the power amp, you were increasing the
volume level without overloading any circuits.

I still agree though that the most likely answer is that the gain knob
on the mixer is more sensitive than the attenuator on the power amp.

ulysses
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