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#1
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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? |
#2
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#3
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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 |
#4
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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." |
#5
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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. |
#6
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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 |
#7
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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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