Tech: Anti-feedback notch filters?

Subject: Tech: Anti-feedback notch filters?
From: Willie-O
Date: 23 Jan 03 - 10:38 AM

My Peavey Ecoustic 112, a 2-channel amp which I am generally very happy with, has a "feedback control", notch filter type. It consists of a horizontal slider marked "Frequency" with a range of 40 Hz > 2K, and a dial-type "Notch" control with values of -27 up to 0.

What I would like to figure out is: how do you actually use this thing to reduce/eliminate feedback? Documentation is completely unhelpful. The only thing I have figured out, is that if you sweep the slider across the frequency range while playing, there is a point where the volume drops perceptibly, then it goes back up. OK, is that the notch? Is there a feedback-prone frequency or something that I should be tweaking? I was having feedback problems at my last gig, at which I didn't bother using pickups since constant instrument-switching made it handier just to mike the guitars/mandola.

This amp has other controls that drastically affect the sound, especially a switch for either active or passive pickups. I usually set it to passive, which gives much more volume, so I don't have to use a separate preamp.

I have yet to find a description of notch filters that is written for musicians rather than for electrical engineers. Anyone want to have a go?

W-O

Subject: RE: Tech: Anti-feedback notch filters?
From: mooman
Date: 23 Jan 03 - 12:28 PM

Willie-O,

I have a similar acoustic amp setup, except mine's a Laney, with a similar notch filter combination.

This may sound a bit crude and unscientific (coming from a scientist as well!) but what I tend to do is to stand with guitar plugged in immediately facing the amp (which is on a stand), crank up the gain and master volume until feedback starts to occur and then twiddle the frequency knob until this feedback is minimized. I then crank it up a bit further and further refine using the level.

Perhaps this will help. If anyone has a better why I'd like to know as well!

All the best,

moo

Subject: RE: Tech: Anti-feedback notch filters?
From: treewind
Date: 23 Jan 03 - 01:28 PM

Mooman, you've cracked it.
There is no more to it than that.

The only further refinement might be to do the adjusting with your guitar held by ypu standing in the position where you are going to play it. Just in case the feedback frequency is different from when the guitar is on the stand.

And don't bother with it unless you need it, because it can make the sound a bit strange.

Anahata

Subject: RE: Tech: Anti-feedback notch filters?
From: Willie-O
Date: 23 Jan 03 - 03:34 PM

Well that gives me something to try...first make feedback happen, adjust frequency (as I mentioned its on a slider on this one), and then adjust level...which is all done in the negative range.

So to not use it, as Anahata suggests, the level should be set to zero?

Thanks both. I have asked Claymore for comment too.

W-O

Subject: RE: Tech: Anti-feedback notch filters?
From: Willie-O
Date: 24 Jan 03 - 01:57 PM

refresh.

Subject: RE: Tech: Anti-feedback notch filters?
From: Bev and Jerry
Date: 24 Jan 03 - 02:47 PM

As both an Electrical Engineer and a musician, let me try to explain the notch filter in English. When you perform, you're creating sounds over a wide range of frequencies. Low pitch = low frequency, high pitch = high frequency. People measure frequency in cycles per second abbreviated cps. Engineers measure frequency in Hertz abbreviated Hz. They're exactly the same.

Your amp takes the small signals from microphones or pickups and turns them into large signals to drive the speakers. This is called amplification or gain. Ideally, all frequencies are amplified the same amount.

The notch filter reduces the gain or amplification of your amp in a very narrow range or band of frequencies. The slider adjusts where this range occurs and the knob adjust how much the gain is reduced. So, zero is no reduction in gain, 27 is maximum reduction in gain.

So, why would you want to do this? Depending on your system, the size and shape of the room, and about one hundred other factors which you can't control, it is possible for some of the sound coming out of the speakers to find its way back into the microphone(s). It then goes through your amp, out the speakers, back into the microphone and around and around until all you hear is a loud squeal called feedback. If this is happening at all frequencies, the amplifier gain is too high and you need to turn it down. But, most of the time it is happening only at a single frequency or at least a narrow band of frequencies. Your mission, should you choose to accept it, is to find that frequency and reduce the gain there and only there. Enter the notch filter.

To find the frequency, do exactly as mooman suggested. Set up whatever conditions are most likely to create feedback (experience will help you here), set the knob near the middle (say -10) and move the slider until the squeal goes away. If there's no squeal, try setting the knob closer to zero until you get one. If you can set the knob all the way to zero with no squeal then there's no feedback and you don't need the notch filter tonight.

Once you find the feedback frequency, turn the knob as close to zero as you can without getting feedback and then turn it down (towards -27) a little to keep you away from the edge.

Many sound systems (like ours) have an equalizer. This is basically several (usually 5 to 9) notch filters at different frequencies which you cannot adjust. What you can adjust is the gain of each of them. So, you reduce the gain in what ever frequency band you need it reduced in. This has several advantages one of which is you can eliminate more than one feedback frequency at a time. We have played in rooms where this is very helful.

Hope I have shed a little light on this subject.

J

Bev and Jerry

Subject: RE: Tech: Anti-feedback notch filters?
From: GUEST,Slickerbill
Date: 24 Jan 03 - 03:28 PM

hey this is great. Question: I have a Prefix Blender system on my guitar. You've been talking amps above, but my guess is the notch filter/frequency features on my guitar are exactly the same, right? sb

Subject: RE: Tech: Anti-feedback notch filters?
From: GUEST,Claymore
Date: 24 Jan 03 - 03:59 PM

Exactly so... In short, set the "Notch" or Gain to a negative number down from the detent center position. Set up the feedback as described above, then sweep the frequncy knob back and forth until the feedback starts to fade. At that point you are trying to localize the exact frequency that is feeding back, then adding or subtracting the Notch or gain at that frequency to reduce the feedback.

Be advised you may have more than one frequency that is prone to feedback, due to harmonics or room frequency or number of people in the room, temperature, humidity, or windage (no I'm not kidding). The best thing to do is to catch the earliest feedback with the notch, then use the amps EQ to try and catch the others as they become apparent. You need only do this up to the point where your overall volumn is sufficient to play out your gig. Unless your amp is in the one location in the world you must have it to play your gig, consider moving it as well (they always sound better up off the ground anyway - which is the reason they make amp stands - but two chairs placed together will do as well). Good luck!

Subject: RE: Tech: Anti-feedback notch filters?
From: GUEST,Al
Date: 25 Jan 03 - 12:35 AM

Where this stuff comes in really handy is eliminating feedback from monitor speakers. After all, they are pointed right at the mics, the worst possible situation for feedback. Just when you start to be able to hear yourself through the monitors, they start to feed back. Using the multiband notch filters gives you a little help with this, at the expense of making it sound a little weird. That't the tradeoff: volume vs. natural sound quality. Al

Subject: RE: Tech: Anti-feedback notch filters?
From: GUEST,Richard H
Date: 25 Jan 03 - 11:30 PM

Willie-O,
I'm also using the Peavey 112 mainly for mandolin, sometimes guitar.
With a magnetic p/u mandolin, I have no feedback problems even at high volume.

With the Rigel which I believe amplifies vibrations from the body, feedback was a problem but I never got it sorted out. Even bought the Baggs Para-Acoustic DI preamp which also has a notch filter but wasn't happy with the results.

Now I'm using a Godin A8 (under the saddle p/u). I crank the knob back to -30, hit the G strings and adjust the slider until the hum is minimised. This has worked on gigs so far but, judging from above posts, I need to get more scentific.

It seems too that many of these devices work better in guitar range than mandolin.

BTW, you mentioned that you used a mic at your last gig and had feedback problems. The Ecoustic's 2nd channel does have inlets for guitar jack or microphone. But, as far as I know, the notch filter control works only for the 1st channel which has no mic inlet.

Subject: RE: Tech: Anti-feedback notch filters?
From: Willie-O
Date: 06 Mar 03 - 10:26 AM

Hmm, I should have checked back sooner after the last refresh. All my questions seem to have been answered--especially by Jerry's clear and detailed treatise.

So if I might paraphrase, the notch filter is really a parametric EQ control, with only negative gain settings.

It's really unclear from the panel layout and manual whether the single notch filter on the amp works on channel 1, channel 2, or both. An aggravation, especially if it doesn't work on the channel with the XLR input, since we all know most feedback comes from mikes.

Fortunately this amp has a separate 5-band EQ for each channel, so that should help somewhat.

Thanks everyone.
W-O