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