The ACR5 Notch Filter is a tool to help you remove feedback.
Feedback is that annoying howling sound that you get when the sound from a loudspeaker is picked up by a microphone connected to the speaker, re-amplified, pick-up again, re-amplified… and so on, so that an acoustical loop is created. The same phenomenon can happen with your electric acoustic since the pick-up system and wood of the instrument become a transducer themselves like a microphone.
The Notch Filter is a “cut only” equalizer that attenuates a narrow band of frequencies. When you set the Notch Filter to the feedback frequency, you can cut that frequency to eliminate the feedback. And, since the bandwidth is narrow, the cut has little effect on the tone of your instrument. By using the Notch Filter you can get the maximum level out of your amplifier without feedback.
The SWEEP control is used to set the specific frequency that the filter will cut from 50Hz to 720Hz.
To set the Notch Filter, connect your acoustic electric guitar to the GUITAR INPUT, turn the amplifier up by raising the MASTER volume and the channel VOLUME control to a level right as feedback just occurs.
Now, turn the Notch filter on by pressing the NOTCH button. Turn the Notch Filter FREQUENCY knob slowly and stop as soon as you dial in the feedback frequency.
If you hear the feedback get lower in volume but not go all the way off, lower the volume of the acoustic guitar. If the amplifier isn’t feeding-back and you have enough volume, you’re done.