Hartke ACR5 Mode D'emploi page 10

Setting the Notch Filter
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.
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