Setting Up and Using Your Hartke AC75 and AC150 Acoustic Amplifier
Setting the Notch Filter The AC75 and AC150�s NOTCH FILTER is a powerful 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 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 NOTCH filter has two controls; FREQUENCY and DEPTH. The FREQUENCY control is used to set the specific frequency that the filter will cut from 55 to 720 Hertz. The DEPTH control is used to set the amount of cut accruing at the FREQUENCY point. The higher the setting, the more cut. Follow the steps below to set-up the NOTCH FILTER � For our exercise set the NOTCH FILTERS� DEPTH control to "6 - 7". � With your acoustic instrument connected to the Channel 1 input, try to turn the amplifier up by raising the main VOLUME and the Channel 1 GAIN control to a level right as feedback just occurs. � Now, turn the NOTCH FILTERS� FREQUENCY knob slowly and stop as soon as you dial in the feedback frequency. � You may hear the feedback get lower in volume but not go all the way off. If the amplifier still feeds back, increase the DEPTH and repeat the above step. If the amplifier isn�t feeding-back and you have enough volume, you�re done.
