Notch/Low Cut Filters

[Bob Branstetter]

Thanks Ken. I'll give it a try and will let you know how it goes.

[Phil Maneri]

On the AI heads the notch and the low cut filters reverse the phase of the input signal as part of what they are doing. The result is getting to move the speaker out of synchronicity with the top. This is useful in avoiding feedback loops where the top and the speaker feed each other into a howling frenzy.

The Low cut or variable center frequency shelving switch (labeled cut) removes everything below what you can hear to the frequency the knob is set at. That ranges from below the E string fundamental (41.5hz) and goes up off the notes on the bass (800hz or something useless) The meat and potatoes of the bass notes from the E 41.5hz to the G string low A around 110 hz are in the first half of the knob's throw.

When you are in a boomy room or find yourself with more low end that you can handle turn this on and set the cutoff frequency knob all the way down at 30hz (all the way counterclockwise). Turn it up in frequency (raising the overall volume if need be) until that nasty boomy thing goes away. Won't take much movement. Most of the junk is at the bottom.

The notch filter picks a frequency by knob position and does a fairly narrow band cut at that frequency. It's Q or bandwidth is fixed and narrow, the cut volume or db is fixed as well. The cut radiates out in both directions from the frequency delineated by centerpoint on the knob in a quickly tapering pattern. usually within a small number of frequency points.

This is very useful when the bass or the room has a wolf or sympathetic radiating tone (like an open A string) that just goes wild when you play it. Turn on the filter and sweep up and down until you find the offending frequency goes away and you're done.

Overall they are elementary filters with lots of fixed positions that keep you out of trouble but also limit the flexibility. The lower the volumes you play at the less they are needed.

To sum up:

The Cut filter is a high pass filter and phase reversal that you set to remove boom from the room or your sound. Turn the knob from left to right to the point when it goes away, turn up your volume to taste and you're done. Fiddle back and forth with the frequency and volume knobs to get the desired sound.

The Notch filter gets rid of wolf notes and reverses the phase. Turn the knob from left to right until the note goes away and you're done.

Simple.

I use the Notch quite a bit as a high pass filter/phase reversal when the Cut seems to drastic. Engage the filter, set on notch, turn knob all the way left and your done. With the full circle its nice and clean and tight on the bottom that way.

On occasion when the room just booms like crazy I'll switch the switch to cut leave everything else alone and keep playing.

Oh and all these adjustments assume the EQ is set flat. I'd adjust my filters first before touching the EQ. Use EQ only after that if you need it.

[Bob Branstetter]

Originally Posted by Phil Maneri

Simple.

I use the Notch quite a bit as a high pass filter/phase reversal when the Cut seems to drastic. Engage the filter, set on notch, turn knob all the way left and your done. With the full circle its nice and clean and tight on the bottom that way.

On occasion when the room just booms like crazy I'll switch the switch to cut leave everything else alone and keep playing.

Oh and all these adjustments assume the EQ is set flat. I'd adjust my filters first before touching the EQ. Use EQ only after that if you need it.

Thank you Phil for giving me exactly what I was looking for specific to doublebass and AI amps. Fortunately, I don't run into situations where filtering is necessary very often, but now I have a practical guide for using filtering when the occasion demands it.

[David Powell]

I have used the notch filter on a Focus head to get the "C" out of a room. We had a few jazz gigs in this place that had awful acoustics but still we had to play at a relatively high volume level. Everytime I hit a "C" anywhere on the instrument I had howling feedback. So I put the notch filter on and rolled it over until I had the notch on the "C" frequency, somewhere around 125 Hz. In this type of use the filter is like a narrow parametric EQ cut. Really all you have to do is put the filter on and sweep back and forth until you find where the culprit frequency is. It's one of the best and most useful features on the AI. It won't cure every situation, but it's a great feature for those situations that it will handle.