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Old 02-19-2007, 01:52 PM
Bob Branstetter Bob Branstetter is offline
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Default Notch/Low Cut Filters

My Series III Acoustic Image Coda amp has switchable notch/low cut filters that are adjustable from 30 to 800 Hz on both channels. However, I seldom use them because I don't have a systematic method of adjusting it. What is the difference between notch and low cut? I would appreciate someone giving me their methodology for using the notch/low cut filters on the job.
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Old 02-19-2007, 03:30 PM
Daniel Yeabsley Daniel Yeabsley is offline
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Here's my (uneducated) answer: The notch filter 'slices out' a narrow section of your sound. It can be useful for removing feedback or resonance at specific points in the audio spectrum.

The low-cut filter, on the other hand, removes everything below a certain frequency. Very useful for clearing up a muddy sounding room, or taking away 'rumbles' if you're using a microphone.

I just wish my iAmp came with a low-cut filter!
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Old 02-19-2007, 08:46 PM
Bob Branstetter Bob Branstetter is offline
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Thank you for that info, but I still need a methodology to actually use it. My trial and error methods have been less than successful.
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Old 02-19-2007, 10:52 PM
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Ken Smith Ken Smith is offline
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Cool Notching...

Quote:
Originally Posted by Bob Branstetter View Post
Thank you for that info, but I still need a methodology to actually use it. My trial and error methods have been less than successful.
Bob, here is how I see it as we use this idea in our Electric Basses. If all bands, Bs., Mid., Treb. are the same flat level you have the un-EQ'd sound. If it sounds a little mid-rangy when you boost the Bass and Treble the way to tighten that up is to 'Cut' the mids until you have your sound. Think of a 'V' being the 3 bands of EQ. Draw a line 'across' the center of the 'V' to mean the flat response un-EQ'd level. The left top point is the Bass, the right top point is the Treble and the bottom center point is the Mid which is Cut the same level that the Bass and Treble thereby 'scooping' out the mids to accentuate the Bass and Treble. This is known also as 'notching' the EQ. How does this work frequency wise? When you boost the Bass, the top of the Bass 'shelf' filter is actually hitting the low Midrange. The botton of the Treble 'peak' filter is actually the upper Midrange. When you boost either Frequency, you are also bringing up the Mids which may be more audible than the actual frequency you want to hear. When you cut the Mids which is the average hearing range for us and what the speaker puts out you are cleaning up the 'edges' of the upper Bass and Lower Treble separating these two frequencies so they don't 'bleed' together.

Let me know if this works for you mentally. If you call me, I will pick up one of my Smith Basses and demonstrate it more drastically for you on the phone. Maybe when this Forum gets a little more advanced I can post some sound clips of the various EQs for my Electric Basses and also of the DBs as well. This these words will have some music to go along with it.

To demo this yourself, set the mids at 500, Bs/100, Tr. 3.5k. Boost the BS/TR +10db and Cut the Mids to -10db and see what I mean. That will give you the 'V' I described. Then, dial in the sound you need depending on the Bass, the Pickup, the Amp, the room, the weather, the music, the mood or the time of the month.... Or the key!.. Also, when playing thru the amp with the Bow, you would want less Treble and more Mids or, Mids flat, Treble cut and Bass slightly boosted. That should sound like '~' with the Treble on the left. Can't make the sign I need..
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Old 02-19-2007, 11:06 PM
Bob Branstetter Bob Branstetter is offline
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Thanks Ken. I'll give it a try and will let you know how it goes.
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Old 02-20-2007, 01:17 AM
Phil Maneri Phil Maneri is offline
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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.
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Old 02-21-2007, 12:14 AM
Bob Branstetter Bob Branstetter is offline
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Quote:
Originally Posted by Phil Maneri View Post
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.
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Old 02-24-2007, 01:34 PM
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David Powell David Powell is offline
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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.
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