Author Topic: All-pass filter in a phasemanipulating circuit need no quality?  (Read 194 times)

KarenColumbo

All-pass filter in a phasemanipulating circuit need no quality?
« on: February 24, 2021, 05:17:18 AM »
Am I correct assuming that there is no use in putting a second order circuitry in place of a first order all-pass filter in a phaser circuit? Because it has no audible "filter curve" and all it should do is shift the phase?
(is there a subforum for n00b questions like this?)
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11-90-an

Re: All-pass filter in a phasemanipulating circuit need no quality?
« Reply #1 on: February 24, 2021, 07:36:22 AM »
well a phaser essentially shifts phase so...

a 1st order stage shifts signal up to 90 degrees, a 2nd order 180 degrees... so 1 2nd order can be replaced with 2 1st order filters, and vice versa...

Is that the answer to your question or did I misunderstand?  :icon_lol:
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noisette

Re: All-pass filter in a phasemanipulating circuit need no quality?
« Reply #2 on: February 24, 2021, 08:17:57 AM »
You´ll always need two AP stages for a phaser mixed 50/50 with the original signal.

At the "cutoff" freq the signal will get shifted 2x 90 degrees=>180 degrees=>out of phase with the original signal, but only exactly at the "cutoff" freq, and only there they cancel out and create a small notch, that´s what a phaser is, one or more small (moving) notches in the signal... *feedback is another can of worms*

Hope it makes sense ;)
This is standard reading:http://www.geofex.com/article_folders/phasers/phase.html

Using a single modulated AP stage you get a vibrato effect, the Escobedo Wobbletron comes to my mind...(click on image)


« Last Edit: February 24, 2021, 08:23:06 AM by noisette »
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Mark Hammer

Re: All-pass filter in a phasemanipulating circuit need no quality?
« Reply #3 on: February 24, 2021, 10:51:42 AM »
I've seen 2nd-order allpass phasing demonstrated, and it has the interesting property that the notches are  - or at least can be - much closer together.  Very interesting sound.

Rob Strand

Re: All-pass filter in a phasemanipulating circuit need no quality?
« Reply #4 on: February 24, 2021, 06:18:01 PM »
The obvious thing is you need to use two of the common first orders to get the same response as a single second order.   When you think of second order all-passes you need to bundle pairs of first orders.

The total phase shift has to do with the total number of stages.    However, where the notches end-up depends on the shape of the phase response.

Second order stages are not all the same.  When you have a second order stage you get an extra parameter you can control, the Q.  You can actually choose the Q with two first order stages by using different caps in each stage, for example like the univibe.   When the caps are equal the Q is 0.5.   When the caps are different the Q is less than 0.5.    The second order circuits let you set the Q more than 0.5.  That's something you can't do with a first order.

The higher the Q  the quicker the phase changes from 0 to the maximum.   If the phase changes quicker it will hit the notch points (where the phase is -180deg, -540deg, -720deg, -900deg) at frequencies closer together.

Here's the phase response of a low-pass filter as the Q is varied.
** You need to double the phase on the left axis to get the phase for  second order all-pass. ***
http://www.analog.com/-/media/images/analog-dialogue/en/volume-41/number-4/articles/phase-relations-in-active-filters/ad41-10_17.jpg?la=en

The main thing to notice how quickly the phase changes as the Q is varied.

If you have ever removed the dry path from a phaser you will notice you hear a vibrato effect.    There are no notches when you do this.   So that means there's more to the sound than just notches.   When you use a higher Q value the amount of pitch shift increases.

Many second order circuits don't allow you to sweep the frequencies, you need to choose the right type of second order all-pass.
« Last Edit: February 25, 2021, 05:29:48 PM by Rob Strand »
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