C.A.P.S Filter
« on: December 30, 2018, 10:16:41 PM »
Im reading Steve Dove's series of articles on "Designing a professional mixing console", overall I find it very informative, however his somehow convoluted and embellished (almost poetic) way of writting makes it difficult to follow for me, a non native english language speaker. Anyway, in parts 8 and 9 he mentions a type of filter similar to a state variable filter, but instead of using integrators, it uses all-pass sections, the author calls this "CAPS filter" (Constant Amplitude Phase Shift) but I have never seen such filter before, nor I can find any info on the web, the article does not describe the circuit very well. In part 9 he provides a complete EQ schematic using these type of filters, so I would like to know more about the CAPS filter, do you guys know of any source that describes the operation of this filter?
« Last Edit: December 30, 2018, 10:38:20 PM by Dualflip »


abbey road d enfer

Re: C.A.P.S Filter
« Reply #1 on: December 31, 2018, 03:26:33 AM »
Im reading Steve Dove's series of articles on "Designing a professional mixing console", overall I find it very informative, however his somehow convoluted and embellished (almost poetic) way of writting makes it difficult to follow for me, a non native english language speaker. Anyway, in parts 8 and 9 he mentions a type of filter similar to a state variable filter, but instead of using integrators, it uses all-pass sections, the author calls this "CAPS filter" (Constant Amplitude Phase Shift) but I have never seen such filter before, nor I can find any info on the web, the article does not describe the circuit very well. In part 9 he provides a complete EQ schematic using these type of filters, so I would like to know more about the CAPS filter, do you guys know of any source that describes the operation of this filter?
I've also been intrigued by this design. I've simulated it and it seems to work very well. However, I'm not aware of any commercial implementation in audio mixers.
For some reason the Wien-bridge and state-variable implementations have been favoured by console designers.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

L´Andratté

Re: C.A.P.S Filter
« Reply #2 on: December 31, 2018, 02:30:44 PM »
Fred Forssell has something about that topology.
https://tinyurl.com/y7gfotv7

Quote
In a nutshell, what the CAPS circuit does is produce a phase shift at the desired frequency (and with the desired bandwidth or Q) that is summed with either the input signal or the feedback signal. Think about what would happen if the phase is 180 from that of the input or feedback signal; it cancels it out, effectively attenuating it. That is (very basically) how the CAPS circuit works. However with the CAPS circuit, the two all-pass filters introduce a 90 degree phase shift at a frequency set by the RC value network connected to the non-inverting amplifier in each all-pass network. Because there are two these filters, the second all-pass filter inverts the phase shift of the previous stage. The results in a +90 and – 90 degree phase shift at the frequency to which the filters are set. That is exactly what we need for a bandpass filter. The first opamp in the series of three “closes the loop” around the filter, allowing both positive and negative feedback. This allows the damping (Q) and the gain to be adjusted in a manner that allows low gain change as the Q is adjusted. The rest of the details of the CAPS circuit can be obtained from Mr. Dove’s excellent article series.
I had a band on the breadboard and, well, it worked
the exotic log/antilog dual pot for the q control can be easily replaced with dual linear and still works with acceptable resolution
« Last Edit: December 31, 2018, 02:34:18 PM by L´Andratté »
Strictly amateur since 1973...

5v333

Re: C.A.P.S Filter
« Reply #3 on: December 31, 2018, 05:02:57 PM »
I've also been intrigued by this design.

me too!

Gold

Re: C.A.P.S Filter
« Reply #4 on: January 01, 2019, 01:28:25 PM »
Fred Forssell has something about that topology.

Isn’t that a “swinging input” graphic EQ topology?

abbey road d enfer

Re: C.A.P.S Filter
« Reply #5 on: January 01, 2019, 11:30:37 PM »
Isn’t that a “swinging input” graphic EQ topology?
That's only part of the description; it is indeed a swinging input topology. But the topology here refers to the filters themselves, or rather the NIC's (negative impedance converter) that simulate a series RLC network, which presents a dip at the tuning frequency. The most common topologies here are, of course real inductors/caps, and so-called gyrators, based on a Sallen & Key structure.
The other types of filters are real band-pass filters, most often MFA (Multiple Feedback Active) for fixed frequency/BW, Wien Bridge for variable frequency/fixed BW, and State-Variable for variable frequency/BW; they cannot be used in the swinging-input topology. They must be used in an extended Baxendall EQ structure.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Gold

Re: C.A.P.S Filter
« Reply #6 on: January 02, 2019, 12:05:12 PM »
Thanks, that’s a good summary. I guess the CAPS topology is a clever way of  extending the capability of a swinging input type EQ to make it behave more like a state variable EQ.

Re: C.A.P.S Filter
« Reply #7 on: January 02, 2019, 07:54:32 PM »
Thanks, that’s a good summary. I guess the CAPS topology is a clever way of  extending the capability of a swinging input type EQ to make it behave more like a state variable EQ.

What I find interesting is that the CAPS filter is basically the same as the state variable, each integrator in the state variable shifts 90° constantly over BW, in the CAPS filter the 90° shift is at a certain frequency when using the All pass filters. However in Steve Dove's article the CAPS filter is used a resonator (like a gyrator) rather than a filter with an input and an output.

squarewave

Re: C.A.P.S Filter
« Reply #8 on: January 03, 2019, 12:43:30 AM »
What I find interesting is that the CAPS filter is basically the same as the state variable
I just simulated this and I don't see anything spectacular going on here. Noise is marginally better in the passband and marginally worse out-of-band. It basically is a state variable filter. Its just using the cap in an RC instead of as an integrator. And I cannot help but think that there is probably an advantage to using the caps in integrators. Or, maybe distortion would be better because the voltage across the caps is less in the CAPS variable filter.

Re: C.A.P.S Filter
« Reply #9 on: January 03, 2019, 12:53:53 AM »
I just simulated this and I don't see anything spectacular going on here. Noise is marginally better in the passband and marginally worse out-of-band. It basically is a state variable filter. Its just using the cap in an RC instead of as an integrator. And I cannot help but think that there is probably an advantage to using the caps in integrators. Or, maybe distortion would be better because the voltage across the caps is less in the CAPS variable filter.

Did you simulate it as a resonator or as a filter? An interesting thing would be to try and use the regular state variable filter as a resonator.


squarewave

Re: C.A.P.S Filter
« Reply #10 on: January 03, 2019, 01:23:43 AM »
Did you simulate it as a resonator or as a filter? An interesting thing would be to try and use the regular state variable filter as a resonator.
Yes, I used a 4th op amp with swinging input. But I would not say one is a resonator vs a filter. They are both resonators and they are both filters. The overall behavior is very similar (but different enough to warrant study).

L´Andratté

Re: C.A.P.S Filter
« Reply #11 on: January 03, 2019, 03:18:54 AM »
As I understand the advantage is to change q without changing passband gain.
(While with adding a fourth amp,  that is also possible with STV filter iirc).

Strictly amateur since 1973...

abbey road d enfer

Re: C.A.P.S Filter
« Reply #12 on: January 03, 2019, 06:10:49 AM »
Thanks, that’s a good summary. I guess the CAPS topology is a clever way of  extending the capability of a swinging input type EQ to make it behave more like a state variable EQ.
Agreed; the usual problem with swinging-input topology is that most of the resonators do not lend themselves to easy "parametric" control. Not the common "gyrator"  and of course neither real inductors. CAPS seem to me to be the only serious contender.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: C.A.P.S Filter
« Reply #13 on: January 03, 2019, 06:14:08 AM »
Yes, I used a 4th op amp with swinging input.
Did you actually make a SVF act like an RLC network? I would think it's quite difficult and impractical...
Or did you inject a BP into a swinging-input EQ?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: C.A.P.S Filter
« Reply #14 on: January 03, 2019, 11:30:12 AM »
From what I can see gain does change with Q whereas in the SVF it does not.

LTSpice files are attached (rename from .txt to .asc).

But here are images of the CAPS circuit:



and here is a typical SVF:



Both of these are adjusted for ~+12dB at ~715 Hz and same Q.

squarewave

Re: C.A.P.S Filter
« Reply #15 on: January 03, 2019, 11:30:58 AM »
Attached is the SVF sim file renamed .txt to attach. Rename to .asc to load in LTSpice.

abbey road d enfer

Re: C.A.P.S Filter
« Reply #16 on: January 03, 2019, 01:48:15 PM »
Attached is the SVF sim file renamed .txt to attach. Rename to .asc to load in LTSpice.
So I checked; it appears the input of the SVF also acts as an RLC series resonator. In the particular case, it is the equivalent of an RLC circuit with a Q of about 2500, showing a minimum of ca. 1.4 ohm at resonance.
So I changed the topology to a swinging input type, and it appears to work.
I would need to spend some more time to see if the BW controls act as they should and if noise is correct.
I don't expect it would change the world; it's just out of curiosity.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: C.A.P.S Filter
« Reply #17 on: January 17, 2019, 01:14:18 PM »
Just found out by accident today that the Rane EQ3B uses the CAPS principle.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: C.A.P.S Filter
« Reply #18 on: January 17, 2019, 09:40:49 PM »
Just found out by accident today that the Rane EQ3B uses the CAPS principle.

Nice!, do you happen to have a schematic?

abbey road d enfer

Re: C.A.P.S Filter
« Reply #19 on: January 18, 2019, 01:56:13 AM »
Nice!, do you happen to have a schematic?
I could only find a retro-engineered schemo there
http://mojobrazac.com/52nng08/euh3fut.php?uluzthrrm=diy-parametric-eq-kit
As I'm writing, the site is down for maintenance, but it should be back soon.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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