Script

SC Filter Invert -- HP to LP
« on: December 30, 2020, 09:42:57 PM »
I want to use a 'subtractive filter' in a SC filter. From what I understand this only works for first-order filters, cos with higher orders it will always only yield a 6dB slope (messing with the overall filter phase), unless phase compensation is used -- which makes it more complicated. Anyway, it's for a GSSL SC filter board designed by Steffen. It's several RC filter settings plus four active filters in this case.

The idea is to add an AUX channel and then 'sum' the high-pass filter signal with the uncompromised polarity-flipped signal to swap the filter: that is, convert high pass into low pass. Overall polarity at the output of the SC filter should not matter cos it will be full-wave rectified anyway. Would the atttached schematic work ?

Is it OK to derive the AUX channel from after the input buffer as drawn (question mark, red line) ?
Is it OK to insert a switch to simply cut the AUX line before the output buffer without grounding ?

Any comment is highly appreciated. I am not good at filter design.
 
« Last Edit: December 30, 2020, 09:53:03 PM by Script »


abbey road d enfer

Re: SC Filter Invert -- HP to LP
« Reply #1 on: December 31, 2020, 12:30:55 AM »
Would the atttached schematic work ?
Yes.

Quote
Is it OK to derive the AUX channel from after the input buffer as drawn (question mark, red line) ?
No. There is no signal there (inverting input of an inverter).

Quote
Is it OK to insert a switch to simply cut the AUX line before the output buffer without grounding ?
It would work, but it's not recommended to switch there (very low level). It would be better to install the switch on the other side of the resistor. No grounding needed, but there are risks of clicks when switching. Not an issue if you don't intend to switch in action.
« Last Edit: December 31, 2020, 01:10:31 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: SC Filter Invert -- HP to LP
« Reply #2 on: December 31, 2020, 12:41:11 AM »
what is a SC filter?

JR
It's nice to be nice....

abbey road d enfer

Re: SC Filter Invert -- HP to LP
« Reply #3 on: December 31, 2020, 01:10:59 AM »
what is a SC filter?
Side-Chain filter.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Script

Re: SC Filter Invert -- HP to LP
« Reply #4 on: December 31, 2020, 01:45:11 AM »
Thank you for the fast reply.

So taking the AUX signal from the output of buffer 1A is the way to go. Good, as it keeps the polarities as needed.

I intend to switch the AUX in and out 'in action', but only for auditioning, not for recording (as a click in the sidechain can trigger the VCA). For layout reasons, how about moving the switch even further 'back' to inbetween the output of buffer 1A and the 10K input resistor of inverter 2A ? No difference ? All on veroboard, so fewer dangling wires.

abbey road d enfer

Re: SC Filter Invert -- HP to LP
« Reply #5 on: December 31, 2020, 01:55:48 AM »
Thank you for the fast reply.

So taking the AUX signal from the output of buffer 1A is the way to go. Good, as it keeps the polarities as needed.

I intend to switch the AUX in and out 'in action', but only for auditioning, not for recording (as a click in the sidechain can trigger the VCA). For layout reasons, how about moving the switch even further 'back' to inbetween the output of buffer 1A and the 10K input resistor of inverter 2A ? No difference ? All on veroboard, so fewer dangling wires.
No difference. The only thing that would make a difference regarding clicks is installing coupling caps and ground-leak resistors.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Script

Re: SC Filter Invert -- HP to LP
« Reply #6 on: December 31, 2020, 02:48:35 AM »
Thanks again. Will vero in the coming days and report back.

Have a happy 31 to 1 transition.

JohnRoberts

Re: SC Filter Invert -- HP to LP
« Reply #7 on: December 31, 2020, 11:19:05 AM »
what is a SC filter?

JR

==
Side-Chain filter.

Thanks... the only significant filtering I've done in dynamics processor side chains was to condition for de-essing. In multiple companding noise reduction side chain designs I used band pass filters and some additional low pass filtering to reduce errors from clock leakage (in BBD delays), etc.

I've found subtractive filters of interest in loudspeaker crossovers because the two stems sum to the original. I imagine this same thing could have merit inside the side chain for a multi-band compressor, but these often have difficulty putting Humpty back together again without anomalies.   

JR
It's nice to be nice....

Script

Re: SC Filter Invert -- HP to LP
« Reply #8 on: December 31, 2020, 12:14:43 PM »
Subtractive crossovers is where I got the idea from. The guy from ESP (https://sound-au.com/articles/derived-xovers.htm#outro) says a phase-corrected subtractive crossover (for a 24dB/octave high and low pass filter network) is a waste of components. Conventional HP and LP networks yielded the same, if not better, he says -- provided components are matched meticulously in either case, I assume.

Anyway, in my case, I have almost a dozen different RC high-pass and active band-pass filters to choose from (one at a time). A conventional approach would mean having to (re-)build those 11 filters in 'inverted' configuration (cos can't easily flip positions of that many R and C components around) -- whereas the subtractive approach promised the need of only one extra stem (AUX line), as all the filters are first-order only anyway.

Useful? I don't know yet  :)

abbey road d enfer

Re: SC Filter Invert -- HP to LP
« Reply #9 on: December 31, 2020, 01:09:03 PM »
The guy from ESP (https://sound-au.com/articles/derived-xovers.htm#outro) says a phase-corrected subtractive crossover (for a 24dB/octave high and low pass filter network) is a waste of components. Conventional HP and LP networks yielded the same, if not better, he says -- provided components are matched meticulously in either case, I assume.
Although I have  a lot of respect for Rod Elliott, he should have mentioned that this comment is valid for speaker crossovers, where the significant phase-shift of the recombined signal is not a real issue, but in other applications, particularly multiband compressors, the resultant change in peak factor can be a serious problem.
I have used extensively 4th-order Linkwitz-riley crossovers for sound systems, but for multiband compressors I had to consider different strategies.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


JohnRoberts

Re: SC Filter Invert -- HP to LP
« Reply #10 on: December 31, 2020, 02:18:49 PM »
Although I have  a lot of respect for Rod Elliott, he should have mentioned that this comment is valid for speaker crossovers, where the significant phase-shift of the recombined signal is not a real issue, but in other applications, particularly multiband compressors, the resultant change in peak factor can be a serious problem.
I have used extensively 4th-order Linkwitz-riley crossovers for sound systems, but for multiband compressors I had to consider different strategies.
I killed way too many brain cells pondering loudspeaker crossovers, the L-R 4 pole alignment is fabulous for high power general purpose sound reinforcement because the steep slopes and -6dB at crossover affords better driver protection against out of passband signal. Back in the early 80s I had some issues making 4 gang alps pot track well enough for crossover frequency adjustment (I had my technician slug them at 50% rotation for better tracking). 

Not to get off too far into the weeds but loudspeaker crossovers are dealing with what is effectively a 3 dimensional problem. Not just how the bandpasses sum on axis, but total power into the room, off axis lobing, etc.

That said side-chain dividing circuits are more 2 dimensional situations so the derived filters that sum back to the original signal seem pretty attractive (to me), despite the modest slopes.

JR

PS: Not to veer too much but the best sounding loudspeaker crossover I ever heard was a staggered pole oddball designed by a golden ear engineer in Peavey's transducer group, that had a one pole response through the critical crossover region, with steeper slopes above and below. I gave him a free hand to show me what he could do, and he did. 

It's nice to be nice....

Script

Re: SC Filter Invert -- HP to LP
« Reply #11 on: January 05, 2021, 08:39:48 AM »
Finished, installed, and works like a charm. Polarity at output never mattered because this filter feeds a full-wave rectifier.

Building it took longer than expected, but only because everything had to sit on a veroboard of 3 by 4 cm that piggybacks on another subboard. It requires 9 cables to connect:
+ - 0
2 x Filter In (L R)
2 x Aux In  (L R)
2 x Sum Out  (L R)

One push button now flips the selected filter curves around, that is from high pass to low pass, from band pass to band stop etc.

Thanks again.

gyraf

Re: SC Filter Invert -- HP to LP
« Reply #12 on: January 09, 2021, 07:14:18 AM »
(..) is effectively a 3 dimensional problem. Not just how the bandpasses sum on axis, but total power into the room, off axis lobing, etc.

Aah thanks - this made some very frustrating misconceptions finally fall into place here. Appreciated!!

/Jakob E.
..note to self: don't let Harman run your company..


 

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