15 kHz 'Brick Wall' Low Pass Filter

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15 kHz 'Brick Wall' Low Pass Filter

Post by reverend » Fri May 07, 2021 3:39 pm

In case anyone is interested, I dug this out the other day. It's a circuit for a 'brick(ish) wall' 15 kHz low pass filter - very necessary if you're running in stereo, and not a bad idea for mono operation too to stop high frequency content from randomly turning the stereo decoder on and off. It has about 0.25 dB ripple up to 15 kHz and then >40 dB rejection from 19 kHz upwards.

All components need to be 1% tolerance. 1% tolerance capacitors can be expensive but thankfully there is only one value used (4.7nF) which simplifies things. All the resistors are from the standard E24 range, making them easy to get hold of.

It needs a low impedance feed, and the output needs to feed a high impedance buffer, I haven't shown any power supply de-coupling or anything of that nature, and you can use +/- other voltages, '8' is not a magic number.

Enjoy,

Rev
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by Zozo » Fri May 07, 2021 5:16 pm

Interesting design Rev, is this one of your own? I'm thinking this might be very useful for a future project.

Any particular preference regarding the 2 dual OP-Amps. I'm thinking you could just use something like the TL074 quad instead?
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by MiXiN » Fri May 07, 2021 7:38 pm

A fella called Katruud from the Netherlands used to sell the 15Khz Brickwall Filters, and they were good.

Anyone know what happened to Katruud as he used to frequent these forums way back when I joined about 10 years ago (old forum)?

When I used to have a NRG Pro3, cymbals & high frequencies made the audio sound dismal in stereo - but the brickwall filter remedied this completely.

If somebody was to make these, I'm sure they'd sell really fast because I know of no other source.

I for one would buy one.

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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by yellowbeard » Fri May 07, 2021 8:06 pm

One of our Dutch buddies recently started selling KatRuud kits again including that filter:
https://www.amateurradioshop.nl/webshop/katruud/
I never built one but I have seen and heard it used, you wouldn't know it was there - no buzz, noise or distortion, just like you would want. It was screened off separate and had its own split rail supply mind you.

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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by Albert H » Sat May 08, 2021 3:53 am

MiXiN wrote:
Fri May 07, 2021 7:38 pm
Anyone know what happened to Katruud as he used to frequent these forums way back when I joined about 10 years ago (old forum)?
Sadly, "Katruud" died a year ago or so. I was in touch with his family a while ago. His designs for filters and stereo coders were generally superb.

The gyrator circuit that "Rev" has posted is very close to my design, though mine has three gyrators, and uses 10n 1% capacitors all the way through. My filter is -58dB by 19 kHz, and has very low ripple. I include an active pre-emphasis circuit on the way into my filter board, and I have a servo-balanced input stage. Its output feeds the stereo coder.

My filter is fitted after the final limiter, and I run the whole mess at +/- 15V for best headroom. The limiter uses an LM13700 in the feedback around an op-amp, and has a soft(ish) clipper to handle overshoots. There is also a composite clipper on the output of the coder to make completely certain that it can't over-deviate.

I also have a more sophisticated limiter - preferred for classical music - that uses a delay line in the audio path, so it uses a feed-forward technique to completely prevent overshoots, and nail the maximum deviation to the required level for the modulator.
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by radium98 » Sat May 08, 2021 7:11 am

OH very bad news RIP him

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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by teckniqs » Sat May 08, 2021 8:10 am

Although I never knew him

R.I.P Katruud.

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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by reverend » Sat May 08, 2021 2:50 pm

Zozo wrote:
Fri May 07, 2021 5:16 pm
Interesting design Rev, is this one of your own? I'm thinking this might be very useful for a future project.

Any particular preference regarding the 2 dual OP-Amps. I'm thinking you could just use something like the TL074 quad instead?
Hi Zozo,

Yes, it's one of my designs but very happy for anyone to use it.

I have an even more 'brick-wall-y' filter design such as Albert describes but I tried more powerful filtering and found that it was finnicky to set up if you wanted left and right channels to match for stereo, and didn't really seem to make much difference as far the effectiveness of killing the 19 kHz area was concerned. Note that sharp filters like this have a nasty tendency to overshoot if you drive them with clipped material, so some kind of 'safety clipper' afterwards is always advisable. Even better, clip the composite signal to remove any spikes from the filter and the mux-ing process.

As for op-amps, I originally used a TL074 as it goes, them or a couple of NE5532 will do the job fine.
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by Albert H » Sat May 08, 2021 5:58 pm

I like the LM833 for the low pass filters. I found a source of them in the Far East for about the same price as a 4558 (about $0.05 each in 100s), and they are as good as the NE5532 in terms of noise and distortion parameters, and they don't suffer from latch-up if over-driven.

Rev - I was trying to work out the "shape" factor for your filter. It's not quite elliptic, is it? I found that the cut-off with three gyrators was about optimum, but two gyrators was about good enough. I tried more stages, but you're right - there's a rapidly diminishing return, and the component values become an order of magnitude more critical. Some commercial processors use as many as twelve sections, but their performance isn't significantly better than the designs you and I have done.

"Katruud" did a multi-stage "brick master" filter, using a combination of Chebyshev and Bessel filters. If built with the best quality op-amps, the results were brilliant, but if you tried to use cheap TL072 and 074 types, it would hiss horribly! I've got the circuit diagram here somewhere, and will post it later.

The latest filters I've been using are "Switched Capacitor" types - high-order lowpass filters, clocked at a multiple of the required turnover frequency. Some of these are quite cheap nowadays, and give great results (and accurate matching between channels) with little hassle. I've been using MAX7410 ICs - two cascaded per channel - with a crystal-derived clock - and getting >-60dB at 19kHz.
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by sinus trouble » Sat May 08, 2021 10:55 pm

So sad to hear about KatRuud :( Top bloke!
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by MiXiN » Tue May 11, 2021 8:11 pm

Talking about Katruud (RIP), I was checking the documents folders of my phone earlier & found a bit of info' including a schematic of his Brickwall filter.

I've found some other bits in my archives, too, but here's a PDF upload of his Brickwall filter - https://docdro.id/rFKVXSy

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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by Zozo » Tue May 11, 2021 10:08 pm

I've been experimenting with a PCB layout for this filter using the TL074 Quad op-amp. Now I'll hold my hands up first to admit the layout is far from perfect with some very long tracks linking up all the parts in the circuitry, but it's my first attempt at getting a basic layout down.

I wasn't going for my usual obsessive for neatness either, which some of you may know by now, but mirrored symmetry seemed the obvious choice for the TL074.

I do plan to incorporate this filter into another project I'm working on, rather than a "standalone" board of Reverend's filter that's present now. But I'll still share one when I think I've got the layout correct.

Possibly someone else may beat me to that, with something much better. ;) ;)
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by reverend » Thu May 13, 2021 8:31 am

Hi Zozo,

Nice work. You might want to add an input and output buffer (if you make the output buffer have 6dB gain, then the overall gain will be 0dB). Something like this...
Rev_FNDR25.jpg
You will need a split-rail power supply, or 'fake' one using a couple of resistors and a capacitor to develop half the supply voltage for the 'gnd' part.

Rev
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Re: 15 kHz 'Brick Wall' Low Pass Filter

Post by Zozo » Thu May 13, 2021 11:39 am

Hi Rev

Thank you for the updated schematic for the filter with the buffer stages. At the moment I've just been trying to get a basic layout.

All understood with the split-rail power. I'll get to work on a updated PCB design sometime during the week and report back.
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