A Universal Sallen-Key Filter Board

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
emrr
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Re: A Universal Sallen-Key Filter Board

Post by emrr »

I've had a couple of fully tested stepped Sallen-Key filter sets ready to go to market for several years, whenever I find the time to learn PCB design. :cry: I only found problems in need of tweaks down below 20Hz, if I recall correctly.
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Doug Williams
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JR.
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

mediatechnology wrote:Thanks ricardo and everyone.

The stop band hump is only in the Low Pass implementation too.
The HP doesn't have it.
yup, due to falling gain at very HF from dominant pole compensation
One of the big advantages of the unity gain Sallen-Key in MS work is that in the pass-band they really are, for all practical purposes, unity gain.
In terms of gain accuracy they win.
One less resistor too.
I don't know about gain accuracy but for absolute lowest cost a transistor emitter follower can be used with that topology (you could advertise it as class A :lol: ).


I did a DAC filter comparison between Sallen-Key and MFB and IIRC the Sallen-Key was indeed quieter.
I still worry about the demands that puts on the op amp output speed and output impedance at very HF. With S&K LPF the HF content coming from the DAC must be absorbed by the cap connected to the op amp output (questionable impedance at very HF).

If serious about using a S&K after a DAC consider making it a 3-pole filter tuning with a real passive pole in front of the 2 pole active stage. The real passive pole front end could scrub off the very HF content I worry about to ground. Scaling the impedance of the first real pole down a decade should reduce interaction with following active stage but i suspect modern filter software could account for that interaction too.

Sorry perhaps esoteric and obscure speculation but DACs can put out some HF steps (in theory at least). Newer DACs are probably somewhat better using decimation(?), or not. 8-)

JR

PS: I used S & K topology for the HP/LP filter kit I sold back in the 80s because it used less parts. In my kit I supplied resistor sets for a few different frequency tunings. Less parts is good when a bunch are just being thrown away. That said I used good bi-fet opamps. I was cheap but not that cheap.
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mediatechnology
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Re: A Universal Sallen-Key Filter Board

Post by mediatechnology »

Doug you reminded me that I should put bigger pads or tie points for people wanting to make stepped filters with switched resistor ladders.
Make before break aka shorting required.

And a note to myself to put DC bias return resistors on the low-pass filters for AC-coupling or floating input.
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

mediatechnology wrote:Doug you reminded me that I should put bigger pads or tie points for people wanting to make stepped filters with switched resistor ladders.
Make before break aka shorting required.
The resistors would be mounted on the switch, right? Wouldn't a 0.1" spaced hole for a header or phoenix connector work?

I've been spending time working out the frequency points for filter set(s) I have in mind. I'm going to have to try. The basic concept of the first one is to have three cascaded 6dB/oct filters. Each on an 11 pos rotary switch. Working out the Q and damping for each and how they combine is going to be the real work. The name I have in mind is "The Bends".
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mediatechnology
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Re: A Universal Sallen-Key Filter Board

Post by mediatechnology »

The resistors would be mounted on the switch, right? Wouldn't a 0.1" spaced hole for a header or phoenix connector work?
Yes, resistors on the FP switch so a header or a larger pad than a resistor would normally have that would be easier to solder to.
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JR.
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

Gold wrote:
mediatechnology wrote:Doug you reminded me that I should put bigger pads or tie points for people wanting to make stepped filters with switched resistor ladders.
Make before break aka shorting required.
The resistors would be mounted on the switch, right? Wouldn't a 0.1" spaced hole for a header or phoenix connector work?

I've been spending time working out the frequency points for filter set(s) I have in mind. I'm going to have to try. The basic concept of the first one is to have three cascaded 6dB/oct filters. Each on an 11 pos rotary switch. Working out the Q and damping for each and how they combine is going to be the real work. The name I have in mind is "The Bends".
-6db/oct filters are real poles so have fixed Q. Cascading 3 one pole filters tuned for the same -3dB point would by definition be -9dB at tuning (ASSuming you put buffers between them to eliminate loading). I don't know if you can make a one pole filter with unnatural Q using SVF, never ever thought about trying to do that.

The concept of mixing damped and underdamped sections is mainly for cascaded active 2 pole per section filters, so under-damped sections can compensate for over-damped sections, but this is mainly used with all cascaded sections tuned for the same*** cut off frequency but with different Qs. Having 3x11 pos switches in series suggests a rather large number of outcomes to anticipate.

You could probably do whatever you can think of inside a DSP (11x11x11 or whatever). Analog filters might get unwieldy.

JR

*** chebyschev actually cascades over-damped and under-damped sections with different tuning frequencies too, which trades higher in band amplitude ripple for steeper transition region cut-off.
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Gold
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

JR. wrote: -6db/oct filters are real poles so have fixed Q. Cascading 3 one pole filters tuned for the same -3dB point would by definition be -9dB at tuning (ASSuming you put buffers between them to eliminate loading).
Or you could think of it as an over damped 3 pole filter. I don't think I've ever heard one. Getting perfect three pole filter response by cascading sections isn't what this box is about. This will be tuned by ear in design and use. I also don't think it will end up exactly like I imagined it would. I have never run across anything remotely like what I am proposing although the building blocks are straight forward. I have wanted this box for a long time. I don't imagine a designer would think of doing this as it would appeal to a very limited few. A lot of people seem to want tone boxes that are easy to use. I hate that. I wouldn't let a pultec clone in the same building.
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Re: A Universal Sallen-Key Filter Board

Post by emrr »

Paul, you might find useful tangent in here if you missed it:

viewtopic.php?f=6&t=576
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Doug Williams
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JR.
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

emrr wrote:Paul, you might find useful tangent in here if you missed it:

viewtopic.php?f=6&t=576
At least I was saying the same stuff (same stories) back in 2013.

JR
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Gold
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

Thanks Doug.
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