A Universal Sallen-Key Filter Board

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mediatechnology
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A Universal Sallen-Key Filter Board

Post by mediatechnology »

I'm always needing simple filters for doing audio experiments particularly for the MS Matrix and Width Controller.
Usually, if I need something, other people do too.


Elliptic Equalization for vinyl cutting, low and high pass filters for width management or transfer, dynamics side chains, test and measurement and simple speaker crossovers are some of the many applications where I've needed a simple filter.

This filter board, a work-in-progress, attempts to be yet another Swiss Army Knife to solve a simple DIY problem.

The filters can be cascaded or operated in parallel.

PC board footprints allow either resistors or film caps to be stuffed in each location.
This allows a LP, HP, All Pass or buffer to be configured for each op amp stage.

When a filter needs to be adjustable, particularly when step switching is required, the cost and complexity can rise quickly.
A tuneable six step stereo 12 dB/octave Sallen-Key Filter requires a 4 pole switch and 24 external resistors.
Higher orders are crazy expensive.
Preventing a stepped switch from clicking either requires a make-before-break switch or always having one value in circuit to prevent loss of bias current.
Duplicate op amps, capacitors and resistors often wind up being less expensive than a multi pole switch.

The board has up to four 12 dB/octave filters.
By switching outputs, lower cost single pole per channel switches can be used.
Four small boards can provide six (or 8) stereo filters.

Image
A Universal Sallen-Key Filter Board for Audio.

The first stage is designed primarily to be an input buffer for the remaining filters.
It can be configured as a balanced input, single-ended input with gain or a fourth filter.

The remaining three filters are identical but can be configured as a low pass, high pass or all pass.
Three different examples are shown.

The first stage can cascade to the second stage and the second stage to the third for multi-order or bandpass configurations.
Buffered 6 dB/octave sections can also be made to provide odd-order filters.

If the idea makes sense it could be extended to MFB filter types which for LP responses may be better in some applications.
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JR.
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

I sold a cheap kit back in early 80s (P91?) that had two 2-pole sallen and key HPF and LPF. IIRC it used one TL074.

I may still have some PCB :lol:

JR
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mediatechnology
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Re: A Universal Sallen-Key Filter Board

Post by mediatechnology »

I think you may have sent me a few of those boards JR.
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

That's great Wayne. This will be useful for all sorts of things. First up using it in conjunction with the Width Controller. I think frequency dependent image control is very useful.
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mediatechnology
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Re: A Universal Sallen-Key Filter Board

Post by mediatechnology »

Thanks Paul. This has been on my list.

Doug alerted me to a Studer All-Pass filter floating around that IIRC used 4 sections per channel.
They did this trick so that the mono sum would only be +3 for fully-correlated inputs.
A pair of these boards could do that trick.

The big challenge is to make it accept Wima film caps having lead spacing ranging from 5 to 22.5 mm.
That in itself isn't too hard but keeping the overall board small will be.
Though for most S-K filters keeping the capacitors under 1 uF keeps the resistor values from becoming too low so 10 mm maximum may be enough.

I could add spaces to make MFB filters but it might just become too confusing once it became time to build one.
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JR.
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

This is a little esoteric but MFB topology may have a slight edge for performance when dealing with line level signals. Since the op amp is operating inverting, the input stage is less exposed to common mode voltage swing.

Actually a lot esoteric. :lol:

JR
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mediatechnology
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Re: A Universal Sallen-Key Filter Board

Post by mediatechnology »

This is a little esoteric but MFB topology may have a slight edge for performance when dealing with line level signals. Since the op amp is operating inverting, the input stage is less exposed to common mode voltage swing.

Actually a lot esoteric. :lol:
With a BIFET maybe not.
The MFB low pass outperforms the S-K in stop band as well.

Though in the applications I'm thinking of it may be esoteric too.
And the signal polarity inversion of the MFB may or may not be a benefit.
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

mediatechnology wrote: And the signal polarity inversion of the MFB may or may not be a benefit.
Maybe add an output buffer that allows a polarity inversion?
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Re: A Universal Sallen-Key Filter Board

Post by JR. »

Gold wrote:
mediatechnology wrote: And the signal polarity inversion of the MFB may or may not be a benefit.
Maybe add an output buffer that allows a polarity inversion?
You generally want to drive filters from a low impedance source, so for a free standing filter block, a differential receiver in front of it can correct for a polarity inversion.

JR
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Re: A Universal Sallen-Key Filter Board

Post by Gold »

I think that's the second time on this forum I've made that mistake. I'm consistent.
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