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Old July 6th, 2013, 11:38 AM   #16
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

If Ken decides to go for a low pass filter option, fitted within the mixer, something along the lines of the attached schematic should do what is required.

In the case of the M267, installation would require the removal of one resistor from the Mixer PCB (that of course is a reversible procedure!)

I would think modifications to the M367 would require a similar approach.

I could probably do a stripboard layout of the attached schematic, if it is of any interest?...
(now added - see post #20)
Attached Files
File Type: pdf LPF schematic.pdf (43.4 KB, 188 views)

Last edited by Roger Shore; July 7th, 2013 at 11:18 AM. Reason: additional info
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Old July 6th, 2013, 02:31 PM   #17
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

I am tempted to wonder if this might be one of those "why don't you just put the elevator on the outside?" issues.

What are the factors that demand A) that you need live sound rather than a narration and B-roll approach. B) that you can't shut the motion detector off C) that you can't simply demonstrate the device by simulation rather than live performance?

None of these may satisfy, but I'd certainly try everything else before I tossed Occams Razor to the ground and presumed that since the tool you're looking at using to fix this is electrical engineering - that this is the only - or even the best way to solve the problem.

It might be. But it might not.

FWIW.
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Old July 6th, 2013, 03:14 PM   #18
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

Bill - my understanding of Ken's problem - as outlined in this (and previous) threads - is that as he is recording legal depositions, he has no choice on:
1) doing it live and
2) doing it in environment where there is no option to either change the venue or turn off the offending motion sensors....

Hence the need for an electronic 'realtime' solution.....
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Old July 6th, 2013, 03:42 PM   #19
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

IIRC the OP is taking legal depositions in locations that he does not select or control. And one of those places is a building that has these ultrasonic motion detectors. Thus the need to deal with the situation as it is handed to him.
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Old July 6th, 2013, 08:19 PM   #20
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

I've added a stripboard layout of the schematic in post #16 in case it's of interest.
Please note it has not been built -- although I think I've checked it OK.
You could probably make it smaller by locating come of the components vertically, but if you're not used to assembling stripboard, it's probably easier to start this way!

Just one possible solution to the problem for Ken to consider.....

EDIT: I found an error in the track pattern layout - which I have corrected There was a missing track cut under R8. If any of the 3 folk who have already looked downloaded the originalPDF, please update with the new file. Sorry about that.
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File Type: pdf 7KHz Butterworth filter mod.pdf (245.8 KB, 158 views)

Last edited by Roger Shore; July 7th, 2013 at 05:42 AM.
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Old July 8th, 2013, 03:21 PM   #21
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Re: How to Build or Have Built a Low-Pass Filter Based On A Custom Schematic

Note that native frequency response of the Shure SM11 dynamic lav mic.
I suspect it has sufficiently poor frequency response to mostly eliminiate the 25 KHz noise.
Furthermore, it is big enough that a parallel capacitor could be easily added right inside the head to further roll off the high frequency response.
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