[sdiy] Good 24db/oct HP-LP-BP-Notch filter Schematics ??

Neil Johnson neil.johnson71 at gmail.com
Thu Mar 6 23:24:52 CET 2014

Hi Tom,

>> Without actually sitting down and working out the schematic we're just
>> kinds dancing round the edge.
> I don't agree.

Fair enough.

> I think if you know what the basic outline of the filter type is and what
> components they used, then the actual schematic more or less drops out.

True, you might get the general idea, but often it is the details that
make the difference.  For example a 4-pole SVF and a 4-pole cascade
filter have roughly the same components, and while you do a hand-wavey
"it's a 4-pole filter of some sort" you won't know exactly what sort
without tracing out the PCB.

> If I
> know it's a Moog 4-pole using a CA3046 and a CA3080, I can draw or find you
> a dozen schematics of that. Likewise if it's a dual-OTA SVF. Much more
> important than the nitty-gritty details of the schematic is the overall
> outline, and you *can* often determine that by reading between the lines of
> the marketing blurb and making a few educated guesses. If you've got photos
> of the PCB so you know accurately what's on the board, I think you can get
> pretty close.

Agreed, but I wouldn't call that reverse-engineering, just making
wild-assed/educated guesses.

> Obviously, at that level of detail, we're not "cloning" the circuit, merely
> implementing something similar. But that's better, since it's not stealing,
> right?!

Who said anything about stealing?

>> Evidence suggests that is the case.  I don't have one to test so I
>> can't say with any certainty.  If someone sends me one I can plug it
>> into the spectrum analyser and confirm its operation.
> Not all the evidence suggests that is the case. Analogue Systems say:
> "The RS110 offers four filter modes. These are 24dB/oct low-pass, 24dB/oct
> high-pass, 12dB/oct band-pass and 12dB/oct band reject (often called
> 'notch') filtering,"
> That doesn't sound like a 2-pole filter to me.

Documents can be wrong, or out-of-date, or simply sketchy with the
truth.  So far the only real evidence I have seen are the photos.

> I doubt they want to share the schematic, but they might tell us if it is
> based on the Moog ladder or not, or some other similar useful detail.
> I'll write to them. I would like to see a definitive answer to the
> conundrum. I'll report back if I get anything useful.

We don't *need* the schematic, we just want an answer to the posed
question.  Schematic is one way to answer that, the maker could
provide an answer probably a lot quicker if they're willing to
cooperate.  Student with the barometer.


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