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

Tom Wiltshire tom at electricdruid.net
Thu Mar 6 21:02:12 CET 2014

On 6 Mar 2014, at 17:26, Neil Johnson <neil.johnson71 at gmail.com> wrote:

>>> Unless the schematic is available it might be fun to reverse-engineer it?
>> Isn't that what we're doing?!
> Without actually sitting down and working out the schematic we're just
> kinds dancing round the edge.

I don't agree.

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. 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.
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?!

>> Do you think it's 2-pole then, Neil?
> 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.

(from http://www.analoguesystems.co.uk/modules/rs110.htm )

>  But maybe an email to the maker would be quicker.

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.


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