ultraflanger schematic error

[J. Luja]

glancing over this schematic today http://www.hollis.co.uk/john/ultraflanger3.jpg
I noticed an odd configuration in the VCVS LP filter following the BBD. I'm certain that the 1n cap connected to the inverting input should actually be connected to the output, starting the rolloff at ~4kHz rather than the ~100kHz as it is drawn.

someone unhappy with a noisy build and want to try this out?

-Jeremy

[Rob Strand]

Well spotted.

odd configuration

It's not common but that configuration is actually valid.  The problem is John's design has the large 100k's in the opamp feedback path which actually stuff-up the filter behaviour.  You would only pick up how bad things are via simulation .

Your suggestion of moving the cap to the output is probably the easiest fix.  I suspect that mod alone will make the sound a tad dull (some may like it).  Slightly better perhaps would be to move the cap,  reduce the first 47k to 39k and the second to 33k.

Edit:
For the cost of an extra resistor and capacitor, a third order filter would be far better!

[J. Luja]

ah, yeah I see what you mean, with smaller resistors it creates a "stairstepped" response. though I can't imagine that's what he was going for  :wink:

I agree with the third order suggestion, and I'd suggest moving the cut-off up to around 8kHz would probably be safe considering the clock frequencies being used.