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March 03, 2011


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George Storm

Too true. The second-order filter is an optimum zero-overshoot design. Perhaps it would be even nicer to use the extra pole to make a third-order equivalent?

Paul Rako

This is a stopband leakage problem. C1 will pass very high frequencies directly to the output. You can fix it by feeding C1 with a buffer amp from the output. See

John Dunn

Stopband leakage arises because C1 does indeed pass high frequencies directly to the output of U1A, but adding a buffer for feeding back to C1 would still require that buffer to be infinitely fast to accomodate a zero-rise-time input from V3. The symptom could be diminished, but not eliminated.

Ralph Williams

This reminds of the issue of slew-induced distortion versus bandwidth limiting when oversampled audio DAC's interface to analog circuitry. Presumably one of the advantages of vacuum tubes is that they bandwidth limit before they slew-rate limit.

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