Low and High Pass Filters

[stompystomp]

Does anyone know if there is a simple circuit around that heavily (ideally completely cut off) reduces the level of the frequencies above and below the standard guitar frequencies. I.e. Say below 70Hz and above say 10kHz. So only signals between 70Hz and 10KHz are passed through. I am guessing this things needs to be active so as to not lower the signal level.
Anybody got any links to suitable circuits or circuits that can be adopted to suit this need?

Stompy.

[Rob Strand]

As far as a simple one opamp circuit goes  try a 2nd order butterworth filter.  You will need a 70HZ high pass and a 10kHZ low pass. Use a Sallen and Key circuit.  The high-pass and low-pass sections are wired in parallel.

Here's a calculator for the job:
http://www.analog.com/Wizard/filter/filterUserEntry/

For "Do you know the required filter response for this design?" enter Yes then select Butterworth


You can get higher cut-off with higher order filters.  Trying to get high cut-off with a single filter give ripples in the response which are often undesirable for audio (unless you know you can put up with it).

[R.G.]

A lot depends on what you mean by "simple". Simple filters don't cut off quickly or completely. This is the persistent frustration of speaker crossover designers.

Filters are distinguished by their cutoff rate and flatness in the pass band. Fast cutoff filters have ripple in the passband. Flat passband filters don't have fast cutoffs. High cutoff filters have convoluted phase responses, which can cause audio problems as well.

If you are interested in more, I recommend the "Active Filter Cookbook", which will make you very informed on the topic.

But for a quick and dirty answer, the best compromise I can recommend for you is a series connection of a high pass filter with a cutoff at 70Hz followed by a lowpass filter with a cutoff of 10kHz. A suitable circuit can be found here:
http://sound.westhost.com/project09.htm
For your purposes, you want to recalculate he lowpass filter(i.e. the bass/low output) for 10kHz, not for about 100Hz, and recalculate the highpass filter for 70Hz, not kHz as in the article. The equations and calculator is given in the article.