EQ - Switchable Frequencies

[JFace]

I would like to make a one band eq with three switchable frequencies, using an on-on-on dpdt switch. These frequencies are far enough apart that I need to use separate gyrators for each. I am using the transistor version of the gyrator.

Image from muzique.com:


My question is, what is the best way to toggle between the three gyrators, with minimal click, and with no damage/oscillation to the gyrators that are not engaged?

A few ideas to get started:

1. The common pole connected to mid pot lug 2, and the three throws each connected to "IN" of the gyrator circuit
2. The common pole connected to ground, and the three throws each connected to the 10k resistors of the transistors, with all gyrators connected to the mid pot in parallel
3. The common pole connected to 4.5V, and the three throws each connected to the R1 resistors, with all gyrators connected to the mid pot in parallel
4. The common pole connected to 9V, and the three throws each connected to the transistor drains, with all gyrators connected to the mid pot in parallel

Thanks for any ideas.

[PRR]

> frequencies are far enough apart that I need to use separate gyrators

Why?

Usually you pick the resistors for happy DC and op-point conditions, and scale the caps for frequency. This only "fails" when the caps get too teeny (<100pFd) or too large (electrolytic?). Usually you can cover the whole audio range just by switching caps.

This is trivial with a DPDT on-off-on switch. Top drawing.

Drawback is that the center position is the high freq, medium one side, low the other side.



If you "must" run three gyrators....

AFAICT the simple way needs a SP3T switch, plus three high-value resistors to keep the input caps charged to the idle DC voltage so they don't pop. Bottom sketch.

[Mark Hammer]

Why not just go quasi-parametric?  That is, sweepable center/resonant frequency, but fixed Q.  More flexibility, and no switch-popping to worry about.

[JFace]

Thanks for the feedback. I hadn't thought of the on-off-on thing, that would work for me. FYI, A dpdt on-on-on switch can be wired to function as a sp3t switch, as shown here:

http://www.sabrotone.com/?page_id=3480

I would want it to be hard switched, and not semi parametric, for a few reasons: the Q's are specifically tuned for each one, I only intend to use the three frequencies, the number of pots vs number of switches is optimal this way.