Audio gain switching using relays

Switching the gain of an audio setup on the fly. Fairly straightforward circuit, lo-noise opamps in noninverting mode, when a gain boost is required a switch parallels another resistor across the ground resistor in the feedback path, increasing the gain of the amp. Gains between 0dB and 12dB, signals about -20 dBu.

Up to now I've been using physical switches, I've trialled automating it by using DOL reed relays instead. With the relays the system suffers from clicks in the output on switching, both ways. I'm investigating obvious causes such as capacitive feedthrough form the coil voltage change, but I'm wondering whether there could be some magnetic effect, where the coil DI/DT induces a voltage *along* the reed, causing the click. The reed is aligned with H, so if everythings 'perfect', this shouldn't occur. In practice, could this be part of the problem?

Reply to
Bruce Varley
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Years ago a place I worked had some integrating charge amps for some piezo sensors. A reed rly was used to discharge the integrator cap. After lots of testing and dicking around one of the guys worked out it was caused by the collapsing / building field of the coil. IIRC it was greatly subdued with a copper foil shield applied to the coil. The circuit used discrete reed relays, the type where the the reed was manually inserted in the coil when the unit was assembled.

I'm not sure that your audio circuit would be sensitive enough for this to be the cause or even if it is related to your problem.

Reply to

"Bruce Varley"

** Not likely.

Make sure the current in the coil rises and falls with 50 to 100 mS time constant and there are no voltage spikes on switch off.

Also, be aware that switching an audio **signal** ALWAYS results in audible clicks.

..... Phil

Reply to
Phil Allison

Thanks Phil. It works a treat now. Your advice was spot on.

Reply to
Bruce Varley

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