Audio gain switching using relays

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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?

Re: Audio gain switching using relays

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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.

Re: Audio gain switching using relays

"Bruce Varley"
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** 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

.....  Phil

Re: Audio gain switching using relays

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

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