Simple circuit to bypass preamp when battery fails

Question

A guitar has an on-board battery powered pre-amp between the pickups and the output jack. This is great until the battery fails, when it goes dead until the battery is replaced. I would like to have a safer failure mode for live performance, whereby when the battery fails it falls back to passive mode, with pickup connected directly to the output jack.

Knowing very little about this, my idea for a solution is simple and probably naive:

There is a connection from the pick-up to the output which passes through some kind of semiconductor device or devices. This device is normally “open” so no signal passes to the output, but it receives an input from the battery which keeps it open. When this signal drops because the battery has failed, the semiconductor switches the circuit from pickup to output Jack to “closed”. Clearly the output would be lower than with the preamp, and without any EQ, but turning up the amp mid song is much easier than changing the battery!

Can anyone suggest a circuit to do this?

Answer 1

This could perhaps be done with JFETs or other analog switches.

JFETs are a (now mostly) archaic solution, but have the advantage that they don't require bias to turn on; they're "on by default", at least up to a modest current limit. Since signal levels are low, this makes an effective normally-closed switch.

^{simulate this circuit – Schematic created using CircuitLab}

There are also depletion-mode MOSFETs with similar behavior, but none are available right now with uncommitted substrate so you'd either have to make your own ASIC ($$$) or use vintage parts. (Even the enhancement-mode ones are poorly available, e.g. SST210 -- definitely available, but specialty boutique stuff.)

This would be paired with a normally-off (enhancement mode MOSFET) switch, or perhaps even a coin cell to bias the gate voltage to a similar end (gate current is very small so this cell can last much longer than the amplifier's will; it could perhaps also be charged by the main battery to extend life ~indefinitely).

The catch is, a battery doesn't die all at once, it just drops below normal operating voltage of whatever it's powering. So you're going to need a battery monitor circuit and level shifters to drive the switches anyway.

This assumes that the circuit in the guitar is open for redesign. If you want to patch something into an existing circuit, you'll need to show the circuit (and the circuit or an equivalent of the guitar itself, and the following pedal/amplifier input circuit(s)). I could draw more circuitry, but without a concrete starting point, or specification, it's as likely to work as not, so I'll keep it to basic starting-point ideas rather than anything more complete.

Answer 2

You could, perhaps, use an SSR (Solid State Relay), but it would need a drive circuit that detects battery voltage dropping below a threshold and switching off the control LED.

The one I've linked to needs less than 1mA to stay off. However that will cut into battery life. It's a changeover (Form C) type.

Analog switches are possible too, but they usually need some voltage to turn on at all.

All in all, I think a mechanical switch would be your best bet. Quick and easy to flip and generally reliable.

Answer 3

Does it really have to be automatic? I would just use a DPDT switch. If the preamp already has a power switch, this would simply replace it.

^{simulate this circuit – Schematic created using CircuitLab}

When the switch is in the upper position, the preamp is switched off and the pickup is connected directly to the output jack. In the lower position, the preamp is on and its output feeds the output jack.

If the battery fails in the middle of the performance, just flip the switch and keep going.