Electronic – Simple circuit to boost line-level audio

You need something like the 4066. Luckily, there are smaller versions with only one switch inside, like a tiny 6-pin SPDT, with 1/3 the circuitry of a 4053.

• NLAS4599: Single SPDT Analog Switch Single Supply
• NLASB3157: Single SPDT Analog Switch
• FSA4157 Low Voltage 1 Ohm SPDT Analog Switch

(I was just looking at these parts for this question.)

You can find examples of how to bias and interface with these kinds of parts here:

• Bypassing and Switching with the CD4053 CMOS Analog MUX
• Audio Switch Notes (also shows a JFET switch)
• "The Juggler 4 is a CMOS based effects order switcher"

Don't use the first circuit. Any noise or spikes on the power supply will be mixed with your signal. Because the bias point is connected directly to the signal, you can't filter out power supply noise without also filtering out the signal.

Do use the second circuit. It produces a mid-point voltage that is tightly coupled to ground, so the DC component is half the supply, but the AC component (noise and spikes) is filtered out by the capacitor. That's not a complete circuit, though, you still need to connect it to your signal.

This is what you're trying to do:

The output is the same as the input, just shifted upward by 2.5 V. The resistor on the input ensures that the input side of the capacitor is already at 0 VDC bias when an external circuit is connected, to prevent "pop" sounds (if the voltage suddenly jumped from 2.5 V to 0 V). The resistor on the output side of the AC coupling cap biases that side to the DC bias voltage. If your circuit already has a clean, low impedance DC bias voltage source, connect to that. Otherwise, you can use circuit #2 to generate the bias, like this:

(The simulation takes a loong time to reach the DC bias value, though. Hit the "Find DC operating point" menu entry to settle it.)

The DC bias voltage is produced by a voltage divider and capacitor to filter out power supply noise. Note that if you use the same Vbias point for multiple signals, they can crosstalk through this point. Larger bias cap reduces crosstalk. Larger coupling capacitor improves low frequency response. But make them too large and they'll take a long time to charge when you flip the power switch.

The 3rd diagram is not a biasing circuit; it's a microphone preamplifier.