I'm stumped on wiring a 10K Ohm audio control potentiometer with spst switch into a LM386 amplifier circuit.
The pot I'm using is very similar to the one in this link:
https://www.radioshack.com/products/10k-ohm-audio-control-potentiometer-with-spst-switch
The LM386 amplifier circuit I'm using is the one in this link:
http://www.circuitbasics.com/build-a-great-sounding-audio-amplifier-with-bass-boost-from-the-lm386/
What I'm doing with this circuit is using it as a simple overdrive pedal for my guitar. Eventually, I'm going to wire this circuit directly into the guitar's pickup wiring.
My circuit works great and I was able to use the above potentiometer as a volume knob with great success.
The only problem now is the pot's switch must be turned on in order for signal from my guitar to go to the amp. When I turn the pot switch to off, the signal cuts off completely.
What I'd like to do is wire the circuit so when I turn the pot switch to OFF, the signal from my guitar gets routed directly to the amp. When I turn the pot switch to ON, the signal from my guitar gets routed through the LM386 circuit.
Is this possible?
I know I could do this with a push/pull type of pot but I like the feel of the rotary switch pot. Plus, I already have a few of them lying around.
Thanks for reading.
Best Answer
simulate this circuit – Schematic created using CircuitLab
Figure 1. A reed relay with changeover contacts provides a fail-safe method of switching using the built-in pot switch.
At the expense of some battery drain a reed relay would provide a good solution with the added benefit that in the event of a battery failure the guitar is still usable and would avoid a cringe inducing silence on stage.
Figure 2. Typical reed relay packages.
Figure 3. Reed relay datasheet from TE Electronics.
A quick web search finds that these are available in 5/6 V DC versions (so a series resistor would be required) with a 200 Ω coil. At 5 V this will cost you \$ \frac {5}{200} = 25 \ \mathrm {mA} \$. Since the current is that high you can add an LED in series with the coil and reduce the resistor value to suit or leave it out for a white LED which typically have a VF of > 3 V. (The relay coil would provide the current limiting for the LED.)
DC thump
From the comments:
It's more usual when the amp is switched on.
Figure 4. The bass boost amplifier. Source: Circuit Basics.
The DC thump occurs because the amplifier is powered from a single rail. Since audio is alternating current it means we need to bias the output to mid-supply or 4.5 V in this design. This should be measurable on pin 5 of the LM386. We don't want to feed DC to the loudspeaker as it will cause unnecessary heating and distortion due to biasing the speaker in or out depending on polarity. The biasing of the speaker means that it has less travel in one direction before it maxes out and distorts.
Looking at the schematic of Figure 4 we can see that this is unlikely to help. The pot is on the input whereas the thump is coming from the amp and C1 on the output.
Since you are only using this as a preamp rather than a power amplifier you could move the pot to the output, after C1, and this might solve the problem for you. It also has the benefit that any noise generated by the amplifier will be attenuated by the pot at lower settings thus giving you a decent signal to noise ratio independent of volume setting. In the original design the amplifier noise will be the same no matter what the volume setting is.