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I'm building a practice amplifier for my girlfriend who is learning to play the bass guitar. I'm a reasonably experienced electronics builder so building something to plug a guitar and headphones into is very simple.

What I would also like to do is add a TRS input so she can play along to her iPod/anything with a stereo output. This signal does not need to be amplified as it will be going straight to the headphones. The amplification circuit for the guitar is based around an LM386, much like this:

enter image description here

But with both headphone cables connected to the 220u capacitor instead of one speaker. It works well. I also need the stereo input from the iPod to go into the headphones, but without any drop in volume for either source. What is the simplest principle to do this?

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First thing: if directly feeding headphones make sure you don't induce acoustic shock by a signal overload - you don't want to damage someone's hearing so make sure the output voltage is suitably limited in level to prevent this.

For the stereo output of the iPod you need to mix left and right via 1 k resistors forming a potential divider with the centre tap a mono signal. Take that mono signal and feed it via a 10k pot into pin 2 of the 386 chip. Use exactly the same technique as you have done with the volume pot in your circuit; one end grounded and the wiper connected to the original wiper on pin 2. Use a 47 nF capacitor like in the original design and feed that from your 1k potential divider that converts stereo to mono.

That should work after a fashion and be reasonable.

I also need the stereo input from the iPod to go into the headphones, but without any drop in volume for either source

If you really want this level of functionality then use an op-amp mixer circuit like this: -

enter image description here

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  • \$\begingroup\$ That looks good but does look like it will convert the stereo signal from the iPod into mono - any way of keeping it stereo? \$\endgroup\$ – TCassa Jan 16 '18 at 10:23
  • \$\begingroup\$ Double up on the driver channels i.e. use two LM386 amplifiers and apply bass guitar to both channels via a mixer. \$\endgroup\$ – Andy aka Jan 16 '18 at 10:24

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