simple component mixer schematic?

What is the simplest wiring diagram / components for mixing two outputs?

Here is my schematic. Esp32 outputting tone on a gpio, and sending tx/rx interfacing play commands to a DF mini mp3 player.

I want to mix the tone and one channel of the player output and feed it into the "in" of the amplifier.

I don't need a pot since I can control the volume of the signals before hand.

I searched and this is what I came up with

  1. What ohm resistors are needed? ( 10k?)

  2. Do I need caps after the resisters? What size?


Background

I tried the pcmtmb library with the hopes that it could handle concurrent wav files. It didn't compile due to timer related issues with the esp32.

There are other chunks of code out there to play audio with DAC outputs of the ESP32 but I have to handle all the buffering, timers, etc. No simple libraries exist yet for the esp 32.

The only clean example I could find for audio over esp32 was a video from pcbreflux which is where the diagram came from.

The DR player doesn't have nearly the functionality of the pcmtmb library (concurrent files playing).

So my work around is to use the tone generator for user inputs and play audio from the DF min player for the rest.

You need an input level that you can't alter with a signal from a channel. Something like this:-

What you have will not isolate the two outputs.

1K - 10K should be fine.

With equal-value resistors, you'll have 50% voltage divider on each side for a loss of -6dB (when there's just one signal).

You want the resistor values to be high relative to the source, and low relative to the amplifier input. So, it's a compromise and there could be more than 6dB of loss but you'll probably still have plenty of signal.

If the amplifier doesn't have a capacitor at the input, it would be a good idea to stick a capacitor (1uF) in series with the input (after the "mixer") to block any DC.

I don't need a pot since I can control the volume of the signals before hand.

Actually... A pot with the center-tap going to the amplifier would be another way of doing it (again with the optional series capacitor). And, that would allow you to adjust the "balance" between the two signals.

ok good. Thanks

So the triangle is an op amp like a lf347?

So the triangle is an op amp like a lf347?

Yes.

Try the resistors first. :wink:

Sure a simple resistor will appear to work, that is the problem.