Splitting a mono line, instrument, or microphone audio signal for two consumers

Question

I am building an audio device that receives a mono input signal from a microphone or instrument via a regular 6.3mm jack. From there it goes into a digital signal processor (a USB sound card, actually) which generates the output. I also want to pass the original input signal through so I can use both the generated output and the original input in the main mix.

Therefore, I need to split the input signal into two outputs: one that serves directly as an output ("through") and one that goes into the signal processor. It's basically where the "?" sits in the below schematic:

                                       +---------+
IN ---?-----------<through>----------->| mixer   |
      |                                |         |
      +-->[ DSP ]-----<out>----------->| console |
                                       +---------+

This video suggest that you can basically just connect the two outputs in parallel.

I would be very surprised if that actually worked well, as the signal processor as well as whatever will be connected to "through" will both have their own, different, impedances (or be disconneted altogether.)

Can someone explain what the right solution for this is? If the solution in the video is the right one, why don't the impedances matter?

It would be extra awesome if the solution works with microphone, instrument, and line level inputs.

Answer 1

I haven't watched the video, but your ASCII art diagram is spot on. Just leave out the question mark. That is, simply connect the one input to the two outputs with a wire or PCB trace.

The reason this works is because the input will normally come from a low impedance source while the two following inputs will usually have a high impedance.

You are dealing with audio, so you won't have any trouble with signal reflections as you would if you were working on RF.

If you are concerned that what ever is connected to "through" may have a low impedance, then you could do this:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

The splitters I have use transformers to split the signal. Jensen has an application note that goes into the details of microphone splitters.

A splitter for balanced lines:

From the linked Jensen application note.