# Mixing analog audio signals with resistors

I am creating a type of mixer for 2 audio inputs into 1 output so i can use 1 pair of headphones with 2 computers without buying an expensive mixer with a amp

Currently i have 1k 1/2W metal resistors on both left and right input signals and nothing on ground

It works great and i can hear both inputs fine though the headphones but it really needs a amp and isnt usable

My question is: What resistors should i be using so there isnt so much volume loss?

• it "works great", but "isn't usable". hmmm. – Tyler Stone Dec 29 '18 at 3:51
• Isn't usable without an amp – alph Dec 29 '18 at 8:35
• What is the load impedance of your headphones? Much of the output level depends on how its value compares respect to the $1\mathrm{k}\Omega$ resistor you have chosen. – Daniele Tampieri Dec 29 '18 at 10:50
• Your method might work OK for series resistor values of about 30 ohms. (Have not tried this). Crude - will give you more volume. Less than 30 ohms may stress the driving amplifiers. – glen_geek Dec 29 '18 at 16:03

I think you'll need more than just resistors to do what you are describing properly. You should probably go active. You could try a simple summing amplifier like this:

This will invert your signal but you can always add another simple inverting amplifier stage at its output (basically the same thing in this schematic except only using 1 resistor at its input).

You will have less loss this way and you can even apply gain if you want to (determined by Rf/Rin). If you add a potentiometer for each input, you'd have individual volume control for each one. You can add more inputs if you like as well.

Hope this helps!

• In audio, I doubt an inverted signal will be much of a problem. – Hearth Feb 13 '19 at 1:35
• @Hearth it can be...especially if the user isn't aware. – Tyler Stone Feb 13 '19 at 1:59
• @Hearth True, I didn't mean to suggest an extra inverting stage was a requirement. There is some debate about whether we can perceive the difference, some people like to make sure to maintain absolute polarity. I've heard of people testing polarity on their speakers by placing a coin on the driver and playing a bass drum hit. If the coin bounces in the air, the driver is pushing air and everything is proper. Probably not too important for this application but I thought I'd at least mention the inversion. – Avid Pro Tool Feb 13 '19 at 2:00
• it's not at all about absolute polarity.... electronics.stackexchange.com/a/341851/156429. – Tyler Stone Feb 13 '19 at 2:25
• @TylerStone true, phase cancellation would be a big issue if he were doing some parallel routing, not sure if that's the plan. The question sounds to me like the goal is to sum the outputs of 2 computers into a pair of headphones if I'm interpreting correctly. Now that I think about it I should mention that any old op amp won't necessarily drive a pair of phones. If you're summing line level signals into a headphone amp, you're good. But if this is 2 headphone outs being summed directly to the phones, you might need more output current than the average op amp. – Avid Pro Tool Feb 13 '19 at 2:35

Most consumer headphones are 32 Ohms impedance. You can safely use 16 Ohm resistors from each computer.

The closest e12 resistor values are 15 Ohms or 18 Ohms.

Quite frankly, most any resistor value from 15 Ohms and uo should work just fine.

You will see a minimum of 8dB or so loss. That can't be helped.