Altoids Tin 1/8" Stereo Mixer

Step 15 FAQ & Design Tradeoffs

FAQ's

1) Do I really need resistors?

Absolutely, Yes.

It's a very bad idea to connect active outputs together without some resistance in between.

Depending on the design of the devices you're connecting together, the best thing that will happen is mild to severe distortion in the sound. The worst thing that can happen is damage to one or more of the devices. Why risk your expensive equipment to save a few cents on resistors?

2) Can I use diodes instead of resistors?

No.

Here's why: Diodes allow current flow in only one direction, but sound is an AC signal that must move in both directions. Also, a diode will not "turn on", i.e. conduct current in the forward direction, until there is a voltage of .7 volts across it. This means that you will lose .7 volts from your signal and for low-voltage signals, you won't get much out at all.

So sound will be either non-existent or very distorted. In fact, diodes are often used to create distortion effects in older analog guitar pedals.

3) Do you have a schematic for the circuit?

Yes, see the first picture below.

4) Can I add a volume control?

Sure, and there are a couple of options here.

- To add a single master volume control, do something like the second picture below, using 10k ohm stereo audio-taper potentiometers. note: audio-taper and log-taper are the same thing.

- To add individual channel volumes, do something like the 3rd picture below using the same type of potentiometer, stereo 10k audio taper pots.

I don't recommend trying to do both a master volume and individual channel volumes because the signal loss will be too great and would require op-amps or other active amplification.

5) Can I Use it as a Splitter?

Yes, with the mixer as built, you could plug one device into the output jack and then connect multiple amplified speakers, etc via the 4 'input' jacks. You will probably lose some volume and it won't work for headphones very well, just line inputs.

However, if you plan to use it always as a splitter and never as a mixer, you might be better off building it without the resistors. While the resistors are necessary for a mixer, they are not at all required for a splitter.

6) Can I Connect a Headphone to the Output Jack?

Not recommended. This circuit performs the best when driving a high-impedance load. As the load impedance connected to the output gets lower (low impedance = heavy load) the volume drop will get greater and headphones have very low impedance (16 or 32 ohms common).

In other words your headphones will not be very loud, and might be very quiet -- that's just the way it works.

If you want to mix into headphones, build/Buy a CMOY headphone amp. The CMOY headphone amps also come in Altoids tin. Do a search on it, there are tons of plans and people selling them out there. You could even put the mixer circuit in front of the CMOY amp circuit if you built one yourself.

Design Trade-offs

This design does sacrifice signal volume for simplicity, battery-free operation and minimal distortion. This isn't a problem in my experience, since my various car stereos, amplified speakers, etc. have more than enough gain to compensate.

There are some trade-offs you can make to reduce this volume loss. Here are the relationships so you can experiment with factors like number of inputs, resistor value and acceptable distortion:

Volume vs. Resistor Value

The higher the resistor value, the more volume you will lose. The lower the resistor value, the less volume will be lost.

Loading vs. Resistor Value

The lower the resistor value and the more inputs are present, the higher the loading on each device's output.

Distortion vs. Loading

The higher the loading seen by a device output, the higher the distortion. Mp3 players, etc are designed to drive 16 ohm (or so) headphones. However, the distortion will be much higher when driving low-impedance (high loading) headphones than when those same outputs are driving a high-impedance line input (high impedance = low loading) of 1 or 2k ohms.

Load Impedance vs. Number of Inputs

Each additional input device will increase the loading seen by the other input devices slightly, but R (the resistor value) will still be the dominant term.

The load seen by each input device's output is roughly:

Zload = R + (R/N)//Zin

Where:
N=number of inputs-1
Zin = input impedance of aux input or amplifier
// = parallel resistance formula: R1//R2 = R1*R2/(R1 + R2)

A Zload of 1k ohm or higher should give fairly low distortion. Anything lower than that will start to get into increasing distortion levels and anything below the impedance of headphones will be dangerous.