Fun, easy project from Instructables. I had it hooked up in my car but some jerk stole it - need to make another one.

Bill of Goods

• Altoids Tins
• 3.5MM Mini Stereo Panel Mount Jack Connector
• Resistors - between 1k and 10k ohms

Schematic

Signal Math

Load seen by each input device

$Z_{load} = R + \frac{R}{N} \parallel Z_{in}$

Where:

• $R$ = mixing resistor value (1kΩ recommended)
• $N$ = number of other inputs ($\text{total inputs} - 1$)
• $Z_{in}$ = input impedance of the amp/aux input
• $\parallel$ = parallel resistance: $A \parallel B = \frac{A \cdot B}{A + B}$

Example

Where: 4 inputs, 1kΩ resistors, 10kΩ aux input

$Z_{load} = 1000 + \frac{1000}{3} \parallel 10000 = 1000 + \frac{333 \times 10000}{333 + 10000} \approx 1322\,\Omega$

Volume loss

$\text{Loss} \approx 20 \log_{10}\!\left(\frac{1}{N_{total}}\right)$

Inputs	Loss
2	−6 dB
3	−9.5 dB
4	−12 dB

Resistor value tradeoffs

$R \uparrow \implies \text{isolation} \uparrow,\; \text{volume} \downarrow$

$R \downarrow \implies \text{volume} \uparrow,\; \text{distortion risk} \uparrow$