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I'd like to build a simple audio mixer which takes signals from two or more inputs, combines them and outputs to the headphhones or speakers. I followed the other question: Making a Simple Audio Mixer [fig 1], but using 4.7kΩ resistors seems too much to drive headphones. On the other hand, I considered using 20Ω resistors instead, but I'm concerned that the inputs feeding high current to each other may damage them. Is my concern justifiable? What would be an optimal resistance in such case?

schematic

simulate this circuit – Schematic created using CircuitLab

Another option I considered was to stick with the high input impedance and use a summing operational amplifier circuit to provide current for the headphones/speakers [fig 2]. Which op-amp would be suitable here? I'm not aiming for expensive amps with excellent sound quality, it's rather a hobby project. I've considered LM386 but since its gain is 20-200, wouldn't it mean that 1V pk-pk would be amplified to 20V pk-pk, so I'd have a lot of signal clipping?

schematic

simulate this circuit

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  • \$\begingroup\$ You could reduce the gain in the summing stage or add a volume pot after it, then feed your signal to the LM386 if you're worried about clipping. \$\endgroup\$
    – Bort
    Dec 19, 2019 at 13:21
  • \$\begingroup\$ Search for an opamp that can drive the required voltage into a 32 ohm load and runs from a single 5 volt supply is my recommendation in the absence of info on part (a) of your question. \$\endgroup\$
    – Andy aka
    Dec 19, 2019 at 13:21
  • \$\begingroup\$ Several audio PAs are commercially available (either class AB or class D) which would be better than a generic op amp for driving a 32 ohm load. Most have BTL (bridge tied load) outputs which can add 6dB to your peak output while eliminating the 220uF output capacitor. \$\endgroup\$ Dec 19, 2019 at 13:43
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    \$\begingroup\$ You want dirt simple? Tie the grounds from IN_1 & IN_2 together. The 32-ohm speaker goes direct from IN_1 to IN_2. In other words, the "optimal" resistors in your circuit are 0 ohms. Each of your signals have their own volume control?...turn the volume down a bit. A disadvantage - both sources have to be active. Beware of Class D drivers! \$\endgroup\$
    – glen_geek
    Dec 19, 2019 at 14:22
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    \$\begingroup\$ I am not an audiophile but here are some general considerations about the simple passive versions.To make the first OP circuit almost "dirt simple":), remove R3; it is not necessary. Do the same with C1 and C3 (if the amplifier outputs allow it). Thus the two (low resistive) R form a "parallel voltage summer" (divider). The speaker load is grounded, the summer inputs can be weighted (scaled). The @glen_geek circuit is a series voltage summer (according to KVL)... more precisely speaking, this is a subtractor. The speaker load is floating; the summer inputs have the same gain of 1 (not scaled). \$\endgroup\$ Dec 19, 2019 at 18:48

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