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I'm working on a low power (portable) audio mixing console, fed by audio signals from MP3-Players and mobile phones.

There should be an option to monitor the sound, but as soon as I route one sound from the op amp's imputs to a headphone, it gets attenuated in the mix (of course).

I thought about having a monitor switch that is DPDT and can switch between a dummy-headphone-load and the actual headphones. Is this possible?

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Is this mixer supposed to be completely passive? If not, why not have a buffer amplifier for the headphones? – Dave Tweed Oct 11 '12 at 21:28
Dave is correct: there should be a seperate headphone amp with a very high input impedance, driving the headphone socket. – pjc50 Oct 12 '12 at 10:19
I would like to run it from a small battery. Because of that using the output power from the input sources should be used to drive the monitor headphones. (This will be mostly earpieces with 16Ω or 32Ω) – superno Oct 12 '12 at 18:11
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@superno: So what's the battery for, if not to power buffer amplifiers inside your mixer? You're going to have to give us a lot more detail about your circuit and what you're trying to accomplish before we can offer any useful advice. – Dave Tweed Oct 12 '12 at 20:42

2 Answers

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A simple dummy-load to simulate a pair of headphones would be a plain-old resistor.

Most headphones are 32Ω-600Ω, though occasionally a pair of "Phat Bass 30000 ultra super mega" or similar headphones will use 16Ω or even 8Ω drivers, to try to get more power from whatever device is driving them, to put out more volume.

If your system is passive, or cannot drive a 16Ω load without the output level dropping, You will have to place at least a unity-gain buffer in series with your headphone output. There is really no avoiding it.

I would recommend just taking 2 16Ω resistors, and soldering them across each channel of a TRS plug. That should work fine for verifying your circuit will work with ~99% of the headphones on the market.

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mixers are usually "unbalanced line impedance" = 10 kOhm headphones are usually 4 ~ 50 Ohm You need a buffer for monitor out that is independent on of any controls. A dummy load is possible it. Reed relays work well for this.

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I'd really like to know how a dummy load would look like. A simple resistor seems to be not sufficient. (The switching itself i'd like to do with DT switches or bistable relais) – superno Oct 12 '12 at 18:14
I think your issue is that the headphone buffer is loading the mixer. But without a detailed block diagram, I can only guess. If you wanted to maintain constant loading I was suggesting a dummy load is possible, but I cannot assume it is necessary. Do you have a separate buffer for headphone monitor? – Tony Stewart Oct 12 '12 at 18:29

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