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Suppose I have an audio signal source.

I want to connect and disconnect it to amplifier electrically (for example controlled by another source, like microcontroller, sensor, 555, etc). I don't want to turn the signal source on or off itself, as it takes some time to start, I just want to connect/disconnect its output. How can I do that?

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  • \$\begingroup\$ Use the output impedance of the source; short the output to ground with a transistor. \$\endgroup\$
    – jippie
    Jun 23, 2012 at 8:48
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    \$\begingroup\$ @jippie - the output impedance may be very low, like from an opamp, and might not like to be shorted. This is not like a 47k output impedance is required to get a maximum power transfer to a 47k load. The load may be 47k, but the output impedance should be as low as possible to get the maximum voltage (not power). \$\endgroup\$
    – stevenvh
    Jun 23, 2012 at 11:48
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    \$\begingroup\$ @stevenh, that's a good point. I imagine that adding, say, a 470 ohm resistor in series with the audio output before the switch transistor will protect the output. The input impedance of most amplifiers is high enough that this series resistance will not adversely affect the performance. \$\endgroup\$ Jun 23, 2012 at 12:38
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    \$\begingroup\$ For muting with a transistor, you need to get a special muting transistor like 2SC3326 that has high reverse hFE, since the signal will go below 0 V, and the emitter is grounded, flipping the transistor upside down. Generally the datasheets show a 1 kΩ series resistor before the muting transistor. Leave the base floating in the unmute case so that the collector doesn't conduct when the signal goes negative and distorts the signal by attenuating differently at different amplitudes. \$\endgroup\$
    – endolith
    Feb 20, 2014 at 15:53

1 Answer 1

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You can use an analog switch like the 74HC4066, or a better specified one from Analog Devices (mainly lower on-resistance).

enter image description here

Another solution used in some Premium HiFi amplifiers is a reed relay. These are electromechanical relays, so they need some power to switch, the analog switches can often do with 1 \$\mu\$A or less.

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    \$\begingroup\$ Thanks for the answer. Could you provide any resource i could read about principles of working of such switches. I probably would use such solution, but i'm more interested in principles how it's done. \$\endgroup\$
    – miceuz
    Jun 23, 2012 at 12:50
  • \$\begingroup\$ @miceuz - the word is transmission gate. I found a brief description here, but while you can use it as analog switch, most references I found are about their use in CMOS logic. \$\endgroup\$
    – stevenvh
    Jun 23, 2012 at 13:03
  • \$\begingroup\$ Here are some guidelines on how to use them as analog switches: geofex.com/article_folders/cd4053/cd4053.htm pic101.com/audiosw/index.html signals can't exceed the voltage rails, split supply is easier than single supply, zero-voltage switching at virtual grounds is easier than switching voltages, etc. \$\endgroup\$
    – endolith
    Feb 20, 2014 at 15:49

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