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I need to buffer an audio signal (feeding into a variable input impedance), and make it mutable with a digital control. The best thing I can think of is an op-amp (maybe TS971? 3×2 mm SOT23) feeding into an analog switch, like NLAS4599 (3×3 mm TSOP-6), but this requires several external components (example circuits).

Anyone know of anything better? A tiny IC that acts as both a unity-gain buffer and a digitally-controlled mute? Supplies are 4.5 V and ground, signal is biased to half that, and the mute and unmute transitions should be pop-free.

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Would any old op-amp/buffer with a shutdown signal be sufficient? Digikey has TONS of options if this is acceptable. MAX9812 is a SC70-6 package, for example. (Not a buffer. LMV341 is a single op-amp with shutdown in an SC70-6 package.)

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  • \$\begingroup\$ That's a fixed-gain 20 dB mic amp, not a unity-gain buffer, but yes, something like that with an internal switch might work. \$\endgroup\$
    – endolith
    Sep 23 '10 at 18:53
  • \$\begingroup\$ LMV341 looks promising, with a high output impedance in shutdown so that the DC bias will remain unchanged until you turn it back on, assuming it's only connected to a capacitor. \$\endgroup\$
    – endolith
    Sep 23 '10 at 20:20
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    \$\begingroup\$ op amp with shutdown. \$\endgroup\$
    – Kortuk
    Sep 23 '10 at 22:48
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    \$\begingroup\$ @endolith, better yet, a resistor from output to the "zero" level will ensure that the signal goes to zero. \$\endgroup\$
    – markrages
    Nov 22 '10 at 20:33
  • \$\begingroup\$ @markrages: Agreed. Something like this: tinyurl.com/26nb4j2 \$\endgroup\$
    – endolith
    Nov 23 '10 at 19:28
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If you really want zero clicks, then you want to fade to zero instead of shutting off suddenly, because that is still introducing a step change to the signal. Or maybe you could shut off on a zero crossing, but I can't think of an obvious circuit to do so.

I think I would feed the signal into a divider made of a resistor and FET to ground, and put an RC on the gate of the FET so it would switch slowly into conduction. This will introduce a severe DC bias in the output, so it could be capacitively coupled into the opamp. If the DC bias is applied and removed slowly, it won't cause a pop. Remind me and I will sketch the circuit up.

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    \$\begingroup\$ neatcircuits.com/audiosw/index.html has a schematic using a JFET for fading in and out as a mute \$\endgroup\$
    – endolith
    Nov 22 '10 at 20:42
  • \$\begingroup\$ Nice, those circuits are a lot easier to read than my description. \$\endgroup\$
    – markrages
    Nov 22 '10 at 21:09

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