From: Kevin McMurtrie
Subject: Re: Help needed with audio mute circuit
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Tue, 17 Sep 2002 08:03:44 GMT
NNTP-Posting-Date: Tue, 17 Sep 2002 01:03:44 PDT
In article ,
"LY" firstname.lastname@example.org> wrote:
>Seems easy, but I've been wrestling with this for a couple of days now.
>to call in the reinforcements :-)
>I need a circuit that would mute the output of an audio circuit, upon power
>on AND power off. Muting on power on is the easy part, but muting it during
>the power off sequence is perplexing. Once the power is turned off, the
>output IC hangs around for a while (10-20ms), then WHAM it crashes to ground
>from its DC bias voltage, thus causing an annoying thump on the AC coupled
>The ideal way to fix this is to use a relay and drive the coil with the
>power. Thus when the unit is unplugged, it would just disconnect the output
>(or short it to ground, depending on the flavor of the relay NC or NO).
>Unfortunately, relays are too bulky and expensive for this consumer
>I was thinking about connecting the output to ground with a JFET. The
>problem is that I don't have the bias voltage necessary to drive it. Only
>have 0-5V. Also, the idea of exposing a JFET to an output isn't very
>appealing to me.
>Hmmm..... am I missing something? Is there a really easy to way to get this
>done with a few bjt's? Any help would be greatly appreciated!
DC biases and AC coupling gets to be such a mess. The problem is that
you have multiple derived grounds and they aren't tracking the power
supply voltage at the same rate. Just trying to keep the power supply
ripple and intermodulation distortion out of the audio can become a
nightmare. Muting power on/off spikes is non-trivial.
All of these problems disappear entirely with a split power supply. You
get a real ground as a single reference for all signals, end to end. It
also eliminates heaps of capacitors, resistors, and regulators.