Subject: Re: Help needed with audio mute circuit
Date: Sat, 21 Sep 2002 16:04:51 +0200
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 21 Sep 2002 14:04:52 GMT
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John Woodgate wrote:
> I read in sci.electronics.design that LY wrote (in <%bLi9.2640$t
> %firstname.lastname@example.org>) about 'Help needed with audio
> mute circuit', on Fri, 20 Sep 2002:
> >> Yes. Several European audio products simply connect a small-signal
> >> transistor across the signal, downstream of a 1 k resistor if the signal
> >> source is low-impedance, and feed a few hundred microamps into the base
> >> to turn the audio off. There is very little non-linearity introduced
> >> when the transistor is 'off'.
> >Can you please clarify this for me. What do you mean by across the
> >signal? Do you mean tie the collector to the signal and emitter to GND,
> >and pump the base to shunt the output to GND to mute the output stage?
> Yes. 1 kohm in series with the signal, then collector to signal,
> emitter to ground. Apply base current to mute.
> Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
> Interested in professional sound reinforcement and distribution? Then go to
> PLEASE do NOT copy news posts to me by E-MAIL!
I've drafted a circuit, can be found here:
One of the main assumptions is that the audio circuit's output DC bias will be
stabilized enough before the +5V rail reaches the threshold value (Vcc trip
point) of the DS1811-10 (4.25-4.49V). After that the DS1811 will allow at least
100ms more to let the coupling capacitor charge (assumed between the DC biased
point and Input1).
To mute additional audio paths, the part R1,R3,Q2 will need to be duplicated,
and the base resistors connected
to the collector of Q1 (the hanging wire).
I have not analysed precisely the distortion induced in mute off state, it
looks like is would be not more than 0.1%.
Of course, R1 increases the output impedance of the device, so it may need to