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Old 2nd October 2007, 05:49 AM   #1
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Default Muting preamp's unused inputs ??

I want to mute any unused inputs to avoid having these extra signals
"floating" around inside my DIY preamp.

1) Would a relay be energized, to create a mute, or would the "off" of the relay, create the mute ??

2) Is it common to ground inputs as a means of muting ?

...My selector switch (non-shorting) has extra decks, for turning the relays on/off.

(maybe you have a link of a diagram ??)

thanks,
=RR=
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Old 2nd October 2007, 12:18 PM   #2
Gordy is offline Gordy  United Kingdom
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If the preamplifier is easy to modify then the ideal solution is to switch the signal and ground of the selected input, while leaving all of the other inputs (and their grounds) disconnected.


If you short unused inputs to ground then be careful...

... if you short them directly to ground then you may overload the output stage of the source component if it is powered-up.

... if you short to ground via a resistor then any voltage received will cause a current which may modulate the ground at that point if the ground impedance is not *very low*.


Good luck.
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Old 2nd October 2007, 03:01 PM   #3
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I follow Gordy:
Use isolated RCA's and disconnect both signal & grond from unused sources.
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Old 2nd October 2007, 04:32 PM   #4
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OK, none of that "shorting to ground" stuff.
This preamp is not assembled yet.
...I'll just "disconnect" (relay) the inputs as close as I can near the jacks.

(I will be using Rod Elliot's power-up/down muting relay as well. It works great in my other preamps to avoid pop/thumps at power-up/off.)
http://sound.westhost.com/project104.htm

I'm going to think out-loud...

At power-up, all inputs are "disconnected" except for the one set by the selector switch...
.... I would I use "normally open" relays, and the selected relay is energized (closing the contacts) thereby making that input active.

Does that seem correct ?

=RR=
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Old 2nd October 2007, 04:43 PM   #5
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Another "soft" mute FET based circuit:

http://www.edn.com/article/CA468422.html
http://www.edn.com/contents/images/468422f1.pdf

..may be overkill for my needs.

=RR=
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Old 2nd October 2007, 04:48 PM   #6
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If you find 4 pole relays, excellent. If not, use 2 DPST for each input.
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Old 2nd October 2007, 08:18 PM   #7
Gordy is offline Gordy  United Kingdom
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Quote:
Originally posted by redrabbit


I'm going to think out-loud...

At power-up, all inputs are "disconnected" except for the one set by the selector switch...
.... I would I use "normally open" relays, and the selected relay is energized (closing the contacts) thereby making that input active.

Does that seem correct ?


That is the simplest way and it will work.
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Old 2nd October 2007, 10:38 PM   #8
sandyK is offline sandyK  Australia
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Default Muting preamp's unused inputs

"If the preamplifier is easy to modify then the ideal solution is to switch the signal and ground of the selected input, "

These days many DVD/CD players use switchmode power supplies. The cases of these items have typically 80-120VAC (240V system) on them due to SMPS front end filtering. Although not dangerous,it results in a low level hum if using an earthed amplifier.This will degrade S/N of the whole system , even if that input is not selected. Switching the earth side of all inputs will stop this from affecting the other inputs which are not using equipment with SMPS supplies.
I have been switching the earth also, on all inputs for many years, and you can readily tell which inputs have higher hum levels.
( Reference- DVD Players:How Good Are They For HI-FI Audio-by Mauro Grassi,Silicon Chip Magazine October 2007)

SandyK
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