muting transistor>relay,can i do this? - diyAudio
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Old 13th August 2004, 04:40 AM   #1
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Join Date: Aug 2004
Location: Halifax N.S
Lightbulb muting transistor>relay,can i do this?

i have a cheaper denon tuner i picked up for almost nothing recently,alltho it works well and doesn't sound all that bad the mod bug is biting

this unit,like many,uses a couple small transistor's across the outputs to mute the output by shorting it to ground...normally removing these is a good thing with regards to sound quality...but i want to keep functional muting....what to do??

so i got to thinking of the best way to do this..obviously a small relay is the best choice and is how it's done in some pricier units...so now i had to figure out how to do this....assuming i can find a suitable power supply tap to run the relay coil...could i simply install a "daughter board" containing the relay[s]and any required supporting devices with the relay contacts wired in where the original transistors short to ground...but keep the original muting circuit essentially intact by moving the final transistors[or a suitable substitute for this purpose,i haven't looked to see what's used yet] out of the audio circuit to the daughter board and then use them to switch the juice to the relay on/off instead of just shorting the output???

hopefully those familier with this common type of mute circuit can follow what i just outlined,if so is there any reason why this wouldn't work?or why it wouldn't work better than the original circuit?the original circuit will be kept more or less intact and alltho physically re-arranged a bit it should still function same as always..the only difference being that instead of the final transistor shorting the signal to ground it will switch a relay that will do the short instead..hence removing the transistor's from the audio circuit and keeping the mute function....but maybe now with a little mechanical "click"

i am not a tech,or even a well informed amateur,i know enough to be dangerous...but this seems like it should work...anyone have any comments? or maybe better or alternative ideas?
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Old 20th August 2004, 12:22 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Default muting relay

Hi. My first contribution so please excuse any little mistakes but tell me about the big ones.
Muting the output to ground will only work if there is adequate current limiting to prevent damage to the output transistor/s.
The hi impedance shorting transistor probably has no effect to the sound quality when effectively open circuit. If the transistor was passing the signal to output then it would affect quality. In the shorting mode quality is not an issue.
The relay idea is good, use a 2pole double throw. connect the output terminals to common. the signal to normally closed and the ground to normally open. this will short the output terminal to ground and leave the signal feeding an open circuit. The signal amplifier should be safe feeding open circuit, valves do not tolerate this. It will also create a short to the next input making it quieter and preventing crosstalk to an active channel.
Do you know how to wire the relay to operate on a high (rated ) voltage to switch on and then to reduce to a lower holding voltage when active? It keeps it cooler especially if active for a long time and reduces loading on the relay transformer. I do not know how to import diagrams, maybe someone else could help. best of luck
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