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Mute for Tube Preamp
Mute for Tube Preamp
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Old 10th August 2018, 12:11 AM   #1
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Default Mute for Tube Preamp

I've built a 12b4a tube preamp and absolutely love it. I think the final piece of the puzzle is the mute switch upon start up. I have a 30 sec timer switch controlling a signal relay on the output. I'm using the attached design I found from an earlier post. It works perfectly and quietly upon startup, but on shutdown I get a pretty good pop. Not terrible, but would still like to stop that from happening. Just for reference, this is installed after my output capacitors and before the final 1M resistor to ground. Any suggestions?
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Old 10th August 2018, 12:28 AM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by madskaman View Post
I have a 30 sec timer switch controlling a signal relay on the output.
on shutdown I get a pretty good pop.
I assume the relay is normally grounding the output when off, that's how it should be connected.
You need to de-energize the relay coil as quickly as possible when switching the power off, to mute.

If you use a 555 timer, add a reverse 1N4148 diode across the timing resistor, so the timer is reset
quickly as the supply decays. You may need to add another reverse diode or two upstream to
discharge other capacitors also, to make the supply decay quickly upon turn off.

If you use some other circuit for muting, post it here.

Last edited by rayma; 10th August 2018 at 12:35 AM.
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Old 10th August 2018, 01:33 AM   #3
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I抦 not sure the schematic from this timer I抦 using. Attached is a picture of it. Maybe I should swap it out for a 555 type.
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Old 10th August 2018, 01:36 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by madskaman View Post
Maybe I should swap it out for a 555 type.
Probably can't do much with that board. If you breadboard a simple 555 timer
with the reverse diode across the timing resistor, that may mute the turn-off noise.

If that still does not work, use a DPDT type for the power switch, and use the extra
contacts to short the timing capacitor when the switch is turned off. That will do it,
since turning the power switch off immediately resets the timer.

Last edited by rayma; 10th August 2018 at 01:51 AM.
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Old 10th August 2018, 11:55 AM   #5
DF96 is offline DF96  England
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If the switch off pop happens when you flick the switch then the cure is an X-rated capacitor (maybe 10nF) across the mains transformer primary.

If the pop is a few seconds later then you need the muting relay to act more quickly.
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Old 10th August 2018, 12:29 PM   #6
jfetter is offline jfetter  United States
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I did a Microchip AC line zero crossing power loss detector years ago using a MCT6 opto coupler.

For this audio app the Pic would supply power up delay/muting and provide single cycle sensing/mute.
Relays are 'slow' to release on power down so that would still be an issue. Muting must catch it.
Triacs are of course out of the question.

I too need this too for my valve/ss hybrid circuits.
( Heater warm up time and total mute on AC single cycle loss.)

Its easy just have to squeeze in the time.
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Last edited by jfetter; 10th August 2018 at 12:33 PM. Reason: add note
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Old 10th August 2018, 12:38 PM   #7
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Project 104 - Preamp/ crossover muting circuit

This works well with delay and instant cut off during power down
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Old 10th August 2018, 02:29 PM   #8
jfetter is offline jfetter  United States
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I have used the relay based circuits, they are too slow and unpredictable.

It may or may not work 100% of the time.

Strictly a mechanical/magnetic problem. I even tried high speed relays.
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