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Old 17th March 2004, 01:42 AM   #1
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Default Protecting speakers against dc with mute?

Heres what I want to do:

I have an LM3886 amplifier, but am worried about DC on the speaker in case something goes wrong with the input resistor or something else goes wrong. Can I make the mute mode activate the second there is DC on the output?

Thanks,
Mike
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Old 17th March 2004, 07:05 AM   #2
djk is offline djk
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"Can I make the mute mode activate the second there is DC on the output?"

And this would do what?

This is a crowbar:

http://www.diyvideo.com/forums/attac...p?postid=49718

If the amplifier is FUBAR the mute circuit or the SPIKE circuit will not protect the speaker.

If you did something to cause the DC that you can correct, the SPIKE circuit in the chip will protect it from the crowbar.
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Old 17th March 2004, 07:34 AM   #3
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Default Re: Protecting speakers against dc with mute?

Quote:
Originally posted by soundNERD
Heres what I want to do:

I have an LM3886 amplifier, but am worried about DC on the speaker in case something goes wrong with the input resistor or something else goes wrong. Can I make the mute mode activate the second there is DC on the output?

Thanks,
Mike

Try out UPC1237. This is a simple speaker protection circuit.

http://www.promelec.ru/pdf/upc1237ha.pdf
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Old 17th March 2004, 08:18 AM   #4
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Default Re: Protecting speakers against dc with mute?

Quote:
Originally posted by soundNERD
Heres what I want to do:

I have an LM3886 amplifier, but am worried about DC on the speaker in case something goes wrong with the input resistor or something else goes wrong. Can I make the mute mode activate the second there is DC on the output?

Thanks,
Mike
You don't have to worry about the input resistor. I think your fears are unfounded as long as you have done the LM3886 connection right.

If you have a 500 W amp with discrete parts I thinks it's more need for DC-protection.
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Old 17th March 2004, 11:41 AM   #5
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No, about the input connection, The resistor to ground is soldered across the end of the input wires, not the board. So, if the wire would break, then the input is floating, and the output on the speaker goes to about 20V. So, when that happens (which it has) I want the mute to flip on to protect the speaker. I know if the amp fails, then the mute pin wouldn't stop the DC, but that I am not worried about.

Actually, if the speaker has a fuse, will it blow if there was any large voltage on the speaker?
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Old 17th March 2004, 03:49 PM   #6
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Default Mute circuit

How about this, connect a relay between the mute pin on LM3886 and the negative supply and activate the relay (to open it) whenever you have a lot of DC show up on the output, like 20V.

Use a relay activated by a high voltage (12-20V) and just connect the ouput to this relay The relay will certainly be activated by 20V, just make sure in normal operation you don't exceed the relay activation voltage. I suppose you could drop the voltage with a resistor to use a more sensitive relay. Now resetting the relay may be interesting but if you choose the right type of relay it may reset itself when the voltage goes below the "set" threshold.
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Old 17th March 2004, 08:42 PM   #7
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ok i am lost, how do i connect it again?
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Old 17th March 2004, 09:30 PM   #8
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Quote:
Originally posted by soundNERD
No, about the input connection, The resistor to ground is soldered across the end of the input wires, not the board. So, if the wire would break, then the input is floating, and the output on the speaker goes to about 20V. So, when that happens (which it has) I want the mute to flip on to protect the speaker. I know if the amp fails, then the mute pin wouldn't stop the DC, but that I am not worried about.
First, why should the wire break? I assume that your amp looks tidy inside. It's not very good the either of the inputs float but see to it that this never happens. If you are worried about this pulldown resistor I'd suggest that you connect it closer to the IC. My advice to you is to make good connections everywhere. The LM38xx is very safe and hard to destroy _if_ it's connected properly.

Quote:
Originally posted by soundNERD
Actually, if the speaker has a fuse, will it blow if there was any large voltage on the speaker?
If would probably not protect for harmful DC
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Old 20th March 2004, 11:41 PM   #9
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"If would probably not protect for harmful DC "

I've replaced enough relays with welded contacts to know that they can't protect against DC either.

Good commercial PA amplifiers have both a relay for quiet on-off use, and a crowbar for protection.
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Old 21st March 2004, 12:00 AM   #10
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Quote:
Originally posted by djk

I've replaced enough relays with welded contacts to know that they can't protect against DC either.
Interesting point, should we understand that as the relays have
been "welded" during normal use (delayed turn on etc.)
so they don't work when eventually needed for protection?
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