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Old 22nd January 2006, 08:03 PM   #1
AndrewT is offline AndrewT  Scotland
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Question amp blown fuse = output offset ?

Hi,
when my power amp blows one rail fuse the output offsets to rail voltage.
Each time this has happened I have had the speakers disconnected. No damage yet.

I have read that some amps can hold zero offset when a fuse blows.

How can one recognise a schematic that can achieve zero offset?

Can a schematic be altered to achieve zero offset?

My only other alternative seems to be to add a dual crowbar to blow the good fuse on fuse failure trigger.
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Old 22nd January 2006, 08:31 PM   #2
Netlist is offline Netlist  Belgium
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Andrew,
You bring the story just as the blowing fuse is the most normal thing in the world. Oops…he did it again.
A reliable amp isn’t supposed to blow a fuse unless you abuse it by either playing too loud or connecting a too low impedance load or…well…we have Cola and beer but I won’t assume this abuse for the moment.
I have no idea what amp you use but converting it is probably not the easiest task.
You could think of a DC protection but usually they are too slow and damage has been done before they kick in.
If I were you, I would ask myself why the fuse blows and if you found the cause, better fix that.

/Hugo
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Old 23rd January 2006, 06:26 AM   #3
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Hi Andrew,

My Simple Killer Amp holds zero if it blows a fuse (output MOSFETs drains). I have to test for such things. Not easy to design for.

Having said that there may be unknown conditions where it won't but I'm still waiting for them.

Cheers,
Greg
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Old 23rd January 2006, 06:39 AM   #4
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Andrew, why do you blow fuses in the first place?
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Old 23rd January 2006, 07:17 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
I know why it blows fuses.

Each time it is when I attach a lead to the RCA input.

Presumably, it is the bad design of the connector that allows the pin to make before the ground that promotes some high frequency oscillation and instantly blows the -ve fuse.
The output goes to +ve rail. I have checked by removing the +ve fuse and output goes -ve.

This has happened twice now even though I have added an extra RF filter in front of the first stage of amplification. It already had a 50kHz (3.2uS) filter before the second LTP stage.

I normally switch off before changing cables but I short cut this process when carrying out testing.

My worry is that a careless duster might just interfere with a connector and I will lose an expensive speaker

I have four of these monoblocks which each have a relay on the output. As Netlist has said a DC detect to overide the relay will cut off but 70Vdc for 500mS is a lot of heating in a voice coil
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Old 23rd January 2006, 07:24 AM   #6
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Does you input have any pulldown resistor or is the input floating when it's unconnected?
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Old 23rd January 2006, 07:29 AM   #7
dnsey is offline dnsey  United Kingdom
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If you want to know whether there's any offset when the fuse blows, why not measure and find out?
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Old 23rd January 2006, 07:29 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi Peranders,
It used to have 4k7 to ground after the 10uF electro and 3k3 to 5534 in put pin.

It now has 51k to ground after 10uF and then 1k0 to input pin with 470pF to ground.

But no earthing resistor before the 10uF DC blocking cap.
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Old 23rd January 2006, 07:32 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi Dnsey,
you misunderstand the problem.
The unwanted output offset is precisely why I started this thread.

I have already confirmed that it goes into offset whichever fuse is removed.
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Old 23rd January 2006, 09:13 PM   #10
anatech is offline anatech  Canada
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Hi AndrewT,
Is there a link to your schematic?

Consider using a switch to short your inputs to ground, that's more convenient than shutting the amp off to change I.C.'s.

-Chris
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