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Old 26th March 2003, 05:32 AM   #1
diyAudio Member
 
Join Date: Feb 2001
Location: Mars
Default Rail Voltage Failure Question

Hypothetical question.

Class AB amplifier, lets say 300 watts
into 8 ohms, 4 ohm stable.

Rails voltage is +75V / -75V
and fused on each leg.

What sequence of events would
happen if you lose a rail voltage ?

Suppose the -75V failed and went
to zero volts or open circuit,
but the +75v is ok.

What would be the typical behavior
of the various stages, input, vas,
output ?
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Old 26th March 2003, 07:34 AM   #2
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Join Date: Mar 2003
Location: Northwest
Most amp topologies I'm aware of do one of a couple things with a rail failure. It depends on the topology. They often distort heavily but manage to maintain DC stability. There may be a transient at the instant the fuse blows (I can't recall that detail) but there's no sustained DC at the output--it just sounds really bad if it will play at all.

I have seen a few amps that will go "open loop" and end up slamming into the other supply rail so you get a large DC voltage on the output. Hopefully these amps have either some sort of DC protection circuit, or at least speaker fuses which will hopefully blow before your woofers (if the DC pulse didn't already damage them with that big of amp).

I've never seen an amp fail (hurt itself) from losing a supply rail.

One common trick with some designs is to fuse the output stage separate from the VAS and front end. So if you overdrive the amp, you lose only the output stage and the rest of the amp (hopefully) keeps its composure.
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Old 26th March 2003, 09:40 AM   #3
Freddie is offline Freddie  Sweden
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Join Date: Oct 2000
Location: Sweden
Quote:
One common trick with some designs is to fuse the output stage separate from the VAS and front end. So if you overdrive the amp, you lose only the output stage and the rest of the amp (hopefully) keeps its composure.
Well, I've had a 200W Class AB amp where the output stage was fused separate from the front end and VAS.

When the outputstage blew up, the fuse at the negativ rail blew but not the fuse at the positive rail. This resulted in 60V at the speaker so the speaker also blew up

So for now on I like to have some sort of DC protection.

/Freddie
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