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Old 5th August 2007, 06:21 PM   #1
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Default DC protection philosophy- need opinions.

Hi
Sorry for covering it once again, but if one has designed the power amp so that at disconnected one rail voltage output stayes close to ground potential, say, below a volt or two, does he need any form of DC output protection?
I guess it is now nothing wrong with rail fuses being tightly tailored to maximum expected current.
How about output relay?

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Adam
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Old 5th August 2007, 06:34 PM   #2
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I'm of the philosophy that you can't foresee or protect against everything, so I build robust amps with no SOA or DC protection. I do use rail fuses, but not of low enough values to prevent anything other than complete destruction of my living room. In 30 years I've never lost a speaker to an amp failure, in fact, I've never had a catastrophic amp failure anyplace other than on the test bench. Had quite a few there though! OTOH, I have had problems with bad relay contacts on commercial amps that used them. I've also seen lots of commercial amps with SOA protection that blew up anyway. That's not to say the theory is bad, just the usual implementation.
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Old 6th August 2007, 03:30 AM   #3
anatech is offline anatech  Canada
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Hi Adam,
I have an opposite feeling on the subject.

I'll start by stating that output relays are easily replaced at reasonable cost. Output fuses are a bad idea.

To design an amplifier without some way of disconnecting or protecting the load (speaker) is irresponsible in my view. Some amplifiers can shut down everything quickly (Carver for example - often only one output gone). Speaker relays are useful for muting thumps and protecting the speaker. You need one rated for the energy you are interrupting. You can also short the output to ground with a triac.

Whatever you do, do something. The only time I don't worry about protection is when I am designing the basic amplifier. Protection happens after I get the rest figured out.

Quote:
In 30 years I've never lost a speaker to an amp failure, in fact, I've never had a catastrophic amp failure anyplace other than on the test bench.
Since I worked in audio service, I saw a constant stream of dead things. Some did cause speakers to ignite. So this is a very real possibility. Very few have expired on my bench thankfully.

-Chris
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Old 6th August 2007, 05:42 AM   #4
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Hi Conrad...

Like anatech, I'm a tech. I see lots of blown gear, and very few bad relays. A speaker output relay is generally one of the most reliable parts of an amplifier in instances where they are utilized.

As far as your statements about relays, contacts can become worn or even pitted (or fused!) when abused or installed in an amp that has been torched a couple of times, but they are easy to source and replace. Thank God most manufacturers have not adopted your viewpoint about thinking their gear is so bulletproof that they are unnecessary.

...not to mention how handy they are for a nice turn-on delay, and immediate disconnection on power-down.

Not being able to foresee or protect against everything is not a very strong argument for making no attempt at all. I wear a helmet when I ride my motorcycle, and a seat belt when in the car...they may or may not save my life if worst comes to worst, but they improve my chances. Same philosophy ought to apply to DC and SOA protection.
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Old 6th August 2007, 10:44 AM   #5
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Thank you for opinions.

Conrad
Quote:
That's not to say the theory is bad, just the usual implementation.
Yes, of course


Anatech
Quote:
To design an amplifier without some way of disconnecting or protecting the load (speaker) is irresponsible in my view.
Generally I agree, but I am asking, because a failure did really happen to my amp, positive rail got disconnected atfer some transport (Polish roads ) and only symptoms were a bit of hum and DC below 2V. Screwdriver cured the problem. Topology is fully symetrical. I guess that disconnected rail voltage is kind of worst case failure in terms of DC.
So my question is: in an amplifier designed THIS WAY to stay close to ground even with disconnected one rail voltage do I still need to worry about speaker protection?

Quote:
Output fuses are a bad idea.
I never used one at output. Only on rails.

Adam
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Old 6th August 2007, 11:40 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I'm agreeing with the majority.
An output relay AND rail fuses seems the safer way to go compared to nothing at all.
Once that output relay has a control circuit, with delayed start and instant off as well as DC detect, it takes just one more relay to incorporate an input mute function from the same control circuit.

After that tiny bit of circuitry is added then consider whether short circuit and/or IV protection is necessary.

BTW,
I have now tested full power from two different power amps on rail fuses that are far too small, they survived.
eg1. 311W into 4r0 on F3.1A, that's 12.5Apk through a F3.1A fuse.
eg2. 453W into 2r0 on F5A, that's 21.3Apk through a F5A fuse.
I wonder just how low these fuses can go to avoid nuisance blowing on full power music?
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Old 6th August 2007, 12:05 PM   #7
Nordic is offline Nordic  South Africa
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Looking at the datasheet, for a common fuse, holds a few answers
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File Type: gif fuses.gif (30.6 KB, 313 views)
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Old 6th August 2007, 02:36 PM   #8
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For a commercial product, you guys are absolutely right. My diy motto is "I built it, so I can fix it." That includes the speakers too. Now, if I had very expensive speakers, I might feel differently. The motto has served me well, since I very rarely have to fix anything. More parts usually equals less reliability. As for the benefits of relays, I'll agree, but they tend to change models too often and by the time the contacts become unreliable, an exact replacement might be hard to find. On the higher end amps with more open layout, they're easy to replace, but I've also seen amps where they were inaccessible without deconstructing half the amplifier. Being basically a contrarian, I've also used speaker fuses successfully, but you have to choose wisely.
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Old 6th August 2007, 03:45 PM   #9
KISS is offline KISS  United States
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Let's break things down in a few categories:

Commercial:
SOA protection. Fuses prevent catastophies.
Consumer:
You have to make it so fuses "can't be upgraded". DC protection.
Knowledgeable audiophile:
A simple approach will do. AGX speaker fuses.

On the bench I was testing a Leach amp with a 20 Amp secondary using a CVT transformer. The cap ratings were not up to snuff and one shorted blowing the rail fuse. There was nothing to shut down the AMP.

I would advocate that if one rail fuse blows the speakers disconnect and the amp shuts down. I do think clipping indicators are usefull as well, but DC protection can be, in esscense shutdown due to clipping.

Thump and pop suppression are essential and they can be integrated into the design.

I fixed a commercial disco amp, about 250 W/channel. The speaker leads were shorted by not paying attention to how they were attached. Basically a bus bar across the speaker terminals. Resistors were puddled on the circuit board. 4 other shops would not touch it. I did modify it, such that when the DC protection kicked in the added speaker relay turned off, I think.
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Old 6th August 2007, 03:51 PM   #10
anatech is offline anatech  Canada
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Hi Adam,
There is a simple protection for rail reversal if one rail goes open. Place reverse biased fuses from supply (after the fuse) to ground. Protected this way a supply rail will only ever go one diode drop reversed.

I have tested some amplifier designs this way and it works well. Also, if your voltage map stage runs off it's own supply, your output voltage will tend to remain close to 0 VDC if a rail fuse blows. If you run your front end off it's own regulator (good idea for sound quality), this happens naturally.

-Chris
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