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Power amplifier safety modifications

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diy didi

I have a Large power amplifier that needs repairing for a client of mine.
It's a no-name brand stereo power amplifier and uses TO-3 lateral mosfets.
It has a large 500VA toroidal power transformer as well as a smaller 60VA toroidal transformer for the relay speaker protect circuits.
The builder didn't add any fusing except for a large 20A trip-switch wich doubles as the power-on switch too. I'm afraid the switch is too big in my opinion and will not trip during a fault situation. Removing it is not an option. Too much metal work.
It at least has a soft start circuit built in for the large transformer.
I would like to make it safer by adding a fuse on the mains side.
My question is does each transformer need its own primary fuse?
Also how does one go about sizing the fuses? I assume they will all be slow blo.
Mains where I am is 230V.


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Losing the protection circuit power without the main power could be a problem so I would suggest using only one fuse.
For 500VA at 230V, the maximum current would be 2.2A so a 2.5A fuse should work.
Yes, slo-fuse would be best to prevent the power-on current surge (due to charging the filter capacitors) from blowing the fuse.

Nigel Goodwin

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For 500VA at 230V, the maximum current would be 2.2A so a 2.5A fuse should work.
That's OK for a pure resistive load, but not for the real world :D

Having a soft-start circuit makes it MUCH better, but even so I would try a 5A fuse - and see how often it blows, then work up from there.

Without the soft-start you'd need a MUCH, MUCH higher fuse - in the UK (with fused plugs) you would need a 13A fuse in the plug for it not to regularly fail.


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The goal is generally "the consumer can't muck with it" and "DC protect on the speakers".

When you decide to complicate it a bit on an amp I built, stuff gets messy. I wanted to fuse each leg of the power supply. (4 x 50 [email protected] 3A), the 120 V mains @ 3A and the speaker at 3 A (AGX fuse). The protection supply is fused off the mains,

If I loose any power rail, I loose all of the rails and I loose a metal oxide resistor. My "protect" isn't "consumer grade", but it works. I revered the NPN and PNP transistors and only lost the resistor. It includes exponential ramping of the audio and slow turn on because of 50,000 uF of capacitance and super fast turn off.
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