Need a bit of help (power amplifier)

[Bikerman]

I'm repairing a chinese amplifier for a subwoofer and don't get it

The amp powers the internal speaker and you can connect (in parallel) an 8 ohm 300W sub.

So I assumed that the amp would give 600W(ish) in 4 ohm, so that the marking on the bok fits (8 ohn 300W external sub)

But the capacitors witch is faulty was 100V (all of them blown), the transformer gives 81V-0-81V witch is about +/- 115V - should be enough to 1500W or there about?

or am I completely wrong?

 

[Hero999]

Is 81VAC the voltage you've measured of the transformer?

115V into 8R will give a theoretical maximum output power of 827WRMS.  You've calculated the peak power which is double.

In reality, the actual power output will be much lower as there will be losses in the rectifier, driver transistors and the voltage across the capacitors will sag under load due to them being discharged and their ESR. If the maximum output voltage is 99V, under full load conditions, the RMS output power into will be 600W. In reality the maximum output voltage will probably be even less than 99V so I doubt it'll really deliver the full 600W.

You're right about the capacitors being underrated, I'd suggest replacing them with 160V or higher, with has low ESR and as higher temperature rating you can get and them not being too large to fit on the board.

 

[Bikerman]

Ahh yes, thankx - Peak power 


Ok, but the 81VAC (600W ish 8R) still seems high to me, as it says 300W RMS 8R on the back, and the external speaker is parallel connected to the internal speaker...

 

[Hero999]

It sounds reasonable to me considering what you've measured is the off-load voltage before all of the losses in the rectifier and driver transistors.