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Old 31st March 2005, 11:05 PM   #1
Bull is offline Bull  United Kingdom
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Question How to figure out unknown amp wattage???

My uncle gave me a power amp he built from a kit ages ago.

It's 3u 19 inch rack mountable,pretty heavy and has 2 very huge capacitors,4x Hitachi mosfet transistors and a large torridal transformer plus other components.

How can I figure out how many watts in rms it gives???

Per channel there are two metal mosfet transistors.
Hitachi J50 [japan 4J2] protected by 3.5 amp fuse and Hitachi K133 [japan 4H1] protected by 9.5 amp fuse.

So could you tell me how much power in rms per channel and @ what ohms it gives it??

Also the torridal transformer sadly has a tiny piece of the insulation [about a few mm hole] on one of the secondary windings[exposing the copper wire itself]. Can I repair this without replacing the transformer instead???


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Old 1st April 2005, 12:53 AM   #2
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From memory Bull, the Hitachi MOSFETS were 100W case dissipation and as such the amplifier would be no more than 50 to 60W at the outside for 8 ohms/channel.

You really need to test the power delivered into an 8 ohm load as the high on resistance and power supply limitations could impact the power output considerably.
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Old 1st April 2005, 01:26 AM   #3
Leolabs is offline Leolabs  Malaysia
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Or use a DC voltmeter and measure the MOSFETs supply voltage
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Old 1st April 2005, 02:40 AM   #4
anatech is offline anatech  Canada
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Supply voltage may be inaccurate depending on the driver voltages and layout. Best measure into an 8R load.
-Chris
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Old 1st April 2005, 02:46 AM   #5
Leolabs is offline Leolabs  Malaysia
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Measuring supply voltage just a quick and dirty trick to know output power,without the need of applying input signal.
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Old 1st April 2005, 03:09 AM   #6
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That is also a poor indicator because the MOSFET could impose large or small voltage losses depending on the output stage configuration. With 1.7ohm Ron and 8V at current it's significant.
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Old 1st April 2005, 11:48 AM   #7
Bull is offline Bull  United Kingdom
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So I recon its btw 100w-200w per channel.



But how can I repair the torridal transformer??? its got a nick in one of the turns of the insulation on the secondary winding exposing a few mm of copper wire.

I was thinking heat shrinking but I don't wanna unwind the whole transformer. Or electrical tape maybe.... Its not high voltage but can still cause a short maybe.
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Old 1st April 2005, 12:50 PM   #8
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Soundcraftsmen uses the J50/K135 output devices, 2 pairs per channel at they produce 205 watts @ 8 ohms both channels driven etc. 300@ 4 ohms.



Zc
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Old 1st April 2005, 01:00 PM   #9
matth is offline matth  United Kingdom
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If its a kit from the UK with the J50 and SK135 then pennies to a pound of salt its the good old Maplin 150W mosfet modules (see below).
These were optimistically rated at 100W @ 8Ohm and 150W max @ 4Ohms with a max supply of 55-0-55v.

If the transformer windings still have the enamel (spelling?) intact on them then I would not worry to much, just tape it up.
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Old 1st April 2005, 01:17 PM   #10
Bull is offline Bull  United Kingdom
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Yes the modules per channel look exactly like that.
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