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Old 30th January 2007, 12:25 PM   #1
matzah is offline matzah  Romania
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Default Please help. Begginer question about an amp.

Hi
Recently bought an kit amp. It uses 2xMJ11015 & 2xMJ11016 darlington transistors being a bridged amp (400W output at 4 ohmi, that's the value entered in it's prospect). A TDA7250 is used as a driver for these transistors.
The problem is that I have only a 2x25V at 400VA toroidal transformer. Altrought the 2x15000UF capacitors would give me some extra volts this woun't be enought. How much power can I get using this transformer?

Another problem is that the amp stops itself for about 2 sec. when playing at high volume on output impedances ranging 4-8 ohmi. I haven't yet figure out what are the exact values for this. It isn't thermal related, from what I think. Being a bridged amp from 2 complementary pairs that would normally give a 200W @ 4 ohm output, I think it would be 8 ohms, the minimum impedance that is needed for the amp to operate propperly. However even at 8ohms these 2 sec stops occur sometimes.
Can anyone assist me with some tips.
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Old 30th January 2007, 01:12 PM   #2
Did it Himself
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You can reasonably guess the peak voltage available at the output stage from the voltage rails. Then it's simple ohms law to find the power.

The caps won't give you any 'extra' volts.
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Old 30th January 2007, 03:09 PM   #3
matzah is offline matzah  Romania
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But what about the aprox 2 sec drop at hight power when connected to impedances from 4-8ohm? it would be pretty resonable to assume why this doesn't work for less then 8ohm but why it still does this for 8 ohm or even more, sometimes.
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Old 30th January 2007, 03:12 PM   #4
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Can't really say without further details of the kit or circuit. Maybe it has an overvoltage problem.
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