4 channel amp power supply?

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Hi folks,

I have a question which hopefully you can help me with.

I am designing a 4 channel amplifier which should be capable of 100 watts into each channel at 8 ohms. I have designed the amplifier to use the Avel Y236903 800VA 40V+40V toroid, but I realize now that the DC voltages into the amplifier rails is going to be way too high. I am measuring 49.5 volts from the secondaries (spot on, btw) which according to my calculations will rectify to approximately 70 volts!

The amplifier is a standard class AB design using complementary MOSFETs (IRFP240 and IRFP9240s to be exact), and for the power output I want I should need a rail voltage maximum of around 40 volts peak for 100 watts into an 8 ohm load.

My question is whether or not this toroid would be acceptable, or should I choose a toroid with a lower voltage and higher current. If I decide to stick with the toroid I have, what would the long-term consequences be? I assume the limited output current of the toroid would simply cause a large voltage drop on the output and would otherwise run hot, but so long as the voltage doesn't drop below the voltage needed for continuous power output, could I get away with having too high a voltage on the rails?

Thanks for you help!:)
For 70V you will want at least 3 output transistor pairs.. to be safe.
By this I'm guessing you mean multiple transistor pairs per channel. With 4 channels, that would be a huge amount of mosfets. Not only would mounting all that be hard, but at around $6 a pop, 24 transistors would cost roughly the same as a proper toroid.

I've decided I'm going to return the toroid to parts express and I will replace it with this: http://www.toroid.com/standard_transformers/rectifier_transformers/solid_amps_pn2.htm#782.302

The additional current should be quite useful to ensure close to full power output from all channels despite the lower voltage.
"Mod" the transformer

I was in the same situation last year when I needed a transformer for my amp. I had 75V caps but my transformer was an isolation unit (60+60). What I did was power it up alone and use my multimeter to poke the secondary wires until I found a better place to tap into the secondary windings. I now have +/-52volts which is fine after rectification. The problem with adding primary windings is that they are almost always the inner winding, so you can't get at them. Besides you would have to add a lot of windings for a few volts decrease in output. Easier to modify the secondaries.
Modifying the primary winding is dangerous - you could easily compromise the safety of the transformer.

Bear in mind that if you unwind the secondaries, you are no longer making efficient use of the transformer core as the windings will no longer be evenly spaced over the core.

Personally, I'd just sell the transformer and get the right one - or keep the other transformer and have an excuse to build a higher powered amp or subwoofer later :)
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