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Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
Mestre Racerxdl wrote ...
Ok, My amp needs Symetrical 55V to do 400W per channel @ 4 Ohms, with 40V RMS I get a 56V peak at the ouput. After the bridge I get 55V. Ok no problem until that. But when I get near the 400W output, the voltage at capacitors drop to about 44V, so I never will get that 400W output. I can buy a 55+55 V Transformer, but I will get a 70VDC at ouput, that will blow a few things on my audio amp.
There's a reason why the voltage drops. With a AC sine wave, the voltage that you transformers rated for is their effective AC voltage, not their peak voltage. To find the peak voltage, you need to multiply the effective AC voltage by the square root of 2, which is about 1.414. So with your 40+40 transformer, your peak voltage is, you guessed it, 56V. The filter capacitors after the bridge rectifier then charge up to this voltage.
However, when the power supply is placed under load, the voltage starts to drop until it reaches the effective AC voltage. This is why linear power supplies without output regulators are not used when there are tight output voltage requirements.
Your best option would be to build a SMPS... but then you might as well build a Class-D amplifier.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
Hmm about the transformer, I know that is a problem of the RMS voltage and the peak voltage. I think a bigger cap can solve the problem too, but I am really thinking about making an SMPS now.
What about the Class-D amplifier? I have seen a few documents arround the internet saying that is not good to use to medium-high frequencies, only is good for subwoofer and something so, because running the PWM frequency at high, it can cause interference on AM Radio or so, that is true?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Class D works fine for audio up to 20kHz if you use a high enough PWM frequency. AM interference shouldn't be a problem, because A) who listens to AM, and B) your output filter should turn the output in to low frequency audio. They're no more disturbing than any other high frequency switch mode device. About making a SMPS, I've built a few and that's enough to last a lifetime. For an amateur (maybe even pros) it's nothing but a headache, and I now use iron cores and linear regs whenever possible. You'll need to set off just as much time for the SMPS as the project itself, at least.
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High Power Regulate Power Supply for Audio Amplifier
