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Old 8th September 2004, 03:37 PM   #1
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Location: Minnesota
Default help with power supply....

here is the scoop:

I have an output stage that I want to run at 20-25vdc at 4A per channel.

(Class A)

The output is a IRFP140 mosfet connected as source follower running into a 1:1 output transformer.

Obviously there is high PSRR here... but I have no clue how to calculate it.

I want to design my PS as simple as possible... no regulation.

But I want to minimize charging currents and possibly go LC regulation...

So... I need the the minimal amount of C possible... but also enough to have a low Q between L and C.

got it?

what transformer should I get? (secondary voltage)...
what C should I start with?

etc...
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Old 8th September 2004, 09:39 PM   #2
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Location: Knoxville
Default Re: help with power supply....

You are using a current source to bias 4A through IRFP140 aren't you? This may help with ripple noise from an unregulated power supply. You WILL get ripple if you are biasing 4A class A.

Anyway, the output voltage from a transformer rectified and filtered is the peak voltage from the transformer. They are rated by the RMS value. Multiply the RMS value by sqrt of 2 to get peak value. Use 15-18VAC transformer to get 21-25VDC. Using a series inductor in the power supply may hinder high frequency current spikes, as may by needed by the amp.


BTW: If you are going to use an output transformer, why not use a step down transformer to match speaker and use a higher impeadence, higher voltage lower current, for the amp circuit? This may be a more suitable situation for class A. Power supply ripple is a fuction of frequency, capacitor value, and current. NOT voltage. Using a higher voltage & less current will yield a smaller ripple voltage. Kinda one reason you don't see to many 2 & 4 ohm home stereo amplifiers. Car amps use an SMPS for power supply and can be designed for lower impeadence.
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Old 9th September 2004, 02:09 PM   #3
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Default actually....

it is a source follower output stage with voltage gain of 1 with a 1:1 output transformer attached.

so PSRR is very high.

maybe go:

4700uf - 2-3mH air coil - 27000uf

with the 4700uf a common cap to left and right channel
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Old 9th September 2004, 09:28 PM   #4
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You may find that using a single power supply for both left and right channels could give you some cross-talk via ground loops. May be more likely with class A. Most high quality amps use separate power supplies, commonly grounded.
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Old 10th September 2004, 03:42 PM   #5
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Default common power transformer?

even go with 2 seperate transformers?
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Old 13th September 2004, 10:53 PM   #6
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Each transformer has its own rectifier and filter caps, just that the ground has to be common not the rails. If your input has a common ground, only three wires, then the input to the amp has to be common save one condition, that input audio transformers are used and the secondary of each are isolated, then you can use compleatly seperate power supplies that are not commonly grounded. Small advantages.
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