Re: Size of power supply capacitors


"Arny Krueger"
>
> "Mark" <
>
>> and don't forget, the bridge rectifier is constantly replenishing the
>> cap, so under heavy load the voltage doesn't really go down that much,
>> the ripple however goes up......
>
> The bridge rectifier is *not* constantly replenishing the cap, below 60
> or 50 Hz.


** Arny is speaking in double Dutch.

The bridge constantly delivers hefty charging pulses to the filter caps at
the rate of 100 or 120 Hz - depending if you are in a 50 or 60 Hz power
environment.

With a steady audio frequency of well above 100 Hz, the filter cap smoothes
the varying load current so it is the same as a resistive one of the same
average value. This results in the least ripple voltage appearing.

However, when the audio frequency is near or below 100 Hz, the smoothing
effect is diminished and the *peak value* of the load current comes into
effect.

For a typical class B amp ( low bias AB in reality ) the load current on a
given supply rail is of half sine shape for sine wave drive - and the peak
value of a half sine wave is 3.1 times its average value.

So, the worst case scenario, with *very* low frequency audio, is that the
supply ripple voltage rises to 3.1 times what is at mid and high audio
frequencies.

This is why most SS amps lose some power output on BENCH TEST at 20 Hz
compared to 1000 Hz - usually about 0.5 to 1 dB.

On music program, with typical speakers connected, it is a whole nuther ball
game.



........... Phil




.

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