Archived from groups: rec.audio.tech (
More info?)
In <1123466858.787501.210490@z14g2000cwz.googlegroups.com>, on 08/07/05
at 07:07 PM, elephantcelebes@yahoo.com said:
>I just ordered a kit for a switching power amp from www.41hz.com, and
>have run up against a conundrum. Suppose, for the sake of argument,
>you have two chip amps, both running on the same regulated power
>supplies, delivering the same output signal to the same load. One is
>Class AB, with roughly 60% efficiency. The other is a switching amp
>with 90% efficiency. But...
>1. Both amps feed the same voltage & current waveform to the load.
>2. Both amps draw the same current waveform from the power supplies.
>One amp dissipates more heat than the other, but it doesn't seem like
>the difference in heat is coming from anywhere else in the system. I
>have to assume that my second assumption is faulty, but I can't figure
>out how.
With an output filter on the Class-D amplifier, 1. could be mostly
true.
If 1. & 2. are true then the amplifier efficiencies are incorrectly
stated.
If you've discovered a simple electronic process that creates or
destroys energy, at least try to patent it. In the past the Patent
office has taken a dim view of such claims, but you could get lucky --
especially if you claim it is a software process.
When the output transistors are switched ON, the amplifier draws
current from the power supply. In the Class-AB amplifier, output
transistor current is always being drawn. In the Class-D amplifier,
output transistor current flows intermittently.
When a transistor is full on or full off, very little waste heat is
generated. When a transistor is partially conducting, quite a lot of
waste heat can be generated. Calculate a few waste heat points by
multiplying the voltage drop across the transistor by the current
flowing through it. When the transistor is full on, the voltage drop is
small. When the transistor is off, current is zero. If the transistor
is half on, waste heat is high. In the Class-D amplifier, the
transistor is full on or off and little heat is generated. (mostly
while the transistor is in transition between on and off)
In the Class-D amplifier a post filter integrates the pulses and the
resulting output can be identical to the Class-AB situation.
Measuring the voltages and currents can be difficult because some
metering systems respond poorly to the Class-D pulses.
---
If you haven't already considered measurement issues, think about the
power associated with a simple sine wave current. Measuring the peak
value is easy enough, but it is not very useful because the dwell time
at the peak is brief. The next obvious thought is "average" over one
cycle. Unfortunately, the average value of a sine wave is zero. While
"average" is a good concept, but we have a computational difficulty.
If you haven't been through all of this, find a good text and do some
reading.
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