Re: mini mosfet audio amp design

In <13uj0q1j8rsikb2@xxxxxxxxxxxxxxxxxx>, RST Engineering \(jw\) wrote:
Somebody is going to have to explain page 3 of this data *** to me.
Please don't tell me it is a misprint, as National has had six years to
correct.

At a supply voltage of 15 volts, the curve shows 10 watts out.

Last I looked no single-supply design can drive to below zero or above
supply rail without the use of some inductive tricks that the test circuit
doesn't show.

So, converting a 15 volt peak-to-peak output signal (assuming that the
device can swing to both rails) gives about 5.3 volts RMS.

The data *** specifies an 8 ohm load, so given that Power = volts^2 / load
yields about 3.5 watts out.

What gives?

"Don Klipstein" <don@xxxxxxxxxxxxxx> wrote in message
news:slrnfug3sc.gjt.don@xxxxxxxxxxxxxxxxx

National Semiconductor LM1875, available from Digi-Key.

http://www.national.com/ds/LM/LM1875.pdf

The basic circuit uses 7 capacitors and 5 resistors, and that is
including .1 and 100 uF capacitors from each supply rail to ground, and
resistors to ground from both ends of the input coupling capacitor. No
additional semiconductors are used in the basic circuit.

That IC uses a split supply, and those curves (I see them on Page 4)
show output reaching 10 watts for 4 ohm load and a little short of 10
watts into an 8 ohm load with +/- 15 volt supply - that is 30 volts
rail-to-rail.

- Don Klipstein (don@xxxxxxxxx)
.

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