DC Power supply question...duh...

From: Allison J Parent <allisonp_at_world.std.com>
Date: Sat Jul 3 10:28:41 1999

<Boy does that sound like spec. inflation! 24 p-p is just 12 Volt peak in
<either polarity with a bridge circuit. That gives 12/8 peak amps and 18
<peak watts (like with a +/-12 volt square wave). The RMS voltage and

no that would be 24/4 (two speakers presuming they are parallel) or 6A.
Assumes a 100% lossless bridge amp. at 24VPP and 6A you have 144W if it's
a symetirc squarewave, less if sine(RMS).

Now reality sets in, using the latest devices (VMOS/hexmos) there will be
a few tenths of a volt loss arcoss the device. So let say the loss is 1V
in that bridge amp of ours. Not allowing for resistance of wire and also
assuming the PSU can easily supply in excess of the required current at

23Vpp/4 = 5.75A that times 23 is only 132.25... reality sets in some.
also the difference 11.75W is heat!!! and thats the squarewave, it gets
really bad for sincewaves (more like 25W as heat!). Can you imagine
trying to get rid of say 40-50W (stereo case) of heat from one of those
tiny boxes?

Other assumptions, an 8ohm speaker is 8ohms. A non truth as it varies
widely with frequency assuming a good encolsure. So at one frequency it may
look like 6ohms and at another 15! Then we have copper losses from wiring
and PC traces (resistance) and what if the 12V is really 11.95V?

<amplifier. Don't tell me these "240 Watt" speakers are powered by 4 "AA"
<batteries ;)... or use one of the many automobile ic amps that give about
<4.5 watts with a 12 Volt supply and a 4 Ohm speaker.

The other reality is it takes very few watts at a sustained level to cronk
ones ears forever.

4AA cells can provide a remarkably large amount of power for and equally
remarkably short period of time. (nominally 3WH) ;)

A real 240W amplifier will cook a hotdog with two nails stuck in the
ends in about 2 minutes at sustained full power! Real power = heating

Received on Sat Jul 03 1999 - 10:28:41 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:32:10 BST