David - power transfer is irrelevant in the context of audio (generally speaking). It is very important in power stations and radio transmitters. Amplifiers and pre-amps have active devices in them - transistors and valves - that amplify the voltages. Power transfer is only relevant where we are trying to minimise losses in "passive" circuits like power lines etc.
In line-level audio, we care about voltages rather than power. Optimum voltage transfer occurs with low source and high sink impedance so that the sink does not "load" the source. For speaker to power amp matching (and this differs slightly between valve amps with output transformers and tranny amps) the issue is that the PSU can only deliver so many volts (say 40) before it distorts. So, the output transistors vary the voltage between 0-40 volts (max). The speaker impedance is say a fixed 8 ohms (lets ignore reactance here) and the output impedance is usually lower - say 2 ohms. The output transistors do the work of varying the voltage to deliver the requisite "power" to the speakers. This is the opposite to mains wiring or antennas where we want to lose as little "power" as possible along the line - there are no "amplifiers" to compensate for the power loss along the way. The downside (if you can call it that) of having a low "internal" or output impedance of an amp is that more power is lost as heat within the output devices...that's why amps get hot. As an example, if your power amp can swing 40V from its PSU rails, has an impedance of 2 ohms and is running into an 8 ohm speaker, the max voltage the amp could deliver to the speaker would be: 40*(8/8+2) = 32 volts The power the amp could deliver into the speaker would be (39.5*39.5)/8=128 watts and the power lost inside the amp by heating up the heatsinks would be (32*32)/2= 512 watts of heat That's roughly 25% efficiency. This is for a Class A amp with full power delivery at all times (not class B!!!) If the impedance of the amp was 8 ohms: volts to speaker= 40*(8/(8+8)) = 20v power to speaker = (20*20)/8 = 50 watts power lost to heat = (20*20)/8 = 50 watts That's 50% efficiency...but the amp can only deliver 40% of the power of the other example. Given that the maximum volt swing from the PSU is a fixed thing and the speaker impedance is a fixed thing, the amp designer only has one choice...if he wants to deliver more power, he has to sacrifice overall efficiency and lower the impedance of the amp... I hope I haven't confused things? ...dons flame-proof suit, waits for attack by real electronics engineers :o) -- Phil Leigh You want to see the signal path BEFORE it gets onto a CD/vinyl...it ain't what you'd call minimal...SB3+Stontronics PSU - Altmann JISCO/UPCI - TACT RCS 2.2X with Good Vibrations S/W - MF X-DAC V3/X-PSU/X-10 buffer (Audiocomm full mods)- Linn 5103 - Linn Aktiv 5.1 system (6x LK140's, ESPEK/TRIKAN/KATAN/SEIZMIK 10.5), Townsend Supertweeters, Kimber & Chord cables ------------------------------------------------------------------------ Phil Leigh's Profile: http://forums.slimdevices.com/member.php?userid=85 View this thread: http://forums.slimdevices.com/showthread.php?t=46347 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/lists/listinfo/audiophiles
