On Thu, 25 Nov 2004 10:15:48 +1100, Tat Chan
wrote:

Jim Lesurf wrote:



The second was that, although measured to deliver 50W into 8 Ohm loads,
this fell to 36W into 4 Ohm loads. Again, it was mentioned that the
distortion tended to rise as this level was approached. This implies that
the available current may also 'soft' clip at a value that may come into
play with speakers that don't maintain an impedance of about 8 Ohms across
the band. (In such cases the rated power may be misleading as significantly
higher powers may be available but with higher levels of distortion than
used to rate the max available power. Alas this is one of the many things
the HFW review fails to specify.)


Jim, I am confused here. I was under the impression that an amp will
deliver more power into a 4 Ohm load than into an 8 Ohm load.

If the amp delivers 50W into 8 Ohms,

then using P = (I^2) * R

I = sqrt (50/8) = 2.5 A


So, using a 4 OHm load, the amp should deliver

P = (2.5)^2 * 4 = 25 W


However if it was measured to deliver 36W into 4 Ohms,

then I = sqrt (36/4) = 3 A


So I take it the current the amp delivers can change with the load?
(apologies if this seems elementary, I haven't done these calculations
in a while!)

It's simply that the amp has limits on both output voltage and ouput
current. 50 watts into 8 ohms represents the 20 Vrms limit of its
output voltage, while the 36 watts rating at 4 ohms indicates a low
current capability of only 3 amps.

--

Stewart Pinkerton | Music is Art - Audio is Engineering