Monday, May 25, 2015

OK Class, how much Class A Power ?

I've mentioned before in this blog how manufacturers routinely inflate Class A power claims by various technically correct but misleading means.  I've also discussed the correct calculation, which is 2RII where I is actual idle bias and R is the load.

The Krell FPB 300 has different idle bias depending on situation.  Temporarily I have seen it consume about 1200W continuously for 30 minutes or so after turn on.  But over a whole day, it can't sustain much more than 700W, which seems to rest on an second bias plateau around 600W.  The bias circuit kicks everything down when the heatsinks start getting above 180F.

For 600W idle, that's 300W per channel, and about 2 amps across the 170V maximum peak-to-peak voltage.  So 2RII gives us 32W into 8 ohms and 16W into 4 ohms.

An amplifier with 1/2 the voltage swing but dissipating the same power would be running 4 amps in consuming that 300W, and therefore have 4 times as much Class A power.  The price for having more maximum power on tap given the same dissipating capacity is that you cannot have as much Class A power.  People would like to ignore this but it is basic math.  If you want more peak watts from the same quiescent consumption, you have to give up the Class A Watts.  Also, Class A Watts do not easily double down as they are limited by current in most amplifiers.  So Class A Watts will halve into each halving of impedance.  The famous Mark Levinson ML2 did not do this because the amplifier was voltage limited at each lower impedance down to 2 ohms--so it did indeed "double down."  But it was not practical for many people, and especially me, because the maximum power into 4 ohms was 50 watts (25 watts at 8 ohms).  And you get that 50W from a single channel mono block about as big and heavy as my Krell.

Here are the Pass Labs .8 specifications.  My Krell can continuously dissipate about the same as Pair of Pass Labs XA60.8's, which have a combined ship weight of 214 pounds.

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