What's better 100w class a or 200w class ab?

I'd reckon that the biggest question is what is the impedance of the speaker? Since this fellow is deciding between two amps from the same mfr, the biggest diff will come IF the "class A" amp slides into AB because of a low Z load. In which case it might just put the same power into the load as the AB amp from the same mfr.

The circuit is likely to be identical, just the rail voltage will change - assuming the description is accurate, the 100w class A amp *ought* to have *higher* rail voltage than the 200w AB amp -as the load causes the amp to slide into AB the advantage of the higher rail voltage will be more output available than with the nominally higher rated 200W AB amp!

:- )

Of course into a calm 8 ohm speaker - go with the 200 watt AB, IF the criterion is only max output with an inefficient speaker...

It also matters what the output devices are. Mosfetters and BJTs act a bit differently in AB vs. A.

On the other hand, with two amps that use the identical circuits, I'd pick the one with the better power supply regardless... :- )

_-_-bear

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Ummm that 72 amp "peak current" thing is a bit, shall we say, "optimistic."

Let's take a look at it. In order to get 72 amps drawn by a load it has to fit into the P = I^2 R formula. So, 250w = 72^2 * R - where 250 watts is the max output of the amp, that gives us a load resistance of 0.048ohms. That's about as close as you can get to a dead short... Ok, ok, so you say that the amp can really put out more than 250 watts? Let's look at what the rail voltage is likely to be. It's rated at that power at 8 ohms, so P = E^2 / R... 250 = E^2/8 or E^2 = 2000 or 44.72volts... that's the RMS value of one rail's voltage. The DC rail voltage is about 1.414 times that or 63vdc. Assuming few losses in the output stage the loaded rail voltage at 8 ohms must be at least that, so let's set it a bit higher to take losses into account... about 70vdc. Now we know something of use. *IF* said amp *could* put out 72 amps for *any length of time* at all, then the max wattage required would be 72amps x 70vdc =
5040watts. Ask yourself how many amps have power supplies anywhere near this size. Even if there is a fairly large power supply bank, this sort of current can only be sustained for an incredibly short time by an amp that does not have a huge power supply. I doubt strongly that the MF line has that much supply in it.

BTW, IF it could handle a 2 ohm load, it would have to chunk out 968 watts, and if a 1 ohm load was there, only a mere 1936 watts. :- )

My Symphony No.1 Amp *does* have a huge power supply. It uses 4kva worth of transformers in the original configuration, along with 500,000ufd of filter caps AND 140 amp Hexfred rectifiers. The amp's power section is fed via 10ga SPC wire. The output stage is modest, using only 6 Hitachi metal case Mosfets per RAIL (12 per channel) each of which is rated at 7 amps continuous = 42 amperes per rail, or 100 watts each = 600 watts max continuous. Hitachi rates them at LESS than that for a 10ms 1 shot with rail voltages above 50 volts! BJTs have similar deratings. This amp is a 180 watt per channel rated unit with 60vdc rails.

This amp weighs 128 lbs., none of which is "filler," it's made from aluminum, not steel. Most of it is power supply components.

You can get more power from a smaller, lighter switching supply, but then you are depending upon the *line* stiffness to control the supply and provide the required current on peaks - there are power factor issues there. Halcro has to deal with that sort of thing...

Anyhow... "peak" anything is a very questionable specification, FYI.