Class A Amplifiers and impedance dips

Do all pure class A amplifiers double in power with each halving of impedance?

No. Only if the power supply is capable - regardless of class - and only to a limited load impedance.
Not only the power supply but output transistors also have a characteristics to react on different loads as well as delivered current limitations.
ALL amplifiers, even cheapo AB's, have more power to throw at lower impedance. Super cheap receivers, ie sony, won't work with even a 4 ohm load. The best way to explain it is it works like the output of a pump. The pump is the power supply. As you lower impedance, you make the outflow pipe bigger, and the pump gets water out more easily. The better pumps can handle pumping more water- cheaper pumps start to burn up. Really good s.s. amps can handle 1 ohm loads. Certain "8 ohm" speakers can dip down that low. Tube amps do not like low impedance, which is why you have to be careful and buy a speaker with a stable load. In the bad old days friends sold the carver "amazing loudspeaker" with impedance dips so low that it fried most amps.

As Gs5556 and Marakanetz have pointed out; the amplifier
class has nothing to do with whether the amp doubles its
power as the impedance is halved.

The doubling is a function of what the power supply and
output stage are capable of. You can have a Class A amp
that does not double - and you can have a Class A/B amp
that does double.

However, you may still see more Class A amps that double
than Class A/B. It's not a function of the amplifier
Class - but due to the fact that a manufacturer that takes
the philosophy of going Class A - accepting the down-side
of extra heat generation, etc in order to gain the
advantages of a Class A - might also provide the beefier
power supply and output stages needed to power double.

Class A and power doubling are not "cause and effect" - but
rather are both hallmarks of a design philosophy.

Dr. Gregory Greenman

I love this website!!! ....What a reservoir of knowledge and help.

Thanks guys for your help.


Is there an agency that dictates what class A or other designation actually consists of? How does one know if a manufacturer claims class A for their amp that it really is and does any agency ever check? I am not doubting any information here just would much prefer to read an authoritive white paper if someone can point us.

F#$k specs! It's all about what an Amp CAN do and what an Amp CAN'T and it's completely upto YOU to judge.
chryslers_rule - You don't want the government in your stereo, do you? Class A amps run the output transistors ALL the time. This allows purity of signal. The disadvantage is the transistor that is left on all the time runs very hot. This means you have to have BIG heat sinks and can't run as much current through the transistor. Class AB amps use a pair of output transistors running in tandem. A turns on, B turns off. B turns on, A turns off. The advantage is you can cool off B while A is running, so you can run B at higher levels while it is on. You don't need the huge metal cooling fins (heat sinks). This means more watts at a cheaper price. The disadvantage is the switching distortion incurred flipping back and forth. As computer technology became cheap and plentiful this has become much less of a problem, because the switching is more precise. Ask morbius for a second opinion. And tell him to WATCH OUT FOR THE ID!