Dynamic Headroom vs Resistance

no other input?

thanks ozfly!
interesting!!
this actually is in particular referring to car audio amplifiers which just about all claim to be able to double power when the resistance is halfed.
Some power
400x1 @4ohm
800x1 @2ohm
1600x1 @1ohm

I'm glad I was sorta on the right track with this (or at least I should say Ozfly's thinking follow mine, albeit his is much more factual!)

In a parallel circuit, the voltage is the same across each resistor. If the resistors are equal in value, the amperage is equal in value (E=IR) across each resistor. So a parallel circuit of equal resistance value will utilize twice the current since amperage is the sum of both branches of the circuit. Since power is the product of amperage and voltage, the power used doubles in that configuration. So the net "load" might halve, but that drives the amperage to double (since the voltage is constant).

The above is true. Here's my interpretation of how it impacts headroom: (folks, please correct me if I'm wrong)

An amp that puts out 50w continuous into 8 ohms should ideally be able to put out 100w continuous into 4 ohms. If it can, that means that it can deliver twice the current with less load. Many cannot. You'll more often see a 50w into 8 ohm be rated as 80-90 watts into 4 ohms. Basically, those amps borrow the headroom available at 8 ohms -- the amp simply can't generate the current. Headroom, as defined, would go down.

Even an ideal amp that can generate 50 w into 8 ohms and 100 w into 4 ohms would face this. The ability to drive 200w for a brief spurt remains constant regardless of the load because the rail voltage of an amp doesn't vary -- it's the current capacity that is the limiting factor to power, even as measured in short bursts. If the rail voltage is 50 volts and the short burst limit is 200w, then the amp can drive no more than 4 amps for a brief period of time. So 200/50 at 8 ohms is greater than 200/100 at 4 ohms.

I would remind him "doubling power" is in an ideal world. In the real world, most amplifiers fail to double their power because their power supplies run out of juice.