Question on amplification

[TankTop]

Amplifiers that double their power from eight ohms and four ohms are generally considered good. Let’s say you have an amplifier that outputs 100 W at eight ohms and 160 W at four ohms. At 50 W at eight ohms would it double to 100 W at four ohms or would it be linear amplification based on max output?

 

[staticV3]

If your Amp is rated for 100W@8Ω, 160W@4Ω, and you plug an 8Ω load into it and adjust the volume to 50W, then without changing the volume you switch to a 4Ω load, assuming near 0Ω output impedance, the Amp will then output 100W.

 

[ahofer]

The important thing is the rated power and stability in the load. “The doubling”, as I understand it, is overrated due to gamesmanship by vendors (Lowballing the 8 ohm number) and the arrival of class D amps that deliver a lot of power but have some (often extreme) load limitations.

 

[Ken Tajalli]

Amplifiers have output impedances themselves. Picture it as a virtual resistor on the speaker wires, through which the power flows.
if this resistor wasn't there, and your amp had beefy enough powersupply inside, then what @staticV3 has explained is correct, well theoretically.
But if the resistor (output impedance) is high in value, then power that flows through it gets shared between the speaker and this resistor. The closer the speaker impedance gets to the output impedance, the more this sharing of power becomes noticeable.
Another related issue is current limitation of the powersupply inside the amp. when it runs out of juice, then the amp can not output anything beyond.
To answer your specific question, it depends!
If the amp in question, has very low impedance, then yes it would double to 100W, if not then it would be 80W. You see the original 160W at four ohms, may be due to PSU's limitation.
For an amp to be able to almost double its output power at 4 ohms, it needs a good PSU and very little output impedance, which costs money and expertise.
Also, consider the above golden reply!

 

[DVDdoug]

Amplifiers that double their power from eight ohms and four ohms are generally considered good

It's not necessarily "good" or "bad". If you've got enough power you've got enough, or you could burn-out your speakers if they can't handle the "extra" power.

But it means it has a well-regulated power supply (the voltage is constant) and the power supply can put-out the additional current (without the voltage dropping) so yes, that could be an indication of a "good design".

But if you lower the impedance enough, you'll run into a point where the amplifier can't put-out the current or it overheats. Some car amplifiers are rated down to 1-Ohm but most home or pro amps can't handle that.


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Resistance and impedance (both measured in Ohms) is "the resistance to current flow". With a constant voltage, current is inversely proportional to resistance so if you cut the resistance (or impedance ) in half you double the current. (Ohm's Law says Current = Voltage/Resistance).

Since Ohm's Law is a law of nature (with man-made units of measure) if the amplifier can't put-out the required current, the volage will drop (the amplifier will clip at a lower voltage with the lower impedance load).

Power (wattage) is calculated as Voltage x Current so doubling the current doubles the power. With infinite impedance (nothing connected) you still have voltage but no current flows so you don't get any power. Zero impedance is a "short circuit" and you get excess current (and almost no voltage) and "bad things" can happen if the amp doesn't shut down to protect itself.