Just a reminder, one of the design goal of a power amplifier is the output "power" rating. To be more meaningful, it really should have been voltage and current. Since voltage, current and power are all related, they used power to keep things simple and easier to understand by the average consumers.
It follows that, if an amplifier's output specs are voltage limited by design, and not current limited at all, then the amplifier can come close to do the so called "double down", from output into 8 ohm vs into 4 ohm.
If it is current limited by design, and not voltage limited at all, then the amplifier will, instead of double down from 8 to 4 ohm load, it will "half down", based on the well known Ohms law that probably 99% of ASR members are familiar with.
That's obviously the extreme case, in reality, all power amplifiers will be designed to have both voltage and current limits. As such, if an amplifier is specified for 100 W into 8 ohms, it cannot be specified for 200 W into 4 ohms, but should most likely be specified for 50 W into 4 ohms, because for the same current output, the power output will be halved when the load impedance is halved based on Ohm's law and the power formula.
Here's simplified numerical example:
Designed maximum output voltage = 28.28 V, Load resistance = 8 ohms, current = 3.535 amperes, Power = 100 watts
If the designed maximum current is 3.535 A, then for load resistance = 4 ohms, Power = 3.535*3.535*4 = 50 watts
That is, when the resistance (same for impedance) is halved, such as from 8 to 4 or 4 to 3 ohms, power output is also halved, not doubled.
One may say, but how do you know the amp is designed for the same current regardless of the load impedance. Just think about it, supposing the amp used in this example is not 3.535 A, but say, it is designed to for a maximum current of say, 2*3.535 = 7.07 A, then of course the amp could be specified to double down from 100 W 8 ohms to 200 W 4 ohms, since power = 7.07*7.07*4 = 200 W, but then if this is the case, the amp should have been specified for 7.07*7.07*8 = 400 W into 8 ohms!!
In this numerical example, the amp is specified for 100 W into 8 ohms, that means it's maximum current has to be in the neighborhood of 3.535 A and it's output into 4 ohms will therefore be in the neighborhood of 50 W, that is, again, half down, not double down.
The main reasons for amps with power output specs "double down" would include but not limited to the following:
- The output into lower impedance such as 4 and 2 Ohms are based on short duration, minutes, or seconds, milliseconds etc., not literally continuous.
- Use power transformers that have above average overload capability. In general, power transformers is an electrical devices that inherently has excellent overload capability.
- Heat dissipation adequately designed, using passive and/or active cooling.
- Use output devices designed for maximum current into much lower load impedance such as 2 ohms or lower (again, duration for the high current flow considered).
The so called "double down" or close to double down output ratings are NOT unconditional, manufacturers just don't tell you the details in the specifications. They don't have to tell you either because they know full well that you will be using their amps to listen to music, movie sound tracks, not test tones so there is no need for them to design for output currents into 4 ohm or less load. Even for those using speaker rated 4 ohms nominal, with dips to 1.6 ohms, amp manufacturers know such low dips will not likely occurred over a bandwidth more than an octave or two, typically you will see such lowest dips over a very narrow range of frequencies such that the amp will only have to drive the resulting high current spike for a very short duration as the music content dictates, that is, such very high current demand periods are short and of intermittent nature.
The double down from 8 to 4 to 2 ohms requirements, similar to the all channel power output requirements are to a large extent, more myth, or ideal, than necessity. Regardless, the ability to do those things, like 120 dB SINAD vs 90 dB SINAD, are nice, whether we can hear the resulting "better sound quality" or not.
It's very early, and I am too lazy to spell check, so if there typos that may cause confusion or result in distortions/mistakes, please kindly alert me, and I'll do the editing accordingly.
