Random thought: Amplifier output power ratings, 4Ω vs 8Ω

Typing the Ω symbol reminded me that I have a lab report to complete. It's week 9 yet I feel like zero hour.

Many if not most amplifiers specify max output power ratings at 4Ω or 6Ω for those that cannot handle the high current, rarely 8Ω. Amplifiers and limited by both the voltage their power supplies provide and the current their output stage can put out. However, in marketing speak amplifiers are always be able to source or sink 1000A of current for a nanosecond. Hence current is not a problem and output power is limited by voltage instead. Since V²/R, the smaller the R, the bigger the power.

Hence the Tripath TP2050 power stage chip is a peculiarity. It is highly current-limited such that it is rated at 8Ω in the datasheet (although it is also mentioned that the minimum it can go is 6Ω).

However, there is something interesting about this chip - in stereo mode, it is a pair of bridged amplifiers. Change some resistors and switch it to mono mode, the two bridged outputs combine together to form a 100W into 4Ω amp that is capable of twice the current. Hooray for FET output stage. These numbers also suggest that the max power into 8Ω in this mode is still 50W, but hey extra headroom is always nice.

Now, this thing has my interest because some completed product amp manufacturers misuse those numbers, dunno if it is by their marketing department, or a careless designer. 100W and 50W output powers were quoted with different combinations of number of amp chips and load impedance. You can see that some combinations don't work.

Official specs for this chip is 50W into 8Ω in stereo mode and 100Ω into 4Ω in mono mode. But nothing was mentioned about power into 4Ω in stereo mode.

So a few calculations using Ohm's Law later,

Current limited = fixed current = power proportional to resistance = half of power into 8Ω = 25W

Well, that is veryyyyyyyy small compared to 50W.

For a few seconds it didn't make sense, until I remember the other form of the formula P = I²R. In paralleled mode it has twice the current capacity. Hence four times the power.

Well, the amount of thinking that was carried out would take just a few seconds, but this reminded me that a good amplifier should never be current-limited. And amplifier power ratings are screwed up.