With class ab amps, the power rating into speakers is often given as, for example 50W into 8ohm and 100W into 4 ohm. Am I right in thinking the latter puts a greater load on the amplifier and if so would the heat output from the sinks also increase significantly [double?]. Given that the most common rating for speakers is 8ohm, the 4ohm rating is not often going to matter but I'm wondering if caution is necessary when using an amp essentially designed for 8ohm, into 4 ohm speakers?
jules
You need a greater load on the amp to get it to deliver current and make max power, but it is the volume levels that determine the actual power used. A 4 ohm speaker adjusted to play at the same volume level as an otherwise identical 8 ohm speaker will use the same amount of power, in theory. Voltage would be lower on the 4ohm speaker but volume would be the same, and power.
Power dissipation (heat output) is different than power output. The power output is always perfectly proportional to the load (before clipping.) But the power dissipation can vary wildly between amp topologies. A 200W Class A SS amp might dissipate 2kW as heat, while only making 0.1W power output on a quiet passage. A class D amp might dissipate only 15W while making 200W power output on loud passage. The amplifier efficiency is determined by the dissipation (heat.) Class AB amps can vary in their efficiency depending on how far into class A they are biased. The more bias current applied to the output devices, the more heat is dissipated, and the better it will sound (usually.) A pure class A amp always dissipates the same amount of heat, no matter how loud or soft the music. All other types do vary the heat with loudness, but it is not proportional, some are only slightly warmer at full power, and others can really heat up after a workout.
4 ohm rating is actually very common these days because for the same signal voltage it sounds louder. Speaker demos switching the same signal between two speakers the lower impedance speaker will sound louder, which sounds better and sells faster.
A speaker has a "nominal impedance," a rating helpful in picking the amp. But the impedance varies because it is frequency dependent, meaning it depends what note the speaker is playing as to what the impedance is, and how much power it takes to play that particular note at a given volume. The power demand varies across the frequencies even at the same volume. You can ask your speaker mfg for an impedance curve of your speakers and you will see how much it varies from lowest to highest frequencies. A nominal 8 ohm speaker (rated 8 ohm) can dip to 4 ohms and demand twice as much power, or rise to 50ohms and demand very low power. The amp should be able to handle that, especially if the impedance dips at a frequency where there is a lot of musical energy, like midbass. The volume would still be the same though, it is the impedance that changed with frequency and makes the current double, if it can. The power rating of the amp means the current limit. It is a function of voltage (how loud you play it) and load (instantaneous impedance of the speaker.) Amps boast "stable into 2 ohms" etc, to let you know low impedance dips are not a problem at normal loudish volumes. But a cheap receiver will say "6 ohms or higher", better obey the warning, or keep the volume moderate and listen for nasty sounds of clipping.
Another factor is speaker sensitivity. More sensitive speaker is more efficient, and requires less power to make the same sound level, regardless of impedance, but otherwise same rules apply.
Rich
