There have been quite a few questions lately regarding the safety (or otherwise) of driving 4 ohm speakers from amplifiers only rated to drive 8 ohm loads. Rather than just add to the mass of unsubstantiated conflicting opinions (e.g. "amplifier XXX is built like a tank, so it must be able to drive 4 ohm loads"), I thought I'd attempt to apply a bit of science to the subject. Hopefully this will make people aware of some of the issues involved in driving low impedances. In relation to ensuring reliable long-term operation, three of the most important parameters for a transistor are it's voltage, current, and power ratings. Exceed any of these three at your peril. Voltage : In any half decent amplifier design the output transistors will have a sufficient voltage rating to withstand all "normal" operating conditions. Only abnormal events such as lightening strikes are likely to cause failures. Current : The transistors need to be able to handle the maximum currents taken by the load. Speaker impedance can (and does) vary considerably with frequency, and often dips well below the nominal 4 or 8 ohm value. Amplifier designers are well aware of this fact and counter it either by using higher rated transistors with large peak current capability (good) and/or by incorporating current limiting circuitry (not quite so good, but better than blown outputs!). In practice a good amplifier design will withstand the ultimate over-current event - the accidental short circuit of it's outputs (but don't blame me if yours doesn't!). Power : Every transistor has a maximum power dissipation rating. This is not a fixed figure but varies with the temperature of the transistor. For example, a transistor might be rated to dissipate 100 watts at 25 degrees centigrade but only 20 watts at 100 degrees centigrade. The amplifier designer should provide cooling, in the form of heatsinks, to ensure that the maximum temperatures are not exceeded. It has been suggested by some here on AK that if you want to drive 4 ohm speakers with an amplifier only rated for 8 ohm loads, then everything will be OK if you limit the volume so that the output current is kept below that which would have been taken by the 8 ohm speakers. The following calculations will show just how wrong this is. :nono: I have used "Excel" to calculate the output transistor power dissipation for a typical 200 watt class AB amplifier when driving an 8 ohm load at full output and 4 ohm load at half output (so that the output currents are the same for both 8 and 4 ohm loads). The following assumptions have been made : (1) The load is purely resistive. (2) The output waveform is symmetrical about zero, so the calculation only needs to be done for the positive half cycle (180 degrees). (3) The amplifier output can swing to within 5 volts of the supply rails before clipping. (4) Bias current is neglected as it's contribution to the maximum power dissipation is relatively low for a typical class AB design. The basic amplifier topology for the analysis is shown below : The voltage, current, and power dissipation waveforms for 8 ohm load at full output and 4 ohm load at half output are shown below (NOTE: The "Output Transistor Dissipation" figure is the total power dissipation per channel, so to get a "per transistor" figure simply divide by the number of output transistors (not pairs of transistors) per channel). The dark blue trace is the DC supply voltage from the main filter capacitors. The sum of the output voltage (pink) and the voltage across the output transistor (green) must always add up to this DC supply voltage. The yellow trace is the current which flows through the output transistor and into the load. Output power (light blue) is simply the output current (yellow) multiplied by the output voltage (pink). Output transistor power dissipation (red) is likewise simply the transistor current (yellow) multiplied by the voltage across the transistor (green). So, for the 8 ohm load we have : Peak Current = 7.5 Amps, Output Power = 225 Watts, Output Transistor Dissipation = 85 Watts. And for the 4 ohm load we have : Peak Current = 7.5 Amps, Output Power = 112 Watts, Output Transistor Dissipation = 198 Watts. Despite keeping the maximum currents the same by halving the power into the 4 ohm load, the power dissipation in the amplifier output transistors is massively increased compared to full output into 8 ohms. The following graph shows how the power dissipation (i.e. internal heating) of this amplifier would vary with output power for 4 and 8 ohm loads. Note that the figures are "per channel" and so the heating is doubled for stereo operation. I have noticed that Pioneer specify output powers for their SX-x3x, SX-xx50, and SX-xx80 receivers into both 4 and 8 ohms, except for the SX-1280 and SX-1980 for which output power is quoted into 8 ohms only. Similarly, the SPEC 4 is rated at 150 watts into 8 ohms and 180 watts into 4 ohms, whereas the output power for the larger SPEC 2 is only quoted into 8 ohms (250 watts). My suspicion is that this is because the cooling on these higher power units is insufficient to allow them to develop the same (or greater) continuous power into 4 ohms as they can into 8 ohms (output power being measured by the old FTC regulations which required 1 hour pre-conditioning at 1/3 of rated output, i.e. near maximum heating of the output transistors). Pioneer could of course have specified an output power into 4 ohms for these models, but how good would it look if the output into 4 ohms was considerably less than that into 8 ohms?? Better for them simply not to give a power rating into 4 ohms. Of course, this is only my opinion and I'm open to suggestions from more knowledgeable AK'ers. :scratch2: To conclude then : For conventional class AB amplifiers the use of 4 ohm speakers greatly increases amplifier heating compared to when using 8 ohm loads - even if the maximum output currents are kept the same by exercising restraint with the volume control. OK guys, I've set myself up to be shot down in flames - who's going to take the first shot?? - Richard B.