4 Ohms Load Stress on Amplifier

There has been a lot of debate regarding driving a 4 ohm speakers from amplifiers only rated to drive 8 ohm load. The lower the impedance of the speaker system, the higher the power stress is on the amplifier. To ensuring reliable long-term operation, three of the most important parameters for a transistor are it's voltage, current, and power ratings.

Voltage: 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 malfunction.

Current: The transistors need to be able to handle the maximum currents taken by the load. Speaker impedance can 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 by incorporating current limiting circuitry. In practice a good amplifier design will withstand the ultimate over-current event.

Power: It’s a known fact that transistor has a maximum power dissipation rating. for instance, a transistor might be rated to dissipate 100 watts at 25 degrees centigrade but only 20 watts at 100 degrees centigrade. A 4 ohm speaker tends to pull more current which generates heat. The amplifier designer should provide sufficient cooling, in the form of heatsink, to ensure maximum temperatures are not exceeded.

In class AB amplifiers, 4 ohm speakers greatly increases amplifier heating compared to when using 8 ohm load, if the maximum output currents are kept the same by exercising restraint with the volume control.

If you are drive a 4 ohm speaker with an amplifier only rated for 8 ohm load, works fine as long as you limit the volume so that the output current is kept below that which would have been taken by the 8 ohm speakers.

very good post Mr.Anil, sure it will help the newbies and even people who are using the amplifier without knowing what it is Do you have any information about parallel and series connection , kindly help me on this topic .

tharun