Ohms And Amp Power Output

[bluegrassphile]

Is it correct to say as ohms decrease amp power output increases (due to less resistance)? For example, if you have an amp that outputs 100 wpc x 2 at 8 ohms, it will effectively be outputting more with a 6 ohm speaker. Another example...let's say you have a Luxman 509x that outputs 125 wpc x 2 at 8 ohms. Will it output more wpc with Harbeth 6 ohm speakers? IOW, is there a power advantage to having 6 ohm speakers? As you can see, I'm a total novice on this subject. But, is the above information accurate?

 

[audio.bill]

Like most technical issues it's not as simple as it first may seem since there are theoretical expectations and then there's reality. It's true that as the load impedance presented to an amplifier by the speaker is decreased it will result in a demand for more current. If an amp has a theoretically ideal power supply with perfect voltage regulation (few amps meet this goal) then a 4 ohm load impedance will draw twice as much current as an 8 ohm, and if the voltage is maintained that would result in power being doubled into the lower impedance. A 2 ohm impedance would again double the current and potential power compared to a 4 ohm load, etc. However in reality there are limitations on how well a given amp's power supply can maintain its voltage under heavier current demands, and if the voltage sags during high current demands then the power (which is voltage times current) will not actually double as expected. So it depends on an amp's design and in particular the capacities of its power supplies and output stages. Things get further complicated when you have amps with output transformers like most tube amps and some solid state designs which tend to maintain a rated power output regardless of the load when the proper output taps are used. Hope this helps and isn't too confusing. You can find several sources with further details with a basic search for the subject of this thread.

 

[bluegrassphile]

Thank you, bill. That was definitely helpful. I was mainly trying to determine if my Harbeth speakers being 6 ohm vs the usual 8 ohms have a power advantage.....and, how much?

 

[audio.bill]

Thank you, bill. That was definitely helpful. I was mainly trying to determine if my Harbeth speakers being 6 ohm vs the usual 8 ohms have a power advantage.....and, how much?

The Luxman L-509X is rated at 120wpc into 8 ohms, and 220wpc into 4 ohms (a bit less than if it had a theoretically perfect power supply which would then provide 240wpc.) So driving the 6 ohm Harbeths the 509x would be rated at about 170wpc.

 

[bluegrassphile]

The Luxman L-509X is rated at 120wpc into 8 ohms, and 220wpc into 4 ohms (a bit less than if it had a theoretically perfect power supply which would then provide 240wpc.) So driving the 6 ohm Harbeths the 509x would be rated at about 170wpc.

That's pretty close to my (amateur) estimate. Add to that, a magazine rated the 509x much higher than the factory specs. Same with the 590axii. It is factory rated at 30 wpc x 2 at 8 ohms and HiFi Magazine (I think) spec'ed the 590axii at 90 wpc x 2 at 8 ohms. Then you add in that my Harbeths are 6 ohm speakers and you gain a bit more output...perhaps a bump to 110 to 130 wpc x 2 at 6 ohms (my estimate).