Calculating output pwoer of amp at different ohm loads.

[valkodav]

Hello guys,

I would like to know how to calculate the amp's pwoer output at different ohm loads.

For example lets say that i have and amp rated at 2x200W @ 6ohm load.

How is this value going to change if i connect speakers with 8ohm impedance.

How to calculate that ?

Thank you.

 

[CnoEvil]

If it's not given in the specs, I suspect you will have to contact the company, as the amp's ability to cope with different impedances will depend on the robustness of the power supply.

 

[Vladimir]

V = I * R, P = V * I, P = V² / R

P - Power, V - Voltage, I - Current, R - Resistance (aka Impedance).

P = V² / R - as you can see, the lower the speaker resitance is, higher the delivered power will be. Ideally an amplifier will double down power output at every halving of the resistance.

100W in 8 ohms // 200W in 4 ohms // 400W in 2 ohms.

As previously mentioned, depending how well designed and robust the amplifier power supply is, better it will maintain this formula without losses. But reality is tough on amplifiers. Everytime a speaker drops in resistance (during large transients peak in music passages), the speaker becomes closer to short circuit and the amplifier power supply must provide more current in order to maintain stable voltage. V = I * R. If the power supply has no current to deliver, voltage will drop, voltage rails will sag, amplifier will clip, tweeters will burn. Amplifier manufacturer will thus declare in its specification at what load their amplifier can deliver a certain amount of power without clipping (1% THD).

Depending on the amplifier design, that 2x200W @ 6ohms may have 2x125W @ 8ohms and 2x250W @ 4ohms, or it could be 2x150W @ 8ohms and 2x250W @4ohms. The manufacturer will have the exact numbers based on measurements of current delivery and THD. Your everyday typical amplifier will not follow the ideal formula of doubling power when impedance halves, therefore it needs to be measured how much it can deliver before clipping. Of course there are big power amps that double down at half load, but that comes at a cost since the manufacturer invested in a better more robust build. Even they don't have ideal doubling down, but with few watts off due to heat losses in the circuit, although manufacturers will round up the numbers so it looks on paper as if they are doubling down with nice round numbers 100, 200, 400, 800.

 

[davedotco]

You can use the formulae V = IR and P = VI, where V is voltage, I is current, R is impedence and P is power.

Strictly speaking, these formulae apply to direct current not alternating current (sine wave or music) but in the scale of possible frequencies, 20 - 20khz is close enough to dc for the sums to work.

In the example given we have P as 200w and R as 6ohms.

Ie 200 = V times I,

but V = IR so substituting V/R for I, we get 200 = Vsquared divided by R. Since R is 6ohms, Vsquared = 1200.

So the amplifier has an output voltage of √1200, about 35 volts.

So into 8 ohms we get 35 = I times 8, so I the current, is about 4.5 amps.

Power is volts V times Current I so 35 times 4.5 or about 157.5 watts into 8 ohms

Into 2ohms we get 35 = I times 2, so the current is 17.5 amps.

Hence power into 2 ohms is 35 times 17.5, 612.5 watts.

That of course is the theory, in reality delivering 612.5 watts per channel, would require a huge power supply, something in the region of 2 to 3 Kwatts, at least 4 times the size of the power supply in the 150 wpc Roksan K3 for example.

This shows why amplifier power output becomes limited into low impedencies.

 

[valkodav]

Thanks guys, helped a lot, now everything is clear.

 

[Andrewjvt]

That some companies measure the power differently like nad and naim

40 w on a naim will in the real world sound a lot more powerful than 40w from another make.

 

[davedotco]

That some companies measure the power differently like nad and naim

40 w on a naim will in the real world sound a lot more powerful than 40w from another make.

This is commonplace.

I recall seeing an early Krell KSA50 on the test bench, quoted at 50wpc 8ohm. 100wpc 4ohm, 200wpc 2 ohm, in reality it was over 100 watts with a 1khz sinewave into 8ohm, distortion still below 0.1%.

There are so many ways to quote amplifier power, it is a nonsense really. You get a much better idea of an amplifiers real power capability by looking at it's power supply capacitors and the size of it's transformer.

 

[andyjm]

There have been attempts to standardise power measurements. It isn't tricky. The 'Andy' test would be:

0.1% THD

All channels driven

8ohm resitive load

Full bandwidth signal

Amplifier must be able to sustain this power for at least 1/2 hour in a normal environment.

This would produce a reasonable measure that could be used to compare amplifiers. A more comprehensive test would be to re-run the test at 4ohm load as well. Given the extensive (?) test facilities that WHF have, they could easily run this standard test for each of the amps they review.

 

[davedotco]

There have been attempts to standardise power measurements. It isn't tricky. The 'Andy' test would be:

0.1% THD

All channels driven

8ohm resitive load

Full bandwidth signal

Amplifier must be able to sustain this power for at least 1/2 hour in a normal environment.

This would produce a reasonable measure that could be used to compare amplifiers. A more comprehensive test would be to re-run the test at 4ohm load as well. Given the extensive (?) test facilities that WHF have, they could easily run this standard test for each of the amps they review.

The US Federal Trade Commision tried to introduce a standard test but manufacturers hated it, many amplifiers failed it and it got quietly dropped.

The 'Andy' test is pretty good though I would suugets 30 minutes at full power is unrealistic, particularly on a steady state wide band signal (pink noise?). For hi-fi use 30 minutes a half power should be more than adequate.

I feel amps should be tested into 2 ohms too, expecting huge continuous output would be unrealistic for most amplifiers but a decent power output with good stability would be a benchmark.

 

[chebby]

I feel amps should be tested into 2 ohms too ...

To allow for poorly designed loudspeakers?

I'd run a mile from 'speakers with impedance that dips that low. (Given how many excellent ones are available that don't.)

 

[davedotco]

I feel amps should be tested into 2 ohms too ...

To allow for poorly designed loudspeakers?

I'd run a mile from 'speakers with impedance that dips that low. (Given how many excellent ones are available that don't.)

There are plenty of 4 ohm speakers about, some of which will drop below 4 ohms even when driven hard, testing at 2 ohms may well be a 'worst case' scenario and I am not asking for massive power at this impedance but any amplifier that delivers reasonable power and is stable into 2 ohms will drive 'real' 4 ohm speakers pretty well.

If you are going to consider all 4 ohm speakers to be 'poorly designed' then I guess you have a point, there are no 2 ohm rated speakers as far as I know.

 

[Jota180]

Manufacturers possibly deliberately understate their amps output into 8 Ohm for marketing reasons. They will say the amp has 100 watts into 8 Ohms and 200 watts into 4 Ohm to give the impression their amp is robust and powerful because look, it's doubling it's power as the impedence halves.

In reality the amp will probably be outputting closer to 115 Watts and while 200 Watts into 4 Ohm is still very good, the impression of doubling as impedance halves is a powerful tool. Since understating specs isn't illegal in the way overstating specs is, they're not exactly committing any offense doing so.