How Can You Calculate How much Watts an Amplifier Produces?

[bones justice 10+ year member]

ok i've seen a MTX 4 channel amps rated at 70x4 at 4ohms, and 105x4 at 2ohms, which is 210x2 at 4ohms bridged. Also a Power Acoustik amp rated at 85x4 at 4ohms, 105x4 at 2ohms and 210x2 at 4ohms bridged. from the 2ohm stereo ratings, can anyone calculate the 4 ohm stereo rating. Or is there another way to gain an idea of how much watts an amp produce at different impedance loads?

Click to read more...

 

[MiniVanMan 10+ year member]

There's no real clear cut way, as amplifier efficiencies vary.

Both of the amplifiers you listed are probably pretty accurate. Almost gone are the days when people believed an amp produced double to power when impedance dropped by half, and likewise power was quadrupled when bridged.

As you can tell by the two examples you gave, the MTX has an increase of 50% when dropping from 4 ohm to 2 ohm. 50% of 70 is 35. Added to 70 is 105.

The Power Acoustik has an efficiency much worse. At least Power Acoustik isn't trying to hide it. They can make good budget stuff when they stay away from the gimmicks (i.e. plasma screens, etc).

The higher the impedance the more efficient your amp will run. Class A/B amps, which encompass 95% of most multichannel amps on the market run with an efficiency of about 50%. Of course this where the "get what you paid for" comes in.

Class D amps have much higher efficiencies, at the expense of useable bandwidth. A class D can have upwards of 75% efficiency at it's lowest impedances. JL is addressing the bandwidth issue as we speak with some full range Class D amplifiers. There are probably other companies as well.

Clear as mud???

Good

 

[regal8r 5,000+ posts]

multimeter and amp clamp/inductive pickup

 

[blackbonnie 5,000+ posts]

ohms law?

 

[azbass 5,000+ posts]

the crude easy way

take the fuse rating

and multiply it by the voltage given..

so 80 amps x 12 volts = ~960 watts

 

[thch 10+ year member]

I had a fairly basic writeup on this.

in short, the DMM + Clamp don't work for this exact context.

basically, unless you're testing amps under equal conditions, you won't be comparing apples to apples. if you compare the amps power output in your setup to the manufacturer's spec, you might be happily or sadly suprised depending on how you test.

this becomes apparent with a DMM+Clamp if you get something like 10V, 2A. this is 20VA, which we're calling power for simplicity (it is not always*). but impedance is 5ohm. if this is a 1ohm nominal speaker, i would have expected to see 10V, 10A, which is 100VA @ 1ohm.

so, do I ignore the current measurement and say the amp COULD have put out 100VA @ 1ohm even though my test setup is testing 20VA @ 5ohm.**

And of course, manufacturer's publish specs with undistored sine waves. you'd need to test in the same manner. meaning some method of determining THD.

which comes down to the context. while this might not get you the number the manufacturer lists, it will get you some number, and you might be able to use it. just keep in mind that by ignoring distortion or impedance you can end up with power outputs that are either too high or too low.

* the RMS measurements of voltage and current seperately mean just that there is fluctuating current and voltage. but there is no garuntee that voltage and current are ever simultaneously non-zero values. to get an average power above zero, you need V*I to be non-zero, which never happens if at all times you have either V=0 or I=0. (guess how class D works)

** energy storage mechanisms tend to allow current to be impeded, giving rise to possible increases in impedance above nominal. in sealed boxes these can be quite high.

 

[Gods_Prototype 10+ year member]

I don't think going by the fuses is accurate at all. My rockford T400-4 says 100A and its like a 600W amp at most I think. It does power a set of components and a 15" type R though its underpowered a bit. It doesn't make it hard to breath like my old amp but it can still hurt my ears, first time I heard two 15" type R's powered correctly I was like shit son I think I should have only bought one.

 

[helotaxi 5,000+ posts]

* the RMS measurements of voltage and current seperately mean just that there is fluctuating current and voltage. but there is no garuntee that voltage and current are ever simultaneously non-zero values.

Care to explain how with a sine wave you could ever have either one equal to zero at all times? last i checked, you can't. Each one will instantaneously go through zero twice per cycle but that doesn't cover but four instants per cycle. Duration of the instants is effectively zero.

to get an average power above zero, you need V*I to be non-zero, which never happens if at all times you have either V=0 or I=0. (guess how class D works)

Voltage induces the current according to Ohm's law but phase delayed. If there was truly no power, no work would be done. Since we can directly observe that not to be the case, we can conclude that average power is other than zero.

The current does the work anyway and in the grand scheme of things the phase relation between the voltage and current does not affect the actual power realized except on paper.

All this still doesn't answer the OP's question though. The drop in power between 4 and 2 ohm loads is a factor of the resistive losses in the output stage of the amplifier. Since you increase the current flow at the lower impedance, you increase the resistive loss according to Ohm's law. If you were bored, I guess you could figure out the resistance of the outptut stage and figure the loss and the relation between the 4 ohm and 2 ohm power. I'd rather just use the specs.

 

[luvinthebass 5,000+ posts]

I thought Class A/B amps could hit over 50% efficiency? my Rf amp said it was 62% efficient

 

[helotaxi 5,000+ posts]

Class A/B amps can hit well over 50% efficiency, but it obviously varies by amplifier. There are some that are only capable of 30% efficiency into their lowest impedance at full load and others that are capable of 75% ...
Most modern A/B amplifiers are between 45-65% efficient.

You also have to realize that a Class A/B amp is most efficient at full tilt with a sine wave. Music is neither full tilt nor a sine wave, so the efficiency drops like a rock real world compared to a test tone. Class D amps' efficiency don't change appreciably from low through full power.

 

[thch 10+ year member]

Helotaxi, this was in response to the comments on measureing power with a DMM and clamp.

using either alone will provide potentially inaccurate results, as the user guesses the impedance of the system. If they are incorrect, the results are also incorrect.

using both gives a better discription, as impedance is calculable. but without knowing if threre is significant distortion will mean getting a number that the amp's manufacturer doesn't publish.

this includes both measurements in voltage and current.

phase can be exploited as well -- the power supply sags the most when there is a large current draw. the amount of tolerable sag is the least when the voltage output is highest. it makes sense to test the amp with a non-resistive load, as this allows the maximum current draw to occur when the amp can tolerate the sagging of the supply.

knowing these tricks allows one to intentionally misrate an amplifier. not knowing these allows one to accidently misrate an amplifier.

as for the "always zero power" comments, the only time this happens is with power supply components that act as switches. (eg, MOSFETs and diodes in a power supply or class D amplifier)