How To Measure "Current" In An Amplifier?

Hi gdhal,

If you imagine a sine wave, you can measure it as peak to peak or RMS.  P-to-P is just that, looking at an oscilloscipe, you look at the peak and valley and measure the difference. RMS (root means squared) tries to calculate the area under the curve, and is what amplifier watt ratings are usually based on.

So what the maker is saying is that if you had a sine wave, at maximum  you'd get 22.5 amps in the + direction, and 22.5 in the negative direction.

I'm not aware of any particular standard for measuring current, so this spec is just as good as any. :)

Of course, what matters in high capacitance loads is the current at the zero voltage. That's why you will read reviewers talk about phase angle. The worst conditions for an amplifier are high phase angle (high capacitance) along with low impedance.

Best,


Erik

A high watts per channel amp does not necessarily mean that the amp has high current.

This isn't actually true.

There is something known as Ohm's Law, which, unlike a speed limit, cannot be broken without the creation of an entirely new branch of physics. From it derives the Power Formula.

Power is equal to Ohms times the amount of current in the circuit, squared.
(P=RxIsquared, P is Power, R is Resistance and I is current)

So the fact of the matter is that high watts requires current, and the current cannot exist if the watts are not there. Let's take the example of 500 watts into 4 ohms. 500 = 4xI squared, solving for I we get 11.18 amps.

So why do we see really high 'current' ratings on amplifiers? Let's take a common rating, 80 amps and see what we get. Giving the amp the benefit of the doubt, we'll use a 1 ohm load so we get maximum current...

Solving for Power we get 6400 watts! Obviously there is something amiss.

What is amiss is that the 'current' rating is not a measure of the amplifier's output abilities. In fact it is the current created in a 10Millisecond period when a direct short across the power supply is made. The output section of the amp has nothing to do with it. To give you an idea of what this is about, we make an OTL that makes 220 watts and is not comfortable driving a 1 ohm load although it does fine into 4 ohms. Its 'current' rating is also 80 amps! This is because we have a substantial filter capacitor bank in the power supply and that filter bank will dump that much current when shorted out. It will make a really loud explosive sound when this is done as well- you don't want to be around **any** amplifier when the power supply is shorted out!!

This is one of the bigger myths in the audio amplifier world, part of the horse race for more power when finesse is not a concern. IOW, the manufacturer **could** express the capacitance of the supply (which is in essence what is happening here) in the form of Joules stored (Joules being the raw number of electrons stored, a combination of capacitance and voltage) which would give you a more accurate number, but the horse race for more power means that it gets stated in current instead, precisely because that is so easily conflated with the power output of the amp.

Its marketing.

Erik, yes I agree that some speaker designers might add components to the cross-over to purposely lower impedance, but not for the reasons suggested, but rather for (as succinctly found in paragraph one here):
http://www.stereophile.com/content/thiel-cs35-loudspeaker-measurements#7ti3uFpWxwLfT1ci.97

Thank you Erik. The difference between plus and minus peak certainly makes sense. But then given what your saying that there may not be (or at least you are not aware) of a "standard" for measurement, then I imagine it should be difficult to compare current specifications among different manufacturers. Hard to tell if a spec should be considered "good or bad".

In my case the entire output specs read 220 Watts per channel into 8 Ohms, Voltage 42 Volts RMS, 20Hz to 20 kHz (onset of clipping), 120 Volts peak to peak, Current 45 Amps peak to peak, Damping factor 180, Output devices 2 pairs per channel.

So would one conclude that given that current rating of 45, it can drive a low impedance load, or can't drive a low impedance load?

@unsound

Thanks so much for using a very specific example, it makes discussion a lot easier. The example you’ve shown for a Thiel is quite benign and an example of thoughtful design.

Please note the minimum impedance is around 4.5 ohms (by eyeball). The Thiel CS3.5 isn’t lowering the impedance to 3ish ohms, but rather it’s lowering what would otherwise be pronounced impedance peaks around 12 Ohms or much higher to be more "normal." This type of network, while drawing a little more power, makes the speaker very tube friendly, while maintaining good performance with solid state amps. The frequency response of tube amps often track the speaker impedance so by flattening the impedance above the woofer's resonance Thiel was making sure the speakers would behave similarly regardless of amplifier used. Good for Thiel for thinking of this in advance.

That’s not the kind of charlatanism I’m talking about though. In fact what I'm describing is the opposite, where impedance is brought to 3ish ohms or less, and reviewers, like buyers, are amazed that the speakers can sense such a difference in amplifiers. The Thiel CS3.5 on the other hand would play very nicely with a variety of amplifiers.

Best,

Erik