How To Measure "Current" In An Amplifier?

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.

Atmasphere, If I may: wouldn't that variable impedance load provide for a less linear frequency response and resulting timber distortion than the steadier impedance load of the Thiel via tube amplification?

You may! :)

The answer though is 'no'. The variable is how much feedback the amp has, not whether its tube or not.

Its not going to take a lot of feedback to get the voltage response needed for flat output on Erik's speaker. My guess in looking at the curves is that about 6 db is all that's needed. Most tube amps with feedback have more than that.

OK, but even with that feedback, unlike many ss amps, the vast majority of tube amps can't double down/halve up with varying speaker impedance and the corresponding varying sensitivity, and I would assume therein deviate from linear frequency response and ergo(?) distort timber, no?

No. What happens, and this is the part where we see that tube amplifier power is more expensive, is that the amplifier makes its full power into the lower impedances, and makes less than that (for example, 1/2 power into 8 ohms as opposed to 4 ohms) into higher impedances.

With a solid state amp, we are used to seeing it double power as impedance is halved. The difference here is that a tube amp is going to make its normal power into the **lowest** impedance, while the transistor amp is going to make its normal power into the **8 ohm** impedance.

I can see how this could be really confusing, so the other way to look at it is that in both cases the amplifier has to act like a voltage amplifier, which is to say that it will be constant power into all frequencies presented. Since the tube amp can't double its power as impedance is halved, instead it cuts its power in half as impedance is doubled.

Either way you get flat frequency response.

The real question is, 'Is flat frequency response important to the human ear?' which is another question entirely and the answer is 'sometimes'. The problem is that the ear converts any form of distortion into some form of tonality. The brain has tipping points where if there is enough distortion it will favor that sort of tonality over actual frequency response (whether flat or not). This is why some amps can sound bright and others not even though on the bench they measure exactly the same frequency response. The brightness can be caused by a tiny amount of higher ordered harmonic distortion or by IMD.

This is why speaker/amplifier matching is still a thing, even though speakers are **supposedly** voltage-driven! IOW you still have to take it home and see how it sounds, even after all the measurements!

@erik_squires

It is NOT tube friendly like the Thiel. Notice the 24 Ohm peak in the bass and 16 Ohm impedance peak where the woofer and tweeter response meet.

http://speakermakersjourney.blogspot.com/2016/05/lm-1-bookshelf-measurements.html

These peaks are what the Thiel crossover was trying to avoid.

Actually your speaker looks a lot more tube-friendly than the Thiel.

The issue here is 'will the amplifier produce a flat frequency response with the given load?" and if the tube amp has a damping factor of 20 or so, which is easily achieved, the answer in your case is 'yes'. In addition, it will do it with lower distortion and greater power than it will with the Thiel. This is simply because the Thiel requires the use of the 4-ohm tap of the amplifier driving it, and the 4-ohm tap is not as efficient as higher impedance taps, has less bandwidth and greater distortion.  Your impedance curves strike me as benign insofar as tubes are concerned.

The amps we make are low or zero feedback, but I would give them much better chances at driving your speakers than the Thiels on account of the impedance. Its more important for impedance to be high than it is to be flat.

Regarding your amplifier, the specs don't add up as presented so there is more to the story. The 45 amp figure seems to refer to output. Converted to RMS as Al did (so it makes more sense) the 15.9amps does not gel with 220 watts. Into 8 ohms at 220 watts the current is 5.24 amps. The 15.9 amp figure makes more sense if the load impedance is slightly less than 3 ohms. At this point the amp would be making a bit over 660 watts. Does that sound right to you?

These reviews indicate the Musical Fidelity can do 440 watts into 4 ohms and 800 watts into 2 ohms. I’m not stating that is or is not true, but IF true, would that support the 45 peak-to-peak amperage claim (or the RMS conversion to 15.9)? And, to the point of this thread, that it can "easily" drive a low impedance speaker?

'Yes' to the latter, no worries. 

To the former, 800 watts into 2 ohms is 20 amps. Thats 56.6 amps peak to peak. If that is so, why would they state that 45 amps peak to peak is the spec?

Your saying that tube amps halve their power as impedance is doubled? Isn't that what ss amps do?

Yes, it is.

Maybe the way to look at this is the 8 ohm power. With a tube amp that is 35 watts, it will also be 35 watts into 4 ohms, and if the impedance of the load is double will then be 17 watts.

With a solid state amp that makes 35 watts into 8 ohms, its output into 4 ohms might be 70 watts, so 35 watts into 8.

This is what I meant about tube amplifier power being more expensive.