High Current vs high power


Hi!

Often you hear/read comments that the current matter more than the power (example Nait) and one should look for high current more than high power etc.?

Can anyone explain that or debunk the myth (my limited physics knowledge tells me that power and current are interrelated for the same voltage and impedance)?

Also, which amplifiers (pre power or Integrated) have 'higher current' than their peers at same power ratings? Is there any specification that shows the current capability of the amp which one can read and compare? like power, THD etc.?

Thanks
K
kelpie

Showing 4 responses by atmasphere

This is a commonly misunderstood concept in amplifiers. What is meant by 'high current'?

Does it mean that the amplifier behaves as a voltage source, and so can double power as the speaker load is cut in half or does it mean something else?

It means both but often not at the same time.

However it might be helpful to run some math once you see the advertised 'current' figures for a particular amplifier. I remember one amplifier advertised 80 amps. That seemed a bit crazy to me, so I ran the math, and gave the amp the benefit of the doubt. Here is what I came up with:

The Power formula is Power=Current squared times Resistance.

So if we have 80 amps and a one ohm load, the resulting power is 6400 watts (the 'benefit of the doubt was that I allowed it to drive one ohm- do the math for four ohms and you see what I mean). The amp in question did not claim to make any such power, so its obvious that the current rating meant something else.

What it means is the amount of current that is seen when the power supply is shorted out for 10 milliseconds. It is a measure of power supply capacity and nothing more. Can this affect authority in the amp? Sure- as the additional capacity can reduce intermodulation distortion at higher power levels. That has a direct effect on impact.

Just to give an additional bit of information- to make 1000 watts into 4 ohms the amplifier will need to make 15.81 amps. That's all!

IOW, most of the time when you see that expression- 'high current' it has nothing to do with the amp's ability to deliver power. And ultimately it is power that drives the speaker, and power is the combination of current *and* voltage.

You also have to be careful of the speaker. Not all speakers like an amplifier with a super low output impedance- some will play bass with more extension and impact if the amp has a higher output impedance (and such an amplifier might not be described as 'high current'). This is because there is more to it that just simple low output impedance in the amp; there is some physics involved.

For more about this see http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
^^ Yes, audiophiles have seen those pesky spec sheets for amplifiers for decades, but so little on them tells us what is going to sound right that you still have to do the audition. More fodder for the objectivist/subjectivist debate...
The Rowland 8Ti is a good example of what I was talking about.

Into 4 ohms it is rated 400 watts, but the current figure states(from the website):
50 amps continuous, 100 amps peak

Its pretty obvious this has nothing to do with power! 50 amps continuous into a 1 ohm load would be 2500 watts, yet we can see from the specs that the amp is unable to double power between 8 down to 4 ohms. Even it if could continue to double power to 1 ohm, the output power would "only" be 1600 watts.

What is being stated here would seems to be that the power transformer can provide 50 amps on a continuous basis. I suspect though that if that were to actually happen it would blow its mains fuse in less than a second.

So what we can conclude without speculation or debate is that this 'current' figure is not a real-world spec so much as it is a measure of the capacity of the power supply. I have seen, FWIW, tube amps with this much and more 'current'.
Here is a link that might help some with this equipment matching issue: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Al is correct- THD is the sort of thing that may well have a reverse correlation with listening experience and for the reason he mentioned. I'm not a fan of *high* THD either, as the ear will convert the experience of distortion into one of tonality (odd orders contribute to brightness, the 2nd harmonic contributing to lushness). If distortion is present, detail can be obscured by the ear's masking principle.

The bottom line here is understanding how the ear perceives sound, and working with those rules rather than against them with the audio equipment design. Because designers frequently do not understand the way the ear hears things, we often wind up with spec sheets that don't seem to tell us anything about how the equipment sounds, even though that is what the spec sheet really should do.

In short, as an industry we often measure things that are not important (and place excess value on those measurements) while at the same time not measuring the things that **are** important (and placing no value on them at all...). Hence we still have to audition audio equipment.