The Most Important Spec for Amps for B&Ws

[Kobus]

Currently going through a sudden costly upgrade, I think the single most misunderstood spec for "our" speakers are the following that is quoted from the B&W website. I am not talking about all of you that knows it already, you are the 1% minority.

Quoted from the B&W FAQ's (which I read before, but did not pay attention to - costly):

"The amplifier should be capable of driving the complex impedance load of a speaker. This can require the amplifier to deliver higher current than it would when driving a simple resistor. A rough idea of the amplifier’s ability to drive a complex load can be gleaned from looking at the power output into different impedance loads. Usually, the power output is quoted into both an 8 ohm and a 4 ohm load. If there were no limit on the current capability of the amplifier, it would be capable of delivering twice the power into the lower impedance. Some amplifiers are capable of this; most are not, but the nearer to this figure you can get the better. Some of the best amplifiers can double the power again into a 2 ohm load and so on. Improved current capability translates into better control and dynamics, particularly in terms of bass attack."

What bothers me is that not all amps do give this spec, including some Rotels, marketed by them.

[akhter]

Interesting...I noticed this as well, but didn't really understand it. The spec sheet of my Arcam AVR300 (drivng a pair of N804) says for 2 channels driven the continous power rating is 120w (4 or 8ohm). What does that mean? Seems like not only does it not double, but it stays exactly the same at 4ohms. What does that mean? I have no idea...

[sascha h-k]

...for ML really no problem

regards,
sascha

[jericho]

For McIntosh no problem, it's got autoformers.

[ChrissB]

It's usually dual mono configurations (talking about integradeds) that can double output in half load. Theoretically it is supposed to be the perfect amplification, but many hi end int. amps dont do it and still can drive difficult loads and sound wonderfull

[bigburner]

Kobus,

Rod Elliott, of Elliott Sound Products, has the following to say on this subject. It's just one man's opinion, but it does seem to make sense.

High Current Amplifiers

There are some who insist that the instantaneous current output needs to infinite (or at worst, half this value), and that amplifiers with limited current sound terrible. This is another piece of nonsense.

Let's assume that a nominal 8 Ohm loudspeaker load has an impedance minimum of 1 Ohm at some frequency. This is a bad design, but a valid assumption. This means that the amplifier must be able to supply a maximum of 8 times the normal current. A 100W amplifier would then supply a normal peak current of a little over 3.5 Amps. At the frequency where impedance falls to 1 Ohm, this becomes just over 28A.

So let's have a look at the very worst case possible, where the load is fully reactive and returns all supplied energy 180 degrees out of phase (at this point, the load is performing no work, so if a loudspeaker, is making no sound). The amplifier now has to deal with two lots of current - that supplied to the load, and that returned from the load. Even it it were possible, the worst case above would require a current capacity of 56A, however a loudspeaker that presented such a load to any amplifier will not last long in the market, since it will blow up nearly every amplifier that is attached to it.

There is no audible benefit whatsoever in creating an amp that can supply 100 or 200A, since the load will never need this current and is incapable under any circumstance of drawing more than the applied voltage and minimum impedance will allow (allowing for the reactive component of the load).

Bottom Line on High Current Amps

Most quality amps will be able to supply sufficient current to drive the loudspeaker load. Any more capability than this is a waste of money, since it will never be used. To achieve these extravagant currents, the output stage and power supply must be far larger than will ever be needed in real life.

Class-A amplifiers are generally capable of a very modest current, usually barely above that theoretically needed to drive the speaker. I have not heard anyone claim they are rubbish, because of the low current capability.

The one exception is with extreme crossover networks or other speaker configurations that create a difficult impedance load. It will often be found that some amplifiers cannot drive these speakers well, and others have no problem. An amplifier capable of high current may sound better with these loads, but I suggest that the speaker design is flawed if the designer is incapable of creating a crossover that cannot maintain a respectable impedance.

[Eliav]

Currently going through a sudden costly upgrade, I think the single most misunderstood spec for "our" speakers are the following that is quoted from the B&W website. I am not talking about all of you that knows it already, you are the 1% minority.

Quoted from the B&W FAQ's (which I read before, but did not pay attention to - costly):

"The amplifier should be capable of driving the complex impedance load of a speaker. This can require the amplifier to deliver higher current than it would when driving a simple resistor. A rough idea of the amplifier’s ability to drive a complex load can be gleaned from looking at the power output into different impedance loads. Usually, the power output is quoted into both an 8 ohm and a 4 ohm load. If there were no limit on the current capability of the amplifier, it would be capable of delivering twice the power into the lower impedance. Some amplifiers are capable of this; most are not, but the nearer to this figure you can get the better. Some of the best amplifiers can double the power again into a 2 ohm load and so on. Improved current capability translates into better control and dynamics, particularly in terms of bass attack."

What bothers me is that not all amps do give this spec, including some Rotels, marketed by them.

As far as I remember, Rotel 1095 is rated 200w at8 Ohms. 330 w at 4 Ohms.
Eliav

[sascha h-k]

as far as I remember, the real good amps always double the power from 8ohms to 1ohm ....

regards
sascha

[Sim reality]

I think the problem here is B&W is talking about "ideal" in a absolute sense... Sort of like "the perfect speaker diaphram is infinite stiffness and have 0 mass". Not phsyically possible but by definition there is nothing better then that.

If you wanted to take the ideal literally, the perfect amplifier would have capacitors that are infinitely large, provide infinite amp at 0 ohm resistance and infinite volts at open circuit.

The more appropraite question is how close to ideal do you need to get before you can't tell the difference anymore... The human ear is far from the ideal listening device and the human brain is worse at processing absolutes...

I mean I spent my teens listening to a $500 stereo and I thought it was the bomb... It's only when I started comparing with what I could build in car audio did I realise I was missing something (then got annoyed with the background noise and now moving to ht).

B&W was just providing a suggestion on what to evaluate... It's like saying speaker sensitivity give an indecation of what detail the speaker can reproduce... It's kind of right in a way but it boils down to let your ears be the judge...

[Kobus]

I agree that B&W was just providing a suggestion on what to evaluate. I am not implying that only amps that doubles the output with halved resistance should be considered, but this type of spec does provide guidence as to why a rotel etc will drive B&W's better that a pioneer receiver.

The problem is that novices look at the published spec of a receiver's output at 8ohm and think that it is good enough.

By looking at the addiotional spec will steer people in the right direction.

[scottielee]

in addition, there are major differences between each class of operation (class a, class a/b, class d...) to take into consideration.

[Pieter]

The more appropraite question is how close to ideal do you need to get before you can't tell the difference anymore...

This about sums up the entire audio game.

[jim777]

And what about the damping factor specification??? If the damping factor is low, the output voltage will depend more on the speaker impedance at different frequencies (because the voltage will be divided between the speaker and the output impedance on the amp).

Maybe this specification is related with the "double-the-courant" spec?

BTW, IMHO, McIntosh on the 4ohm tap or my McIntosh without autoformer but rated at 4ohms is so sweet with B&W

[Kobus]

"damping factor", now I tell you this is something that VERY few peaple know about or understand. Where can one read up on this, or are there an easy explanation.

[Sim reality]

Damping factor try: http://en.wikipedia.org/wiki/Damping_factor

It's exactly related to the "double-the-current" spec... Just no one usually quotes it (namely because not many would understand the repurcussions) as even the factor is really just an estimation because in "real world" if the amp had a really good Damping factor it could change an order of magnitude depending on the load... IE: 2-16 ohm load power change would not be linear.

The problem with the real world is that it never fits nicely into a mathematical formula...

[Sim reality]

Sorry... I just realised I sound doom and gloomish...

Looking as specs might give a indecation of what the amplifier is capable of but you have to take it with a pinch of salt...

If the problem of "quality" could have been solved mathimatically you wouldn't see manufacturors spending decades tweaking their systems and coming out with new components that actually sound better then the ones that they replace even though they have simular specs...

[jim777]

Let's say that even if the damping factor was low but a speaker was a perfect 8-ohm impedance at all frequencies without inductive part or anything, the amp would sound as good as any other. With B&W speakers, that have a port (and a cross-over of course), have uneven response. In theory, you need a speaker with a high damping factor if you want to cancel out the effect of this uneven impendance.

The most important specification that is 99.999% of the time NEVER given (because it would be more of a graph or something) is the transient response and the performance at zero-crossing. A pure class-B amp could have a very good SNR at full power, but perform very bad a zero-crossing, which sounds *very* bad with real music.