Reminder: how to tell current from an amp's specs?

At a given impedance, say 8 ohms, the current of the amplifier is directly related to the specified wattage. If your speakers are rated at a flat 8 ohms across the frequency spectrum, more watts equals more amps, i.e. current.

The issue becomes more complicated when a speaker has a widely varying impedance rating across the frequency spectrum. For example, my Thiel CS6 speakers range from about 2 ohms up to around 8 ohms depending on the frequency. In order for the amplifier to supply the necessary power to allow the speaker to have a flat frequency response it needs to double its output with each halving of the impedance. My Krell KSA 300S outputs 300 watts at 8 ohms, 600 watts at 4 ohms, 1200 watts at 2 ohms, and 2400 watts at 1 ohm. When people talk about a high current amplifier this is what they mean.

Current relates to amps. The more amps your amplifier produces the more "current" it has. You can calculate the amperage of your amplifier from the wattage specs at the rated impedance at those wattages. Amps are calculated by dividing Watts by Volts. 300 watts divided by 120 volts = 2.5 amps. 2400 watts divided by 120 volts = 20 amps. Big difference.

If an amplifier outputs 500 watts at 8 ohms and 500 watts at 4 ohms it would not be considered a high current amplifier - at least in my world. It doesn't matter how big the power supply is or how expensive the parts are. It simply cannot output the amps that a speaker with a demanding impedance curve requires to produce a flat frequency response. There's nothing wrong with that, however, if you are running a speaker with a flat impedance curve and the amp has the watts to drive the speaker to an acceptably loud level. Devore speakers, for example, are designed to have a flat impedance curve so they can be driven by a wide range of amplifiers that don't produce high current.

People get wrapped around the axle about "current" but most speakers present a fairly benign load and do not require the heroic output of a Krell, Pass, D'Agostino, or Levinson amplifier. But if you are running certain Wilson Audio, Thiel, Apogee, or other speakers that present a difficult load then a high current amp is necessary for them to sound their best.

Sorry for the long post. I hope this helps.

I have to disagree with atmasphere on the issue of low impedance Wilsons doing fine with tube amps. Any speaker that has a demanding impedance curve will not give a flat (to the best of its ability) frequency response if it is not driven by an amp that provides high current (doubles output with halving of impedance).

I'm not saying that you can't use a tube amp with a big Wilson but the speaker will not produce the frequency response that it was designed to produce. You are basically introducing an unpredictable tone control for the speaker. You may like the sound or you may not but you are not getting the sound that the designer heard when he voiced the speaker.

There are speakers like Devore Fidelity that are intentionally designed with a flat impedance curve so they can be run by virtually any amp. My Thiels would sound anemic with a tube amp because they drop to around 2 ohms in the bass region.

@invalid First, when I took in my Krell for rebuild I hooked up a few integrated amps and receivers that I had on hand just to see how they would sound. It was unmistakable that these amps produced a different frequency response than my Krell. Specifically the bass was reduced in volume and "anemic." I extrapolate that a tube amp would produce the same result.

Second, there is a very good "Thiel Owners" thread on Audiogon where this phenomenon has been discussed. I partly based my statement on real world experience of several Agon members.

Third, I've been to three different audio shows where they had the larger Wilson speakers. In every case they were driven by D'Agostino amps that double their power down to 1 ohm. I've read many times in reviews and forums that the larger Wilsons sound their best with a high current amp. Reviewers have also noted that when they drive a speaker with a difficult impedance curve with a tube amp the bass is often weak. Check out the review of the Octave Audio Jubilee tube amp in the September Stereophile. JV Serinus describes this exact phenomenon with his Alexia II speakers. Then, in the testing section John Atkinson praises the amp for having lots of power into 8 ohms but he doesn't recommend it for use with speakers where the impedance drops below 4 ohms. I've read similar examples dozens of times. Over and over again reviewers state that the tube amps they are reviewing are not appropriate for speakers with difficult impedance loads.

I can't believe there is even a debate about this. It is well known that driving a speaker that has a variable impedance curve that dips below 4 ohms with an amp that cannot produce the appropriate current at the low impedances will change the frequency response of the speaker. It's a well documented fact. If you can produce evidence to the contrary please do so.

This is an interesting exchange that addresses one of the most important aspects of high end audio: how amplifiers and speakers interact. So I'm going to keep going at the risk of being "outgunned," In my world facts and evidence are more important than the reputation of the participant.

If the amplifier is able to act as a voltage source at less than full power then the contray is certainly possible. For example the Osiris amplifier that @akg_ca mentioned can't double power at full power as load impedance is halved, but at lesser powers it can. In a case like that when the current demand is exceeded, rather than changing the FR the amp will simply clip.

I belong to a Krell Forum where there are several people who have very high levels of expertise, including a former CEO of Krell. I have asked this very question, i.e., Does an amp produce more relative current at low power output than full output? In other words, if a 100 watt amp is rated at 100 watts at 8 ohms and 100 watts at 4 ohms, will the amp perform better when it is running at 10 watts? Could it produce 10 watts at 8 ohms and, say, 15 watts at 4 ohms? The answer was clear and unequivocal. The power output as a function of impedance is a ratio that stays steady throughout the amplifiers power range. Running an amp at low volumes does not make it perform better into low impedance loads. In the Stereophile review I see nothing saying that the amplifier performs better at lower power output. If that the case why did JVS hear the reduced bass through his Wilson speakers? He certainly was listening at levels far below full output. Also, why didn't JA mention this in his testing. He clearly said that the amp wasn't appropriate for speakers with impedances below 4 ohms. Why didn't he put a qualifier on that statement explaining that it would be fine at low volumes? If anyone can provide evidence that supports the idea that amplifiers are more capable of driving low impedances when they are run at normal volumes please provide it. I am more than willing to change my mind on the basis of good evidence.

Just because your amplifier can drive to 2 Ohms and is able to double power as it does so does not mean its sounding its best when doing so. All amplifiers make higher distortion when driving lower impedances! If you think that distortion is inaudible think again- the increased distortion is audible as increased brightness, harshness and a reduction of detail (distortion obscures detail) because most of that added distortion is unmasked higher ordered harmonics.

My Krell KSA 300S was measured to have 0.12% distortion at 2 ohms, less than 0.11% distortion at 4 ohms, and slightly less than 0.10% distortion at 8 ohms. If anyone can hear the difference between these distortion figures then you have a lot better hearing than I have. I would consider these differences insignificant but if anyone can provide evidence that listeners could hear these differences please provide it. Again, I'm glad to admit I'm wrong if I encounter facts or evidence that contradicts my understanding of an issue.

There seems to be contempt on the part of some forum participants regarding speakers that have a demanding impedance curve. My view is that this is simply a design decision on the part of the speaker designer. They felt that it was a necessary tradeoff to achieve the sound they were after. I think it's going to be pretty hard to tell Wilson or Thiel owners that they made the wrong choice and that their speakers are full of distortion because their megabuck amplifiers perform poorly at low impedances.

I'm going to state my basic point again. You can run your low impedance speakers with a tube amp all you want. If it sounds good to you then, Terrific! Just understand that you are introducing an unpredictable tone control into your system and that you will not be hearing the speakers sound the way their designer intended. Jim Thiel used big Krell, Levinson, and Threshold amps to voice his speakers and he specifically used a Krell KSA300S to voice the CS6 which is the model I have. I personally want my speakers to sound the way they were intended to sound. I suppose that I could try a variety of tube amps to see if I liked any of them but I would rather buy an equalizer and just change the frequency response to suit my taste. Fortunately I am extremely pleased with the way my system sounds and I have no desire to deviate from the type of amplifier that Jim Thiel used to design my speakers.

You are correct but my point still stands. Krell amps are better than pretty much anything on the market when it comes to supplying current to low impedance loads and the increase in distortion is negligible at lower impedances.

Krell and other beefy amps may not be perfect but they are lightyears ahead of tube amps when feeding speakers with demanding impedance curves. Their less-than-perfect performance was taken into account by Jim Thiel and David Wilson when they voiced their demanding speakers using these amps.