"High Current"

Keis: Your comment makes it look like high rail voltages have the potential to suffer more than a lower rail voltage. That simply is NOT the truth. ALL voltages collapse under demand IF there is a lack of current. The use of a higher impedance load simply reduces the need for as much current, but one could still run into sag IF the circuit was still not "beefy" enough OR the circuits response time was not quick enough. As such, rail voltages have little to do with sagging whereas they have a LOT to do with the dynamic capabilities / headroom of the circuit.

If one needed 1 amp, 10 amps or even 50 amps of current, so long as the circuit could deliver it on a timely basis, there would be minimal voltage sag. The idea behind a "TRUE" high current amp is that it can handle any load at any time due to always having an abundance on reserve. That "any time" part takes into account that the circuit is responsive enough to deliver the needed power as needed, not lagging and therefore sagging under demand.

This is why i said that an amp with high rails, high current, high speed, high stability and wide bandwidth is the ultimate amp. It can deal with any load or signal related condition thrown at it and do so with finesse. Sean
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Sean,

I really have to disagree with your remarks on the Krell
FPB-300cx. [ Are you sure you are familiar with the "cx"
variant?]

I have access to some pretty sophisticated test equipment -
with pico-second resolution - and the Krell 300cx matches
its factory specs - even with heafty reactive loads.

Dr. Gregory Greenman
Physicist

Morbius: You either don't understand what is being discussed here or aren't paying attention. I never said that the Krell won't meet spec's. What i did say is that we are discussing a different type of spec that Krell doesn't publish. For that matter, most manufacturers don't publish this spec as previously mentioned. While i'm quite certain that the Krell can deliver its' rated output at those impedances, i would be interested to see what it clips at into those loads.

Using the published specs of the Krell as an example, let me explain what i'm talking about. An amp that delivered 400 wpc @ 8, 700 wpc @ 4 & 1250 wpc @ 2, all measured at clipping, it would EASILY meet the 300 / 600 / 1200 wpc spec's. Having said that, it would not be a voltage source or a "true" high current design. It is simply an amp that is somewhat de-rated at higher impedances in order to look "more impressive" or "beefier" on paper.

A voltage source amp ( TRUE high current design ) using the same 400 wpc clipping figure would theoretically produce 800 wpc @ 4 ohms and 1600 wpc @ 2 ohms. While nobody could deny that the "made up" power output ratings at clipping of the amp above weren't "solid", such an amp truly isn't a "high current design". If it were capable of delivering all of the current that it needed, it would "double down" at clipping, not just double the "factory rated specs".

In this regard, the theoretical yet "beefy" amp that easily meets "factory spec's" diverges some 12.5% at 4 ohms from being a TRUE "high current" or theoretical "voltage source" amp. At 2 ohms, the amp nosedives to being 21.9% away from being "theoretically perfect" or a TRUE high current design. One should bare in mind that these "power levels", albeit completely theoretical and made up, are still VERY, VERY respectable compared to most commercially available products. Most amps made today don't come anywhere near these types of figures i.e. they are much worse / further away from being a true high current design.

For sake of comparison to the "theoretical amp" above, here's the factory ratings of the "baby sized" Sunfire amp:

300 wpc @ 8 ohms

600 wpc @ 4 ohms

1200 wpc @ 2 ohms

As you can see, these are the same as the Krell's published spec's that i referenced for the "theoretical" amp referenced above. The actual measured results of the small Sunfire amp at clipping as published in a review dated April of 1995 were as follow:

387 wpc @ 8 ohms

768 wpc @ 4 ohms

1460 wpc @ 2 ohms.

Theoretically, based on the 387 watt clipping figure @ 8 ohms, the "baby" Sunfire should have been able to deliver 774 watts @ 4 ohms compared to the 768 watts that it tested at. VERY close to theoretically perfect as you can see and less than a 1% discrepancy. On top of this, it is 28% more powerful than the published spec, which was already nothing to sneeze at.

As we move to the 2 ohm theoretical output based on the 387 watt clipping figure at 8 ohms, the amp should have been able to deliver 1548 rather than the 1460 that it produced. This is a difference of 88 watts and a divergence of appr 5.6% from theoretically perfect. Obviously, this is still a phenomenal figure but a greater deviation from "theoretically perfect". Even though the amp deviates further from being "theoretically perfect" or a "TRUE high current design", it still surpasses the factory rating by 21.6%. Again, a pretty whopping percentage, especially given the staggeringly high factory rating of 1200 wpc.

Outside of that, one has to wonder if the mains feeding the amp and / or the original small gauge factory hard-wired power cord weren't responsible for the reduced output @ 2 ohms. This is quite possible due to the amount of current draw required from the wall to output this type of power. If the AC system itself were sagging or the factory hard-wired cord were restricting current flow, it would therefore lower the power potential of the amp itself.

Given that these test results were generated on a first generation Sunfire with a somewhat "scrawny" ( 16 gauge ) hard-wired power cord, and the newer models have had several upgraded revisions ( increased power supply reserves ) performed to them with the option to use your choice of detachable power cords, it is possible that they could come closer to being "theoretically perfect" in terms of "doubling down" at clipping.

As such, one can see that most amps truly are NOT "high current" designs. In fact, most are FAR from it. Some amps are "higher current" than others, but as a general rule, "high current" is simply another marketing term used as a buzzword for audiophiles to be seduced by. Sean
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PS... For sake of clarity, the figures used on the first "theoretical amp" were only based on the rated figures of the specific Krell amp mentioned above. The actual figures have NOTHING to do with the actual performance of that specific Krell product. It may perform better or worse at any given impedance, i don't know.

Sean...To put it briefly...we don't operate our amps at clipping. What we care about is distortion-free power, and that's what is usually quoted.

BEAR this in mind :-)

El: I think that you missed the point. The ability to deliver the amount of "clean" power as needed in a timely fashion on a dynamic basis at any given frequency or impedance is what i'm talking about. The closer that one comes to achieving that goal, the more refined, musical and natural their system will perform. The lack of smearing, strain and ringing combined with the improvements in liquidity and harmonic structure that one experiences is an eye & ear opening experience. The music is no longer coming from a stereo system, sounding canned, compressed and "box-like", but actually spreads out and presents a very dynamic yet subtle panoramic view into the recording.

Much of this comes from the improved response times and control that a non-current limited wide bandwidth high dynamic headroom system provides. If you actually measured the peak power required to reproduce specific types of dynamic transients, you might be pretty shocked. When you factor in that the impedance of the speaker varies over the frequency range and that more / less power & ability to control and respond to the signal and load may be required simultaneously, one begins to understand that you can never have "too much", so long as it can respond on a very timely and dynamic basis.

Obviously, one can avoid some of these pitfalls by picking speakers that present a gentle impedance curve with lower levels of reactance, but such designs typically tend to sound somewhat "stifled" to me. That's because the manufacturer has typically added quite a few parts to the crossover to tame specific resonances and problems. This ends up sucking the life and harmonic structure out of the music and reducing the quality of the amplifier / driver interphase.

I guess that it boils down to the fact that we are damned if we do, damned if we don't. There are obviously compromises to be made in all but the very most expensive, well-designed systems out there. I always strive to try and reduce the potential for technically related problems, but when it comes down to it, the bottom line is that it has to sound good. Luckily, resolving most of the technical issues ends up sounding markedly better, so the two goals seem to work hand in hand. Sean
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