If the power rating doubles when the resistance drops from 8 ohm to 4 ohm, that's a good sign.
Many companies will offer current ratings when they are high because it's a good sales pitch.
The instataneous current is also important. Newer amps seem to use a bank of small cap's vs 2 big ones. They supposedly recharge quicker giving better transients as well. Since most musical peaks are very quick, the speed or slew rate of an amp can be just as important as steady state current rating.
Disclaimer: I'm only repeating what I've read. |
I don't think that "high current" describes an attribute or a particular amperage threshold that is to be surpassed. Look at it this way: a 50 wpc that can double down from 8 ohms to 4 ohms will put out much less current than a 500 wpc amp that can also double down. Can they each be a "high current" amp? Maybe I'm wrong, but I think the term is a marketing one. |
Cdc and Gs bring up very valid points i.e. yet should teach us one thing: everything is relative.
Having said that, i think that it is generally accepted that an amp that "doubles down" i.e. produces twice as much power as impedance is cut in half is considered to be "high current" for its' power rating. I would go so far as to say that some amps are "high current" even though they might not "quite" make it to doubling. As Gs stated, obviously an amp that is rated for 300 / 600 is going to produce more total current than an amp rated for 50 / 100, but the ratios of voltage to current amongst the two would probably be quite similar. The biggest advantage to using the larger amp would be that you would have a much greater level of dynamic headroom which in turn would allow one to play louder with less fear of pushing the amp too hard / clipping.
Bare in mind that the above is refering to SS amps as tube amps are a completely different beast when it comes to power output vs impedance. Most tube amps are making use of output transformers and are relatively current limited when compared to a "decent" SS design. In terms of OTL ( Output Transformer-Less ) designs, it would not be out of the ordinary to see an amp that put out less power as the impedance was reduced. This has to do with the output impedance of the tubes / circuitry itself since there is no output transformer acting as an impedance matching device to help the tubes along.
Other than that, there is a LOT more to "good sound" than having high current capabilities. Linearity in terms of THD / IMD, bandwidth, frequency response, transient response, level of bias, etc... are all other specs to take into consideration too. The bottom line though is how the unit sounds to you in your system. I've seen / heard plenty of combo's of "good" gear that simply did not match well together. I'm sure most others here can attest to similar experiences also. Sean
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I think this is simple: there is no answer. I would not even go so far as to believe the power-doubling-into-half-load being a good sign because there is no standard in how the power is measured. There are all kinds of tricks - don't understimate the power of marketing. Just try the amp you think should work and see for yourself its performance. This way, the results are 100% accurate! Arthur |
Thanks to everyone for the quick responses. The points of listening to all equipment and never trusting a manufacturer's figures are well taken. Unfortunately, short of buying each individual piece and then reselling it on audiogon, I will probably never be able to hear most of the amps I would be interested in purchasing.
While I think that what I am wondering about is an A/C, not a DC circuit, could I apply P=I(squared)R to figure out how much currrent an amp will push, given a known speaker resistance and output power? Perhaps this is a gross oversimplification that will lead me down the wrong path. |
For the most part, a (conventional) amplifier's maximum current delivery is dependant on the power rating (of the transformer(s) used, rated in VA or KVA. The higher power transformer rating, the greater current delivery potential (all else being equal).
In non-technical terms, the more difficult it is to physically lift the amplifier, the higher its output current delivery will be!
peter jasz |
Hmm. I have amps with current rating, eg., 45 amps, 40 amps, 60 amps. One of them is not difficult at all to lift. Very light in fact, an old Mission Cyrus One integrated that feels like it weighs a pound or two. Yet it was rated at 40 amps, but only 25 wpc. Now, I'm sure you think that's just marketing talk, but the fact is that I used to drive a pair of KLH 5's with it(about 77db efficiency) and it's never had trouble with any pair of speakers. It is a little light on the very bottom, a little warm in the upper bass and seriously rolled off in the treble, but still a very musicallly dynamic little amp.
In general, however, the heavier amps do seem to be rated for higher current. |
Paulwp, welcome to the world of marketing by specs.
Remember, power = current squared times resistance. Using 40 amps and 25 watts, this says that:
25 = (40)^2 * r where r = resistance. solving for r we get a value of about .015 ohms.
In other words, if those numbers are correct, the amp only delivers 40 amps into essentially a dead short. What is the minimum impedance the amp is rated to drive?
At 8 ohms the amp could deliver about 1.8 amps, and at 4 ohms about 2.5 amps. This assumes the amp delivers 25 watts at both impedances, which is probably not true.
In short, these numbers don't really make sense.
I also like the weight test mentioned in previous posts.
As always, trust your ears. |
I remember reading an ad by a mfg. (Adcom?) who said to remove the top of the prospective amp and look at its main fuses. If the fuse is rated to blow at 20 amps (or whatever) how can the amp put out more? -Valid question... |
Ghostrider, the rating and advertising claim is "capable of 40 amps pk-pk," and the claimed continuous wpc number into 8 ohms both channels driven is 30. This amp was actually one of the first I saw, this was 20 years ago, for which any current delivery claim was made. The point was it actually was a very robust little amp had not trouble driving very inefficient speakers that supposedly higher powered amps could not drive. So there must have been something to their claims. Nowadays, I use larger amps designed for high current delivery and stability into any load designed by people who I have spoken with and trust. |
This is a great discussion. I used to think that if the amp doubles its power from 8 ohms to 4 ohms, it's capable of delivering a lot of current. True, this is a good sign, but there are high current amps that don't exactly follow this rule. If you are familiar with the Electron Kinetics Eagle 2 amp by the legendary John Iverson, it's rated at 125 wpc into 8 ohms and 200 wpc into 4 ohms. Yet, it's acknowledged as one of the real 'high current' amps. It can drive inefficient and difficult speaker loads with ease (such as the Apogee ribbon speakers) inspite of its relatively compact size. If you peek inside an Eagle 2 amp, you'll see some other signs of a typical 'high current' amp: a massive power transformer and two oversize electrolytic capacitors, each one the size of a beer can (80,000 mf x 2). I would also tend to agree that a heavy amp is a good sign because it usually means it has a hefty power supply. This is just my two cents worth. |
Most any amp can provide a high level of "surge current" or momentary "peak to peak" output levels, but it is "long term muscle" that separates the "wanna-be's" from the REAL "brutes". Brief surge capacity can help amps sound cleaner on dynamic peaks, but they may still suffer loss of control / smearing on passages that are both loud and long in duration.
As for the Electron Kinetic's amps, the designer could have easily de-rated the amp to spec at 100 wpc @ 8 and 200 wpc @ 4 ohms. Instead, he chose to rate it more realistically and let the performance of the amp speak for itself. The fact that the Eagles have such large power supplies are what give them a big advantage when it comes to bass control and extended duration low frequency passages i.e. they are able to sustain high current levels longer than many other amps and don't "poop out" easily. Sean
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PS... My favorites are the specs that say something like "2000 watts PMPO" on computer speakers, boom boxes, etc... PMPO translates to "Peak Momentary Power Output" but really means "this is what the device is capable of at the point of thermo-nuclear meltdown" : ) |
Paulwp-
I'm not saying it's a bad amp. It sounds like it was well engineered for its price point. But that doesn't stop the marketeers.
Your last post sounds like the old "Instantaneous Peak Power" ratings that amps got in the 70's, before the FTC stepped in. You had 1 watt amps delivering 2000 watts of instantaneous peak power.
Refering to my previous post, the power delivered to a load of resistance r with a current of 40 amps is 40^2 times r, or 1600 times r.
So if they claim delivery of 40 amps at 8 ohms, that implies 12,800 watts delivered.
At 4 ohms, that's 6,400 watts.
At 2 ohms, 3200 watts.
At 1 ohm, 1600 watts.
In other words, just saying the amp can deliver 40 amps doesn't really tell you anything by itself. You need to know for how long, into what impedance, and over what bandwidth.
Otherwise it might mean that the amp can deliver 40 amps for just the quarter second it takes for the line fuses to blow, or the protection circuitry to activate. |
So does anyone have a rough method of evaluating current without physically contacting the amp. All of the aforementioned techniques are sort of "tire kicking"... |
PS - to Dweller
The fuse argument is a good one, but not as you state it. Remember, the amp's power supply converts power line voltage up or down as required by the design (tube or transistor). You can't compare line current directly to speaker current.
So its more correct to say that the fuses limit the continuous power the amp can provide. Power is also equal to voltage times current, so a 10 amp fuse at a line voltage of 115 would limit power to a maximum of 1150 watts.
This number is an upper bound but not a least upper bound, and doesn't take into account efficiency, which would limit power (and thus current) further.
Of course fuses take time to blow, so the instantaneous power drawn from the AC line could exceed this number. |
The LFD Mistral SE has no current limiting circuitry. It puts out 75 wpc. Does this mean it's a high current amp? I don't know. To me, if it was a true high current amp it should be able to drive <1 ohm Apogee ribbons and the only limitation would be maximum loudness based on the rated 75 wpc.
Coreymccann there really is no way to know all the facts from a spec sheet. Even if you could find out the amp rating, there are many other problems which you'll never know about. For example, many amps produce high order harmonic frequency distortions which can be incredibly unpleasant. I spoke with a top U.S. amp designer and he mentioned something about thermal stability or intermodulation (Aball is the expert on this stuff) which isn't even measureable but can be audible.
Another way to get high current is with a tube input stage because tubes can handle much higher current peaks than transistors.
I understand your problem because I am having the same problem. You could spend $5,000 trying out all the $2,000 amps out there to find the one that is right for you. So we all have to live in our own fantasy world to some extent and figure what we own is pretty good (or good enough). Or else go broke trying everything in existance. |
I agree with Valhalla, the weight of an amplifier is a good indicator of performance (power delivery) however it does not necessarily mean good sound quality. It really comes down to how the amp is engineered but having a big power supply can be a plus in a well engineered amp. By the way Valhalla, how are you? We have had excellent some excellent past transactions. Thanks PHD |
