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

I have a sinking feeling that I've been here before but, as the subject line says, how can I tell an amp's current from its published specs?

Thanks!

39 responses
 Well, peak current is rarely specified.  We can learn the current specified at different speaker impedances, but the amp may be able to deliver more. The power formula you want to use is:   Power = (Amps * Amps) * R or:   square root of (power / R) = Amps where R is the Ohms listed in the spec. Thank you, Erik. I always read your posts with admiration. This time, though, it's going over my head.  Let me ask it this way. I've repeatedly read that some speakers, notably Maggies, thrive on high current amps. (I've also seen that disputed.) I was noodling around a thread this morning on tube amps and Maggies, and a poster wrote that his Prima Luna was great because it was high current. How did he know that? How can I look at an amp's specs and know if it would be considered a high-current amp?  I can't even see that all specs include amps in their ratings. I can see that some amp makers advertise their equipment as high current. Thanks! (I know that I wondered about this a couple years ago but I don't think I posted on this before. I see lots of comments on currents and watts, but nothing that explains how one call tell if an amp is "high current.") So what do you plan to do with the current after you have a number?  It is generally not useful to know. Let me ask it this way. I've repeatedly read that some speakers, notably Maggies, thrive on high current amps. It really doesn't work that way. What is important is if the amp can drive the load with its rated power. If it can't obviously it will clip prematurely. The 'current' is surprisingly small. Let's say you have 4 Ohm Maggies and you're driving them with 200 Watts. The Power formula tells you the current: Power = Current(squared) x Resistance Power is 200 Watts Resistance is 4 Ohms. The current is thus about 7 Amps. Not that much really. So if 400 Watts then 10 Amps; still not that much. Current cannot exist without Voltage; together they are Watts. You might want to read this: Amplifier myths The 'current' you read about so often with much larger numbers is actually something else! Read the article at the link. Simple. I'm thinking about getting a new amp for my Maggies. A number of people recommend a high-current amp for them. (Some say this is a myth.) The repeated line is: "it's not the watts; it's the current." So I'm curious how people determine which amps have high current. It's that simple. I don't need a number and I don't mean to suggest that it would determine which direction I go in. It just makes me curious how people know if an amp is high current or not. (Let me add that I'm not trying to open a debate about Maggies with tubes or Class D or anything like that.) I'll add as well that I'm a humanities professor, not an electrical engineer. I don't even understand what people mean by "power" out here (In cultural studies, we use that term in a Foucauldian sense.) Sometimes that word seems to mean watts, sometimes current, or voltage, or ....  I once set up a model train for my son. It took enormous focus for me to work through these terms, and unfortunately what little I learned is gone. I may be completely wrong, but I've been under the impression that an amp was considered "high current" if it was able to double (or nearly so) it's output wattage every time the load resistance was halved. So, 50W into 8ohms, 100W into 4ohms, 200W into 2ohms. Many amps won't do this, some will. Thanks to everyone! And @atmasphere , I wrote my response before seeing that you had posted your very helpful response. I think I'm getting it! Ohm’s law alone does not apply to power. If you put a 4-ohm resistor across a 9-volt battery, does that mean the resistor is consuming (9*9/4) 20 watts? No. That would mean the battery would have to accommodate the resistor with slightly more than 2 amps, but it the most it can do about half an amp. So the answer is a high current amplifier is one where it stores enough energy to maintain voltage over the speaker’s varying impedance. You cannot tell from the specs how much energy it stores because most of the manufacturers specify the power at only one impedance (an 8-ohm resistive load). A speaker presents an inductive load which varies with frequency. But you can get an indication of the performance if the manufacturer provides power ratings at less than 8-ohms. Levinson, Krell and the like maintain their voltage (energy) down to two ohms. If an amp can do that at a 1-ohm load, then it’s truly a high current amplifier. Literally. Here is another well-written article on this topic: http://education.lenardaudio.com/en/14_valve_amps_7.html You are not wrong, but this general quick reference rule of thumb is a good quick guide , but is not not an absolute bible . Sure …. In many cases, you can recognize an amplifier with ample current delivery by looking at its wattage output as impedance decreases. A great amplifier will double down, so 100 watts at 8 ohms becomes 200 at 4 ohms. BUT ….and its a BIG “but” … There are exceptions to this general “rule of thumb” quick reference guide. The REGA Osiris integrated amp is one of these exceptions. While it is a beefy hi-current and high WPC amp that doesn’t fully double down, it still has LOTS of current because of its high-end build and design. This is provided by those four Sanken output transistors per channel, that insure that no speaker is too hard to drive. I defer to Ralph for, well, everything but thought I’d dumb it down for the rest of us.  To me, a big honkin’ transformer (sorry Ralph) and the ability to double down to 4 Ohms and maybe double again down to 2 Ohms is a good indication an amp has the balls to power most speakers (did I dumb it down enough here?).  I’m thinking Krell kinda stuff I guess.  Then again, there are obviously some tube amps that can drive certain difficult loads (like Maggies?) so the equation would seem to be, er, a bit variable.  Now I’ll let Ralph tell me where I’m wrong. I had an integrated that "only" specified  and additional 90 watts into 4 ohms (225@8) yet it never broke a sweat powering Maggies. 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. The REGA Osiris integrated amp is one of these exceptions. While it is a beefy hi-current and high WPC amp that doesn’t fully double down, it still has LOTS of current because of its high-end build and design. This is provided by those four Sanken output transistors per channel, that insure that no speaker is too hard to drive. This statement is incorrect. The extra transistors make sure the output section can support the current without damage but if the power transformer lacks the current capacity it really won't make any difference.  To me, a big honkin’ transformer (sorry Ralph) and the ability to double down to 4 Ohms and maybe double again down to 2 Ohms is a good indication an amp has the balls to power most speakers This is generally correct but if the speaker does not have any low impedance dips it could be a waste of money, especially on Maggies which do not have such low impedance dips. I should point out also that not all speakers that dip to 2 Ohms need a lot of current capacity in the amplifier; Wilsons are a good example. They can easily be driven by tube amplifiers even though the low impedance dip is in the bass. Generally speaking Wilson loudspeakers have traditionally been easy to drive. If I can point something else out: 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. 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. I have to disagree with atmasphere on the issue of low impedance Wilsons doing fine with tube amps. Yeah, good luck with that.    If I can point something else out: 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. Like I said before, I defer to Ralph always. @northman "I've repeatedly read that some speakers, notably Maggies, thrive on high current amps. " Quick answer: If you want to listen to Maggies at normal levels a couple of hundred wpc @ 8 ohms is about right. If you want to turn it up, start thinking about a strong 400 wpc. My Mac 400 wpc amp was used up pretty quick on a pair of .7s. Good read, thanks, guys. If I can point something else out: 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. Hi Ralph  I appreciate you pointing this out. It’s very often overlooked in these types of discussions. I have always failed to find the wisdom in designing low impedance/high phase angle (Difficult load) speakers that then require/demand massive amplifiers to drive them. An environment for the appearance of high odd order harmonic distortion.  Charles “… The REGA Osiris integrated amp is one of these exceptions. While it is a beefy hi-current and high WPC amp that doesn’t fully double down, it still has LOTS of current because of its high-end build and design. This is provided by those four Sanken output transistors per channel, that insure that no speaker is too hard to drive. This statement is incorrect. The extra transistors make sure the output section can support the current without damage but if the power transformer lacks the current capacity it really won’t make any difference.   I’m no engineer but it’s still a beefy beast, It’s power supplies are big muthas, it’s a no compromise, super high performance, 162 W into 8 Ω (250 W, 4 Ω), dual mono amplifier housed in a custom Rega CNC machined aluminium case. It weighs over 54lb.that provides more than enough current to drive the hardest of loads. The Osiris uses two low noise, purpose designed 400 VA toroidal transformers using high-grade, fully bonded core material. Eight Sanken 200 W output transistors are used in a “triple” high current output stage enabling the Osiris to drive even the most awkward of speaker systems with ease. @8th-note   how do you know the Thiel's will sound anemic in the bass with tube amps, have you ever tried a high powered tube amp with them? There is a guy running tube research labs gt800 tube amps with apogee full range and he said the bass was way better than any solid state amp that he had tried with them, and the list is long. I asked Wendell at Magnepan that same question and he said:   "If the amp doubles its power between 8 and 4 ohms you will be fine."  In my experience, the Maggie .7 sounds best with class-A solid-state or a good push-pull tube amp.    herb @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. 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. 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. @8th-note I’d just stop here if I was you as it will not end well.  You’re simply outgunned here. There are "facts" and there are FACTS. Personally, I appreciate a civil exchange. I've found this thread to be quite edifying. P=IE.  Power = Current * Voltage.   Problem is that amplifier rail voltage varies and is seldom specified.   Pick an amp that increases output as the resistance is lowered and is stable at loads as low as possible- e.g. 2-ohms. 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. Krell amps don't really double the power when impedance is halved, krell understated the 8ohm power ratings, check out stereophiles measurements. I have that same amp you have it' definitely has an overbuilt power supply, but to double it's power with each halving of impedance it would have to have a perfect power supply which doesn't not exist in the real world. 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. Some Thiel owners say tube amps sound better, with more natural bass than solid state amps, just goes to show you don't know just going by the numbers, listening is the best policy. The upcoming KRELL KSA i400 amp was measured recently with the following numbers according to my dealer. - 400 Class A at 8 Ohm - 800 (400 Class A) at 4 Ohm - 1600  at 2 Ohm - 2200 (I think) at 1 Ohm When I had my Thiel CS3.7 (until 2 months ago) I used the CODA #8 and the KRELL DUO 175XD. Both amps were able to work in the 2 Ohm region. I could definitely hear the difference compared to an amp that could not deliver continuous power at the 2 Ohm region. I felt it was in the area of bass weight at all volume levels.  All that gear is now sold  (KRELL sold today). Some Thiel owners say tube amps sound better, with more natural bass than solid state amps, just goes to show you don't know just going by the numbers, listening is the best policy. in statistics, we call those people outliers. Maybe they are outliers because they are the only ones to give it a try. Maybe they are outliers because they are the only ones to give it a try. +1 Charles Outliers can be cranks as much as the ones on the middle, but the outliers tend to stick out more. They can be wrong, but they can be right. Only time tells, if anyone is still paying attention by then. Normal is for Average. The middle of the herd is almost never paying attention, unless the signaling is gross and overwhelming. Human nature. My particular point to make in all of this, panel speakers being excepted to some degree due to fundamentals of the technology, is that complex expression low impedance speakers are just bad design, IMO and IME. If it exists at all, it is generally there to make some specific kind of point about some aspect of audio quality, or it is just ignorance/illiteracy/incompetence dressed up in marketing - gaming you for some money, in an aura of self importance. And that was pretty well the peak of the pairing of high end solid state and high end speakers in the 90’s, in a nutshell....where most of the peak speakers of that time, the big monstrosity speakers... had impedance plots/curves that looked like a seizure victim trying to draw a earthquake seismic record out for you on a sheet of paper. A downright human embarrassment. Where they had the gall to almost have this complex curve as a badge of honor, which is even worse, as it propagates the mess to some given next level, where the next domino of foolishness comes into existence and is knocked over: The super high current amplifiers of very dubious sound quality due to the designs being too robust, like tractors being used at attempts in subtle deft motional finesse. Where we need a true current unlimited voltage source type of amplifier to drive these low impedance designs and that is very tricky at best. Bad combination to have to deal with. One that tends to exacerbate the differences in amplifiers due to the amplifier stressing playing out openly, due to said complex impedance load presented by the given speaker. This empathically does not say if the low impedance speaker is actually any good or not. But, tellingly, Mr middle of the road hearing ability, who happens to have lots of money, gets to play dress-up, and play golden eared audiophile, as the differences in pairings... are easily heard. And that was, again, the peak audio world of the 90’s to mid 2000’s. It still happens but the money was more free then, and the time was right. So it spun out of control. Eg, Stereophile magazine was closing in on 300 pages in some issues in that time period, with advertising revenue to die for.. In my experience, marketing trumps innovation, every single time. Getting to a better place in audio can be a complex path, but that’s the name of the game for the folks here, so have at it... Some of these hard to drive speakers were made because people wanted full range speakers that took up less real estate.