Amplifiers store charge in the capacitor reservoire to permit short-term high current output. If an amplifier really needed a 30 amp draw from the wall, it would have a different power cord. Most wall outlets are limited by wiring and breaker to 15A.
15 amps at 115 volts represents 1725 watts...more than your toaster! Think what this would do to the tender voice coil of your speaker. Obviously the average power delivered to the speaker is only a few watts. Speaker sensitivity typically runs about 90 dB for 2 watts. 90 dB is quite loud.
If you stand on a wet concrete floor and place a finger over the positive and negative poles of capacitor of a recently turned off amplifier you will find (via your hair being on fire) that energy is, indeed, stored in capacitors, not transformers (except as the transformer acts like a capacitor as a by-product).
I am not aware of any home audio transformer that requires 30 amps steady state(roughly 3700 watts at 115v).
If you are curious about your audio current draw buy a "Kill a Watt" digital meter through Ebay which will give you a real-time amperage/wattage reading.
I am not connected to "Kill a Watt" in any way or have one for sale.
Don't confuse average power with peak power. Peak power comes from caps and can be more than 15 amps but you are playing really loudly by that point. Average power will be A LOT lower -hard to say exactly because it depends on lots of variables.
Amp efficiency also plays a part, most class AB amps being 40% efficient or so. My McIntosh amp meters rarely average 30W with pretty sensitive JM Lab speakers. Factor in efficiency and the amp draws about 75W from the wall (which is less than 0.7 amps). Arthur
All good points. Remember, as I said at the beginning, if the amp has a standard wall plug, it was designed to expect no more that 15A from the wall!
The power cords on most amps are based on their rated power output at 8 ohms. Even then, this doesn't mean that the cord that was supplied is adequate for what the amp is capable of.
As speaker impedance is dropped, most amps will draw more power as they try to put out more power. As such, it is quite possible for a good sized unit to draw more than 15 amps. As an example, let's use the very efficient Sunfire amplifier as a reference point.
At 8 ohms, the "standard" 2 channel Sunfire is rated at 300 wpc @ 8 ohms, 600 wpc @ 4 ohms and 1200 wpc @ 2 ohms. When tested by a third party reviewer, this amp actually delivered 387 wpc @ 8 ohms, 768 wpc @ 4 ohms and 1460 wpc @ 2 ohms. These readings were taken on a steady state basis i.e. clipping and not a temporary "peak" reading. Peak power would be even higher but for a much shorter duration. As a side note, power output at clipping is the truest way to tell just how well built and stable an amplifier is. This one obviously passes with flying colours.
Obviously, these figures are quite respectable and "almost" a perfect voltage source. Given that this is a stereo unit, it is actually putting out twice that amount of wattage as you have two channels. This means that the amp can, if called upon to do so, deliver almost 3000 watts of output at low impedances. Since all amps draw more power than they put out, one can see that if one were standing on the throttle of a very low efficiency low impedance speaker that could handle gobs of power, it is possible to pull WAY more than what a 15 amp circuit is rated to deliver. Granted, this would be a rare occurance to ever need this much power, it doesn't mean that the amp couldn't take useful advantage of having this much power available to it at any given time.
What happens when we need high current from the wall and we can't get everything that we need? It's simple. The voltage rails on the amp sag, power output falls and distortion occurs. This is a form of dynamic compression and is a non-linear distortion. By non-linear, this means that the distortion doesn't occur all the time, only during given conditions ( like peaks ). As such, if we can provide the amp with a very high and steady voltage, we can count on the amp to deliver the goods in the best way that it can and do so in both a steady state and peak manner.
Now take all of this a step further. What does one get when one runs a high powered multi-channel amp for HT and / or multiple amps for two channel? You can end up with quite a bit more draw. On top of this, most amps aren't anywhere near as efficient as the Sunfire amps, making them "gulp" down power whereas the Sunfire would be "sipping" from the AC line as needed.
As as side note, the "Kill A Watt" device mentioned above works only when testing devices that pull a steady state load. In other words, one can test their system at idle OR when playing music IF the amp is of a true Class A design. Given the dynamic nature of music and the dynamic draw of power from the wall to reproduce it, the "Kill A Watt" isn't fast enough to actually measure and respond to the changes as they occur. As such, if you use one of these devices to check what your system is playing as you normally use it, the readings will be measurably lower than what you are actually pulling. As such, if one were to test the Kill a Watt with a light bulb, the readings would be close enough for a guesstimate. With an audio system under a dynamic load, the readings are near meaningless. This is not to say that you can't check individual components to see what they are pulling at idle, but that one shouldn't rely on this device for accurate "in use" readings while "jamming". Sean
PS... I had driven my Sunfire Signature, which is rated for 600 wpc @ 8, 1200 wpc @ 4 and 2400 wpc @ 2 ohms into thermal shutdown on a couple of different occassions. I was using a 4 ohm speaker with multiple woofers. As such, one can see that it is possible to pull GOBS of power, not only from the amp, but from the wall too. After sending the amp back to Sunfire for some upgrades and circuit changes, i've yet to have the amp shutdown on me again.
You are perfectly correct, of course, and one can draw more than 15A from a 15A line for varying periods of time. My point remains that the performance of any amp from an honest manufacturer is specified on the basis of a 15A line (whether they cut corners on the power lead or not).
The preactical limitation on performance is whether, considering everything else that may be in the system or on that power line, there are 15A available for the amplifier.
One thing to keep in mind:
A 15A branch circuit is designed to provide somewhere in the neighborhood* of 15A under steady load. Circuit breakers are thermal devices - if the load on the circuit increases above the max capability for a significant length of time the circuit will heat up and the breaker will trip.
The breaker will allow instantaneous loads of much higher than its nominal capacity to flow through. Thermal circuit breakers are relatively slow to react, so an amp can draw a large amount of current when necessary without causing a breaker trip. Only when the overload is prolonged will the breaker trip.
Therefore, most power amps made for home use will do fine on a 15A circuit. If more is required, special circuits and power cords will be mandated by the manufacturer (e.g. Levinson No. 33 monoblock).
* The actual current available is dependent on several factors, including temperature/humidity, age of the circuit breakers, condition and size of wiring, utility power output, etc.
Feedback around an audio amplifier enables it to properly follow the signal input even if the "rail" voltages fluctuate. If the rail voltage diminishes, the transistor (or tube) is just turned on more. I am sure everyone is familiar with the way that a power amp will continue to play for several seconds after it is turned off. During these several seconds the rail voltage "sag" is catistrophic, but there is little immediate audible effect.
Sean has a point, but... If an load device has the ability to draw more than 15 amps (steady state) it will not be certified by UL to be furnished with a standard Nema 5-15 (straight two prong one ground)power plug. If your product is designed to draws more than 15 amps on anything more than a transient basis and you equip the product with a 5-15 plug you are violating UL and NEC rules and leave yourself open to both civil and criminal liability if injury or property damage results.
There is no doubt that a load higher than 15 amps may be drawn for very brief periods(such as motor start or musical peaks)but, as noted earlier, these loads are transient. HD motors and audio-amplifiers usually have capacitors specifically to cover this temporary situation.
If you think your Sunfire can draw more than 15 amps steady state (no matter how inefficient the load) just hook it up to a Kill a Watt and use a test record for whatever frequency you want for as long as you want. I would be amazed if you show a 15 amp + draw and/or you pop a 15 amp breaker.
The point is this is why G-d and/or the EE wizards have created power inlet/output plugs rated for more than 15 amps. My air compressor and air conditioner have a NEMA 5-20 plug and require a twenty amp 5-20 outlet and circuit breaker.
This question sometimes goes into la-la land where there is a "mine (dedicated circuit) if bigger than yours". So, put in a 100 amp 4ga circuit if you must for your 100/100 watt amp. It might make you feel good but the electrons won't care.
Whilst the above discussions are all technically correct concerning wall plugs, UL, current draw by the amp, etc., has it occurred to anyone that Doc777 maybe confusing the current capability often quoted by big muscular power amps ?
This includes Sunfire, Musical Fidelity, Harman Kardon etc who frequently quote high current capability to drive difficult loudspeaker loads.
Currents way in excess of 30A are often quoted which is not so difficult to achieve with the higher power solid state amps.
With a typical 50Vdc rail voltage, and an extreme steady state case, this 50Vx30A=1500Watts and still within the 1725Watts of your typical wall plug.
Kal: If what you say is true i.e. that all "legit" manufacturers power ratings are based on a 15 amp AC line feeding them, there are quite a few products and manufacturers that are either lying or lack ethics.
Rex: Your response about products and manufacturers providing what is needed to operate them is all well and good. One question though. Do you think that all manufacturers provide what they should when one is operating a product at capacity under the worst conditions? On top of that, do you think that the bean counters at these companies think about this when they are trying to cut production costs to the bone and increase profit?
As far as breakers go, breakers tend to deteriorate with age. As such, a 15 amp breaker can become "soft" with age and become marginal in terms of performance with as little as 10-12 amps of current drawn through it. This is not to mention that connections develop corrosion with age, reducing current potential. Rex did correctly point this out in his post above.
El: I don't know what you are thinking here, but it surely doesn't have anything to do with audio. If voltage sags during a high duress peak, leaving the transistors on for a longer duration will not correct for the lack of dynamic impact that was lost. If the power is not there when it is needed, you can't correct for that later. Loss of amplitude is a distortion of the original signal.
As far as your example about an amp "playing on" without an AC feed present, how long and how well an amp will do this is directly related to the power supply reserve ( capacitor bank ) and how efficient the amp is.
To shoot a whole in that theory as far as audibility of distortion goes, if one has the system cranked and AC sags drastically, the available power falls drastically as the distortion increases. The sound will only remain satisfactory so long as the peaks are reproduced at full amplitude. Once peaks of sufficient amplitude are encountered that tax the power supply reserves, clipping and the associated distortion and power drain set in rapidly.
One has to remember that most amplifier designs are NOT regulated and that many of the newer designs use "power tracking" circuitry that directly follow the rails of the amp. Sagging of the rails will basically cripple this type of amp. The earlier Carver Pro amps ( which aren't actually Carver designs ) are of this variety. They become useless when AC starts dropping rapidly. I know this as i've worked with them in small clubs and had them "browning out" on me due to limited AC supplies.
Zorpman: As i mentioned above in response to Kal, either a lot of manufacturers of BIG amps are lying about their low impedance power potential or the UL is slacking to a very great extent by allowing a lot of "unsafe" products to be marketed wearing their badge of acceptance.
As i mentioned above, the Kill A Watt is NOT accurate under dynamic conditions. I've called the company and discussed this with their "technical department" ( ha ha ha ). Their response was that if you measure a light bulb, it will give a reasonably accurate reading. If you pulse power on a dynamic basis, the circuitry may not have the required transient response to follow the actual electrical demands of the DUT ( Device Under Test ). As such, the Kill A Watt should be used for power averaging and may not even be accurate for that. Having said that, it is a handly little device when properly used and i've done so many times. Sean
PS... Many of my examples in this thread take things to the extreme. Under most cases with most amplifiers operating under what most would call "normal use", a solid 15 amp line will be more than enough power to feed a two channel amp. Then again, the amp is going to pull what it needs, so if there are other components on the line, the amp may end up "robbing Peter to pay Paul" in terms of starving those components. This is why many folks run dedicated lines for the amps with separate AC feeds for the line level gear. This equates to reduced voltage sag for the entire system.
If you've got a big high bias amp, like to listen loud in a big room with low impedance low efficiency speakers, you might want to think about running a heavier line. I have six amps running in my main system and they are all "high bias power suckers". My HT system is rated at 6400 watts RMS. My office system uses 800 wpc monoblocks. My bedroom system is bi-amped. Now you know why i consider "weak AC" a REALLY bad thing.
NEMA 5-15R, 15 amp receptacle, two or more can be on a 20 amp breaker. Wire size #12 or larger.
Maximum continuous full load current on a single nema 5-15R receptacle is 12 amps.
Maximum continuous full load current on a sigle NEMA 5-20R is 16 amps.
Maximum overcurrent device, breaker, for a NEMA 5-20R is 20 amp.
This is according to NEC, National Electrical Code.
This also parallels UL standards.
A 20 amp cord cap,NEMA 5-20P plug, has one vertical straight blade, the hot ungrounded conductor.The other straight blade is horizontal,the neutral, the grounded conductor.This would be looking at the wall with the ground prong down.Hot on the right, neutral on the left.
30 amps, no way on a NEMA 5-15P. I just can not believe it could get a UL listing.
Chipman...A good observation. I suppose one could have an amp with ten volt rail voltages that could put out 150 amps, which would be 1500 watts. It would probably work better as a welding machine than an audio amp.
Chipman: The figure that you quoted would be power output related, not power consumption related. On top of that, 30 amps per channel is not a huge amount and you would have to double the figures that you quoted for two channels. One should keep in mind that most current ratings are based on peak capacity. Some manufacturers "cheat" and quote peak to peak values, which would be twice as high as the peak capacity. Sean
Many thanks for all the help. Initially I felt my question might have been too "newbie" but I find my others concerns were answered too, like a dedicated 20 amp line etc. I love this room!
Sean, I totally agree with your comments. If you read carefully the first sentence of my response that is exactly what I said and thats why I think the whole discussion concerning 30A draw from the mains supply is a red herring !
In my example of 30A OUTPUT current, equating to 1500W - in my book this is big whether peak to peak or rms unless you have some very power hungry speakers!
However Power output is directly related to power input.
You can never output more power than the mains can supply, and is actually usually less because of ineffiencies of amplfier designs. Amplifiers (and mains supply) can on an instaneous basis supply more than rated current and manufacturers use this quoting music power, pk-pk etc for marketing purposes.
No sane manufacturer is going to commercial a domestic amplifier that needs more than 15A of continuous current from the mains.
sean...About circuit breakers...when a tree next to my house was struck by lightning almost every fuse in the house blew, but not a single breaker opened. (Give me fuses any day).
About rail voltage sag...if the audio signal peaks at 35 volts this can be output by a 70 volt rail voltage with the transistor conducting 50 percent, or by a 60 volt rail voltage and the transistor conducting 58 percent. Because the rail voltages are heavily filtered in the power supply (and sometimes regulated) the "sag" (as the name suggests) will be at very low frequency, which any feedback loop can easily follow.
Not sure if this is true but I read that if you wind the power cord or extension cord in a circle, say 6-7 times, when lightning strikes, the cord will melt like a fuse.
No charge for that one :)