House Amperage vs Stereo Amperage

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!

Kal

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
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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.

Sean-

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.

Kal

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.