The All Channels Driven (ACD) Amplifier Test - page 3
So how could some amp companies circumvent this? We have seen a 7CH linear amp tested in a magazine where they claimed it generated more than 315wpc x 7 ACD, which implies a sustained output of more 2200W requiring more than 40Amps through a single power cord!
APC AV: Circumventing the Rules…
It is ok for product to slightly exceed the branch rating for non-continuous and abnormal circumstances for brief periods. Nonetheless, there are many products in the AV market that lack any sort of NRTL mark - high end, expensive, and well known products lack UL, CSA, VDE, etc approval. Obtaining a NRTL mark does not guarantee performance nor absolute safety. It certifies the manufacturer took the time to design a product to meet commonly accepted standards for safety and regulatory practices.
There are many products that lack a Listing mark altogether or may use terminology like 'Adheres to', 'Designed to', or 'Meets standard'. I'm not sure why they chose to break the law (in some states & localities) and offer a product without a Listing mark. Perhaps their products cannot meet the standards or the company is too small pay for the testing. It is scary to think that if my house burned down and a non-Listed product caused it, the insurance company may not honor the claim. Further, I may be held liable for damages and injuries by using a non-NRTL approved product. The dealer, installer, distributor, and manufacturer will certainly be held liable/negligent for so grossly ignoring code & regulatory requirements.
APC AV: 315wpc x 7 ACD….
Watts are Watts are Watts. Conservation of Power applies to all things electrical - If the amp is supplying 2200W to the speakers, 2200W must be supplied to the amp. Neglecting efficiency calculations, power supplies, etc, an amplifier supplying 2200W continuously will require 18.3A @ 120V - it will blow a 15A breaker...no bones about it.
Let's put this into some context with some definitions of power:
Volt-Amps (VA) - the apparent power is the product of the voltage being supplied to the equipment and the actual current being drawn by the equipment. VA determines the wire and circuit breaker size. The wire and circuit breaker determine the branch circuit rating.
Volts x Amps = Volt-Amps (VA)
Power Factor (PF) - determined by the phase angles between the current and the voltage. It is expressed by a percentage. It does not have any units and defines the relationship of VA to Watts.
Power Factor = Watts ÷ VA
Power Factor = VA cos α
Watts (W) - the actual power drawn by equipment and it determines the power purchased from the utility company. Also known as REAL power, watts do the actual WORK in electronics, motors, light bulbs, etc.
Watts = Power Factor * VA
Editorial Note about Power Factor from APC AV
Most consumer amplifiers utilize large transformer followed by a full wave bridge rectifier and bulk supply capacitors. The current drawn is out of phase with the voltage - the Power Factor (PF) will be less than 1. This is called a non-Power Factor Corrected (PFC) power supply. A PFC power supply has a PF that is nearly 1.
The PF for most consumer amplifiers is 0.65 - 0.72. This means only 65%-72% of the available branch rated current can be turned into REAL power. The larger the amplifier, the worse the PF issue becomes. A light bulb however, does not have any reactive components - nothing changes the phase angle between the current and voltage. It is a called a resistive or linear load. It's PF=1. A light bulb that consumes 1A @120V is a 120W light bulb. (120W = 1A x 120V x 1).
Some consumer amplifiers and most high power professional audio amplifiers use switching power supplies to power the amplifiers. These switching powers supplies may or may NOT employ PFC.
Engineers and power minded readers will note that the AC line feeding your power amplifier will distort and may increase the power factor. The slight increase in power factor will not yield enough of a corresponding increase in REAL power to over come a low PF.
Following the NEC & UL requirements:
Volts x Amps = VA
120V x 12A = 1440VA
Volts x Amps x PF = Watts
12A x 120V x 0.65 = 936W
12A x 120V x 0.72 = 1036W
A non-PFC transformer based amplifier can only draw approximately 1000W of usable power. Ignoring the NEC & UL (consuming 15A), that number increases to approximately 1200W.
Using your 315Wx7wpc channel example, let's look at the actual AC line current consumption (ignoring power supply and amplifier efficiencies):
2200W ÷ (0.65 x 120V) = 28A
2200W ÷ (0.72 x 120V) = 25.5A
Clearly 25-28A grossly overloads a standard '15A or even '20A' line cord and branch rated circuit.. The amplifier would need to be outfitted with a 35A or larger AC connection. Might I suggest a 120V dryer-style plug?
Is it accurate to determine an amplifier's true power rating by only knowing the VA rating of the transformer? Or can this be fudged if the transformer isn't being rated in the linear portion of the VA curve?
APC AV: The straight and easy answer to your question is 'No'. It is not possible to determine an amplifier's true rating by only knowing the VA of the transformer. It may give you a rough approximation, but nothing more.
A transformer's rating is a moving target that encompasses regulation, thermal, frequency, duty cycle, and load information. For example, different regulation specifications are typically provided for different load points. The regulation specification tells the designer what to expect when the transformer is 'fully loaded' given a certain load current wave shape. The load current wave shape will vary depending upon the application - most transformers are specified with sinusoidal current draws.
A step-down transformer whose secondary (output) may supply 59VAC unloaded. This will drop to 55VAC when loaded with a sinusoidal current draw (resistive load). An amplifier power supply will have a completely different load current wave shape and the loaded voltage may be significantly less.
The secondary (output) voltage will always decrease with an increase in load. However, the application, design, and COST determine the amount of voltage drop. Transformers can be designed such that the secondary voltage varies little with load. This is called the 'regulation percentage' or percent regulation. A good transformer should have 3% or less regulation percent. Transformers that have approximately 5% or more are considered 'loose' and have poor regulation.
Regulation Percent = (V no load - V full load) ÷ (V no load)
For example, a theoretical 100W, 4ohm amplifier requires 30V at full power. The output current is 7.05A. Size, weigh and cost are limiting the transformer size, so the designer decides to squeeze as much out of a transformer as possible.
Regulation Percent = (35V - 30V) ÷ (35v) = 14.2%
An amplifier's power supply transformer that drops by 14% under full load implies the transformer is loose. Note: Some call this specification the 'percent regulation' and will specify it as follows:
Percent Regulation= 1 - (V no load - V full load) ÷ (V no load)
Percent Regulation = 1 - (35V - 30V) ÷ (35v) = 85.8%
Like all things electrical, the relationship between the thermal and electrical designs must be fully understood. This relationship will determine the peak vs. continuous current rating of entire system. Transformers will have similar ratings - one transformer may have lots of thermal mass and be able to withstand full load indefinitely with excess capacity for peak loads. Another transformer may heat up very quickly a during the same loading and have no margin for peak loads. Things like heat generated from copper losses (I^2*R), core losses, primary/secondary voltage, number of taps, current, loading, and load wave shape all affect a transformer's rating.
Inexpensive low frequency transformers limited by cost will have less core material and smaller gauge copper windings. For the same load, smaller gauge windings will generate more heat than large gauge windings. The amount and type of core material will also vary with cost. However, transformers, if designed properly, can transfer power at efficiency levels exceeding 98% (or more!) such that the secondary voltage varies little with load.
The transformer is only one part of the amplifier power supply. The amount of bulk capacitance will determine the amount of ripple on the power supply rails. Bypass capacitors and EMI filtration will determine noise immunity. Some amplifiers have regulated power supplies which add a level of complexity and cost.
It is important to understand not one electrical or mechanical component will provide a definitive rating for any electrical device, let alone an audio amplifier.
