The All Channels Driven (ACD) Amplifier Test - page 4
An Overview of Power Supply Basics
Editorial Note on Transformer Basics from APC AV
The most basic transformer is comprised of two wires lying next to each other. Applying a voltage to one of the wires - the primary wire will create a magnetic field. Altering the voltage changes the magnetic field. This magnetic changing field induces a voltage into the secondary wire. Increasing the primary wire's exposure to the secondary increases the efficiency of the induced voltage. Wrapping both wires in a coil increases this exposure. Adding a magnetic material, like iron, will concentrate the field and increase the efficiency even more. Connect a load to the secondary wire and the field will transfer power. The field within the magnetic material (the core) is called flux. The amount of flux in the core is called flux density. The voltage present on each turn is called Volts Per Turn. Different windings can be added and power can be transferred to different loads at various voltages. (The amplifier rail might require 35V, but the logic supply requires 5V. A second transformer is not necessary - just a winding).Transformers become hot because of two factors: the current flowing through the wires (I^2*R) and core losses. Every time flux is induced the core, the material heats up slightly, just like current through a wire. Changing the flux creates eddy (stray, non-productive) currents that do nothing but generate heat. Core material and geometry can minimize eddy currents and core losses. This means most transformers will consume a few watts just be being connected to the wall. Inexpensive transformers use higher loss core material that generates more heat. Selecting larger diameter wire for the turns, lower loss core material, and improved core geometry will lessen the amount of power wasted.
Editorial Note about low frequency AC power supply transformer saturation from APC AV
The term transformer saturation is thrown around too lightly. Transformers typically saturate under a few conditions: over-voltage, too low a frequency, and too light/not loaded. Transformers with too small or under designed cores will saturate when the flux density is too high. Flux is created by the voltage on each individual winding (wire wound around the core) or the Volts-Per-Turn. The amount of flux in the core is called the flux density.
The core typically made of laminated steel or iron is capable of withstanding a specified flux density, and if the flux density it too high, the core becomes overloaded. The magnetic field will collapse and the transformer will act like a partial short circuit. The current will begin to increase exponentially. The transformer core must also 'reset' within a given time period otherwise the flux density will continue to climb and the magnetic field will collapse. This is why DC is never used with low frequency AC transformers.
If the input voltage is too high, the volts-per-turn will be too high. Therefore the flux density will be too high and the core will eventually saturate. If the core is not allowed to reset within a given time period (using a 60Hz transformer on a 50Hz grid), the transformer will also saturate.
Theoretically, a transformer can saturate while under heavy loads, but this is not likely. Increasing the load, increases current and the Volts-Per-Turn will begin to drop. The losses (resistive) and the magnetizing current (current used to magnetize the core) will begin to dominate. The flux added by the primary is 'drained' by the secondary and it would be difficult to saturate a transformer with too high a load.
In summary the type of core material and winding gauge affect the transformer's performance at any load. A transformer without enough core material can saturate readily. A simple experiment will demonstrate transformer saturation. Increase the input (primary) voltage on an unloaded/lightly loaded inexpensive 60Hz transformer. Observe the input current on an oscilloscope with a current probe. The large current peaks is the magnetizing current gone awry - the core has saturated because there too much flux in the core. Transformer cores saturate when the flux density becomes too high - flux density is determined by voltage, loading, and frequency.
APC AV: A great example of a weak transformer - or one that has poor regulation - is a smaller variac similar to those used to evaluate amplifiers. In this example, the variac was setup to boost the output voltage from 108V to 120V. In an unloaded state, the Variac was boosting the voltage by 12V RMS. The variac began to lose regulation as the load was increased to 50% of its rated load. This particular test was only run at 50%, imagine how much worse it will be at 100%!
V no load RMS : 120V
V 1/2 load RMS: 112V
½ Load Percent Regulation: 7%
The clipped waveform also changes the behavior of the power supply. The peak voltage available for the bridge rectifier has been reduced from 170V to 158V - a whopping 12V drop in the peak voltage output. (120*SQRT(2)=170V)
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Variac is unloaded. Ch1: Input Voltage Ch2: Output Voltage |
Variac is 50% loaded Ch1: Input Voltage Ch2: Output Voltage |
The following example is based upon a commercially available stereo audio amplifier. The power ratings were taken from the manual.
100W RMS into 8 Ohms
150W RMS into 4 Ohms
Using only basic power equations, I back calculated the required amplifier output voltage and current to satisfy the loads.
100W @ 8 Ohms - > 28.3VAC RMS, 3.54A
150W @ 4Ohms - > 24.5VAC RMS, 6.13A
These numbers indicate the transformer and power supply are pretty weak - the power supply voltage rails sag sufficiently enough to prevent the power from doubling with half the resistance. If the power supply transformer had sufficient regulation and the rest of the amplifier was designed for it, the output power should look like this:
100W @ 8ohms - > 28.3VAC RMS, 3.5 Amps
200W @ 4ohms - > 28.3VAC RMS, 7.05 Amps
Recall that Power =( V^2 ) ÷ R, P = V x I
A properly designed power supply is going to limit itself to prevent damage to itself and to the load. So any good power supply should prevent excessive current and voltage. Too much current can over heat components. Too much voltage can cause capacitors to vent and components to explode!
APC AV Additional Comments on Amplifier Ratings
APC AV: Consumer amplifiers are rarely if ever called on to produce continuous power - far from it. The duty cycle (ratio of on-time to off-time) is much less than 100%. Different types of audio have different duty cycles and will load the amplifier differently.
Take a look at the chart below from Crown Audio. These numbers are common throughout the professional audio world. Numbers similar to this can be found in the SAMS Professional Sound Engineering Handbook as well.
See: http://www.crownaudio.com/pdf/amps/138350.pdf
Duty Cycle for different program materials:
|
Speech |
10% |
|
Acoustic/Chamber Music |
20% |
|
Full-Range Rock Music |
30% |
|
Compressed Rock Music |
40% |
|
Pink Noise |
50% |
As illustrated above, an amplifier's duty cycle is quite low. So your 315W x 7 channel amplifier may have a worst case duty cycle of 30-40%. With that in mind, let's re-evaluate the amplifier example (neglecting efficiency, power supply design, amplifier topology, etc):
# Channels x Watts Per Channel x Duty Cycle = AC Power
315W x 7ch x 30% = 660W
315W x 7ch x 40% = 882W
Max Average Power Draw
(660 W + 882W) ÷ 2 = 771 W
Max Average Power Draw ÷ (PF x Voltage) = Current Draw
771W ÷ (0.65 x 120V) = 9.88A
So your theoretically, your 7-channel behemoth could be supplied by AC power from a 15A branch rated circuit. However, that is theoretical and not reality. Factor in the amplifier efficiency of 35-60% (topology dependent) and power supply efficiencies of 60-85% and suddenly the amplifier powered by a cord mandated 15A branch circuit may not be sufficient. (Packaging a consumer device with a NEMA 5-15P limits the device to 12A continuous draw which is all a 15A branch circuit should provide to a single load).
An unregulated power supply (transformer, bridge rectifier, and capacitor) can be quite efficient -upwards of 85%. The bridge rectifier losses will scale linearly while the transformer may not. Moderate to high quality capacitors will add little to the overall power loss. Traditional regulated power supplies using linear regulators are horribly inefficient. The quality of the transformer and regulation scheme can easily reduce efficiencies to 60%.
Assuming the best case scenarios for traditional amplifier and power supply design:
System Efficiency = Power Supply Efficiency x Amplifier Efficiency - % Ancillary Loads
System Efficiency = 85% x 60% - 2%
System Efficiency = 49%
Continuous Power Draw = Max Average Power Draw ÷ System Efficiency
Continuous Power Draw
771W ÷ 49% = 1573 W
Continuous AC Power ÷ (PF x Voltage) = Current Draw
1573W ÷ (0.65 x 120) = 20.1A
As explained above, 20.1A, 1573W, 2412VA is too much for a 15A branch circuit limited to 1440VA (12A x 120V). It is too much for a 20A (1920VA) branch circuit! A 30A (2880VA) circuit must be used for this scenario to be correct.
Music has a very high peak to average ratio - otherwise known as crest factor (CF). This CF will ultimately determine the electrical rating of an amplifier. If an amplifier is expected to deliver 90dB @ 1m of sound from a 86dB/1watt speaker, the amplifier will need to produce approximately 3.5W of continuous power. But 3.5W of 1KHz tones isn't pleasant to listen too. A Symphony will have a tremendous loudness range from whisper quiet to shouting/subway platform levels. The peaks will be much higher. That same 86dB speaker would need to produce 60dB (~0.05W) all the way to 100dB (~30W) at your listening position. Those 30W amplifier peaks will be short lest you damage your hearing. Kick drums, movie explosions may drive this number higher, but the other factors being to surface such as speaker abnormalities, room acoustics, and human ear sensitivities name a few.
The bulk capacitance within the amplifier should be more than capable for supplying peak musical demands in consumer audio applications. Thermally, I should hope the power supply and amplifier would be capable of handling 30-50% of its rated RMS output. Electrically, I should hope power supply would be capable of supporting the thermal limits as well as be capable of handling 2-3 times rated RMS output for short periods (kick drum, movie explosions, T-Rex, etc).
The maximum power that can be extracted from the wall is limited by the power factor of the amplifier. Modern sound reinforcement amplifiers have PFC front ends - they draw a PF approaching 1. Therefore, they can extract the maximum power from the wall - all of the power used is REAL power, not Reactive or Apparent VA power. While most consumer amplifiers may ELECTRICALLY be able to sustain rated power output (on a channel by channel basis) for a few moments, THERMALLY they cannot.
The peak power demands are satisfied by internal energy storage (big caps). For the rest of the time, the amplifiers only produce a few watts RMS. An astute observer will have realized long ago a simply fact: if your 315wpc x 7ch amplifier can function well off of a 15A branch rated circuit, it must not be doing much WORK.
Watts are Watts and energy does not appear out of thin air. IF it is coming out the amp, it has to be coming out of the wall and then some.
