Tuesday
Aug102010

What's a watt? How to rate amplifier power.

Back in the day, James Watt was developing his steam engine everyone knew that power was  rate of doing work or using energy. Time was part of the unit. Confusion was minimal.

Flash forward to present day audio specifications and confusion reigns. Power in watts is the go to number for amplifier buyers. Power is good and more power is better. Marketing types know this and  some products are specified in marketing watts or paper watts. 

Short of outright fraud, the lever used in these calculations is that the RMS voltage content of an audio signal is lower than that of a sine wave. The rule of thumb is amplifiers are used at 1/8th power. 

So should an 800 W amplifier run continuously at 800 W , 100 W or in between? If it's a PowerPhysics amplifier it will run for a minute or longer at its rated power of 800 W and forever at one quarter to one third. These numbers will vary if custom heat sinks are used. 

Amplifier run time should be independent of frequency. PowerPhysics amplifiers will output full power for the same amount of time at 20 Hz or 20 kHz. 

Amplifiers that require a Zobel network to control ringing without a load may  limit run time at high frequencies. PowerPhysics amplifiers do not use Zobel networks because unless the components are very large they can melt at high frequencies. Be warned.

The bottom line: Run your own tests. Do not buy paper watts.

Tuesday
Aug102010

One World, One Power Supply? Universal AC input voltage.

Yes!

PowerPhysics' switch-mode power supplies feature automatic input voltage selection. This circuit adjusts the power supply to accept an AC line voltage anywhere in the range from 88 to 275 Volts.

And better yet, the user doesn't have to do anything. Just plug in the power supply and it does the rest.

Tuesday
Aug102010

What is Power Factor Correction (PFC)?

PFC is required by the European Union CE mark for amplifiers greater than 600 Watts*. It makes sure the power supply does not create cycling currents in the AC power grid.

The PFC circuit is in the input stage of the power supply. It aligns the phase between the AC voltage and current to use all of the available circuit breaker current for amplifier power. With PFC you can run more amplifiers from a given circuit breaker.

With PFC the power supply becomes a resistive load which does not force the AC wiring in the house or building to carry cycling or "imaginary" currents. These currents are labeled imaginary by engineers because they never generate power for the speaker. However, they do cycle through the building's AC circuit breaker so a small non-PFC amplifier can trip a large breaker.

 

* The actual specification is for harmonic content and you have to calculate the amplifier power.

Tuesday
Aug102010

Why are Class-D amplifiers so efficient?

Class-D amplifiers transfer power directly to the load. The output switches (MOSFETs) are either ON or OFF. 

In the ON state the switch is saturated or "shorted". It can pass lots of current with very little loss because there is only a small voltage across the switch.

Power = Voltage x Current. If Voltage or Current are zero then the power (in this case power lost to heat) is also zero.

In the OFF condition the switch is an open circuit and there is zero current flowing through the switch.

In a linear (Class A, A/B or B) amplifier the output devices have both voltage and current across them at the same time. This creates large losses and the requirement for low bus voltages and/or massive heat sinks.

Monday
Aug092010

What is special about One-Cycle Sound?

One-Cycle SoundTM is the fastest reacting PWM control law. It compares the audio input directly to the PWM waveform using non-linear feedback. This creates high-fidelity, transparent audio amplification. 

A typical PowerPhysics One-Cycle SoundTlM amplifier has 300,000 switching cycles per second. OCSTM makes 2 corrections per cycle or 600,000 corrections per second! 

One-Cycle SoundTM corrects:
  • Audio signal changes
  • Power supply noise and ripple
  • Amplifier power stage (MOSFET) errors
  • PWM rise and dead time errors

And it happens every 1.7 micro seconds. 

No other amplifier works as fast to deliver high quality audio.

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