I have tested a few power amps under various load conditions and have found that most appear to have a PF of the numbers quoted above at idle and low power conditions. However, when the load increases the PF gets gets higher (up to approximately 0.8 to 0.85). This is due to the rectifier diodes being "on" for a longer period of time. The article appears to differ with the measurements I have taken.
Why?
d.b.
Dan here is the response to your question directly from Mike from APC
The PF strongly depends on a number of factors including the Diode Conduction Angle. The PF and Diode Conduction Angle are affected by:
AC Voltage Waveshape (Voltage on the Primary)
Governed by: Impedance of the Line & Other Electronic Loads on the AC Line Secondary (Transformer Output) Voltage Waveshape
Governed by: Regulation of the transformer -> Impedance of the transformer, winding resistance. etc
The impedance of the line and the regulation of the transformer are the main factors which will alter the power factor. The softer the line, the more the sine wave will flattop and become distorted. Diode conduction time (angle) is governed by the electrical properties of the diode. A diode acts like a one way valve such that the output voltage must be lower than its input voltage. Once the Anode is lower than the Cathode, the diode will conduct. The flatter the AC waveform, the longer the conduction time. This has the effect of increasing the power factor.
If the transformer has poor regulation - if the output wave shape is severely flattened ('flattopped') by the diode bridge, it also has the affect of increasing the power factor. In other words, the transformer is soft and has a high impedance, the PF increases.
One trick used by some designers is to increase the impedance - to force the PF higher with added series impedance. Those familiar with old tube power supplies will note that many have an inductor in series with the output of the transformer. This has two relevant benefits - it increases the PF and decreases in-rush currents. The designer may purposefully design a high impedance transformer too.
The bulk supply capacitance also affects the diode conduction angle and hence the PF. Bulk supply capacitance determines the amount of ripple on the amplifier supply rails and the amount of peak power available to the speakers. Decreasing the capacitance increases the PF while loaded, but increases the ripple on the amplifier supply rails.
Another pitfall when measuring PF occurs when using a VARIAC. Take a look at the VARIAC plots. A moderate quality variac was loaded using a simulated audio amplifier under load. The input was generated by a 15kVA AC source that provided a very stiff and pure sine wave. The output (ch2) of the VARIAC was flat-toped and more trapezoidal in shape. Therefore the PF of the amplifier changed with the addition of the variac.
