Thoughts on power factor and DC windings

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
high power factors have greater chance of being achieved in classA amps where the load is fixed and not varying....

No that is not a correct interpretation for the linear power supplies normally used for amps.

You can have the same linear power supply with the same B+ and DC current for a class A set up as a class AB setup when the amp is delivering a rated audio output power. Both setups will have the same PF value. The PF is likely to improve with the class AB as it is throttled back to idle, as the peak value and crest factor of the current waveform will reduce.

What you don't have comparable is the audio output level.
 
ok i have a class AB amp to test. let us find out if theory holds...

edit: how easy it is for the theory to fly in the face of reality...

with class AB amp, power factor varies with music, going as high at 1 to a low of 0.7...

my 2A3 set was a constant 0.92 with music...

so what now?
 

Attachments

  • IMG_0947.jpg
    IMG_0947.jpg
    684.7 KB · Views: 69
  • IMG_0951.jpg
    IMG_0951.jpg
    667.7 KB · Views: 57
i do not have access to those, can't afford....do you own one?
getting the power factor can be done the hard way with an ammeter and voltmeters and true power meter, but that is hard work...

what my results showed, class A is a constant load, class AB is a varying load in terms of power...
 
Ex-Moderator
Joined 2011
As trobbins already pointed out, it's not the load, it's the topology of the power supply that matters. Typical linear power supplies have high distortion in its current waveform, so without factoring in the harmonic content as in the professional power meters, you can not get an accurate reading of the PF using just voltmeters and amp meters.

But as PRR also pointed out, for us consumers, it does not matter a bit, because we get charged the same by the electric company, no matter what our household PF is. The discussion here is just for our own amusement. :D
 
Tony, I'm hoping your PF value for class AB at idle is higher than PF value when thrashing the speaker. That is consistent with the mains AC current pulses being a higher peak amplitude, and higher crest factor, when supplying more load power - the PF changes due to the changing distortion level of the current waveform. Jazbo's link shows that diagrammatically.
 
you can not get an accurate reading of the PF using just voltmeters and amp meters.

You can if your "current meter" could measure true RMS and had bandwidth up to about the 15th harmonic. Well, at least the 7th. Any idea where you might be able to find such an instrument?

The discussion here is just for our own amusement. :D

It might also matter if you want to be sure your transformer isn't being overloaded. Especially for high draw applications like class A and tubes - where that trafo doesn't get to rest much.
 
It might also matter if you want to be sure your transformer isn't being overloaded. Especially for high draw applications like class A and tubes - where that trafo doesn't get to rest much.

it does matter, since i design and build all the traffos i use in my amps, this is a very easy thing for me...

and i follow PRR's advice, build it like a tank, within reason of course...:D
 
You can if your "current meter" could measure true RMS and had bandwidth up to about the 15th harmonic. Well, at least the 7th. Any idea where you might be able to find such an instrument?
Even a true rms current meter has some 'doubt' as to accuracy, especially for 'high C' filtering were the current pulse peak is relatively high and narrow, and the crest factor exceeds most meter accuracy capabilities.
 
Even a true rms current meter has some 'doubt' as to accuracy, especially for 'high C' filtering

I think I'd trust my DSO, and the accuracy of some quick numerical integration in Excel. It really is as simple as adding up all the little time slices. Fourier? Again, Easy peasy for a computer. Phase shift at the fundamental? Can measure that with the cursors directly. I think that covers all the components needed to calculate power factor. Everything you need is already in your own back yard. Don't be afraid of a little math.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.