F5 power amplifier

[labjr]

I would think reliability is an issue with switching supplies. Too many parts. I imagine if this was a good idea for Class A amps then the master would be doing it already.

[bobodioulasso]

Low efficiency simple amplifier / high efficiency sophisticated PSU.
What a strange idea?

[Melon Head]

It would be nice to get Nelson's comments on this.
If all things are considered and Nelson says it is a complete waste of time.
I will forget about it.

[Nelson Pass]

I don't think it's a waste of time at all. My take on it is
that if you use it, some sort of good passive filtering is
probably a good idea.

Maybe I'll buy a pair of those nice open switchers and try it.

[Melon Head]

Tomorrow I am going to do some noise measurements on my switch mode powered B1 buffer before and after the rc filtering, to see what the difference is.

[chrismercurio]

I like my battery powered B1. (try that too)

[Russellc]

I'm only to page 222, so sorry if I havent got there yet, but was wondering what everyone is going to do now that FQA19N20C and FQA12P20 are discontinued? I ordered a parts kit from Tech-DIY, but was wondering if there is a replacement?

Thanks, back to the thread,'


Russellc

[fritz39]

Speaking of the Fairchild parts...

I was looking at the threshold voltage of the FQA12P20 vs the FQA19N20C and
noticed there is about a 1V difference. That wasn't the case with the original
MOSFETs. When you adjust the bias to get 1.3A one pot will be higher than the
other. Doesn't that make the gain on one side of the input fets higher than the
other? Does that make any difference or does the feedback take care of it?

I also noticed on one of the pictures from an F5 review that a 2.2 ohm resistor
was put in parallel with the 0.47 ohm resistor on the N-Channel side. Does this
have anything to do with the difference?

P.S. I haven't matched anything and the output on an oscilloscope looks very
symmetric. I just don't understand why.

Thanks.

[Nelson Pass]

Is it the output or the distortion that looks symmetric?

[Beftus]

Originally Posted by Russellc

I'm only to page 222, so sorry if I havent got there yet, but was wondering what everyone is going to do now that FQA19N20C and FQA12P20 are discontinued? I ordered a parts kit from Tech-DIY, but was wondering if there is a replacement?

Thanks, back to the thread,'


Russellc

You can use IRF9240P's and IRF240N's instead.

Originally Posted by Nelson Pass

The output MOSFETs are the IRF240 N channel type (Q3) and the IRD9240 P channel type (Q4). They will want to have a voltage rating in excess of 50 volts, a current rating of about 15 to 20 amps, and a dissipation of about 150 watts. Comparable parts are widely available, and here I used Fairchild FQA12P20 and FQA19N20C.