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Woody1911a1 28th February 2012 11:23 PM

Fetzilla Build Documentation
5 Attachment(s)
Here's what I've collected and to the best of my knowledge it's all the latest renditions .

The BOM I used as a general guide and ordered resistors and other components to my preferences . not quite sure who that came from . Use at your own risk .

Enjoy , Woody


My sincere thanks to Woody, this is the complete FetZilla documentation endorsed by me, now made 'sticky'.
I have also appended here a complete distortion analysis of the FetZilla. Two highly competent EE friends of mine here in Melbourne took distortion measurements of the FetZilla on Monday 27th Feb 2012. They are taken at 1KHz, into an 8R resistive load, at outputs respectively of 1W (+10dB), 13.3W (+20dB), and 26.5W (+23dB):

Fundamental 10.00dB Fundamental 20dB Fundamental 23dB
H2 -93dB 5.012E-10 H2 -78dB 1.585E-08 H2 -78dB 1.58489E-08
H3 unmeasureable H3 -93dB 5.012E-10 H3 -88dB 1.58489E-09
H4 unmeasureable H4 -105dB 3.162E-11 H4 -93dB 5.01187E-10
H5 unmeasureable H5 -102dB 6.31E-11 H5 -103dB 5.01187E-11
H6 -96dB 2.512E-10 H6 -106dB 2.512E-11 H6 -110dB 1E-11
H7 unmeasureable H7 -110dB 1E-11 H7 -101dB 7.94328E-11

H8, H9 and H10 are all unmeasureable

THD by voltage 0.00274% (1W, +10dB); 0.01284% (13.3W, +20dB), and 0.0134% (26.5W, +23dB)
Measurements below about -80 dB can be assumed to be insignificant except in ultraclean designs

These figures are surprisingly good, but with output bias set to 370ma the first 5.5 watts of output are in Class A, so perhaps not so surprising after all.

I would add that the THD does not correlate well with the sound qualilty; rather, it is the profile of the distortion artefacts that affect the perceived sound quality for the human ear.

Not bad for a jfet, two mosfets, and two lateral output fets - only five active devices in total!

My thanks to Graeme and Paul for generously giving their time to measure the FetZilla!



Woody1911a1 29th February 2012 12:18 AM

1 Attachment(s)
sorry turns out you can't upload an excel file here , so i .zip ed it :)

RonR 9th March 2012 08:33 PM

Hi Hugh,

I think the latest Schematic for the PSU can be seen in post 383 of the "Swordfishy/ASPEN FETZILLA power amp" thread Here, where R5 = 10 Ohms rather than the 100 Ohms in this thread. I'm not sure if there are other differences between the versions there and here.



P.S. My own Fetzilla build has hit a bit of a roadblock at the moment, but more of that later....

AKSA 10th March 2012 11:07 AM


You are quite right; in the later version, post #383, I moved to a lower value because I found that the bass was more prominent this way.

No other changes to the sound however, but no one I know passes up more bass!!

You are no doubt reserving your build for the warm summer months!!

Good luck with the project, and thanks for your post,


AndrewT 10th March 2012 11:43 AM

I changed C12 to maintain time constant.

AKSA 10th March 2012 08:07 PM

Not sure why you bothered, Andrew, makes no difference whatever to the sound quality.....


AndrewT 11th March 2012 10:26 AM

why bother fitting C12 or even the RC filter if it does not clean up the power rail.

AKSA 11th March 2012 10:32 AM

Because we are endeavouring to reach 'better sound', according to subjective measures, NOT 'clean up the power rail' if this appears to reduce bass.

Remember, this was designed by dedicated subjectivists with absolutely no technical clue, my friend....... go ahead, clean up the power rail, it will undoubtedly improve the specs, after all!


AndrewT 11th March 2012 10:35 AM


Originally Posted by AKSA (
........ NOT 'clean up the power rail' if this appears to reduce bass.

Where did this come from?

AKSA 11th March 2012 10:10 PM


If you increase R5 to match the 100R on the other rail, you reduce the half wave rectified 'ripple' and thereby improve the quality of the DC, clean it up, if you will.

Correspondingly, if you leave R5 at 10R and boost the associated capacitor by a factor of ten to preserve the time constant, again, you clean up the rail, no question.

BUT, in both cases, for reasons uncertain, you lessen the bass delivery. Hence the decision we took.

Another reason for this misunderstanding might be that your comment here:


why bother fitting C12 or even the RC filter if it does not clean up the power rail
looks very like you are laying down the gauntlet, and I reacted accordingly.

Unfortunately, I say what I say, without ambiguity, and where doubt exists about why, I also say so. I'm certainly not the smartest sausage on the barby, but I strive to do my best.



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