Son of Zen

Son of Zen

Front_Plate_LEDI am delighted to introduce this project from Brendon to the site.  Named the Son of Zen, this is a true Behemoth of an amplifier.  The original design is by Nelson Pass, a very well known figure in the DIY Audio Scene.  More details can be found at his website.  This amplifier design is a flexible hexfet based design that requires little adjustment and no capacitors in the input line.  Brendon kept it simple and true to Nelson’s original design.

This article was originally published on Brendon's Geocities website, it is with his kind permission that it is re-published here to preserve it beyond the closure of Geocities.

Completion Date: April, 2001    Revision Date: October 7th, 2001


Rated Output: 0 - 30 Watts x 2 @ 8 ohms - fully adjustable using a Variac
Power Supply: 2000 VA Toroidal
Transformer, 224,000uF total capacitance (112,000uF per rail)
(updated to a 3mH 10 awg 2.65mm size inductor)
Silicon: IRFP-240 Hexfets - 2 per channel (single gain stage)
Dimension: 275mm x 450mm x 630mm (11" x 18" x 25") H x W x L
Weight: over 75kg (165lbs)
Inputs: Balance Line XLR (RCA optional with soldering)
Voltage Rails @ 230 VAC: +/- 24.4 VDC

Front_Plate_LED Front

Power switch and a 10mm size red LED. I used one of those big donking power switch that is rated 15A @ 240Vac. Both the front power switch and the IEC main socket (in the back plate) came from a power supply out of an old 486 computer.


Back_Connectors_1I thought all of the connectors on the back were nicely laid out. Could not move the amp without the handle bars. The ONLY fuse used was a slow blow main line fuse. It's the black round thing above the IEC plug in.

I was quite impressed with the polishing - I give thanks to Reese (my uncle's friend) for letting me use his great machine workshop.


Layout_1 As you can see, the size of my toroidal transformer is quite impressive. Having a weight of 18kg, It was custom made to fit taller and skinnier rather than the standard generic flat and wide toroidals. A 5mm thick copper strap bolts the transformer in place (very solid).

The whole bottom plate has countersunk holes so the plate lays completely flat and smooth.

image I've tried to mount the Hexfets as close as possible to the speaker terminals. Looking closely you can see all the white heat-sink compound that has been squeezed out around the transistors. What happened was I could not find any TO-247 size mica insulator cases so I ended up using the larger TO-264 insulators. Lower left you can see a big black round thermistor used for power turn on surge. I must say it was very effective. All the green wire leads are star grounded.


A Variac is a very useful tool for testing electronics. You can set it to output any AC voltage you want by simply turning the dial. However, when in use you can notice an electrical hum coming from this box which can be a bit annoying. But I would say less annoying than the noise of some electrical fans make. This particular Variac is rated 0 - 260VAC and rated 10A @ 260V.

image The 4 black electrolytic capacitors are made by Panasonic. They are the snap-in type each rated 56,000uF @ 35V for a total of 224,000uF which can cause quite a surge of power when the amp is 1st turned on. Each electrolytic capacitor is shunted with a 4.7uF 100V Phillips metalized polypropylene capacitor. The 2 inductor coils are rated 2mH but will be soon replaced with a larger 3mH coil (made with 2.65mm (10 awg) gauge wire). I hope that would solve the mass heat problems that the original coils had. The resistors did not give off the heat that I thought it would - however, it could be the inductor robbing the heat away from the resistors.

At 240 VAC input the voltage rails measure 24.4 VDC


In the middle of the centre heat-sink is where I put my bridge rectifier. It's rated at 35A @ 400V. Again I used thick OFC gauge wire with 60/40 tin/lead solder. Some people prefer to use exotic wiring like Cardas hook-ups and fancy WBT speaker terminals and etc. I chose not to use any of such kind because I was sure that the cost wouldn't show much improvement in sound.
There are 7 heat-sinks on each side of the amp. The little gold things are wire-wound resistors. The 8 ohm resistors are configured in series (5 + 1.5 + 1.5) each are rated 50 watts with 5% tolerance. The 1 ohm resistors are 25 watt 5% wire-wounds. All made by Arcol out of the U.K.

image The heat-sinks on each side were not easy to construct. I had made countless trips to the local hardware store buying the same drill bits and tapping threaders just to get 4 holes out of each heat-sink. Never the less the hard work was well worth it. Looking closely each side had a right angle aluminium strip that holds them perfectly parallel. For the top cover I used a 10mm size Plexi-glass with the bottom edges being countersunk. This covers the heads of the screw that hold the heat-sink in line with the right angle strip.


I brought this monster to my friend's place who's into high-end sound. We had the following equipment to compare:

CD Player: Nakamichi CDX (manual loading tray)
Pre-amp: Integrated Denon AVCA-1
Speakers: B&W 6P
Amps: Integrated LFN 60watts x 2 special edition
Custom made tube amp 8watts x 2 using Tesla valves

The 1st thing we've noticed was the absence of no hiss or hum. We really had a hard time wondering if the amp was turned on or not. I think if I used a fan to cool my SoZ, I would not realize how noise free this amp really is.

Sound wise, I am very impressed. The amp was wired single ended which is a bummer since more distortion is added going RCA. However, I will say that I personally didn't like the sound of the B&W speakers. They are a great speaker playing classical music passages where it's quiet. But when it comes to playing complicated parts and modern rock / pop music, they were quite stressful to my ears. The LFN sounded quite horrible when playing pop/rock music. It lacked considerable amounts of bass. The tube amp gave the smoothness as expected. But when we played the same songs through the SoZ amp, we were shocked to hear how much bass it produced. The fast transient response and sharp attacked my amp had was no match to the other amps.

I will give a proper sound evaluation as soon as I find a true balance line CD player and pre-amp and a good set of horn speakers. Re-wiring the amp back to balance line is something I really want to do as everyone else is still stuck with single ended RCA.

Many thanks to Nelson Pass for making it possible for me to build my very first do-it-yourself power amplifier. I totally agree that simplicity is best!










Original MCM inductor measurements using my LCR meter. They came out to be 2.07mH with a DC resistance of just under 1 ohm. The wire gauge was 1.1mm (18awg) having a dimension of 7cm (2 1/2") diameter and 2.2mm (7/8") wide and with a weight of about 600g. Maximum continuous DC current rating of around 300 watts was definitely far too small.

The problem with the original inductors was simply - TOO MUCH current and what happened after 20 - 30mins of operation for the first time produced an unpleasant odour in the room. I was sure that the enamel coating on the inductors was getting cooked like a hot potato - so hot that you couldn't touch them for a split second.

Therefore, I needed to build my own air core inductors that could take 1000 watts of continuous DC current. The problem with making larger inductors is the rise in DC resistance. Smaller gauge wires have higher DC resistance so in order to make nice inductors with large values - you have to use large gauge wires.

The end result was making this gigantic coil using 2.65mm (10awg) size wire. It measures 22cm (9") in diameter and 4cm (1 1/2") wide. I intentionally over-wound them to give a 3.18mH rating. The DC resistance of 0.6 ohm was even lower than the original ones.

No more stinky smell floating in the room with less heat coming off the inductors means better listening to my ears.


The inductors are mounted on a wooden base which is tightly secured to the bottom of the chassis.

imageBigger than a soccer ball.

I wonder if donuts come in this size?