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I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
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Eternally Penultimate Zen - mod #4

Posted 11th May 2013 at 08:32 PM by alexcp
Updated 27th April 2015 at 12:29 PM by alexcp (Added pictures, links)

My first solid state amplifier build, in 2009, was The Penultimate Zen, aka Zen Variations Part 4, aka ZV4. It was built with the parts I could find back then, without knowing where to look. It ended up big, heavy with two 400VA toroidal transformers, ugly inside with some electrical tape here and there, without proper grounding, etc., etc. I was as disappointed with the sound as my wife was disappointed with that heavy black metal brick gathering dust.

My first mod on ZV4 came after I turned my disappointment into finding better part suppliers. I returned to my ZV4 and replaced the output caps with something marginally more decent from eBay. The sound improved a notch. Aha!

My second mod was to rebuild the power supply. I threw away one of the toroids, added a softstart circuit from Hypex, and put in a CRC filter with some Mundorf HC caps. One power supply for two single ended channels forced me to learn about grounding and add input transformers and balanced inputs. The sound improvement was there, but it was almost negligible. Oops.

My third mod was to replace the only important MOSFET in ZV4 with a power JFET from Semisouth (now, sadly, defunct). I actually rebuilt each PCB with the best parts I could find - Nichicon FGs and KGs, 715P Orange Drops, etc. The sound with the JFET got a BIG improvement! The measurements confirmed that, too - I reported on that earlier in this blog.

My most recent mod, #4, was today, and again I worked on the power supply. I replaced the 400VA 2x18V toroid with a 500VA 2x36V R-core, so that I can again enjoy separate supplies for each channel and remove the input transformers. The new transformer is heavy and sits in the middle of the chassis, so I added a separate foot to support it, hoping it may also help with the sonics.

More importantly, I replaced a single CRC filter with two CLCs, one per channel, each consisting of two 22000uF Mundorf HC caps separated by a 10mH/5A Hammond 159ZJ choke. My simulation with PSUD2 promised a substantial decrease in the post-filter ripple, which I did not believe I need, as the filter is followed by a voltage regulator anyway. Nevertheless, I had the chokes and had fun cramming them into the enclosure.

After working on this, I had no time to do the measurements, and only a little time to listen. The system was the Audio Analogue Crescendo CD player, a First Watt B1 clone, the ZV4, and a pair of vintage Celestion DITTON XR 15's. The music was Buena Vista Social Club and The Dark Side of the Moon.

This was not a proper double blind test, and my memory of how the amp was performing before the mod is distant, as I was listening to my other amps recently. However, I do believe that this time a better PSU did make a difference.

The bass improved vastly, and the spatial definition of the modded ZV4 was much better than I can remember from any of my amps. I suddenly realized that I hear the details I've never been aware of in these familiar recordings. I found myself listening to the music, although my intention was to listen to the amp. Good!
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