ZenV4, take four (ZenV4-J+CLC)

My first solid state amplifier build, in 2009, was the Penultimate Zen, aka Zen Variations Part 4, aka ZenV4. 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 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 second mod was to replace the only important MOSFET in ZenV4 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 third mod was again in 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.

With the new PSU, the bass improved vastly, and the spatial definition of the modded ZenV4 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.

The system was the Audio Analogue Crescendo CD player, a First Watt B1 clone, the ZenV4, and a pair of vintage Celestion DITTON XR 15's.

 

 

ZenV4, take three (ZenV4-J)

By 2011, Nelson Pass published the details of the JFET upgrade to First Watt F2, Newark was selling power JFETs from Semisouth, and there was no excuse not to upgrade my ZenV4 with the new active device.

I replaced Q1 (see the original schematic) with SJEP120R100A; increased R5 to 130 kohm to bias the JFET correctly with Vgs of approx. 1.5V; and reduced R8 to 22 ohm. I also rebuilt the PCBs with better parts.

The measured result is a nice reduction in THD+N; attached are the graphs for THD+N @ 1W into 8 ohm vs frequency, before and after the upgrade. With IRFP044 as Q1, the distortion was mostly 2nd harmonic (at -65dB), plus some traces of the 3rd harmonic. With the JFET, it is still mostly 2nd harmonic, which is now at -80..85dB, with the 3rd below my measurement floor.

Subjectively, the improvement is remarkable! There is additional detail and depth of the scene.

The THD+N @ 1W into 8 ohm vs frequency with a MOSFET (IRFP044N) as Q1:

 

The THD+N @ 1W into 8 ohm vs frequency with a JFET (SJEP120R100A) as Q1:

 

ZenV4-J PCB, take 3, now with better parts:

ZenV4, take two

In 2009, Nelson Pass' Penultimate Zen a.k.a. ZenV4 was my first solid state build. I used the parts that were available, did not know much about making it work well, and did not have test instruments beyond a cheap multimeter. Also, I did not have a preamplifier to work with it or speakers sensitive enough to enjoy its 25W. As a result, the amplifier turned out not quite satisfactory and for two years has been gathering dust and my family's complaints. I was thinking about scrapping it and reusing the enclosure for something more useful. Instead, I reworked it.

I removed one of the two power transformers (moving from dual mono to stereo configuration), replaced my original dodgy PSU filter caps with Mundorf M-Lytic HC, replaced internal low voltage wiring with thick speaker wire, reconfigured grounding (so it finally was safe!) and installed balanced inputs with input transformers. What a change in sound! With my Exposure CD player, B1 clone preamp (see a later post) and Heresy III speakers, the amp provided enjoyable listening to Placido Domingo's arias. It also had (some) bass now, which had been sorely missing before. Frankly, I did not expect such a change from apparently minor amendments.

Also, I installed a commercial soft start module, which also provides a pushbutton on/off and works with PTC thermistors to protect the amplifier from overheating.

 

ZenV4, take one

My first attempt at solid state amplification was Nelson Pass' Penultimate Zen, a.k.a. ZenV4.

ZenV4 proved trickier to build than the simple schematic suggested. I used a chassis from designbuildlisten and PCBs made with the gerbers then posted on passdiy.com. For my first take, I used the electrolytic capacitors that were easily available and some electrical tape 🤦. The sound was disappointing, so I made a number of revisions to my build, to be posted separately.