Tube-rectified power supply for Oto’s phono stage

I recently finished an interesting project I would like to tell you about – a tube rectified, regulated power supply dedicated to Oto’s phono board.

Shortly after converting my Oto Phono SE integrated amplifier into a power amplifier, I built a phono stage using the original parts: a phono board featuring a 12AX7 and a 6DJ8 double triode with a passive RIAA equalization network in between, a power supply based on solid-state rectification, C-R-C-R-C filtering, and the mains transformer. I can tell you that moving the phono stage out of the amplifier’s enclosure improved its performance considerably. There was more silence between the notes, and listening became easier, more relaxed, and more enjoyable. Not only did the power supply have a bit more headroom since the line stage was no longer drawing power, but the adverse effects of electromagnetic interference caused by the close proximity of components inside a single chassis were also minimized.

As a further step in refining the phono stage’s performance, I was playing with the idea of trying a tube-rectified power supply. During my research, I stumbled upon the M2 power supply circuit diagram in the Audio Note Kits L2 Line Preamplifier manual. This tube-rectified and regulated power supply board has gained enormous popularity and has been copied and sold on the internet as a kit ever since it found its way into the public domain. The circuit stems directly from the Audio Note UK line of products, which is an exception rather than the rule. It uses a 6X5 tube as a full-wave rectifier and an ECL82 tube as a voltage regulator. The reference voltage for the regulator is set via a Zener diode. An article describing a similar one-tube regulator based on the ECL82 can be found in Sound Practices magazine here. Another one is described in issue 17 of Vacuum Tube Valley magazine.

I decided to build this power supply using Audio Note UK parts and point-to-point wiring. I prefer to use point-to-point wiring because I find that it always sounds superior to printed circuit boards. A printed circuit board is a serious compromise made to facilitate series production, but it sounds vastly inferior to a well-made hard-wired network. That’s the reason why, in the upper echelons of Audio Note gear, one hardly sees printed circuit boards.

Some conceptual work was done, and a two-board layout was drawn — a large board for the high-tension supply and a smaller one for the filaments. The mains transformer, choke, tantalum resistors, and copper foil capacitors were sourced from Audio Consult in Denmark. Standard Audio Note electrolytic capacitors, tube sockets, and hookup wire were ordered from Acoustic Dimension in the Netherlands. Small parts like the trimming pot, disc capacitor, Zener diode, and bridge rectifier were sourced from Reichelt in Germany. With all the parts collected, what really impresses me is the AN TRANS-0103 preamplifier mains transformer — a big and heavy chunk of iron, the kind one expects to see in power amplifiers.

One very nice feature of the M2 power supply is the possibility of adjusting the output voltage in the range of 250V to 300V on the fly via a trimming pot. As I discovered later, this is important because even small B+ increments in the range of a few volts make a profound difference to the sound. One doesn’t need to swap capacitors or resistors to achieve this effect, but simply turn a screw. I find this feature indispensable since it allows for precise adjustment of the B+ voltage, whose optimal value might depend on the particular tube specimen it feeds.

I can say that the regulated power supply brought a considerable improvement to the phono stage’s performance. I was able to hear more of what was on the record—it was as simple as that. What followed were months of spinning records.

I usually spend extended periods listening either to records or to CDs—that’s simply how it works for me. While the turntable sits on the top shelf of the rack, the CD player stays packed away in its box, or vice versa. Eventually, when I grow tired of the selection of records at hand, I swap the turntable for the CD player, and another long listening period begins. By then, I’ve usually forgotten most of my CDs, which makes rediscovering them all the more enjoyable. I find this habit of not mixing formats to be a positive thing. It provides a refreshing change, not only in sound but, more importantly, in the music itself. It marks the beginning of a new period of discovery and learning. It also prevents me from growing tired of both the sound and the music I have. Otherwise, without change, we gradually become accustomed to both, and eventually we stop appreciating them. Passion and excitement slowly give way to routine. That’s when switching formats becomes especially rewarding.

Another aspect is the cognitive effort involved every time we have to make a choice. The so-called Hick’s law states that the time required to make a decision increases with the number of available options. In other words, the more alternatives we have, the longer it takes to decide. “What should I play this evening — CD, vinyl, or should I stream?” It’s frustrating enough to choose a record from an ever-growing collection, but adding the question of which format to listen to can easily lead to decision paralysis. The fewer choices we have to make, the happier we tend to be. Limiting ourselves also brings a renewed sense of appreciation after a long period of abstinence. I’ve been on an analog diet for several months now, and I can feel that it’s almost time for a change.

Switching between analog and digital playback is a special occasion for me because it happens so rarely. This time, I decided to take the opportunity to try the line stage I had removed from Oto’s innards some time ago. It is a classic one-tube line stage using an ECC82 miniature double triode in anode-follower configuration. The day finally came, and the turntable was replaced by the CD player. The phono board was disconnected, and the line stage, with the 100 kΩ potentiometer in front of it, was connected to the power supply. The power amplifier’s attenuator was turned all the way up, and the valves began to glow.

With the first few bars of music playing, I was literally blown away. It is unbelievable what Oto can deliver when driven by a dedicated one-tube line stage fed by a regulated tube-rectified power supply. I expected to lose some degree of transparency with the addition of another stage, but to my surprise, the opposite happened. Not only did the underlying drive, momentum, and energy suddenly propel the music with unexpected vigor, but the see-through transparency reached a new level as well. Deep notes began to make an impact, and the amplifier was literally brought to life. Of course, the remarkable Audio Note CD 1.1x also played an important role in this transformation — a device that knocks my socks off every time a well-recorded CD is played. Thank you, Audio Note!

A suggestion for jazz guitar lovers: Attila Zoller — Lasting Love, 1997, Acoustic Music Records. I love this CD. The sound quality is outstanding, and the musicianship is from another planet. I often find myself forgetting that I am listening to a solo guitar. His tonal palette is so rich and his rhythmic integrity so strong that I feel as if an entire band is playing. Highly recommended!

That’s it for now, fellows. Thank you for stopping by, and as always, happy listening!