sound practices reading club /29

Today’s system is already the last one we will cover. The article is Homebrewer of the Month by Thomas Mayer of Munich, Germany from sound practices issue 16. This one is different in that it is not so much a tour of his system, as it is the the story of the development if his personal style of building ultra-fi. From early beginnings, via ever-beefier driver stages, to 3-phase power supplies.There are also two schematics (signal part only, no PS), one for the 211-211 amp and one for a 801A line stage.

teaser quote: ‘I read about Sound Practices in their ad and ordered all issues they had (SP1-5) at the time. This was around 1994. I remember how I was fascinated by all those circuits in this mag. I knew nothing about tubes back then, but I got really hooked. I bought some literature like the RDH4 and started with small steps.’

my take

There is hope. I personally know Thomas, and I know him as a push-the-envelope type of guy, always trying in innovative way to get the most out of tubes and iron. But there was a time, ‘around 1994,’ when he started from zero. We can see where he was by the time SP16 came out—that was at the end of 1998 (I gather). Yes, it takes determination to get there, but it is certainly possible.

Just in case this article gives you the impression that Thomas was sort of almost done at that time, here are some later developments:

• filament bias. This cleverly uses the filament current to bias the tube via that resistor, which will be so small in value (Ohms instead of kiloOhms) that it no longer needs to be bypassed with a cap. Notice that choke in there? Thomas moved to LCLCL supplies for his filaments, (logical, it is a current source then), which other people are also reporting as ‘the best.’
• capless parafeed stage. Very creative (ab)use of the output transformer: originally a beefy step-up interstage, it is ‘turned around’ to be a 2:1 line out transformer, and the high-ish DCR of that winding (1600 ohm) combines with the low-ish B+ to ‘not short out’ the power supply. The kathode resistor is kept out of the picture by the choke.
• another capless creation: an all DHT (!) phono stage. Makes use of the fact that 01A tubes need ‘only’ 0.25A filament current, so that 145V / 2A (!) B+ supply is for 2 channels. Thirty watts of dissipation in each 480 ohm resistor, btw, so not a bad idea to use 120W jobbies for these. I remember hearing a DHT phono stage (not necessarily this one) from Thomas in a casual comparison with a few others and suddenly understood the difference between listening to music (through his) and listening to an electronic device (the others).
• and a last way to get rid of caps: forced symmetry. All the chokes in this schematic make sure that no signal current flows into either ground or B+, eliminating the caps of the power supply.

And just when you could be overwhelmed by this stream of ideas to do more with tubes and iron, here is something that Thomas told me: ideas ripe at their own steady pace. Between figuring that the above filament bias could work and actually trying it out, more than a year passed. So we do not have to feel bad about not trying that great new plan this month.

These days, Thomas has made his profession out of his hobby. He is building amplifiers for a living. Here is an impression of his efforts and a glimpse of his current system.

practical note: I once asked Thomas about what looks like filament bias in the 801A line stage schematic. ‘What filament bias?’ Thomas said. Turns out that the schematic is slightly misleading, an arrow above the 1k5 resistor is missing for the return of the filament supply current.

bonus tracks

Thomas also wrote a very interesting 3‑Phase Power Supply article in sound practices issue 17. update: as a gift from Joe Roberts, SP editor, from the sound practices CD, here is the original 3‑phase article.

update [3-1-15]: Thomas now has a great blog.

That’s it for our systems topic. Next time we will return to power amps (part 2).

Now go and read the article, see you next week.