Monday, 14 July 2014

The Aa Line Preamplifier I – The Mechanical Construction of the Enclosure

Hello to everybody reading my blog,

now after the end of the soccer world championship it might be time to start a new series of entries about the building of a DHT preamplifier as central component of a future orientated system. Several times I have quoted my extremely positive experience from my Aa Line preamplifier, which I do use since 2004. This was my first realized preamplifier incorporating directly heated triode tubes in the small signal section of my chain, namely the line stage of my set up. When I realized this concept almost ten years ago, I did not expect to be so much convinced one day that such a preamp will go to be my final word in the line amplification stage.
At the beginning it formed a extremely well sounding approach towards refinement, but in 2004 my chain was not in that developed stage, where it is today. The first remarkable side effect came when I started to rediscover my digital signal sources again. For more then a decade my Meridian CD player was into storage. With my Platine Verdier Record Player and my time living in the US, created a concentration into vintage vinyl as foremost signal source. But around 2005 the digital storage of music attracted my attention. I did start to record my vinyl for digital storage at harddisks. The combination of "lossless" storage and Apples hard- and software (Airport Express w. digital out, lossless format to be supported), formes a easy access to digital systems within my analogue tube amplified audio set up. Together with my new Aa-line amp it created a extremely refined sounding system, with lots of merits I didn't know from my analogue arm. In these days I did use my audio mainly to listen to my analogue jazz record collection, from time to time I made some recordings from vinyl just for storage issues. I didn't take the digital side to serious, I was a lot more shaped by my early experiences from the 1990ties, when early digital recordings and cheaply made transcriptions from vinyl to CD created my worse experience with digital and formed the decision to leave it for the "consumer".

When I had to move my studio in 2010 I had to adapt new listening conditions. At the beginning I did start just to set up a digital streaming chain as first set, in order to have some music by hand. It came as is always happens, new room acoustics did create new courtesies, I started to like the digital arm of my set up quite well. Together with a new interest into actual and therefore newer recorded music, I realized a unknown conservated audible quality. It was very obvious that in the recording part of audio in the last decade a huge quality step was realized. Completely digitally recorded, mixed and finalized takes showed up with a remarkable close and accurate preserved soundstage, in a way of perfection I rarely didn't know from vintage vinyl. All the typical aural fingerprints, made by every analogue component in the recording chain, the microphone, its input transformer, the preamp, the mixer, the tape recorder, the amplifier and serveral other chained components seem now not any longer exist in a conspicuous way. Instead a much more homogenous and digitally corrected sound image seem to be the new standard of actual audio preservation today. A better focused staging with unknown lots of captured air is the typical side effect of modern well made recordings. All the former unavoidable phase shift effects seem to be gone with digital shaping, instead a wide, dynamic and refined window to lively recordings seem to be now some sort of standard. With the exceptional resolution of my line preamplifier, these qualities were the foremost sensual facts to be experienced, which I rarely did only know from a handful of my best analogue recordings. The DHT tubes combine a extreme musical coherence with absolute linear resolution in a way, indirect heated tubes don't show. The Aa Weitverkehrsröhren are exceptional top performers within the common known DHT's for small signal amplification. I did try other tubes as well like the widely used 26 tube ore the TM2 with similar success, as well with transformer coupling, but I will stay with the German Post Tube, since I like its extremely open, transparent and dynamic sound better than any other. They are made once for 10000 hour service in professional installed telephone relay systems and have some other mechanical advantages the most other DHT's don't show up with, because these typical US-american tubes once were made as consumer products for radio receivers in the 1930ties. The availability of German Post tubes is far better in my culture than any US-DHT and there is the variety of three different brands with different properties to find. The rare and expensive original early (from 1927 on) tube from Siemens (with tip and glass tree inside), the common Valvo type (globe type with mica spacers) and the last version made in the former East Germany by Neuhaus Röhrenwerke (with brass socket and coke bottled glass shape and ceramic spacers).

So I started after almost 20 years to buy again some CD's, giving me a new unknown listening pleasure on the first hand, but as well as streamed data beam digital sets a comfort and usability I never could establish with vintage vinyl recordings. Vintage vinyl is still unique in different fields, it wonderful large size covers, the printed informations and a typical liquidity in the timing of their sound, digital formats do not cary. Today I use both systems side by side with mating success, but the operability of streamed data sets the pace for the years to come. So the quality of the line stage is the key for that kind of use and needs to be of exceptional ability.

First  evaluation stage of my Aa-Line preamp in 2004, equipped with battery clips for the heaters and here showed with rare Siemens Aa tubes, identified by the tip.

Solid mechanical construction with complete relying at oil caps and solid silver wire for the signal path. On the left: the variacs for the lead batteries and a stepped attenuator for volume control.

But I will not conceal that there are some problems with the use of thoriated tungsten filament direct heated triodes, other tubes don't show. With directly heated triodes the cathode acts as heating element for the tube, which means the heater is part of the signal path. The most people know this problem from power tubes, where it creates sometimes hum problems, which can be solved quite easily, because of the higher signal to ground ratio. But at a lower signal levels and following further amplification, any little hum sign will be amplified up to solid levels. Here the problems needs to be solved completely. To get it work, it needs advanced solutions for the heater supply. In the beginning I did start with classic regulators (LM317), these bring a lot of radiated heat to the amp and don't sound very good. After burning my fingers at the chassis of my preamp I did decide to look for other solutions and ended up with a very uncomfortable solution, I did use lead batteries first. Now I had to charge every day my batteries in order to have enough energy for some hours of listening pleasure. I did try several different battery types, since the cheap Chinese lead batteries didn't last very long, after 100 loading cycles they lost  energy to be stored. So I switched to nimh-batteries. I got a quite good sound but only for two hours time with one set of batteries. So I had to buy several sets, which needed to be exchanged after a good year of use by charging them every day. I liked the sound qualities a lot, but hated the charging practice and the all around contact problems in practice. Always some hiss or hum happened needed some helping hand to fix the contact problems for further listening. Not comfortable.

In 2010 I switched to Rod Colemans CCS boards, which make life a lot easier. These constant current supplies are sounding as good as battery power and do not burn tremendous amounts of heat like other regulators do, when well implanted. These can be used with almost any DHT triode, independent of current amount with good success. So they are as well a good solution to be used with power amps.
Now after almost 10 years of use with this preamplifier and after final evaluation comparing it to other options with my new speakers, but as well in other chains, I decided to bing this concept into the next level and final stage. This will be accompanied with several blog entries.

It needs a well made mechanical design to find the long term usability. 

Now, in the final stage the line amp is going to be the switching center for all types of sources and needs several in- and output options. Only the best components are prefered for the job to be a final preamplifier, here with Lundahl iron, NOS Daven stepped attenuators and a Elma switch. Classic WE components are a good guide how to make a well organized amplifier.

For my new approach the Line amp becomes a central position in my chain. Beside the analogue system arm with my vinyl record player, the dac will feed the line stage with digitally streamed music using the comfort offered by wlan and with the iPad as a remote system, since all data are stored at the hard disk of my computer. For the future this is a important and foremost feature to be extended as main source? As well a recording option has to be kept be in mind, which will create other problems to be solved with tube amps.
I did try to listen again to some conventional line amps in the last couple of month. After this exceptional evaluation with very different amplifier concepts of advanced stage I was restless convinced about this preamplifier and therefore I did decide to bring it into a final state. For absolute perfection it needs a much lower output impedance to work flawless universal with every tube amp. With the high impedance of the Aa (30 K) it is almost impossible to match this preference, so some tube amps are not happy with the high load and connecting cables are getting a difficult choice. So, staying with the Aa tube and relying to its extreme unique sound qualities make a fundamental change of the implementation necessary. Cutting the internal resistance into half as a result of the doubled tubes into parallel mode, might be the way to get half of the output impedance. It should sound as good as before with much improved load into the most input stages of tube amplifiers.

Read on soon about the next steps, Volker

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