Saturday, 16 August 2014

The Aa Line Preamplifier III – The Completion of the Amplifier Part

Hello to everybody,

today I want to show the completion of my amplifier chassis with the constant current filament regulator boards. As already stated I did use the predecessor of this preamp for several years with different battery types, since conventional dc regulation showed a lot of problems and did not sound as good as batteries. So I had to struggle instead with contact problems, broken cables and dying batteries for years, which was a real pain. I had started with lead batteries, later did I use conventional AA Nimh's, a pack of three makes 1.5 hours of listening possible. So did use several packs, the everyday use and charge cycles made life for them short, almost every year I had to exchange the most packs.
In 2010 I did learn about Rod Colemans Constant Current regulator boards, which have been designed for this particular dedication, the filament supply of DHTs and for the use in pre amplification stages as special option. These boards can be configured for almost any triode type, even the big ones like 211, GM70 or 845 with high current draw figures can be used. For each triode one board is needed separately, which means here in this case with two Aa's running in parallel mode as couple will be fed by one regulator. They don't run very hot, so they are quite easy to use in the amplifier enclosures.

The Coleman regulator boards are mounted to both chassis sides to dissipate the heat and to be very close to the filaments of both Aa couples, here shown with one board.

The regulators are dead quiet and don't show any deterioration of the sound structure, comparing them to batteries. They will be available though Rod Colemans web page and are a great help for everybody using directly heated triodes in audio amplifying devices. They need to be implanted with a raw power supply each, which should be designed to match the final preferences set by the tubes itself. Any extra power will be burnt down into heat by the regulators itself. As example for the Aa with 3.8 volts for their filament a transformer with separate windings for each raw supply with minimum 11 volts output is needed. Each Aa draws 0.5 ampere of current, together it is 1.0. The boards allow the exact setting of the current to match the variations of such old vintage tubes.

Since the Daven attenuators  are extremely difficult to find nowdays, which means it is almost impossible to find them as stacked stereo units in values like 100K or 250K, I did use one mono type for each channel. With everyday use of such a preamp, separated attenuators for each chanel would drive me crazy. With any control of volume you need to match the stereo balance again, a night mare if the amp is not positioned exactly in the middle of the speakers. In order to make the use more comfortable, I introduced a coupling system. With a o-ring both shafts are coupled together by wheels from a toy steam engine. The o-ring keeps both attenuators in balanced mode since I need to adjust one of them. If, I can hold one since I turn the other one where need. We will see if that system will do comfortable within everyday use, otherwise I have to design a spring supported clutch system…

O-ring coupled shafts for the Daven attenuators.


The back shows several sockets for different dedication. From left there is the power input socket (6 pole) where both raw voltages for the Coleman regulator boards will be supplied. The other two contacts are for the filtered anode supply. Next to this socket are two output xlr-sockets, the two main output terminals of the amplifier, these can be switches for the two corresponding rca sockets as alternative outputs at the other end of the back panel. All paired rca sockets in-between are four different input contacts, which are connected to the source switch, operated from the front panel. Four inputs should be enough nowadays, since the most sources are fed through one dac, like web radio, streaming clients, computers and similar. So there are three inputs for other sources like phono equalizer or a recording unit and here the second set of output sockets will be helpful.




After mounting the two potted chokes on top of the amplifier case and after connecting the tube sockets with four Aa tubes, the amplifier chassis is complete and is waiting for its complex dedicated power supply. When the power supply is completed and will work with the dedicated smooth anode supply of 200 volts and the two raw supplies for the regulator boards, I need to adjust these with a dummy load instead of the real tubes to prevent an overload of their filaments. But till there is quite a long way… As almost everybody knows, a good power supply is by far more complicated than the amplifier part. I will show it with an option of two different versions, a quite simple tube rectified supply with the three different voltages to keep this preamp a simple stand alone solution.

For the aficionados the series of articles will be continued showing a quite unusual pre-amplification concept, which will have the future in mind. Similar to german broadcast preamplifiers of the 1950ties I will show the build of different stages with different amplification factors. A gain amp (+50 db), a catch-up-amp (+25db) for compensation of losses and the already shown line amp for impedance matching to the power amp.
With these modules and a dedicated power supply almost any use in a modern audio set can be managed. For example a typical dac output stage will need roughly a amplification of +50 db, almost the same than a good phono stage before RIAA equalizing. When equipped with a input transformer with different windings and different ratio, like 1:10 or 1:25 (just to mention the extremes), the same amplification stage can be used for both sorts of applications. Since the former digital signal is now strong enough for the impedance amp (line), the phono signal needs further equalization with a RIAA filter with typical loss of -25db at a following stage. So another amplifier with +25db gain is here needed for compensation of the losses to level up for the line stage.

A multi voltage power supply will be needed for such a amplification chain. Such complexity needs to match highest standards and components, potted oil filled transformers and chokes would be first choice, but it is already difficult to find this stuff nowadays. The anode supply develops with tube rectification at the beginning, ideally made with a classic mesh anode rectifier tube lie RGN1054 or 1064 as perfect partner of the Aa tubes, to be continued with several chokes for HV and LV: 1. a well filtered B+ for the DHTs in the line stage; 2. two well filtered low dc-voltages for the Coleman Regulators; 3. a further filtered and tube regulated anode supply for the gain amps, and finally 4. a well filtered dc-supply for their filaments of all indirect heated triodes. I assume the weight, complexity and size of a well made power amp to get an idea about the case.


Read on soon,
Volker