Wednesday, 15 June 2022

Back to stage – a PSU for a known singing bird

Hello everybody,

in loose sequence I am posting articles about vintage audio equipment. In the last couple of month I have spend my time more with listening to music and with the support rearing of songbirds, in particular of black birds,  than with building audio equipment. To listen to the very different sound of birds is as well a exceptional training for the educated ear...

Today I am talking again about building a power supply for a British tube preamplifier with RIAA-equalizer stage. A familiar customer asked me a while ago if I will be able to make him a power supply for his Concordant Exelsior preamplifier. He pretended to have a schematic for it and as well the schematic of the amplifier itself. I said that it would not be to difficult to make one. Next he asked me if I will be able to make one with tube rectification and better components. I said I need to check, because some components are hard to find in the meanwhile and that I would need to check my stock of parts. The schematics turned out to belong to a completely different version of this preamp. As the most people might know mains transformers with split balanced HT-output and two different filament windings "don't grow anymore on trees" as it would be needed for this! A quick look in my storage gave me some overview to pulled transformers from old equipment, foremost from old radios, some were missing parts, others where winded for 4 volts filament, etc. I didn't find the right part to do it well. Next question is to find new parts, Lundahl might here the right solution, but nothing under 200 €, which was expected a lot to expensive. Next problem would be to find a matching case for all, matching in visual terms, but as well big enough to harbor all components. Not easy, but I decided to get forward with a black painted Hammond casted aluminum case of the same height and depths of the Concordant. My customer limited the cost range for all to 250 €, which is an almost impossible sporting competition to stay beyond. A rough calculation showed immediately that such amount will pay the needed parts but not the work making it. On the other hand this guy never had a chance to listen to this preamp before, since he bought it without power supply. This person normally listens to a Shindo Monbrison and other well priced and established products, so he does not know what he will become at the end, just to explain his tight money framing. To be honest with this limit a simple PSU with silicon rectification and simple filter stage for the anode rail and silicon 12 volt IC regulation will be the solution to bring the package back to life within such financial limitations. I told him about and he pleaded to get something better, smoother sounding. I told him, that there will be no guaranty that even a perfect made PSU will bring this preamp to the level he ideally might expect.

Ok, I started some research at the internet and opened the Concordant case with the result that I realized that this version is equipped with ECC82 in the output stage and ECC88 in the RIAA-filter stage. This means with ECC88 it needs a 6.3 volts filament supply – no way make a cheap silicon based regulation with penny parts only ugly rails of hated caps and resistors to drop the voltages. 

So it gets back to the start with tube rectification with EZ81, choke regulated loading and filter stage for the anode supply. I looked again into my storage list for mains transformers, where all transformers are listed, which are useful for power amplifiers with their exact output data. The most of the transformers are collected as pairs, I am always afraid to use one of them for a single project, so still a lot of them is untouched since years. So I found the right type with the right taps, just a little bit oversized for such a project, but otherwise perfect. I decided to use it because I am not getting younger and the future projects might get countable. Now I needed to order a Hammond 157G 30H/40mA smoothing choke and I found a supplier who offered a bunch of NOS double capacitors 2x100uF/400V of ideal physical size to match my selected case. The Concordant sports a luxury, a mirrored double filament support for each channel, so it says at the front plate double mono preamplifier. At the back it has two identical unusual German DIN three pole jacks for this reason. I found in my storage the matching plugs, otherwise nowdays hard to find. So I collected all necessary parts from my storage together, – switch, fuse holder, cables and wires, resistors, tubes, silicon rectifiers, oil caps, supports, screws, nuts, bolts and more. When I first time tested the size of the case with the real components, I realized that it will get tight, – exceptional tight. At the end there was not the space for one more resistor inside. I needed several times to regroup the components inside to make it work!

At the end of the day all parts were in place and the case could be closed without pressure. Before I needed to isolate delicate exposed contacts. For example the tip of the EZ81 just matched the close case because I drilled ventilation wholes into the cover, the middle whole takes the tip, underneath the octal socket I needed isolation, because there was only 1mm air left between HT contacts and the base plate!

When I connected the new preamp with PSU to my setup, I was really astonished. The sound was exceptional refined, with wide open smooth and liquid soundstage, exactly so I do like it a lot. Of course I don't have any comparison to the original version, but I must say this combination is very very good. Yes the PSU uses some basic ideas which have been established in my equipment for years. For example the push-pull choke for the filament, the low oil capacitance in the loading stage for the rectifier tube to make it fast and smooth. 

I am looking forward about the ruling of the owner when he does the compare it with his Shindo Monbrison. It might get difficult... This is definetly under all tube preamps which had a chance to listen to one of the rare singing birds.

And who is now going to pay my work?