Hi Pavel,
To save people the time, here's the basic schematic
This is basically either a
Raven, or the
K&K linestage and can be built either way. The difference between those two is the type of output iron used, and there are costs and benefits to each. The Raven uses standard push-pull iron with DC through the primary while the K&K is parafeed with a blocking cap. In theory, the K&K can use higher quality iron with better coupling between the primary and the secondary. But, it does need the cap which will impact on fidelity.
That said, the basic issues to building one of these are output iron, input iron, and what tube to use. And, to a lesser extent, power supply -- to a lesser extent because half the point of the design is that the PS is less of a factor than for a single ended amp.
For input iron, you basically get what you pay for. The Edcor WSM's in the schematic are about $10 each, and sound and work fine. They lack some dynamics and may have a little distortion. Good for breadboarding, though. For a little more money (between ~$50 and $80 each), you can get Cinemag's or Jensens or Sowters. They will all be a significant step up from the Edcor. Or, you can splurge and get some amorphous core Lundahls. These are the best input transformers I've used, hands down. But, they aren't free.
For tubes, anything with decent gain and Gm -- 6H30, ECC99, 6N6p, 5687. Or, use a quad of something like 5842, 6688, D3a, C3g, etc. I use this circuit as the
output stage to a DAC with 6688's and it works great. Lots of good options.
That leaves the output iron which is the trickiest. For Low Z phones, you are basically looking for something with a big enough ratio and with decent primary inductance. The power toroid in the schematic is actually great for breadboarding. I don't think you'd want it long term, but it is a good and cheap way to get the circuit working. The won't deal well with DC though, so they are parafeed only. Note, too, that the dual windings make the schematic look nicer, but are not necessary.
Single winding parafeed transformers will work fine, too. A decent option is the Lundahl LL1930. These are 5.8+5.8:1+1. This means that you can get a Z ratio of ~4500:32. This isn't ideal, but the inductance of these is such that it is a little better than that number suggests. Unfortunatly, that's about the best I am aware of unless you get something custom made. To that end, Electra-Print, or perhaps Intact Audio, are good sources.
For standard push-pull iron, well, there still aren't a lot of choices. Something like a Lundahl LL1630 or LL1689 might be a good choice. Otherwise, again, custom might be the order of the day.
Finally, for the PS, the K&K version has CCSes to isolate the PS from the circuit (in addition to the fact that the circuit is isolated anyway.) So, PS hardly matters. Use decent diodes to keep out the switching noise, and a small filter -- CLCRC would do just fine. For the Raven-esque version, sticking with a shunt regulator of some sort, especially one fed by a CCS would be a good choice. The shunt element can be a glow tube, or a zener, or some other clever concoction. It probably won't make a huge difference.
Oh, all of those tubes will run happily with about ~160V on the plate, less for some of them. With the Raven-esque topology, you would thus need a ~160V PS. For the K&K, you'll need to drop some voltage across the CCS, so ~220V is probably a decent spot. Also, if you use the K&K topology, note that while the blocking cap can see full B+ at startup, if the tubes are perfectly matched, there will be 0V across it at idle. The whole point of it being there is that perfectly matched tubes is unlikely, but it is basically plate to plate, so the voltage across it is actually minimal. You still need to account for startup voltages.
One last thing, due to the high voltages involved, you really need to ground one side of the output transformer secondary. I know this is no fun as it is no longer "balanced," but this is the reality of high voltages. You might find a transformer with a center tap that can be grounded and run full balanced, but that will require something to be custom made.
If I ground the one of the taps on the output transformer, it's no longer balanced or do I not understand how you want to actually "ground" it.
