300b lovers

Yes, we are pretty much back to an updated Karna circuit with more modern power supply topology, and custom wound iron everywhere.  I listened to every possible variant of the circuit and it is obvious that removing all coupling caps and going with all transformer coupling walks all over any other variant in pretty much every way.  We are now using custom wound Monolith Magnetics iron for power and output transformers, and Cinemag interstage transformers.  The chassis is much wider than the shoebox amps displayed at the Pacific Audio Fest in Seattle.   Spatial will change the look slightly, but the layout and size will be the same.  There are reasons for the layout, which makes for very short signal path at all sections and also has complete isolation of the power supply from the signal part of the amp.  They are just about ready to build in quantity and I would expect them to be available in November or December if all the vendors meet their production schedules for parts.  I will say that I cannot listen to anything else now.  I am spoiled.....  everything else sounds dull and coloured to me now and I have pretty much cleared my house of all other amps and preamps.  We shall see what others think!

I cannot seem to post a photo of the final prototype, but will upload to my old website and see if I can post a link to that photo eventually.

This thread, with over 500 posts I think, has allowed interested people to understand the evolution of their 300b amps design compared to other designs.  A rare insight into the machinations of amplifier design, which to be honest Roger Modjeski from RAM was stellar at doing as well.   Lynn astounded me with an earlier post that some designers, maybe it was with speakers, design to specs and that's it.  What Don and Lynn have done is an endless process of swapping not only components, but topologies, a very interactive process, subject to both measurements and equally or more important, endless comparative listening observations.  This to me is the optimum process to design gear and the process has to be exhaustive, but both Don and Lynn have done  this for decades... this iterative process.   And, to wit, they have shared their excursion on this thread in masterful detail to inform all of us.   

My pre-pro 300b amps are a glimpse of heaven with my Cube Audio speakers and it is hard to imagine how much better they will sound when Don updates them to the final version, but  I am certain his promises will be fulfilled.  

I managed to update my ancient website with the beginnings of 300b project page.  There is a photo of the final prototype of the Blackbird 300b monoblock amp at the bottom of this page.  Spatial will change the look a bit, but dimensions and layout will be the same.  The amps are optimized for sonics and short signal path.

This is very close to the production version. It is simpler and more straightforward to build than the shoebox format shown at the PAF show ... more spacious, more direct layout, and the power supplies are confined to their own section of the amp, on the right side of the chassis. The vent holes for the twin B+ regulators are visible on the right side of the amp, next to the VR tubes.

The audio-only circuit is on the left side, with very short signal paths from 6SN7 -> interstage 1 -> matched balanced 6V6 -> interstage 2 -> matched balanced 300B -> Monolith output transformer -> speaker jacks. The input selector switch bypasses the input transformer when XLR is selected. Compared to the show amps, there are actually fewer parts in the production version, with a very simple signal path from input to output. From input to output, there are only wires, transformers, and triodes, in a fully balanced circuit. No coupling caps, no plate-load resistors, no plate inductors, and no dynamic loads.

One subtle difference is each single grid is driven by a pair of balanced plates, so distortion and noise are minimized in every stage of the amplifier. In the show amps, each 6V6 grid was driven by the corresponding 6SN7 plate. In the production amp shown here, each 6V6 grid is driven by a balanced pair of 6SN7 plates, thanks to interstage 1. I was doubtful a good interstage could be made for the 6SN7, but our transformer designer came through with performance from 18 Hz to 32 kHz. Close collaboration with modern transformer design is what made this possible.