Spatial Audio Raven Preamp

The transformers in the Raven preamp and Blackbird amps are all custom designed to work perfectly with the tube sets chosen.  They don't require loading resistors, and there is no need for a grid resistor on any of the tubes.  The number of parts in the direct signal path is very low and all are of very high quality.  You cannot get an off the shelf transformer to achieve these specs or operate that way.  You have to work with a very talented transformer designer to get what you need.

@donsachs Is this meant to answer my question?

I used to think that rules-of-thumb applied to transformers. Well, no. Maybe in the Fifties when they were designed with slide rules, but now you work closely with the designer and their simulation software, followed by a sample build. Model, build, put on the test bench, send the complete set to Don, he re-measures and auditions, and repeat as often as necessary.

The Raven is now on the fifth version of the custom transformer set. As noted by Don, they are optimized for the 6SN7 in balanced mode and the most common range of loads presented by solid-state and vacuum tube power amplifiers.

The secret of transformer-coupled audio design is a close working relationship with the transformer designer. In effect, the circuit is co-designed with the transformer designer.

One of the great things about working with Don is he has good working relationships with key vendors, so we can get custom designs on a timely basis, and we know exactly how they were designed. This lets us further optimize our circuits around the custom parts, rather than work around an off-the-shelf part.

To further the transformer discussion.  We don't need to name companies, but many take a sort of swiss army knife approach with multiple windings that allow for many connection schemes.  Sometimes they work, sometimes they don't.  But they are not optimized for the task.  I auditioned several with exotic core materials and found them to be a bit tipped up in the treble to give hyper detail, but they didn't sound right.  Of course, they look fine on the scope.  Interstage transformers from well known companies would ring at 15KHz or so, or others would run out of steam at 50 Hz and below in our circuit.  We are not talking Hammonds and Edcors here, but very well respected higher end manufacturers.   Our preamp output and amplifier interstage transformers are custom wound and use interleaved laminations of nickel and steel in a proportion that gives very even frequency response and hits the sweet spot balancing detail with proper tonality.  They have full frequency response without any oscillation.  This is based on years of experience by Dave Geren, the designer at Cinemag.  Dave tells me there are only a few sources in the world who can make the cores to his specification.   So a lot of design experience has gone into the transformers used in the 300b project, and as Lynn noted, several iterations for each transformer to get it right.  

Good cores are essential for low distortion.

Capacitive coupling exists between windings regardless of the quality of the transformer. For this reason alone its important for the transformer to be loaded correctly so it expresses its winding ratio without inter-winding capacitance as a parallel element. Otherwise the frequency response won't be flat.