Spatial Audio Raven Preamp

300 ohms sounds about right. The output transformer is a step-down type, with ratios of 4.5:1 and 9:1.

The previous Spatial preamp, the DS, uses a cathode follower instead of an output transformer. The output impedance is about the same, but a transformer multiplies current in proportion to the turns ratio, while a cathode follower has the same current availability as the tube ... which for a single 6SN7 section is around 8 mA (not much). The Raven multiplies that 8 mA * 9 = 72 mA, thanks to the transformer.

If you have planar headphones with a 8 to 40 ohm impedance, a low power (and very very quiet) power amp is ideal. Only 1 or 2 watts are needed, and Class A operation is highly desirable. Something midway between the Raven and Blackbird, perhaps a two-stage integrated amplifier with DHT PP power tubes and about 3 watts of Class A output. Aside from headphones, the market for 2-3 watt amplifiers is pretty small.

@soix 

+1

A $250 pair deserves a dedicated headphone amp. Typically headphone outputs, even on really expensive preamps, are poor quality after thought. Not always but typically. 

Well, our circuit is not an afterthought. The Raven is a small power amp, with no resistors in the signal path, but it is limited in scope to medium-impedance headphones. Supporting 8 to 40 ohm headphones would require a complete re-design and a substantial increase in price, with no sonic gain for use with power amps and loudspeakers. Our primary market are people using the Raven with power amps.

The multiple taps of the Raven’s output transformer gives us the freedom to support medium-impedance headphones with no degradation in quality. No series resistors, no attenuation network, and no coupling caps, just a connection right from the transformer secondary.

If we retain the same Class A balanced vacuum tube circuit, but scale it up to support low-impedance headphones, the price would end up midway between the current Raven and the Blackbird power amps.

Thanks for all the info, Lynn. I'm looking forward to trying mine with good headphones. It's an amazing pre-amp with speakers, I can tell you that.