Spatial Audio Raven Preamp

I expect my Raven will be delivered in about 5 or 6 weeks. I already have Don's dual mono 300b amps and the newest rendition of his Model 2 preamp, so I will get to compare them side by side.

That should be a synergistic combination so I’m anxious to hear your thoughts as well as the comparison between preamps.  Thanks!

I hope you folks like the Raven (which are built and sold by Spatial Audio Labs). It’s a miniature version of the power amp, with one stage of gain instead of three. It has both RCA and XLR inputs, with the balanced input going straight to the 6SN7 grids, and the RCA going through a studio-quality input transformer (which converts the signal to balanced). The volume control is a special-order Khozmo dual mono unit, with volume and L/R balance on the remote control. The output is also transformer coupled, with both RCA and XLR balanced outputs.

Don Sach’s previous preamp had a special SRPP circuit, which used clever noise-cancellation techniques to reject power supply noise. The Raven similarly uses inherent circuit balance to also reject power supply noise ... although there isn’t much, since the power supply regulator itself has 130 dB of noise rejection.

Probably the biggest sonic difference is the previous Don Sachs preamp used a cathode follower and a very high quality coupling cap in the final stage, while the Raven uses transformer coupling to accomplish the same thing.

They are custom-design Cinemag, optimized for our circuit. They are not off-the-shelf parts, which have bandwidth problems with the output impedance of the 6SN7.

The 6SN7 is one of the best tubes ever made, but is not easy to match with most transformers. That’s where Cinemag came in, who saw the 6SN7 as a fun challenge. Several prototypes later (design, computer model, build, measure, listen, and repeat the cycle), we arrived at the production models we’re using now.

The Khozmo volume control is also optimized for this preamp, with a different signal path than most preamps. The Raven is balanced throughout, from input to source selector to volume control to vacuum tube to output.

One nice thing about transformer coupled balanced construction is the risk of sending DC pulses to a delicate transistor power amp are greatly reduced. (Even when a transistor amp is switched off, a DC pulse of more than a few volts can damage the input transistors.)

P.S. How does the output transformer protect a power amp? First, there’s a 4.5 times step-down ratio, reducing unwanted transients by a similar ratio. Second, the circuit itself is balanced, instead of a single-ended cathode follower exposed to a hundred volts or more. Circuit balance is typically 3% or better, reducing potential transients by a similar amount (about 30 dB). Third, and most important, transformers can never pass DC, unless the windings themselves have failed. By contrast, capacitors may pass "leakage current" and gradually short out as they age (a well-known problem when restoring vintage electronics).