Spatial Audio Raven Preamp

Both the preamp and power amp are so symmetric we have to take extra care during assembly to make sure the phase is correct at the output. Multi-color wire comes in handy here.

For that matter, the circuit is so robust it still amplifies with one side non-functional. Distortion is higher, of course, so it’s another thing to be checked during assembly and test. Both phases need to be present and accounted for, polarity correct in every stage, and both channels pair-matched.

The Raven requires a 6sn7 tube on each channel that has matched sections.  Really, a difference of 1 mA is no big deal, but tubes with wildly mismatched sections will not have good bass response.  Any modern 6sn7 you buy will have sections within 1 mA of each other.  The two channels don't have to be matched to each other, just the tube on each channel needs proper section matching within that tube.   The Blackbird amps require similar matching between pairs. 1 or 2 mA difference is fine.  Any more than that and bass response will suffer.  Of course both units are supplied with matched tubes and it is not hard to buy them.  

If you want to run NOS tubes or ANOS, you should ensure the matching is as above.  Smoke won't come out of the preamp or amps, they will work with mismatched tubes, but they won't sound as good.

@donsachs @lynn_olson I have been looking at maybe purchasing the  preamp for a couple months and have a few questions. I have a nice SS state amp but it is not balanced or have zero feedback.  What will be the benefit of the Raven Preamp if  the amplifier is not balanced or has zero feedback.  Does this negate all the benefits of the Raven or is it a cumulative thing?  

@brbrock It is a cumulative thing.  Especially if your source is a DAC with balanced output.  The Raven will happily communicate with all things RCA on both input and output.  The gain is slightly less because XLR is a 5V swing and RCA is at most 2.5V.  But the Raven will work fine with anything.  That said, it can be in the middle of a fully balanced system, which has advantages.   I am pretty sure Spatial has a 45 day return policy with little or no restocking fee.  So you would just be out the shipping to try one.

I would say if your SS amp can be driven to clipping by 1.5 V or less, you will be just fine.

The Raven accomplishes several things at once:

1) Moderate voltage amplification (from the 6SN7).

2) Substantial current multiplication (from the internal step-down transformer).

3) Signal conversion from either RCA or XLR to RCA, XLR, and headphone outputs.

4) Volume control via stepped resistor array, with L/R balance control on the remote control, as well as volume and input selection.

5) Signal conditioning, with removal of DC offsets *and* RFI interference, and breaking of ground loops between components (via transformer coupling).

So it’s not just a preamp or passive volume control. These benefits extend to all types of power amplifiers ... Class A or Class AB transistor, Class D Mosfet or GanFET, or triode or pentode tube amplifiers.

RFI break-in is the bane of modern hifi gear, since most homes are bathed in microwave signals from WiFi, Bluetooth, and RFI noise from multiple switching supplies in TVs, computers, various gizmos that use ARM processors, etc. etc. Just scraping off all this RFI cruft before it gets to an analog circuit can make quite a difference in low-level sonics ... no more barely-audible buzz or hash getting into the power amplifier.

The classic tube preamps of the Fifties and Sixties were designed at a time when nearly all homes were RF silent. No computers, WiFi, Bluetooth, or switching supplies. No wall-warts. None of that. The only RF-noisy places were TV studios (15.75 kHz TV sync buzz is everywhere), AM and FM transmitters, microwave relay towers, or military installations ... where isolation transformers were routinely used to isolate and suppress RFI incursion into audio signal paths. We are applying the same isolation technology used back then, with custom transformers that are designed with modern computer modeling software.