Spatial Audio Raven Preamp

I should go into regulators and their sonics a little. Yes, regulators have "a sound". Regulators are amplifiers that feed amplifiers, with the difference the "amplifier" amplifies incoming audio, while a regulator amplifies a DC reference voltage. But it’s an amplifier nonetheless.

Most "linear" type regulators use an internal servo feedback loop to maintain a steady output voltage ... a regulator basically simulates a perfect battery, using feedback to get as close as possible to the ideal. But ... that is an approximation, not the real thing. There are very slight delays responding to a change in current demand, and that is where coloration enter into the sound.

Some audio amplifying circuits have a steady current demand on the supply, and others bounce up and down, following the audio signal. A single-ended audio amplifier, whether tube or transistor, will have a current demand that mirrors the audio. You could put a current sense probe on the supply rails and hear perfectly good music (along with some buzz).

A Class AB amplifier, by contrast, will have quite distorted music on the power supply rails, because it is switching between (B) the upper device, (A) both devices at once, and (B) the lower device. This changes the efficiency of the output stage as the different operating modes change with the music. The switchover between modes can either be hard or soft, depending how the amplifier is biased and how the devices enter the AB cutoff region.

When the load is a Class AB device (like an output stage or an opamp), great demands are placed on the regulator. If it is not a perfect regulator (instantaneous and distortionless), coloration enters the picture. This is why regulators sound different, because a nonlinear load (such as Class AB) then exposes nonlinearities in the regulator.

A balanced Class A amplifier has the great advantage that the load looks pretty much like a resistor at all times, short of heavy clipping. By contrast, the load of a single-ended stage looks like the music it is playing, always varying, while Class AB is quite distorted thanks to a pair of devices switching on and off as the music goes through it. Only well-balanced Class A has a steady draw that doesn’t vary with the music, whether loud or soft, all the way down to zero.

Unfortunately, opamps are limited in not being able to dissipate much heat due to the small package size. Very few opamps are designed to be used with heat sinks. So the only way to keep heat emission low is efficient Class AB output stages, relying on feedback to linearize the crossover region (opamps typically have very high feedback). Higher powered transistor and tube amps also use Class AB to keep heat emission to acceptable levels, at the expense of higher distortion in the Class AB transition region.

The nonlinear load challenges the regulator design, and regulators for the output stage of transistor and tube power amps can be as large and heavy as the output stage they are powering. In effect, one amplifier driving another. This is why it is very rare for medium or high power transistor or tube amps to have regulated output stages. Usually they have a simple lowpass filter with no regulation, saving a great deal of cost and weight compared to the regulated alternative. With no regulation, the sound will always change, depending on the incoming voltage fluctuations, the AC waveshape, and the noise riding on top of the AC power.

The rigorous solution is fully balanced Class A operation for every stage of amplification, not just one or two, and low-noise precision regulators for each of those stages. This keeps the workload of each regulator to a minimum, and the current draw on each regulator is constant regardless of audio signal. It also maximizes isolation between the AC power line and the incoming audio signal.

The Raven also uses an isolation and phase splitting transformer for unbalanced RCA inputs, while balanced signals go straight to the 6SN7 tube grids. Regardless of the incoming signal, whether balanced or unbalanced, the stepped-resistor volume control and internal electronics are always in Class A balanced mode.

What Lynn said is very audible.  Both the Raven preamp and Blackbird amps use this approach to power supply and balanced circuits.   Once you start listening to circuits built and powered this way it is very hard to go back to conventional approaches because they sound just a bit cloudy or muddy.  It is like a veil being lifted.  The constant merry go round of trying different coupling caps and other things to color the sound in a way you prefer comes to an end.  Instead, once you understand what is going on, you spend a year or two eliminating every bottleneck you can so that the circuit can perform at its best.   What becomes evident is that the circuit and the approach are incredibly transparent.  If you change anything that supports it, you hear it instantly.  Cloud, the main tech at Spatial made the same comment.  You can instantly hear any subtle change you make.  The type of wire becomes very noticeable.  Tube choices are very audible.  Of course you hear these things with other more conventional gear, but not to the extent you do with this circuit and power supply architecture.

Obviously, there are lots of very nice preamps and amps in the world that sound very good.  I used to make some of them myself.  But they don't sound like this.  When you eliminate a lot of the "grunge" that you didn't even know was there, you get a very spacious and airy sound, with incredible detail that you have never quite experienced before.  That is what I hear, and most others who have heard it have made similar comments.  It is not so much about what the circuit sounds like, but rather what the music sounds like when you eliminate a lot of the coloration and distortion that you were never really aware of.  For example, we touched on the idea that there is very subtle spatial information in the signal that is partially obscured by other circuits.  These things are hard to measure, but they can certainly be heard.

I understand that two more preamps have just shipped, so we should get some more reports from owners here fairly soon.  I know it is hard for people because you cannot just go to a dealer and hear the gear, and there are only a few of these in the world, and most are prototype versions.   The production versions are entering the market now, so when people post, the rest of you will get a better impression of how the preamp sounds in a variety of systems to a variety of ears.

Is it recommended to reverse polarity at the speaker terminals with the Revelation?

@marantz2270 

No.  The Revelation preamp and the Blackbird amps both preserve phase.  So just hook them up straight.

Thank you, Don.