Spatial Audio Raven Preamp

@donsachs 

I'm looking to purchase the pre within the near future, it seems to be the top option in my price range. Just looking at the Raven on the Spatial site I saw that it uses two 6AX4 rectifiers. If you don't mind me asking, why that rectifier and not one of the units that are popular with tube rollers, such as the 5U4G, 274B, or the GZ34?Would they not work with the design, or too expensive, etc?  Thanks.

@charliee

The damper diodes are extremely tough, they are very quiet, and we are running them VERY conservatively. They have tons of headroom and will last a long time. That, and you can buy lots of them for very little money. So a customer can buy a couple of sets for the price of one decent quality conventional rectifier, and FAR less than an NOS Mullard GZ34, or even new production high quality 5U4 or 274b types. Truth is the damper diodes sound better anyway. Also, in this preamp you can use the 6W4 and there tons of those out there as well for under $10 per tube.

My previous preamp sounded best with the 6BY5, a dual damper diode. So that was my first experience listening to them a lot in a preamp circuit.

I’ve been using damper diodes (from old TVs) since 1997. I’m frankly surprised why people are still using the audiophile favorites. Damper diodes have (much) quieter switching, have substantially higher peak current, and sound noticeably better. The only downside is they consume a lot of heater current and require 6.3V heaters. The majority of damper diodes also use unusual sockets, so they are not pin-interchangeable with the standard types.

The parameter I care most about is smoothness of switching. This is hard to get right, with most vacuum diodes having sort of a Class AB switching region. This can be examined by using a scope probe with a 100X internal attenuator and a safety rating of 1kV or better, connected to the secondary of the power transformer. Voltages are very high, so great caution should be exercised while making the measurements.

The worst diodes have a rough transition between 0 and 50 volts, with holes chewed out of the waveform (generic solid-state bridge). The OK quality ones are fairly smooth but the zero crossing region (measured at the power transformer secondary with special probes) is quite obvious, with small variations between the usual audiophile favorites (which is where the famous tone color comes from). The best diodes almost look like Class A triode, with very smooth transitions that are complementary. Only damper diodes do that. They also have peak current capability that is 2X to 5X higher than any standard 5V heater diode.

With skill, snubber circuits tuned to the transformer, HEXFREDs and high-voltage Schottky’s can approach damper diodes in quietness, which makes them useful for power amps that have to handle a lot of power.

Why the obsession with switching noise? It’s much easier to reduce noise at the source then attempt to filter it later. The harmonics from the 100/120 Hz switch noise sneaks past regulators, magnetically induces noise in nearby circuits, and radiates back out the power cable. Better to minimize it right at the source, which is a function of the transfer curve as the diode is switched on and off.

At Spatial, we use a belt-and-suspenders approach to power supply design. We select the quietest diodes for the application, use CLC filtering as a pre-filter, then apply that to a precision regulator with 130 dB of noise rejection. The regulated output is then applied to a balanced audio circuit with another 35 dB of noise rejection (due to inherent balance). The servo circuit in the regulator has very little to do since the current draw from the audio circuit is very nearly constant, thanks again to the inherent balance of the audio circuit.

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.