Spatial Audio Raven Preamp

Heard from Spatial that my Raven should ship tomorrow.  I am very excited to hear this preamp in my system.  I will try to refrain from posting initial impressions until adequate burn-in (20+ hours).

@jc4659   Well, it's ok to post an initial impression:)  But then please follow up with the 20+ hour report.  You get most of it within 20-30 hours, but there are subtle improvements out to 100 or so as the big cathode bypass caps run in.  Most of it is there by 20-30 or so though...   Enjoy.  Hope it works well in your system.

@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.