Spatial Audio Raven Preamp

@donsachs Youbetcha. Initially will use a Lumin X1 w/ Khorns. Was going to treat room before this (major) Amp/pre upgrade but a windfall happened and rather than a spiffy wardrobe (who me?), tango lessons or going to the Tour de France this was the best option. Room treatment on tap. Excited!!! Whee Ha, Daddyoh. 

@wsrrsw Fasten your seatbelt, that will be a VERY dynamic system.   Have you rebuilt the crossovers on the Khorns?  If not, there is improvement to be gained there.   The Khorns are a wee bit bass shy below 50 Hz or so, but the amps will give you everything those woofers are capable of.

Let us know when you have a few hours on it all.

@donsachs I have gone about this room and system a bit bass ackwards. The walls do have resilient channels and quiet board on top. The room isn’t a 100% listening space as I’m a retired (but still doing some work) photographer. It’s a multi purpose room. Currently a small set is up blocking the left speaker a bit and the tunes still sound good. Big imaging not so much.

The Klipschorn’s AK6 sound good smack dab in the middle of the room as well as the far end where I have an editing station. If space would have allowed speakers that were several feet out in the room I would have not used the Khorns AK6es. I have a 9x12 north window that makes for a great natural light source. A daylight studio is tops. The (now translucently shaded) small high windows make it so the room has two light sources as a room with just one light source is off-putting. (A PATTERN LANGUAGE Towns, Buildings, Construction By Christopher Alexander, Sara Ishikawa and Murray Silverstein is a great book)

This is to scribe I know my light stuff but maybe not sound stuff as much.

To divert. Many years ago I had a big party and rented a friends brother’s Khorns. When it was time to take them back his wife told me I could buy them. I said, ‘I could not afford them”. She said, “yes you can”. She was very pissed off at them in their small living room. I paid scant little for them. He got back as his wife with voluminous electro static speakers that went to the ceiling. They are still married. I sold the Khorns to a client and over thirty years later they still are in use.

Diversion #2. Close to fifty years ago my parents had a marital separation and my mom was righteously mad at my father. He moved out and she gave away his beloved chest high Tannoy speakers, MC 275, records and turntable. They reconciled but no more big rig came back. I asked Mom why she didn’t give them to me and she said she never thought of that.

This is the background stuff that forms us. Dad was into stereo. I have been since I can remember ( but with out funds to indulge as I now do until a few years ago ) and now my grown sons are too.

The “plan” was to do sound treatments and then get serious about amplification and a pre. I have been a 300B lurker for years and the Raven/Blackbird’s are on order after an unexpected windfall and well you know the rest.

I never planed to use the Lumin AMP long term but it’s what I had (snarfed up on Agon) with the Khorns but it’s a lot more than Aok as a place holder.

Should I better the cross overs in the AK6’s and if so with what? It took three of us to get the Khons up on special sound dampening blocks and as they still smell new. (a lady bought them and retuned as she didn’t like the color!!!!) Will I void the warranty if different cross overs are put in?

Room treatments should make a considerable difference in quashing the bit of echo. Next week I’m off to find a big rug remnant and get it bound and will order 24-30 panels. VERY dynamic requires that too.

Quote the Raven

No one here can love or understand me
Oh what hard luck stories they all hand me
Make my bed and light the light, I’ll arrive late tonight
Blackbird, Hi. Hi. Helllooooowwww.

If the Khorns are basically new models from Klipsch, I wouldn't dig into the crossover because, yes, you will void your warranty.   Just enjoy them.  I modded Cornwall IV and they had cheap caps in them and very cheapo sand resistors, including one in series with the midrange horn.  Changing to good caps and path audio resistor on the horn made the speaker much better than stock.  I sold them to a guy who loved them, so the warranty wasn't an issue for me or him.   If the khorn has similar caps and resistors in it as compared to the Cornwall IV, then you can definitely get better sound, but at this point, get your system set up and just enjoy it.  You may go in a different direction on speakers eventually, who knows.

There is a whole subculture of modding the classic Klipsch products ... Cornwall (probably the easiest to tame), La Scala, Belle Klipsch, and of course the Klipschorn. Look up "Klipsch Forums" and off you go. Classic Klipsch are famed for dynamics but the frequency response can be sort of uneven in the 1950’s manner.

Aside from response irregularities, the quality of the factory crossover parts is kind of marginal. But there are at least two different vendors selling premade souped-up crossovers for each of the classic Klipsch models ... no soldering needed.

The KHorn definitely requires corner placement in order for much bass below 60 Hz ... the corner of the room completes the horn mouth, expanding it several times. Some folks actually use false corners, which work fine, provided the false wall is reasonably rigid and doesn’t vibrate.

I use loudspeakers that are designed to be close to room corners, but the room in our new home could not accommodate that setup. While losing the corners decreased bass and the overall energy, I did gain some detail as a benefit. After doing a lot of research, I wound up adding, in my case, custom open baffle subs even though my loudspeakers go down to 30Hz as-is. It’s an unconventional setup as the subs are in-between my loudspeakers and my centered audio rack. However, this not only solved my bass/energy issue due to the loss of corners, but the subs strengthened the system’s foundation, and now it has more detail, sounds bigger, and retains the relaxed sound that I love.

I am also very interested in the Raven preamplifier.

Kenny 

The Raven design is stabilized and they are in production at Spatial Audio Labs in Salt Lake City. Don Sachs lives in British Columbia in Canada, I live on the outskirts of Denver in Colorado, and we both communicate with Spatial on a regular basis.

I am hoping to receive word that my Raven will be shipping by the end of this week.  I plan to post my experiences here after at least 20 hours of burn-in.

Hi remember me, I started this thread. 😁 Anyway, I finally got my Coda s5.5 amp introduced into my system. (In user systems) I replaced my VAC Renaissance 70/70 amp and VAC Cla1Mk III preamp. I use the Legacy Wavelet II as the preamp/crossover/DSP room correction now. With very little time on the rig it sounds very good but I think I would like to add a little "Tube magic" back into the system's midrange and soundstage if possible. So the question is for those that now have the Raven, does the preamp have that characteristic at all? I remember before when I had to run my Focus XD's full range the VAC Preamp added just the right amount of tube flavor to satisfy, so looking for a bit of that. I don't see any reviews about the preamp in this thread so far, just checking.  Thank you!

BTW, I am saving for the Raven, but had a few set backs that detracted from my savings for it, but back on track now.

@fthompson251 Lynn and I designed the Raven and I have lived with all the prototypes and final version for many months and it has all the tube things you like, but is far better than any tube preamp I have ever built or heard.  It throws a sound cloud with very detailed pinpoint imaging, provided the rest of the system can do that.  Plus, it will drive the XLR inputs on your Coda amp, so if your source is balanced, then you will have a completely balanced system.  Of course the Raven will also handle rca inputs and outputs, but there advantages to a fully balanced system from source through the power amp output.  So I hope a few folks with Ravens will give their impressions on this thread.  There are a couple of prototype Ravens floating around and those folks have chimed in above I think.  There is also a Raven thread on the spatial audio lab audio circle.  I know that Spatial will be building the next 5 Ravens within 7-10 days, so hopefully a couple of the new owners will also chime in and you will have a basis for a decision based on an opinion other than mine, since I am incredibly biased:)

Thanks @donsachs  I appreciate the response. Sounds like a good match for me. 

I am looking forward to hooking up the Raven to my Odyssey Audio Khartago monoblocks. The Model 2 sounds great with them as it is. However, I do plan on getting the active ATC SCM 40A v2 speakers as they have the XLR connection.

I heard the powered ATC monitors at the last Rocky Mountain Audio Festival, and they were some of the best speakers at the show. The ATC midrange driver, in particular, is a legend in the speaker industry, and ATC did a really good job with the active crossover and the internal power amps.

The Raven can easily power 20 to 50 feet of balanced cable with its 4.5:1 step-down output transformer, so it should be an excellent match for the ATC monitors. Studio-grade Mogami or Belden XLR cables should be quite good, but there’s Cardas, Dueland, or Anticables if you want to spend more. I would stay away from high-capacitance cables that look like garden hoses. Capacitance is not your friend when it comes to long cable runs.

Thanks for the information, Lynn, especially on the cable recommendations. I can't wait to hook up the preamp. 

Yeah, stay away from the garden hoses and faux snakes, no matter what the reviewer says, or in what magazine. Simple is good, less capacitance per foot is better. The studios don’t use garden hoses or faux snakes, why should you?

Many of the reviews of $5000 cables are using them as system-wide tone controls. Wrong approach. Cables just need to get out of the way, nothing more. If EQ belongs anywhere, it's in the speaker crossover, where it can do the most good.

You can make very nice xlr runs out of Duelund tinned stranded, but it is spendy these days and you are talking long runs.   I use Paul's anticables in my system, but they are only 3-4 ft runs.  If you are running 8-10 ft over to a powered speaker that could get expensive as well.  Belden wire is quite cost effective usually if you hunt around for sources.  There are a million pro audio cables in the world, and that is essentially what you are doing.  I would start there and see how it sounds.  Like Lynn said, avoid super thick "audiophile" cables.

Thank you for the advice Don and Lynn. The Paul anticables do sound enticing.

As Don mentioned, avoid the thick and crazy-expensive audiophile cables. You have to remember most audiophiles have noticeably colored transistor gear, and use aftermarket products like power conditioners and $5000+ cables to minimize glare, grain, and excessive HF output in their systems. The better approach is using low-coloration electronics and loudspeakers with a smooth response, especially above 2 kHz.

The key spec in any cable, much more important than any other, is capacitance per foot (or meter). Capacitance should be well under 100 pF/foot, preferably much, much less. Inductance *does not* matter unless you are running an AM transmitter (those are RF cables). Inductance does not load down the preamp, but capacitance does, occasionally causing transient instability in a preamp with high feedback. The Raven has zero feedback, but solid-state preamps typically have very high amounts of feedback (40 dB or more), resulting in load sensitivity to the preamp/amplifier interconnect.

The quality of the insulator (dielectric) in the cable also affects the sound, and Teflon is not necessarily the best. It’s the first choice for aerospace applications, and has exceptional DF and DF measurements, but in my experience, may not the best for audio. Various types of plastic all have their own colorations, and the process of fabricating the cable applies mechanical stress to the plastic, which changes its dielectric properties. The more complex the construction, the more complex the coloration, and the longer it takes to settle down (possibly never). Most of all, DO NOT TRUST the reviews you see in magazines or on the Internet.

This question may have been covered, but I have not seen it.  Is the preamp part to part wired? Combo of circuit boards and PTP wiring? Are the tube sockets mounted to circuit boards? 

Most interesting product and thread.  
 

@grannyring The preamp, and the matching amps are point to point wired, yes. The tube sockets are all mounted directly to the top panel in both pieces. The only pcbs are there to hold large film capacitors to facilitate mounting them securely. The regulated power supplies, both high voltage and tube filament, are built on small pcbs, which are point to point wired to the rest of the components. Again, all signal path wiring is point to point with very high quality wire. The power supply wiring is all teflon insulated mil spec copper. Signal path is all copper as well, but there is no teflon insulation in the signal path. There are NO electrolytic capacitors except in the regulated DC filament supplies. The entire high voltage B+ supplies in both preamp and amps are built wtih all film capactitors, all resistors are wire wound.

All parts are chosen so that they are run very conservatively. No part exceeds 60% or so of its voltage or current or temperature rating. For example, the DC filament supplies can deliver 3A of current and in the preamp they provide 1.2A to the pair of 6SN7 tubes. Resistors are run at no more than about half of their power rating anywhere in the circuits. The preamp and amps are designed to last a long time and to survive the odd tube failure without damage to the unit.   You can put your hand on the power transformer of the preamp or amplifiers after hours of use. Even though both circuits are class A, they run at reasonable temperatures and do not stress their transformers at all.

Impressive build and handling of the important details. Thank you for such a detailed answer. I’m very impressed with the quality and design of these products as well as the transparency and comprehensiveness of the responses. You and Larry are to be commended. Thank you.

Let's just say that before building my own tube creations I restored probably 300-400 pieces of vintage tube gear, including about 75-80 citation II amps, over 40 citation I preamps, and many Scotts, Fishers, Sherwoods, Macs, Marantz 8b, etc... I saw what were clearly high quality build practices, and cheap ones too.  I always appreciated the gear that was easy to work on, and hated designs that were difficult to rebuild.   So everything we build is designed so that it is easy to work on should anything ever go wrong.  It is designed so that things are never run anywhere "near the ragged edge".   Tubes are in class A, but still conservatively run and should have long lives.  I want the customer to enjoy the gear for years, and should there ever be a problem, I want the tech to easily be able to swap out a part and have it running again. 

I remember working on Marshall guitar amps for musician friends.   They have a dozen little pcbs, tied together with jumpers cables that all have the same terminals, so that you have to mark each board and cable so you know how to put it together again.  The problem would always be on the bottom board!   So we avoid construction like that.  This gear is a bit complex, but very modular, the layout is neat and clean, and it is designed to be trouble-free.  For example, all AC is on one side of an internal shielding bracket, and all signal path on the other in both preamp and amps.  Star grounding, with strong attention paid to current loops.  No hum!

Wonderful! Have my eye on the preamp! 

Part of the reason for the sonics (aside from the circuit) is the physical simplicity of the preamp. No circuit board is needed because there really isn’t that much to the audio signal path. Input selector -> balanced switched-resistor volume control -> balanced 6SN7 vacuum tube -> output transformer. That’s it.

There are no coupling caps, no multi-transistor current sources with Zener-diode references, no muting relays with time-delay logic, and no DC-balance servo circuits ... so there’s no need for circuit boards to contain all these secondary functions. Just very short point-to-point wiring.

The same is true of the Blackbird power amp, as well as the Raven. The audio path is surprisingly simple.

Indeed the signal path on both the preamp and amp is very simple.  That said, the power supplies supporting it are rather complex and they are kept well away from it.  Also, the preamp output transformer and amplifier interstage transformers took over a year of prototyping to get right.  So the circuits are "deceptively" simple, but there is a lot supporting them, and as Lynn stated, these circuits are kept well away from the signal path, both electrically and physically.

Also, if you spend time reading this thread and the very long 300b lovers thread, you will see that the "deceptively simple" circuits are also cleverly designed to eliminate distortion at its source as well.

I have word from Spatial that the next batch of 5 Ravens is in production, so those folks awaiting builds should have them pretty soon.  Then some of you can post your impressions here!  It will be good to know how people like the preamps in a variety of systems.

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.

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. 

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.

RFI = Radio Frequency Interference

EMI = Electromagnetic Interference (includes magnetic fields)

15.75 kHz (or close to it) is the horizontal scanning rate of 525/60 NTSC (color or monochrome) analog television. The 625/50 PAL or SECAM rates are similar. Analog television environments were notorious for high interference levels, as well as electrical noise from early SCR light dimmers for on-set illumination.

Modern television is digital from camera, to signal processing, to transmission or storage, to decoding and display. No more sync noise, just computer hash at MHz frequencies.

My Raven preamplifier from Spatial Audio Labs arrived today and I thought I would post initial impressions.  More folks should be receiving theirs any day now so additional comments from them will be helpful.  The packaging was first rate and the box itself looked like it had been hand carried from the next town over.  Everything was pristine inside the double box and the finish on the preamp was perfect.  The only change in my system today was swapping my Ayre K-1xe preamp for the new Raven.  Everything else is the same.  The Raven has a cherry base and I upgraded the standard 6SN7 tubes to the Linlai elite version.  I let the preamp idle for an hour before listening to female vocals, then instrumental, then male vocals, female, etc... After only a few minutes listening the word that struck me first was REFINEMENT. There was no grain as one might expect with only one hour burn in.  Initially, I was pleasantly surprised how similar the Raven was to the Ayre; clearly better but not by a wide margin.  However, the gap began to widen over the next several hours.  Music was fluid, clear (like a sense of zero distortion), natural, well balanced top to bottom, and became more beautiful and emotionally engaging with every song I played.  I am astounded how good this thing is after only 3 hours.  I will provide more detail next week after spending more time listening but there is no question in my mind that this is an amazing preamp and I will be keeping mine.  I look forward to getting 20+ hours on it and reading the impressions of other new owners.

@jc4659  Glad it arrived safely and fired right up.  Thanks for your initial impressions!  It will become more 3D, with more sound stage depth and pinpoint imaging as it runs in for another 50-100 hours.  Tone colours will become more vivid as well.  I am sure the Ayre is a very nice preamp, but it should sound a little flat and two dimensional compared to the Raven, once it has run in.  That eery sense of space will develop on the Raven over the next week or two.  We call it the "trippy" sound:)

@donsachs Soundstage depth, image depth and pinpoint imaging is already better than the Ayre which itself was an improvement over the K-5xeMP in these qualities. 

@jc4659  I know Spatial runs the tubes at least 20 hours before they send things out, just to weed out any failures.   A 6SN7 is pretty well run in by 20 hours.  Yes, there are subtle changes in them out to 50-100 hours, but most of the sound is there in 20 or so.  What you will hear is the cathode bypass caps running in.   They are very large film caps and they are idling along at literally about 2% of their voltage rating.  So they take a while to settle down.  When they do you get that last 10-20%.   Where the real magic lives if the rest of your system can present it.

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.

AB amplifiers have no problem with the crossover region and do not rely on feedback to sort this problem out. Its a matter of the output devices being properly biased rather than anything to do with feedback. Of course, feedback is helpful (if applied properly) to improve the overall linearity of an A or AB amplifier. As Norman Crowhurst indirectly pointed out, most of the time feedback is incorrectly applied.

@lynn_olson Since you are using an output transformer, there is an ideal load for which the transformer is optimized. As you know, transformers transform impedance and  this goes both ways, so if the load at the output is higher, the load on the output tubes is higher too. But transformers can ring if the load is too high and conversely, roll off if the load is too low. Traditionally, balanced line transformers are designed for low impedance operation (in the old days 600 Ohms, hence the dBm rating). Ampex provided a switch with a 600 Ohm resistor on the back of their tube tape electronics so if the electronics driving something with a high impedance input, the load could be provided to prevent the transformer ringing.

So I'm very curious how you handled this issue- did you design the transformer for a low impedance and simply installed a loading resistor?

I saw a comment from you about no servos in the context of noise which I assume was a typo since as you know, servos do not introduce noise. You might consider one, since your circuit is direct coupled from input to output before the transformer. Offsets (possibly at the input) can result in an sizable imbalance which introduces distortion, even if you run matched tubes. A servo can be easily used to correct this issue, allowing the output transformer do have its lowest distortion and they are inexpensive to implement.

lynn_olson

Unfortunately, opamps are limited in not being able to dissipate much heat due to the small package size.

That's why so many good balanced components use discrete circuits for the operational amplifiers.

I own a Geshelli DAC and their new balanced headphone amplifier. Although value-priced, both products use the Sparkos (made in Colorado!) discrete op-amps, with a very large Class A operating region. If you’re going solid-state, this is an attractive approach.

The Raven is kind of the opposite: Zero feedback, all-triode balanced, using some of the lowest distortion vacuum tubes ever made (6SN7). What makes it possible are the custom-designed input and output transformers ... before that, Don and I were limited to off-the-shelf transformers, which in turn limited the selection of vacuum tubes that would be compatible with the circuit. The original Raven, designed way back in 1998, used tubes in the 5687/7044 family. These are very good, but are not as linear as 6SN7 family, and the subjective difference is pretty noticeable.

The zero feedback approach brutally exposes the sonics of every single part in the signal path, right down to the sonics of the volume control. Our custom transformers have outstanding sonics (subjectively), and a flat response to 30 kHz in-circuit. The Raven has a signal path of: a discrete-resistor volume control, wire, a balanced 6SN7 dual triode, and a custom output transformer. The unbalanced RCA inputs have their own custom input transformer for phase-splitting and ground isolation.

Zero feedback vacuum tube circuits are not for everyone. If tubes make you nervous, I can recommend the Geshelli with Sparkos discrete op-amps (if the low price doesn’t offend you). If you are very price sensitive, there are lots of Shenzen-made products for $200 or less, with ESS converters and pretty decent op-amps.

It might sound odd, but both the Geshelli and the Raven have a similar parts cost to retail price structure. Solid-state isn’t really that expensive if you buy the parts in hundreds to thousands quantities. And the expensive, heavy case with the 1/4" front panel adds nothing to the sonics ... that’s 100% marketing.

Tube circuits, though, get expensive very fast. Top of the line transformers are NOT cheap. Precision regulated 300 to 450 volt supplies aren’t cheap either. Tubes, by themselves, are kind of mid-price, but again, the top-of-the-line models aren’t cheap either. Vacuum tubes have always been handmade, even in their heyday in the late Fifties. Same for transformers.

None of this should be surprising. The majority of the solid-state market use parts that are made in quantities of hundreds of thousands to millions. This drives down costs. By contrast, the tube sector uses parts that are made in quantities of hundreds to a few thousand, several orders of magnitude smaller than the solid-state sector.