Oh, Amtrak is still around, just with fewer routes. The Denver/Portland route went away about fifteen years ago. However, as part of the Inflation Reduction Act, Amtrak is getting some routes restored, which I look forward to.
As for airports, DIA is truly gigantic, with six enormous concourses, all radiating from a rattly and jerky central subway. United changed the gate THREE TIMES on the trip to Seattle, all on very short notice. By the time I got my seat on the 100% full 737, I must have walked two miles, with no place to sit and rest, just a nonstop slog the whole way. Although I felt sorry for myself, I felt more sorry for the exhausted single moms carrying infants.
This is gross violation of the ADA act. There are old people (me), folks on crutches, and single moms force-marched to distant gates through an immense facility with nowhere to sit.
I think it’s absurd the wealthiest country on Earth has only two modes of transportation, with the airlines steadily degrading service year after year, and the Interstate Highway System no better than it was in the Seventies. Of course, if you are uber-rich, you just take your limo to the private-jet-only airport and stroll out of the limo straight onto the private jet a few feet away.
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lynn _olson
I hear you, airports and airlines can be overcrowded, and who knows what is floating around. Sorry to hear you need the rolling assist to get around in airport, those walks can be brutal in some airports. Is Amtrak really no more…?
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I got whacked with Covid after going to the Seattle show last year. Both the Seattle and Denver airports were overcrowded zoos, so I’m pretty sure that’s where I got it. Fortunately, Paxlovid knocked it out in about 36 hours.
This time, I will NOT fly Economy Class, will ask the airline for a wheelchair to traverse the miles-long concourses at Denver, and will wear a 95% filter mask until the A/C on the plane comes on.
For older folks like me, the airports are an exhausting trek without some kind of assistance. They are not ADA compliant unless you tell the airline a day in advance. Otherwise, you walk, with no place to sit and rest.
It’s unfortunate there are only two transportation options between the Pacific Northwest and Denver: a very tedious drive through the wastelands of southern Wyoming, or a United flight on a 100% booked 737 Max. Well, yes, I could rent a private jet, but that’s $20,000 per trip. Don’t think the IRS would accept that as a tax deduction.
The most pleasant trip was taking the Amtrak with a private cabin, but that disappeared ten or fifteen years ago. Sure was nice, though, eating in the dining car while watching the West roll by.
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Wear a mask at the airport? Why, you plan on robbing the place😁
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Yes, I plan to fly to Portland for the Spatial presentation and demo. This time I'll be sensible and fly Business Class, so I get decent legroom and seat width. And wear a mask at the airport.
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@abd1
As far as I know, Spatial still plans on going to Portland in the fall sometime. They are busy with the launch of the new smaller S6 speaker and the newer Q3 as well, plus they are ramping up the Raven builds. I will post here, and of course on their board whenever there is a trip scheduled. I know there are lots of folks in the PNW who would make the trip for a two day showing of all the gear...and Lynn and I would love to renew some friendships and meet new folks.
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The Blackbird amps are indeed the next evolution of the Karna amps. Same circuit, but updated power supplies and simpler tube choices. All tubes are in current production with multiple manufacturers so there is plenty of choice. The only exceptions are the voltage regulator tubes, but there are literally thousands of them out there to buy for under $10 per tube. In the Raven, the damper diodes are no longer in production, but the same is true. Many to buy and they last for years.
As for the coupling caps, yes I have heard most of the great ones. I prefer a custom designed transformer. To me, the transformer has basically no sound. Not if they are custom designed for the circuit. The caps all sound very different from each other, plus they take at least 20 hours to run in, with subtle changes for 100 hours. The transformers sound the same from the first minute. I was on the cap side of the ancient cap vs. transformer debate for years. Then I got custom designed transformers.... Now I don't use coupling caps in any of my builds.....
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Links to the aforementioned articles:
The Amity, Raven, and Karna Amplifiers
These articles date back to 2004 or earlier, and some of my opinions have changed since then. The availability of custom transformers opened a broader palette of tube selections, and Don’s work on precision power supplies offered a wider range of possibilities in the overall design.
I find it interesting that my objections to the sound of coupling caps is as strong now as it was back in 1997. Once you accustom to the sound of no coupling caps, adding them back in the signal chain is very noticeable. Much of what we expect of "tube sound" is nothing more than the sound of XYZ capacitors.
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@williamdc
Glad you are enjoying your new Raven. It does seem to tick all of your boxes! We arrived there after much experimentation and listening. I have always loved damper diodes, so I knew we were starting there. The transformers took a few rounds to get exactly the performance we sought. I had used the Khozmo attenuators for years, but I had to think hard about how to get them to do what we wanted. It all came together. You will find the bass improves over the first 100 hours as the very large cathode bypass film caps run in. You get most of the sonics after 10-20 hours, but it subtly improves. Enjoy!
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Lynn,
Yeah, no negative feedback and single stage tube design should be added to my checklist.
I recall first reading your articles on your nutshellhifi.com site -- "The Amity, Raven, and Aurora" and "The Karna Amplifer" close to 20 years ago.
The schematics made a big impression on me back then - quite foreign but so inventive - a modern take on an old Western Electric brain trust concept.
After finding this Raven thread recently, I then found the related Audiogon thread "300b lovers":
https://forum.audiogon.com/discussions/300b-lovers?highlight=300b%2Blovers
which I thought in some ways was as significant for tube amplifier design as your "Beyond The Ariel" discussion on diyaudio.com was for speaker design.
Anyone interested in the Raven preamp or Blackbird monos and also interested in amp design might consider reading the 300b lovers thread carefully.
After my own dilettante experience building low WAF tube preamps, owning various commercial tube, transistor, and passive preamps, and my search for an appropriate commercially produced tube preamp, then reading these two Audiogon threads, and recalling your Nutshellhifi writings, it really came together for me that I’d be able to own a pretty flexible natural sounding balanced preamp.
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Nice that the Raven ticks all the boxes! Seriously though, you’ve pinpointed (better than I could) what separates the Raven from a conventional Marantz 7C re-creation.
The preamps from the Fifties and Sixties followed a pattern of 2-stage 12AX7’s for lots of gain, a 12AX7 or 12AU7 cathode follower to knock down the output impedance, and plenty of feedback around the whole thing. Look at an Audio Research SP-3A or SP-6 and you see the same circuit. This is what most audiophiles think is "tube sound" for the simple reason that hundreds of thousands of preamps and integrated amps were built just this way, so it’s a very familiar sound.
But there are other ways to build a preamp, borrowing from studio electronics going back to the 1930’s. That’s the Raven, with no coupling caps, zero feedback, and a fully balanced circuit all the way through.
More technically, when the Marantz (or similar) circuit is compared to the Raven, there are 1, 2, or 3 coupling caps in the signal path. "Tuning" the sound of a classical preamp is usually little more than a cap swap, leaving the circuit itself untouched. The classical preamp is single-ended, relying solely on feedback to get distortion down to acceptable levels. And the sonics of cathode followers remain controversial, depending on the nature of the cathode load (resistive, inductive, active current source, etc.).
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My (opinionated) tube preamp checklist:
* Must take full advantage of balanced DACs
* No Balanced to Single Ended input transformers
* Must support XLR balanced out and RCA single-ended out for use with balanced amps or single-ended amps.
* Balanced XLR out and single-ended RCA out must have equally good sound quality
* Not capacitor coupled
* Much prefer 6SN7s. Would consider 6SN7 family or DHTs. But balanced DHT preamp is large, heavy, pricey, with possible microphonics, etc.
* No small signal tubes
* Stepped attenuators with a good remote. No potentiometers. No TVCs or AVCS. (I do like autoformers combined with transistor buffers. I think resistor attenuation is a better match with vacuum tubes)
* Must look good in living room. Prefer wood chassis. Conversation starter preferred.
* Tubes proudly displayed, not hidden in chassis
* Medium voltage gain
* Damper diode or solid state rectification. Damper diodes preferred. (I bought a bunch of 6AX4 damper diodes in 2015 in anticipation)
* State of the art power supply regulation
* Must be good value for money
More in next post...
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Is the Oregon Triode meetup still in the plans? I live in Portland, for now, and I'm interested in the Raven pre and maybe the amps at some point. Hoping to still be in Portland if this meetup happens.
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I received my Raven preamp a week ago. I've got about 80 hours on it now.
It's great! I'm really digging the expanded tone colors, instrument textures, sense of space, and natural non-mechanical sound.
My system is all digital using a Holo May DAC into the Raven. My main amplification is via a pair of DIY First Watt M2 monoblocks. I also own a pair of Hypex Nilai 500 monoblocks that I have been using with the Raven. I am running balanced from Holo May to preamp, and then out single-ended out to the M2 monos. With the Nilai monos the system is fully balanced. My speakers are Pure Audio Project Quintet15 open baffles.
I'm finding that I'm listening more to whole albums, or parts of albums, instead of jumping around to different selected tracks and playlists of selected tracks. The music on the album sounds excellent, so I just keep going. This is how I used to listen to vinyl before I developed my unfortunately terminal record mold allergy.
I found out about the Raven from this thread a few months ago. I've had a checklist of features, getting larger over the years, that I wanted to see in a commercial vacuum tube preamp. The Spatial Audio Lab Raven amazingly met my requirements. I immediately ordered one.
More in my next post...
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The Raven has split windings on the output transformer so both RCA and XLR’s can be used at the same time (the RCA output uses one-half of the secondary winding). So feel free to use to use the RCA output to power the active subs.
@lynn_olsen This suggests that there is a grounded center tap of the output transformer. Normally a balanced line output (if it supports AES48) is floating (this practice is to prevent ground loops). Did you provide a switch to break the center tap's ground connection if the RCA is not used? I don't see it in rear panel photos.
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When I was playing around with Cornwall IV I pulled all the cheap $4 MKP caps out of the crossover and replaced them with vcap odam. I put path audio resistors in there as well. The main resistor directly in series with the midrange compression driver was a $1 sand resistor on a $5000 pair of speakers. I was not impressed. The sonic improvement was quite staggering. I then used a pair of decent quality Hsu subs run with their speaker level inputs with pigtail cables from each speaker. I simply ran a stereo pair of subs, one by each speaker. Much better.... Finally sold it all and got a custom pair of open baffle speakers made with a horn as the top end. Embarrased the Cornwalls and sub combo ..... Klipsch are lots of fun, but they are not the top of the heap when stock. They can be improved. Hence the millions of Klipsch modding threads in the world.
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A pair of subs don’t need to be exotic or anything, just fill in the gap below the horn cutoff. And two are definitely better than one. I prefer stereo bass, but the home theater folks use a mono input to drive 2, 3, or 4. I don’t care about sound effects on movie soundtracks (who cares), but do care about wideband sound.
One thing about the Blackbirds is they will tolerate absurdly reactive loads, since they are push-pull Class A and do not use feedback. No protection circuits are needed.
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@curiousjim
The answer is probably yes. We have run them with 86 dB 4 ohm speakers and it worked just fine. What kind of load is your speaker? Can you tell me the speaker so I can look up the spec? Unless it is some sort of terrible reactive load and you are trying to create rock concerts in a cathedral they would probably work just fine.
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@donsachs
Quick question. My speakers are 90dbs, can your 300b mono blocks give me, say 85db of sound?
Thanks in advance.
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@wsrrsw
Good, sounds like you are on the right road. Subs are tricky. Good luck. I am sure you are getting some nice imaging and very real sounding instruments with great dynamics. Klipsch and bass... well...... that is not their strong point (but I didn’t say that out loud)
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@lynn_olson
Bam!
Thank you! One sub added. (Old one kept just in case) and will get another. Much better sound experience right away. The pre/system sounded aok with no discernible difference after putting cable into Pre’ Amp’s RCA output. Thought I could do as much but waited untill one of the birth fathers chimed in.
Sub set around 50Hertz. Will mess wi/ setting up/down.
Another round of thanks.
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I should add that all folded bass horns are tricky beasts. They only exist because a true straight bass horn that is flat to, say, 35 Hz, would be the size of medium-sized car. In other words, the size of an adjacent room. Two needed for stereo, of course.
PWK compromised with the real world by folding the horn (which creates internal reflections) and using the room corners to expand the size of the horn mouth. The internal reflections create ripples in the response above 150 Hz, and the cutoff region has +/- 5 dB ripples in the response, which interact with the room modes.
This is why adding a subwoofer is kind of tricky. You have to integrate not two, but three things: the Khorn response in its cutoff region (which is definitely not flat), the built-in filter of the subwoofer amp, and the room modes. Having two (or more) subwoofers is very useful because the room modes for one subwoofer will be at different frequencies than the other subwoofer, which smoothes out both of them. It’s also why multiple small subwoofers, in widely spaced locations, is a much better choice than a single subwoofer.
I should add the Khorn horn cutoff might be a lot higher than Klipsch says it is. 60~70 Hz would not surprise me.
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The Raven will have no trouble driving an RCA output at the same time as XLR. This has been tested by Spatial. You should have no problem except that the XLR output is higher level than the RCA. That is as it should be. If your subwoofer amps have gain controls you should be able to match levels. Subs are always tricky. The other option is speaker level outputs to drive stereo subs near the main speakers. That is the way I have always done it, but thankfully, my current speakers easily play 30 Hz in room so I no longer use subs.
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The degree of overlap between the horns and the subs (at least two) will be entirely subjective. Unlike closed and vented boxes, horns do not have a smooth, predictable cutoff region (which is 12 dB/octave for all closed boxes, and 24 dB/octave for vented boxes). Instead, they drop like a stone, and the octave just above cutoff can be pretty rough as well.
Combining the horn with the subwoofer will require judicious use of the "phase" control on the subwoofer plate amplifier, and messing around with the lowpass filter. The magic spot might be anywhere from 30 to 60 Hz.
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The Raven has split windings on the output transformer so both RCA and XLR’s can be used at the same time (the RCA output uses one-half of the secondary winding). So feel free to use to use the RCA output to power the active subs.
However ... the added cable capacitance on one-half of the winding, might, in principle, unbalance the XLR output a little bit. Maybe. Most likely not. Use low capacitance cable for the sub output, if possible.
Mostly, try it and see. The Blackbird power amps have pretty good common-mode rejection, so unbalance gets washed out in the input stage.
I keep hearing good things about the Rhythmik servo subwoofers, so you might check out getting a pair (using stereo bass, with the lowpass filter set to 40 Hz).
Although horn speakers have gratifyingly low IM distortion, they drop off very fast below cutoff. It's basically a brickwall cutoff, so "pushing" them below cutoff has limited utility.
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was just thinking sub(s) and your post said it better. I'm not sure how to attach a sub(s) to the Raven Pre? Must be an easy way?
@wsrrsw It appears there is a single-ended output in parallel with the balanced output. You'll have to check to see how this works but it might be quite simple- the main amps on the balanced outputs and the subs connected to the RCA outputs. I would expect to see a switch that grounds pin 3 to pin 1 so the RCA connections will work properly but they may not be connected to the output transformer driving the XLRs, negating the need for a switch.
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@donsachs Yahsureyoubetcha I'm using Hijiri XLR's from a LuminX1 streamer into the Raven PRE and Cardas Clear Beyond XLR's from the pre into the Blackbird Monoblocks.
I boosted the base a bit more w/ the roon parametric equalizer at 20 and 40 hertz.
Rather than take the pre and monoblocks 200 feet or so to the other speakers up and over hill and dale, I used the Lumin AMP w/ X1 streamer and compared that to the Blackbird Monoblocks and the Raven Preamplifier. Apples to apples; heck no. The Lumin AMP is 160W per side into 8Ω. It's a class A amp. The tubes vs solid state sounded too close to call. Take that SS.
@atmasphereI was just thinking sub(s) and your post said it better. I'm not sure how to attach a sub(s) to the Raven Pre? Must be an easy way?
T H A N K you both.
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I would be really interested to hear any reactions/reviews of anyone using the Raven preamp with the praised Class D monoblock amps such as the Atma-Spheres or the AGDs.
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@wsrrsw You have an ideal situation for using subwoofers. You might look into something called a Distributed Bass Array which is the use of 4 subs placed asymmetrically in the room so as to eliminate bass nulls and peaks in the room. This is often particularly noticeable at the listening position if the room has regular dimensions.
My main speakers are flat to 20Hz yet I have no deep bass at the listening position in my room. So I use a pair of subs to break up the standing waves that cause this problem. One is to my left and the other slightly to the right and behind me. Both are as close to the wall as I can get them.
In your case since your speakers don't go much below about 45-50Hz, you are an ideal candidate for all 4 subs. The Swarm from Audiokinesis is an ideal candidate for this since they are meant to be placed directly against a wall so as to be out of the way as possible. The subs are designed to roll of at 3dB per octave starting at 100Hz going down. The room boundary effect provides a +3dB/octave boost, so they are flat to 20Hz. They are 1 foot square and 2 feet high. I have the 10" drivers actually facing the wall so as to maximize the room boundary effect.
The idea here is to run them no higher than about 45-50 Hz, at which point its very easy to get them to blend seamlessly with your Klipsch speakers since below 80Hz in most rooms the bass is already entirely reverberant before your ears can sort out the bass notes being played. As long as the are not allowed to go too high they won't attract attention to themselves- the main speakers will convince you the bass is coming from them via the harmonics of the bass instruments.
It will be easy to get this setup to integrate with the Spatial amps and preamp. Then you will hear how much bass is really there and not only that, you may find that the mids and highs get more relaxed and detailed due to how our ears alter the tonality we perceive when part of the spectrum is missing. So getting the bass right calms down the mids and highs.
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@wsrrsw
Also, yes, in my experience the streamer has a huge effect on things. So play around. The Raven will happily use RCA inputs, but your best performance will most likely be with a DAC that has XLR outputs. Then you have an entirely balanced system through the amps all the way to the speaker.
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@wsrrsw
It will not be the amps unless something is wrong with them or you have a bad tube. The easy test is to move them to your other system. Your Sonus are 86 dB, 4 ohm. Not the best load, but the blackbirds will drive them and that will tell you if the amp has issues. Khorns have limited bass below about 45-50 Hz. It falls off like a stone. They are what they are..... They do other things quite well. The crossovers can be upgraded if you have not, but the physics of the speaker in that cabinet are what they are.. the spec is 33 Hz, but that is - 4dB. It is an incredibly easy load for the amps. If you have the patience, put the preamp and amps in front of your other speakers and see if you are satisfied with the bass. I suspect so. If not, then you may have a tube that didn’t ship well. My guess is it is a combo of the Khorn and the room they are in.
The first thing people say when they hear the Raven and Blackbirds is how amazing the bass is.... and your speakers are trivial to drive. The amps hardly know they are connected.
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@donsachs @lynn_olson The Blackbird Monoblocks and Raven Pre have over thirty hours on them and how sweet they are. Great job gents.
But darn it Don right you are about the Khorns and lower base. Running Roon I have even gone into the paramedic equalizer (a sin?) and boosted the lower hertz but still compared to my other system w/ a pair of Sonus Faber Guarneri Evolution’s speakers (faster sounding than Khorns) the base is flat verses the aliveness and vibration of a nice walking bass line. Low piano notes should vibrate too. “Grandmama's Blues” by Cyrus Chestnut on the "Earth Stories” album is a great test track https://open.qobuz.com/track/2003813 or even “Ham Hocks and Cabbage” by the Christian McBride Trio https://open.qobuz.com/track/10462550. Lower Tom drums also not quite there.
The low mids..heck all the rest (room still not treated and it’s boomy ) get ‘er done. Your gear is great and my contact over at Spacial has provided great service.
I have to consider if’s my streamer adding/subtracting to the bass quandary. I’ll swap out my other streamer and dac.
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Good cores are essential for low distortion.
Capacitive coupling exists between windings regardless of the quality of the transformer. For this reason alone its important for the transformer to be loaded correctly so it expresses its winding ratio without inter-winding capacitance as a parallel element. Otherwise the frequency response won't be flat.
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To further the transformer discussion. We don't need to name companies, but many take a sort of swiss army knife approach with multiple windings that allow for many connection schemes. Sometimes they work, sometimes they don't. But they are not optimized for the task. I auditioned several with exotic core materials and found them to be a bit tipped up in the treble to give hyper detail, but they didn't sound right. Of course, they look fine on the scope. Interstage transformers from well known companies would ring at 15KHz or so, or others would run out of steam at 50 Hz and below in our circuit. We are not talking Hammonds and Edcors here, but very well respected higher end manufacturers. Our preamp output and amplifier interstage transformers are custom wound and use interleaved laminations of nickel and steel in a proportion that gives very even frequency response and hits the sweet spot balancing detail with proper tonality. They have full frequency response without any oscillation. This is based on years of experience by Dave Geren, the designer at Cinemag. Dave tells me there are only a few sources in the world who can make the cores to his specification. So a lot of design experience has gone into the transformers used in the 300b project, and as Lynn noted, several iterations for each transformer to get it right.
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I used to think that rules-of-thumb applied to transformers. Well, no. Maybe in the Fifties when they were designed with slide rules, but now you work closely with the designer and their simulation software, followed by a sample build. Model, build, put on the test bench, send the complete set to Don, he re-measures and auditions, and repeat as often as necessary.
The Raven is now on the fifth version of the custom transformer set. As noted by Don, they are optimized for the 6SN7 in balanced mode and the most common range of loads presented by solid-state and vacuum tube power amplifiers.
The secret of transformer-coupled audio design is a close working relationship with the transformer designer. In effect, the circuit is co-designed with the transformer designer.
One of the great things about working with Don is he has good working relationships with key vendors, so we can get custom designs on a timely basis, and we know exactly how they were designed. This lets us further optimize our circuits around the custom parts, rather than work around an off-the-shelf part.
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@donsachs Is this meant to answer my question?
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The transformers in the Raven preamp and Blackbird amps are all custom designed to work perfectly with the tube sets chosen. They don't require loading resistors, and there is no need for a grid resistor on any of the tubes. The number of parts in the direct signal path is very low and all are of very high quality. You cannot get an off the shelf transformer to achieve these specs or operate that way. You have to work with a very talented transformer designer to get what you need.
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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.
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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.
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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.
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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.
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@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.
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@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.
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@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:)
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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.
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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.
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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.
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@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.
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@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?
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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.
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