Speaker Active Matching (SAM) Technology in the Devialet Expert 220 Pro Integrated Amplifier
The French company Devialet has introduced a number of amplifier innovations since its founding in 2004. It is most famous for its hybrid output stage, which uses a Class A amplifier for voltage gain and a Class D amplifier for current gain. Devialet’s amplifiers look and operate differently than conventional products, combining sleek industrial design with very advanced software-driven features that both improve the sound and make the products easy to use.
The Expert 220 Pro integrated amplifier reviewed here incorporates many of the more than 100 patents the company has been granted over the past decade. One of these patented technologies is of particular interest to me: Speaker Active Matching or SAM.
Addressing a fundamental problem of loudspeaker bass behavior, the SAM system is a new way of correcting transducers in the lower frequencies. A truly exciting and spectacularly effective device, it is one of the most musically intriguing I have encountered in many a year. The problem of correct bass timing in speakers has been known for a very long time—decades really. But addressing it was a difficult matter in a purely analog world. In the DSP world, things are different. The details of all this will require some explanation, and, in fact, there are aspects of this device that I have not yet been able to clarify completely in technical terms. But in listening terms, the results are remarkable.
So before explaining what SAM does and why such correction is necessary, let me describe in summary what the sonic effects are before I go into the underlying technical considerations.
How SAM Sounded on Speaker X
I tried the SAM system on a medium-sized stand-mounted speaker of high sonic excellence. Which one, you may ask? Pardon me if I do not say. In my many writings about DSP room and speaker correction, I have found that any speaker I describe as benefiting from these adjustments is almost automatically devalued. The immediate effect on many readers is: “So that particular speaker needed help, but that is because it is not such a good speaker. My marvelous speakers do not need help like that.” This is always—or almost always—wrong. Your marvelous speakers do need help. Virtually all speakers do. And I definitely do not want to let anyone think that the fact that the SAM system improved the particular speakers I used as criticism of those speakers. In fact, the medium-sized speaker I am describing is one of the great speaker designs of the world. Reputable reviews from other people have called it “one of the best,” and so it is. Call it Speaker X from here on.
Speaker X is almost flawless from the mids on up. But because of its medium size, it lacks deep bass extension. And because it lacks extension, it also lacks a certain precision in what bass it does have. I shall explain later why this is inevitable. No passive speaker that lacks full bass extension—and even more than nominal 20Hz extension is needed—can do bass timing correctly. This is just a fact, even if it is an unfamiliar one for people not acquainted with filter theory.
On to the sound with SAM: I have been enjoying listening to the Telarc Ravel-Borodin-Bizet CD. This is attractive music, which bears repeated listening, and it is very well recorded. But the music is not so profound that one is reluctant to listen to it over and over in quick succession and with attention in good part to the sonics. (One really should not use Beethoven’s Missa Solemnis as an audio-test item.) On Speaker X, it presented a quite convincing picture of an orchestra, something that I did not feel foolish listening to coming from my Wednesday rehearsals with a large orchestra (Santa Monica Symphony). But the bass was somewhat limited and not entirely precisely defined. Things such as the low string pizzicato passages were inclined to be a bit blurry compared to reality. And the blats of trombones and the like were slightly less blatty than they should have been.
Enter the SAM system. All of a sudden—no ifs, ands, or buts—the bass acquired much greater perceived extension and a lot more additional perceived precision. Trombones blatted as they should, when appropriate. Low string pizzicato passages acquired the resolution of real life. One could hear what went on in the lower frequencies much better than before. Indeed, the whole sonic picture was cleaned up. The result was a sound that was far closer to a real orchestra—among the top echelon in reproducing orchestral sound of any audio systems in my experience (within reasonable dynamic limits).
Let me be careful here. Speaker X is a medium-sized speaker. It is not going to be able to pump out the huge dynamic levels of the Carver ALSes (a large line source with a high-powered subwoofer) or the Cerwin-Vega CLS-215s, with their two 15″ woofers per side. Speaker X did acquire enhanced dynamic range, playing louder than before without audible distortion, but no electronic pre-processing or active management could make a speaker of this size and nature fill an auditorium. In a domestic living room, however, volume was more than sufficient, and accuracy was superb.
Audio reviewers are an excitable lot on the whole. But my long-term readers are well aware that I am more along the lines of “the ice-cold bastard of audio reviewing,” to borrow Janos Starker’s phrase. (I once congratulated him after a deeply romantic performance of Dohnányi’s Konzertstück. He said thanks and then remarked with a wry grin: “That ought to show those people who call me the ice-cold bastard of the cello.”) Almost everything calls out my critical instincts rather than wild and uncontrollable enthusiasm. The SAM system is an exception. This is really something extraordinary and revolutionary, and I don’t mind saying so. It is musical magic. But it is not literally magic—no magic signs, no mandrake roots dug up at the dark of the moon and dried pointing exactly north. This is a serious technical design that works in an explicable if somewhat complex way. What the device does is measurable, as I shall get to later. (At least I could verify part of it in measurements. Other claims made by Devialet seemed correct in listening terms, but I could not check them all with the measuring methods I had available.)
Why SAM Is Needed
A speaker is in effect a band-pass filter—it is flat (more or less) from a certain low frequency on up to some high-frequency limit. But it rolls off below that low frequency and above the high one. We are only interested here in the lower-end roll-off. So, in effect, we are looking at a speaker as a “high-pass filter.” Now high-pass (minimum-phase) filters are never phase linear unless the low-frequency roll-off frequency is extremely low—in principle all the way down to 0Hz, so there would be no filter at all. This is a mathematical thing about filter theory, which we don’t need to go into. But the effect is that there is always phase non-linearity—timing error, if you prefer—in the lower frequencies of a speaker. Ported box speakers, which roll off at 24dB per octave are worse for this, for a given roll-off frequency, than sealed boxes, which roll off more slowly at 12dB per octave. But some phase non-linearity is always there in all cases. (There have been a few attempts to correct this at least partially via an external all-pass analog filter, e.g., the KEF Kube. But such things are speaker specific—one cannot use them across the board.)
In general terms, audio people know about this and have for a long time. Nearly everyone has noticed that speakers that go deep into the bass usually have more precise bass further up. And the same is true of amplifiers—flat to DC or nearly so gives bass precision up into the audible range.
It’s easy to underestimate the significance of this time error in speaker roll-off. This would be a good moment to recall what ought to be one of the most famous demos in the history of audio, but has seemingly been almost forgotten. It is mentioned briefly in High Performance Loudspeakers by Martin Colloms and Paul Darlington (Seventh Edition, p. 236) with a reference to Laurie Fincham’s AES paper of 1983, but it is not trumpeted there nor anywhere else, as it should have been! This demonstration involved taking a speaker, which already had considerable bass extension but rolled off by 20Hz and EQ’ing it electrically (with minimum-phase EQ) to be flat to 5Hz. Of course, the speaker could not produce substantial output at 5Hz or anywhere close. Something like 40dB of EQ was required. But it was EQ’d flat to 5Hz in principle. Then material was played—in public, these were public demos—which contained little content much below 40Hz. And at 40Hz on up the EQ was doing nothing substantial in terms of frequency response.
Nonetheless, the result was startling. While there was no substantial increase in bass content on account of the limited bottom-octave content of the material, there was a very large increase in bass clarity and definition. One observer (in Hi-Fi News and Record Review, as I recall) described how a rapid alternation from B flat to B in the doublebasses in a Sibelius symphony, which had been an undifferentiated rumble in the non-EQ’d speaker, turned into a clear and defined trill with the EQ switched in. This was all the effect of timing correction, of removing the time errors introduced by the non-EQ’d speaker’s roll-off. Amplitude change was minimal. The listening effect was from timing correction.
I am indebted to Paul Miller for bringing to my attention that Michael Gerzon, in connection to the BandW room-correction system (which never, in fact, appeared as a commercial product, more’s the pity) also worked on this issue. (One can find his work discussed at www.stereophile.com/reference/706deep in a survey by Keith Howard, which also gives detailed references. And you can find Paul Miller’s own listening impressions, which are much like my own, of the SAM system at www.devialet-store.nl/sites/default/files/Hi-FiNews-May2014.pdf.)
That the bass would be made more precise by the EQ in the KEF experiment was clearly based on filter theory. But the size of the audible effect was surprisingly large. And perhaps even more surprising—and highly relevant to the present situation—the effect was clear in spite of room effects. The definition of the “first arrival” of the bass was highly evident in spite of what the room around it may have been doing.
Now we come to a moment that is a bit disconcerting. No matter what speaker system you are using, the restoration of bass resolution by the EQ in this experiment is part of your sonic picture. You may have gotten used to it, but there is no way around the fact that the blur in time is there. Even if you have a subwoofer that goes down super-low, the blur in your main speakers is still going to muddy the sonic picture.
Are there not any speakers that are immune to this bass blur? Well, if I EQ the Cerwin-Vegas (which go down easily into the low 20s) flat down to around 16Hz or lower, I can get pretty close to the definition that ought to be there in, say, the 40Hz-up region. I can do similar things with the sealed-box AR 303, which also has deep bass extension. And doing the appropriate EQ to the big Wilson models would probably line them up well in timing. But on the whole, there are very few speakers that go down far enough to get close to what ought to be there. And even the ones that do would probably be even better with SAM. (How much even nominally “full-range” speakers roll off in the bottom may come as a surprise. Scan through the NRC anechoic measurements at www.soundstage.com/index.php?option=com_content&view=article&id=16&Itemid=140. No wonder people feel the need for subwoofers!)
Let me emphasize again that I am not speaking here only—nor even primarily—of bass extension as such. Most music has little energy below 40Hz (which is the bottom of the usual orchestra and also of the electric bass used in rock). But the definition from phase linearity, from correct timing, is the issue here. Bass clarity is important—really important as it turns out.
Why don’t people talk about this more, since those in the know have known about it for a long time? The reason, I am afraid, is the natural human impulse to ignore what you cannot fix. But with SAM, you can fix it and, once you have, you will realize how much you have been missing.
How to Fix Bass Timing
In digital signal processing, one can control phase behavior and amplitude response separately. So a DSP speaker can be phase linear down to the bottom of its operating range. And some of them are. But with analog speakers, you were, pre-SAM, stuck. There was no practical way you could EQ an ordinary speaker flat to the 5Hz of the KEF experiment in the real world. If you did, and then by chance put a really low note through—KEF was allowing nothing much below 40Hz, remember—your speaker would probably destroy itself, so much would the EQ have boosted the signal in the really low frequencies.
But the SAM system operates a different way. First, speakers are measured electrically and, for driver movement, with a laser interferometer, and then a model is constructed of the behavior. This is done for various speakers (a lot of them by now—see the full list at Devialet’s website) and one can download the correction algorithm specific to that speaker on an SD card that one then plugs into the SAM device, which is also an integrated amplifier. I used the Devialet Expert 220 Pro amplifier, which is incidentally a super amp. This is really a review of the SAM system, but the Expert 220 Pro is superb even when SAM is not used. It is absolutely in the top echelon and at a very reasonable price for an amplifier of this performance. (It also has a large number of inputs, including a phonostage—which I hope to review separately later. It is effectively a universal integrated amplifier.)
Supplied with the correction information on the SD card, the SAM system “watches” the electrical signal being sent to the speakers and corrects it 192,000 times a second to make sure that the corrected signal being provided will make the acoustic output of the speaker match the input to the amplifier (the system operates in the lower frequencies, fading its correction out by around 150Hz). The system is vaguely reminiscent of the Philips Motional Feedback idea of decades past (somewhere around I have an LP called “Music to Listen to with the Motional Feedback System,” or something like that—all right, so I am a pack-rat). But the SAM system is much more sophisticated and is not a feedback system as such.
Analog die-hards and technical troglodytes are perhaps going to say, “Oh but there are going to be digital artifacts.” All I can say to that is that I did not find it to be the case; plus, I might suggest, for people who have the idea that vinyl is somehow better, that if you played your records with a Moerch DP-8 ’arm (which I helped design) you would have a much better chance of hearing something like correct bass than if you did not—for the same kind of minimum-phase reasons. With most tonearms, anything like phase linearity in the bass is just not happening in vinyl playback.
But there is more than phase correction involved here. And the more is, to my knowledge, unique. The SAM system also EQs the bass so that your speaker has flat response down much further than it did before. But it watches and does this only as long as it is safe to do it. It restricts the lowest frequencies to what the speaker can manage without distortion or damage. This means that your speaker has in effect a lot more bass extension than before, as long as extreme demands in dynamics in the really low frequencies are not made.
And, oh yes, I should mention that it also watches for distortion and corrects it, to the extent this is physically possible. This system makes the bottom end sound really clean.
I have no accurate way to measure phase behavior for timing; I am relying on listening impressions. But I did measure frequency response. At moderate pink-noise inputs, Speaker X when SAM’d really did measure flat down to effectively 20Hz (as promised on the Devialet website). But as I pushed the signal level up, the SAM system gracefully reduced the 20-to-40Hz region, so that the speaker never distorted nor damaged itself. This bass extension business really does work.
Why the Extra Extension Is Good
One can check the bass timing aspect directly in part, because the SAM controls allow turning down the amplitude correction while retaining the timing correction. In practice, this control did not seem to entirely defeat the amplitude part of the SAM operation, for reasons that are unknown to me, but it reduced the amplitude change quite a lot. For speakers that have reasonable extension already, the timing and distortion corrections are likely to be the most important parts of the show. But the extra bass extension is really useful, too.
Now you might say, what is the use of extension to 20Hz or close to it if the level is limited? The answer to that requires some thought about musical instruments.
Think of a concert grand piano. The bottom note is 27Hz (leaving aside the Bösendorfer Imperial). But the sounding board is a little less than 5 feet wide and it emits sound as a dipole radiator. At 27Hz, the wavelength of sound is 42 feet. A piano is not going to produce a whole lot of sonic energy at 27Hz, on account of dipole roll-off. That 27Hz note is heard mostly via energy in its overtones.
This sort of thing holds for many musical instruments, though not all. Large organ pipes produce substantial energy in their fundamentals, and of course electronic music can contain arbitrarily large amounts of energy at very low frequencies. But for many instruments, the amount of energy below 40Hz is actually within reach of even a moderate-sized speaker. (Interestingly, Fincham in his aforementioned AES paper discusses this point and observes that many instruments in fact roll off much the same way a speaker does, so that one can track the actual response with the speaker.) The SAM system’s extension of frequency really is useful.
Speaker X, SAM’d, presented Freddy Kempf’s piano on BIS CD-1042 (Freddy Kempf Plays Rachmaninoff) with extraordinary solidity and realism. This really sounded like a concert grand, with none of the tendency to make the big grand sound like an upright that so often arises. Here was a smallish speaker producing a big sound thanks to SAM.
SAM with a Larger Speaker
I also tried the SAM system on a larger speaker, a floorstander with two eight-inch woofers. Call it Speaker Y. As it happens, this speaker does not have a lot more bass extension than Speaker X, but it does allow somewhat higher playback volumes without stress. Speaker Y is not quite flat from 500Hz on up, so I EQ’d it a bit in the higher frequencies. Then with the SAM system correction data inserted in the Expert Pro 220 amplifier via the SD card, I was ready to go. And again the results were spectacular in listening terms. Orchestras sounded really convincing, as did the large grand piano. This was superlative sound, far beyond what the speaker alone could offer. The precision and transparency in the bass were extraordinary. This speaker too offered a really convincingly realistic sound, comparable to—or truth to tell, superior to—what most much more expensive speakers can offer, as far as bass performance is concerned. This was really something special—bass that was truly reminiscent of the live experience. (I listened to this setup with orchestral music on the morning after a Nutcracker rehearsal. Nutcracker is not the biggest, loudest, bass-iest Tchaikovsky score, but it has big moments, and the SAM’d Speaker Y was bringing out something very like those moments, even in what was nearly direct comparison to live.)
Spatial impressions were also unusually good, as they were with Speaker X. Much of the sense of the space of the recording venue is carried by bass information. We recognize that a place is large in good part by the reflection and reverberation of the lower frequencies. Better low-frequency extension and timing translate into a greater perception of space. And so it was with the SAM system. Expansiveness was the rule. Recordings made in large places really sounded convincingly large, adding an unusual verisimilitude to large-scale music.
Properties, Concerns, Competition
To my mind, the Devialet SAM system represents a major advance. Most DSP room- or speaker-correction systems operate as fixed-setting EQ devices, or combined EQ and phase-manipulation devices of a fixed setting. The settings are determined by measurements, made assuming that the speaker or speaker/room combination is a linear system. Their behavior is independent of volume. To the extent that the speaker actually is a linear system at all the levels to be encountered, this works well. But there is no possibility of accommodating changes associated with different volume levels. And there is no possibility in particular of allowing more EQ in the deep bass at lower levels (where the speaker will take it well) and limiting this extended bass EQ at high levels for the safety of the speaker.
There are systems that do both amplitude and phase. DEQX offers this, and Uli Brueggemann’s Acourate system does phase correction of in-room behavior. And here, of course, an issue arises: Does the ear hear the phase behavior of room-plus-speaker or does it “hear out” the behavior of the speaker as such? My guess is it probably does both. Never underestimate the power of the brain’s auditory processing. The SAM system aims to fix the speaker. And the effect of this is really so convincing that one cannot escape the idea that the ear is in fact “hearing out” the speaker behavior separately.
This is a complex issue, with roots going back at least as far as the work on gestalt-processing and the “association model” of the ear/brain by Gunther Theile (www.aes.org/e-lib/browse.cfm?elib=14192, and references therein). One of Theile’s arguments is intuitive but also really convincing: If the ear/brain is completely fooled by two-speaker stereo, if only the literal signals in the ears are considered, why is it that the front of the “soundstage” is associated to the plane of the speakers? And why does a close-miked mono vocal not appear to be in your head or at most a few inches in front of you? Food for thought.
In any case, one cannot do both—correct the total arrival and the speaker behavior (the direct arrival) as well. SAM opts for fixing what the speaker does. You are on your own via placement and so on for taking care of room effects. One thing worth noting in this connection, however, is that, as long as room effects are not too extreme, the amount of (minimum-phase) EQ needed to fix the “usual floor dip” between 100 and 300Hz will not be enough to destroy the benefits of correct phase behavior of the speaker. This is so, at least in my experience. (I tried out concatenating minimum-phase analog EQ with the SAM system, EQ’ing the analog input to the Expert Pro. This worked very well in practice. One could get almost the best of both worlds, room sound and speaker sound at least in the range relevant to the floor dip.)
These various deep waters of psychoacoustics arise inevitably with the SAM system. The full-on SAM system pushes up the steady-state in-room response (the RTA measurement) quite a lot in the bass. Part of this is the system extending the bass response. But part of it happens at frequencies, say around 100Hz, where the speakers themselves are already basically flat. The SAM’d speakers had more bass not only where they were lower-octave deficient but at other frequencies as well. And yet, somehow the SAM’d speakers did not sound bloated. The bass was strong but precise. This is a different effect from what happens if one takes a minimum-phase device and pushes up, say the region around 100Hz. There is some interaction of timing and perceived bass level and overall behavior. Again Colloms and Darlington (op. cit.) discuss this in some detail (p. 184 ff). But this is an area where exact answers are not yet known, I think.
Still, one cannot help wondering how much of what you are hearing with SAM—which sounds really good, no qualifications, at least with the two speakers I tried—has to do with amplitude response and how much to do with timing as such; Devialet is quite definite that the SAM system is not minimum phase and that timing correction happens beyond the improved phase behavior simply because of improved bass extension.
One could get some insight into this question if one had some information on the literal improvement of phase linearity. The white paper at images.computeraudiophile.com/graphics/2014/1216/Devialet-PHANTOM-White%20Paper.pdf shows an illustration of the system lining up speaker output with the input signal. (Devialet assures me the illustration is an actual measurement not an artist’s conception.) But how to connect this to more usual phase-linearity measurements is unclear. Similarly, Devialet claims reduction in distortion, but no measurements are supplied. This seems to me somewhat odd. The things being claimed are quite explicit in technical terms. If you’ve got it, why not flaunt it? Why not show people measurements of how well your system works? I cannot imagine why the engineers do not want to strut their stuff by revealing extensive measurements.
I am hoping as a follow-up either to persuade Devialet to provide some conventional measurements or to do some myself. (Accurate phase measurements are not too easy to arrange in the low frequencies, however—one really ought to have an anechoic chamber or a way to get the speaker up high out of doors.) Meanwhile, the waters remain a bit murky in measurement terms. The increase in bass extension, which diminishes with level to protect the speaker, is definitely measurable, and it definitely has desirable effects. But the source of other audible improvements is not so easy to pin down. Everyone knows that small changes in bass amplitude response can have large effects on perceived sound. And so it would be good to know what parts of the sonic effect are from the bass boost (which is there) and which parts are the other factors—improved timing, phase linearity, lowered distortion, or what.
The Big Picture
I had a fine time with SAM musically. I have seldom been so excited about an audio development as I was hearing that middle-sized Speaker X putting out such a convincing facsimile of an orchestra at full tilt. I was just jumping around. Whatever SAM was doing, I liked it. I really would like to know exactly what it was doing. But whatever it was, it was producing extraordinarily realistic music.
This was true to the extent that if I were starting anew and willing to spend the price on a SAM-equipped Devialet integrated amplifier, it is hard to imagine buying anything else. SAM opens up new prospects, and even in the unlikely event you did not like the SAM system, you would still have a superlative piece of electronics. The Expert 220 Pro is not inexpensive but in the current pricing climate, it is surely reasonable for a full-function integrated amplifier that performs at the highest level. And when you add in SAM…what else could be in the running at the price is a question that comes to mind.
It seems to me all but certain that the idea of actively controlling the interaction of electrical input to achieve the target speaker output is a major advance and that it should become universal in audio.
I think of an automotive analogy. I thoroughly enjoyed the experience of rebuilding a Rochester four-barrel carburetor years ago (fortunately assisted by a friend, Ritchey Hume, who really understood the thing). What a miracle of ingenuity it was, with all sorts of thermal and mechanical sub-devices to deal with all the variable conditions it would need to handle in operation—the choke adjustment controlled by a device which changed position according to temperature on account of expansion or contraction when temperature went up or down, the “accelerator pump” which detected a sudden depression of the gas pedal to spurt some extra fuel into the carburetor, the float assembly to detect the level of the fuel in the reservoir—all marvels of engineering ingenuity. But when the chips are down, and down they are on a cold morning when you need your car to start or on a really hot day when you need the same, or when you need to pass the California emission test (the infamous “smog test”)—well, you just have to swallow your nostalgia for the old ways and admit that electronic fuel injection, a system that is watching everything that is going on and adjusting many parameters on the fly on the basis of these detailed observations, is just better. The carburetor senses operating conditions but it senses them too slowly to get everything just right all the time. Electronic fuel injection works better because it’s sensing what is needed so much faster. The result is not so romantic perhaps, but it is superior.
The situation for speakers seems to me similar. Already twenty-five years ago, the Spendor SP1/2, designed by Derek Hughes, was so accurate that DSP correction of it in-room by Sigtech or of the speaker itself by Arion/Essex did very little because there was so little to correct. Still, there was the phase non-linearity in the bass (inevitably as already noted), a little bass distortion at high levels, and bass extension limits. Being able to fix these things and especially to, in effect, extend bass response without having to worry about damaging the speaker would definitely be an improvement. This additional extension method is simply not available in the analog world. As Devialet says somewhere on its website, before SAM, speakers had to be as large as bathtubs to have good bass. With SAM, not so any longer.
Technologies like SAM that monitor the speaker’s behavior and correct that behavior on the fly seem to me the way to progress even beyond the almost miraculous performance of the best passive speaker designs. I am still working on what exactly SAM does in measurement terms. But I can tell you for sure SAM sounds fabulous and, perhaps more importantly in the long run, represents to my mind what is needed in audio. It has been more than twenty-five years since I first started writing about the marvels that DSP devices would produce in audio. Wonderful they were and are. But they are static—they do what they do without concern for what the speaker is actually doing except to the extent that this is embodied in linear system measurements (frequency response and phase response). But the Devialet SAM system goes beyond in its dynamic signal-following behavior. It was time for the next big step. And here, I think, it is.
Specs & Pricing
Type: Integrated amplifier (including phonostage) with SAM correction system
Inputs: Line, optical digital, USB, coax digital, WiFi, phono (mm/mc), AES/EBU, Ethernet
Supported digital formats: Up to 192kHz/32-bit
Power output: 220Wpc into 6 ohms
Distortion at full power: 0.0005% THD+N
Signal-to-noise ratio: 130dB
Dimensions: 383mm x 40mm x 383mm
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