VTL S-400 Reference
Contents
1. Electronically reprinted from December 2005 Brian Damkroger Tubed stereo power amplifier Inputs 1 pair unbalanced RCA 1 pair balanced XLR Out puts 2 pairs binding posts Tube complement two 12AT7 two 6350 twelve 6550C KT88 Rated power output at lt 3 distortion 300W into 8 ohms 24 8dBW 400Wpc into 4 ohms tetrode 23dBW 150Wpc into 8 ohms 21 8dBW 200W into 4 ohms 20dBW triode Frequency response 10Hz 125kHz 0 3dB Output impedance ca 1 1 5 ohms Input impedance 57k ohms Input sensitivity 2 2V for full output Power consumption 480W at idle 2500W at full power 24 615mm H by 11 5 295mm W by 24 615mm D Weight 220 Ibs 100kg net 330 Ibs 150kg shipping 04376106 auditioning 05146432 measuring Approximate number of dealers 47 Warranty 5 years nontransferable VTL Amplifiers Inc 4774 Murrieta Street Suite 10 Chino CA 91710 Tel 909 627 5944 Fax 909 627 6988 Web www vtl com Email mail vtl com www Stereophile com December 2005 EQUIPMENT REPORT VIL S 400 Reference POWER AMPLIFIER VTL S 400 Reference power amplifier hen I reviewed VT s MB 750 monoblock amplifier in the December 1997 Stereophile Vol 20 No 12 it was a transitional time for the company Luke Manley had recently taken it over and he and his wife and partner Bea Lam were aggressively retooling They introduced new business systems including rigorous inv2. with the supplies in older VTL amps such as my own Ichibans Luke said With the older amps the power sup ply floated with AC voltage in addi tion to sagging There was no way to keep the tubes at their optimum point We always knew that precisely regulat ing the supply was better but we didn t have the technology to manage the regulation Beneath the S 400 s top cover The green LED lights beside each tube confirm status and blink to indicate a bias adjustment is underway Engineers love gadgets but the proof is in the pudding I spent the better part of a year with the S 400 during which time it shared power amp duties with the Simaudio Moon Rocks Mark Levinson No 20 6s and my much updated VTL Ichibans I mated the S 400 with sever al different preamps and CD players and a wide range of cables Two con stants throughout were my Thiel CS6 loudspeakers and my analog front end a VPI HR X turntable and tonearm and a Lyra Titan cartridge First I can attest that VTL has suc ceeded in their goal of making the tubes invisible After the novelty wore off of watching the blinking biasing LEDs and accessing the diag nostic displays I never gave the S 400 and its wizardry a second thought Pd push the Power button watch 240 appear on the front panel display and start counting down the seconds and go pick out a record Based on my year or so with the amp I d pro nounce it both plug n
3. A Go Go LPs Brown Eyed Handsome Man from Johnny Rivers at the Whisky A Go Go Imperial LP 12264 is one of my favorite cuts with any good system it will trans port me to a front center seat at the club But the first time I heard it through the S 400 I was stunned I actually caught myself looking around to see which of my fellow audience members was talking and singing along Incredible The third of the VTLs attributes its transparency is the easiest to describe in audiophile terms I could simply hear much farther into the soundstage with the S 400 than with the Sims Ichibans or Levinsons And with the S 400 the limit wasn t the system running out of resolution at the rear of the stage or losing ambi ent detail into a background texture Instead I could either hear whatever surface bounded the recording space or hear the microphones sensitivities reach their limits And up to either of these points I heard nothing other than the recording venue This transparency favored live recordings especially more simply recorded performances where the miking patterns had been integrated into a seamless envelope The Johnny Rivers and Ramsey Lewis albums sounded coherent immediate and alive and these are standard buy at the corner store LPs Simple well done recordings given a modern audiophile makeover such as AcousTech s reissue of Lightnin Hopkins Goin Away LP Prestige Bluesville Acou
4. anced through the S 400 I was initially suspicious of the S 400 s apparent neutrality Given its rel atively high output impedance 1 1 5 ohms I expected to hear at least some frequency response anomalies due to the amp s interaction with the speaker load Plus John Atkinson s measure ments always show this sort of behav ior in VTL and other similar ampli fiers I dug into this a bit however and discovered one possible explanation Between 50Hz and 10kHz JA s simu lated speaker load fluctuates between 4 ohms and gt 20 ohms with huge peaks around 70Hz and 1kHz www stereo phile com reference 60 index1 html In contrast his measurements of the Thiel CS6 showed a much flatter impedance curve that varied by only about 1 ohm over the same range and within an even narrower band for much of that span see the March 1998 Stereophile Vol 21 No 3 p 96 Another component of the S 400 s presentation and its lack of a sonic thumbprint was its clarity In addition to the changes I describe above I noted that switching the VTL to triode mode or swapping in the Sims Ichibans or Levinsons wove a liquid texture into the sound and fine details spatial tem poral and tonal weren t quite as clear Going back to the S 400 or switching from triode mode to tetrode was akin to opening a window that is not quite optically perfect With the S 400 in tetrode mode fine details were much easier to identi fy and
5. follow but a more significant effect was the realistic presence and immediacy that imbued every compo nent of the performance On Hang On Ramsey it was there as much in the applause and bassist Eldee Young s singing along with his own solos as it was in Lewis piano I went through box after box of my favorite opera sets just to hear them come alive with the S 400 s clarity I m sure that there was fine detail in spades the soundstage was appropriately large and the image dimensionality and ambience cues were beautifully rendered but I really didn t notice any of that I was too busy sitting in the audience of a virtual opera house alternating between quickly reading the libretto and closing my eyes to melt into the performance Perhaps my favorite example of the S 400 s stunning clarity was Jimmy Smith s Hammond B 3 organ on his Who s Afraid of Virginia Woolf LP Verve SMAS 90577 a wonderful Rudy Van Gelder production from 1964 The organ s slightly buzzy electronic tex ture was perfectly reproduced by the S 400 Combined with Smith s mes merizing expressive playing the VT s clarity seemed to erase the recording playback chain altogether plopping Smith his organ and the backing players right in front of me Most spectacularly voices lit up and came alive with the S 400 I con stantly found myself digging out older more simply recorded live recordings such as Johnny Rivers Whisky
6. in triode but transients were slightly dulled giving me a sense that the top end wasn t as open and extended as it could be Plus the piano s notes sounded homogenized and slightly thick with no obvious inner structure or temporal evo lution And although I was never aware of a distinct change in amplitude as Lewis played down into the instrument s lower ranges there was a richness to the notes that seemed to increase as the pitch dropped In tetrode mode on the other hand Lewis piano was dead on Each treble note was clearly a changing mix of har monics that went through a distinct evolution from a sharp cutting tran sient through a quick bloom and decay Similarly Red Holt s cymbals were slightly muted with a thick metallic hiss in triode mode but a startlingly clear mix of components in tetrode mode The differences were even more obvious moving down through the midrange and bass Going back to Lewis piano running the S 400 in tetrode mode resulted in an even more complex evolution of the sound after the initial transient an obvious pro gression of string vibrations building in complexity and increasingly exciting the instrument s soundboard Ray Brown s bass on the Pure Audiophile reissue of Soular Energy LP PA 002 was another great example warm and full to the brink of being overstated through the Moon Rocks or my Ichibans still highlighted but more clearly detailed and realistically bal
7. speakers At other times I detected a politeness less than explosive dynamics and a very minor softening of transients which reminded me a lot of how Michael Fremer described his impressions of the latest Lyra Titan car tridge in one of our e mail exchanges Pd never heard these subtleties before but does that link them with the S 400 or merely suggest that removing the other amps colorations allowed the speakers and cartridge s to be heard I suspect the latter but if that s true what was associated with the VTL Neutrali ty Clarity Transparency In the case of neutrality the answer is yes absolutely I ve heard a lot of amps including a few really superb ones but none that sounded as tonally neutral as the S 400 in tetrode mode There was a simple almost stark honesty to its por trayal of instruments tonal structures that in comparison laid bare the arti facts that the other amps contributed to instruments and voices In fact merely switching the S 400 between its tetrode and triode modes provided a wonderful example of this Switching to the S 400 s triode mode or to the Simaudio Moon Rocks that I reviewed in September for that matter added a touch of warmth and sweetness to the sound and seemed to gently roll off the highest frequencies Ramsey Lewis piano on his trio s 1965 live recording Hang On Ramsey LP Cadet LP 761 was a good example It sounded pretty good with the VTL
8. standard against which I ve compared preamps and CD analog stages Until the TL 7 5 nothing had come close but the VTL really seemed to combine the best of both worlds the Placette s neutrality and transparency with the abili ty to accurately reproduce dynamic transients from the smallest to the largest and from the bottom to the top of the frequency spectrum Recalling that Paul Bolin had raved about VTL s TL 7 5 when he reviewed it in the Octo ber 2003 Stereophile pulled his piece up from www stereophile com tubepreamps 938 to see what specifically he d had to say about it nearly fell out of my chair Not only did his impressions and conclusions about the TL 7 5 mirror mine they sounded eerily like what I d heard from and written about the S 400 amplifier It is no easy thing to attempt to describe the sound of a component that had less intrinsic sonic character than anything else have ever reviewed The TL 7 5 had such an infinitesimal sound of its own and did everything so supremely well that found nothing to rationally criticize Sonically it simply did not exist in the signal chain Well done Paul couldn t have said it better Having spent a year with VT s S 400 and TL 7 5 my conclusion is that they are indeed a matched pair but not because each synergistically complements the other s signature Instead VTL has effectively removed the colorations from both and lets the music and the rest of the syst
9. triode which will be subjec tively innocuous especially at the low static levels featured in these graphs At high powers the S 400 s low loop feedback means that higher order harmonics make an appearance fig 11 tetrode fig 12 triode though the fact that the harmonics decrease linearly in level with increasing order works against their audibility especially in triode mode Note the Stereophile VTL 5 400 Tetrode SOHz spectrum at 85W into 8 ohms dBr vs Hz 0 0 m OANA NAAN lok iN 500 0 Fig 11 VIL S 400 ae canes nee of 50Hz sinewave DC 1 kHz F 85W into 8 ohms linear frequency scale Stereophile VTL 5 400 Triode SOHz spectrum at SOW into 8 ohms dBr ve Hz 0 0 An ah mila Ata Fig 12 VIL S 400 eon ade eat of 50Hz sinewave DCI kHz at 50W into 8 ohms linear frequency scale commendable absence of power supply related spuriae in these graphs The big VTL did less well when it came to high power intermodulation test ing fig 13 tetrode fig 14 triode both taken just below visible waveform clipping on the oscillo scope screen which is what expected from the decreasing linearity shown at high frequencies in the THD plots figs 7 and 8 VTL s S 400 is a thoroughly worked out tube design whose computer monitored operating condi tions should ensure many years of service Its design er s decision to go for low levels of loop negative feedback does compromise the amplifier s high power linearity but th
10. ctions From here a user can turn various displays and indicators on or off and display total tube or amplifier hours input voltage or voltage set point and rear heatsink temperature These buttons also allow the fault indi cators to be cleared The user can also display the bias level for each tube then use these data to juggle tubes to create the most closely matched pairs Because bias THE S 400 BIASES ITSELF FOR YOU CHECKING AND SETTING EACH TUBES BIAS DURING THE POWER UP CYCLE level is an early indicator of tube degradation plotting bias levels against time can help determine when its time to retube When you do retube be sure to reset the tube time to zero and clear the preset bias set tings You want the S 400 to gently raise the bias level to find the new tubes optimal points rather than risk damaging them by starting at the old tubes bias levels If by now your eyes are glazing over and you re starting to think that this would be a lot easier if you could just hook up a laptop VT s engineers have beaten you to it An RS 232 con nector on the rear panel allows you to read log and control all of this from your computer All of the displays codes functions and software com mands are explained wonderfully in VTUs comprehensive user s manual New cosmetics more buttons and features same old circuit Not even close Making the tubes disappear was one of VTLs goals for the S 400
11. different sample of the S 400 to the one BD audi tioned My sample had been used at the Home Entertainment Show last May and came to me following the Show before its return to New York dealer Sound By Singer With its choices of unbalanced and balanced inputs and triode or tetrode output stage opera tion VT s S 400 is four amplifiers in one To make measuring its performance feasible carried out complete sets of tests for both triode and tetrode operation in balanced mode spot checking some measurements via the unbalanced input All the inputs preserved absolute polarity the XLRs appearing to be wired with pin 2 hot Before did many tests the amplifier was run for an hour at one third power in tetrode mode into 8 ohms The voltage gain into 8 ohms was the same with both balanced and unbalanced drive but differed between the two output modes tetrode was 27 2dB triode lower at 25 3dB The unbalanced input impedance was a moderately high 74 2k ohms at 1kHz dropping at the frequen Stereophile VTL 5 400 Tetrode Freq Response at 283W dBr ve Freq Hz 10 100 ik 10k 100k 200k Fig 1 VTL S 400 tetrode mode balanced frequency response at 2 83V into from top to bottom at 2kHz simulated loudspeaker load 8 4 2 ohms 1dB vertical div right channel dashed Stereophile VTL 5 400 Triode Freq Response at 283W dBr vs Freq Hz Fig 2 VTL S 400 triode mode balanced frequency response at 2 83V into from top to botto
12. e tradeoff is probably the very low levels of static distortion when the amplifier is giving out less than 10W Even so it is probably best to avoid speakers that plunge much below 4 ohms to get the maximum dynamic range from this ampli fier even though note that BD had no problems driving his Thiel CS6es which remain between 2 5 ohms and 4 ohms over most of the audioband see www stereophile com floorloudspeakers 218 index4 html John Atkinson Stereophile VTL S400 Tatrode 19 20kKHz2 al 140W into 8 ohms dor vs Hz 0 0 k 4 Ok l Fig 13 VTL s 400 E e HF e T DC 24kHz 19 20kHz at 140W peak into 8 ohms linear frequency scale Stereophile VTL S400 Triode 19 20kHz at 30W inio 8 ohms dBr vse Hz 0 0 LELI LHJ a LHE amil Hie fH mil HI LOLI H LIE Uh Pam bl Li Fig 14 VTL S 400 triode mode HF intermodulation spectrum DC 24kHz 19 20kHz at 90W peak into 8 ohms linear frequency scale T s TL 7 5 Reference line preamplifier was designed by the same team and around the same philosophy as the S 400 Reference power amplifier and from the begin ning the two models were intended to be used together Typically when a design team creates matching products much attention is paid to ensuring that their charac teristics at least complement each other and if possible achieve a positive synergy The Burmester 011 preamplifier which reviewed in the July 2005 issue and 001 CD player are great examples of this excell
13. em of course speak for itself Just as the S 400 s 20 000 price is a lot to spend on a power amp the TL 7 5 s 13 500 tag is a lot to spend on a line stage but I ve heard nothing else like it These are true benchmark products Brian Damkroger Posted with permission from the December 2005 issue of Stereophile www stereophile com Copyright 2005 PRIMEDIA Inc All rights reserved For more information about reprints from Stereophile contact Wright s Reprints at 877 652 5295
14. ent alone magical when used together But given that I had a hard time pinning any sort of sonic signature on the S 400 couldn t help but wonder what that suggested about its companion preamp the TL 7 5 Ref erence line stage so asked to borrow one and hear for myself Because had several CD players capable of driving the S 400 directly tried to isolate the 7 5 s sound by switching it in and out of the signal chain and by comparing it with the Burmester 001 and the Placette Remote Volume control As with the S 400 found it nearly impossible to ascribe any specific sound to the TL 7 5 Inserting it between an amplifier and a CD player any CD player even the Burmester 001 enlarged the scale of dynamic transients somewhat and made microdynamics and inner detail much clearer though not unnaturally so My conclusion after extended listen ing trials was that the VTL was removing a slight dulling and veiling of the sound rather than artificially goosing it up The same was true for other subtle aspects of the sound With the TL 7 5 in the system the soundstage was larger and airier and both clarity and transparency were improved Comparing the VTL to the little Placette reduced the differences quite a bit but they were still there particularly the improvement in dynamics and image dimensionality and to a lesser extent inner detail The Placette s transparency really impressed me when first heard it since then it s been a
15. entory and quality control rebuilt VT s dealer network around top rank dealers and systematically upgraded the products themselves to improve their consistency reliability manufacturability and performance VT s goal Luke explained to me at the time was to build amplifiers that competed with the very best and to make the tubes invisible to the customer The plan rolled out just as Manley had envisioned and VTL amplifiers evolved through several iterations the MB 750 saw both Signature and Reference edi tions and the flagship MB 1250 Wotan set a new standard in tube amplifier per formance However having to bias the tubes manually in the case of the Wotans 24 per amp didn t exactly make them user friendly So in the background a mul tidisciplinary team of VTL engineers was already working on a completely new generation of products The first to appear was the sensational TL 75 line pream plifier which Paul Bolin raved about in the October 2003 issue The next to break cover were two power amplifiers the 800W Siegfried monoblock and the subject of this review the two channel S 400 Reference Talkin bout a g g generation we ve not seen before It s hard to know where to start in describing the 20 000 S 400 so differ ent is it from what s come before You could start by listing everything you might want a tube amp to do and that s just what VTL did Then they turned their engineering team loose
16. m at 2kHz simulated loudspeaker load 8 4 2 ohms 1dB vertical div right channel dashed Data in Volts 0 10 Time in ms Fig 3 VTL S 400 tetrode mode small signal 1kHz squarewave into 8 ohms cy extremes to 56k ohms 20Hz and 44 4k ohms 20kHz As expected the balanced input imped ances were twice these figures The S 400 should not load down the output of any preamplifier with which it is used The output impedance also differed between the two operating modes ranging from 1 82 ohms at 1kHz to 1 9 ohms at 20Hz and 20kHz in tetrode mode and from 1 66 ohms at 1kHz to 1 7 ohms at the frequency extremes in triode mode As a result of the Ohm s Law interaction between this source impedance and the manner in which a loud speaker s impedance changes with frequency Data in Volts 0 10 Time in ms Fig 4 VTL S 400 triode mode small signal 1kHz squarewave into 8 ohms Stereophile VTL 5 400 Tetrode THD N ve Power W into 8 42 ohms 10 eet are Fig 5 VTL S 400 tetrode mode distortion vs 1kHz continuous output power into from bottom to top at 1W 8 4 2 ohms there was a significant 0 6dB modification of the amplifier s frequency response by the magazine s simulated speaker in both modes fig 1 tetrode fig 2 triode Note the more extended ultrasonic bandwidth of tetrode compared with triode in these graphs 3dB at 115kHz vs 3dB at 87kHz respectively The unbalanced responses we
17. n tube amplifier design and the first of a new generation Today my rec ommendation is that you find one see and hear what it can do and get out your checkbook bal Burmester 001 Cary 300 303 CD players VPI TNT HR X turntable amp tonearm Lyra Titan car tridge Sutherland PhD Ensemble Fonobrio phono stages VIL TL 7 5 Sutherland Director Placette active line stages Placette Remote Volume Control Sonic Euphoria PLC passive line stages VTL Ichiban Simaudio Moon Rock Mark Levinson No 20 6 all monoblocks Thiel CS6 Interconnects and speaker Nirvana S X Ltd Audience Au24 Nordost Valhalla Silversmith Silver AC Audience PowerChord Synergis tic Research Designers Reference MIT Z Center Audi ence AdeptResponse AC condition ing amp distribution systems FIM 880 AC outlets Finite Elemente Refer ence equipment stand amp Ceraball footers Nordost footers Audio Tools equipment amp CD storage racks VPI 16 5 record cleaning machine VPI amp Disk Dr record cleaning fluids Immedia SPT stylus cleaning fluid Zerostat antistatic gun Nordost ECO3 amp Audience Aural Illuminator CD cleaners treatments Brian Damkroger Posted with permission from the December 2005 issue of Stereophile www stereophile com Copyright 2005 PRIMEDIA Inc All rights reserved For more information about reprints from Stereophile contact Wright s Reprints at 877 652 5295 or logistical reasons measured a
18. on the project A few things are obvious balanced and single ended inputs a rear panel 0 180 phase switch fully differential input low idle current standby and mute and a soft start mode to extend tube life And like most VTL amps the S 400 can be run in tetrode 300Wpc 8 ohms or triode 150Wpc 8 ohms modes It also has extensive protection circuitry with a fuse for nearly everything and even a rear panel socket and switch for test ing the fuses The S 400 s sleek tower style shape echoes the TL 7 5 s and its component quality fit and fin ish are superb The list goes on The S 400 biases itself for you checking and setting each tube s bias during the power up cycle then tweaking it whenever the amp senses that no signal is present It also constantly monitors the tubes and if a fault is detected indicates which tube is ailing and whether or not the amp is in danger If it is the S 400 shuts down if it isn t the problem tube is effectively removed from the circuit and the amp rebalances and continues without it The S 400 also checks for overheating indicates if there s a problem senses and displays several types of power supply faults and shuts down the amp or not accordingly Pretty impressive but we re just get ting started Removing a small panel reveals a cluster of six buttons and LEDs that provide still more information and access to additional diagnostic and pro gramming fun
19. play and absolutely bulletproof In fact the only thing that reminded me the VTL was in the system was its no extra charge space heater function a result of its dissipating 480W at idle and a whopping 1700W at full power To put this in perspective the Sears Deluxe Digital Vortex Heater I was looking at last winter has settings of 800W and 1500W and efficiently heats a 16 x 16 room When Trish and Luke Manley were discussing my then new listening room over Christ mas break Luke asked if I d installed any sort of heater Just your amp VE HEARD A LOT OF AMPS INCLUDING A FEW REALLY SUPERB ONES BUT NONE THAT SOUNDED AS TONALLY NEUTRAL AS THE 400 IN TETRODE MODE Luke she responded but that seems to be enough I m teasing but it s true the heat it threw off was the only thing that consis tently reminded me of the S 400 s pres ence I tried in vain to isolate its sonic thumbprint by installing it in familiar setups Each time I replaced another amp with the VTL a distinct coloration was removed from the system s sound but I could never definitively identify what if anything the S 400 had added What was the VTLs character In some cases of recordings or partnering gear I noted a faint coolness suggesting a slight frequency response dip or reduced dynamics in the upper bass and perhaps the opposite in the upper midrange but that s an awful lot like the character of my Thiel CS6
20. re identical Both bandwidths are more than is nec essary to handle audio signals but correlate with Stereophile VTL 5400 Triede THD N vs Power W into a 4 2 ohms 10 0 1 1 10 100 500 Fig 6 VTL S 400 triode mode distortion vs 1kHz continuous output power into from bottom to top at 1W 8 4 2 ohms Stereophile VTL 5400 Tetrode THD N s vs Frequency Hz at 2 83V 1 Fig 7 VTL S 400 tetrode mode THD N vs frequency at 2 83V into from bottom to top 8 4 2 ohms superb reproduction of a 10kHz squarewave fig 3 tetrode fig 4 triode However tetrode mode reveals some damped overshoot on the square wave s leading edges this increasing in level into higher impedances Channel separation not shown was better than 90dB in the midrange decreasing slightly at the top of the audioband Background noise levels were low with the A weighted signal noise ratio ref 1W into 8 ohms measuring 91 4dB in tetrode mode 89 3dB in triode The wideband unweight ed figures were still good at 78 4dB and 80 2dB respectively Figs 5 and 6 plot the THD noise percentage present in the S 400 s output against output Stereophile VTL 5 400 Triode THD N va Frequency Hz al 2 43W 1 Fig 8 VTL S 400 triode mode THD N vs frequency at 2 83V into from bottom to top 8 4 2 ohms power into 8 4 and 2 ohm loads for tetrode and triode operation respectively As is usual with an amplifier featu
21. ring low loop negative feedback the distortion rises linearly with increasing power We usually assess an amplifier s maximum power when the distortion in its output reaches 1 but as shown by figs 5 and 6 the S 400 s power deliv ery is limited when this criterion ts applied VTL specifies the amplifier s power at a more relaxed 3 THD N and at this level the S 400 gave out Data in Volts Fig 9 VTL S 400 tetrode mode 1kHz waveform at 1 5W into 4 ohms top 0 098 THD N distortion and noise waveform with fundamental notched out bottom not to scale l 5 E a 1 0 Time in ms Fig 10 VTL S 400 triode mode 1kHz waveform at 1 05W into 4 ohms top 0 107 THD N distortion and noise waveform with fundamental notched out bottom not to scale 325W 25 1dBW into 8 ohms in tetrode mode 200W 23dBW in triode mode Into 4 ohms the maximum powers available were 405W 23 1dBW and 220W 20 4dBW respectively The amplifier works very hard into 2 ohms how ever with 220W available in tetrode mode 17 4dBW and 200W in triode mode 17dBW VTL specifies the S 400 s optimal load as 5 ohms This is also revealed by the plots of the S 400 s small signal THD noise against frequency fig 7 tetrode fig 8 triode where the 2 ohm per formance is less good than that into higher imped ances However the VTL s small signal distortion is heavily second and third harmonic in nature fig 9 tetrode fig 10
22. sTech 1073 were amazing and closer to fulfilling the promise of recorded and reproduced performances than Fd ever heard before How do you put the puzzle together when you can t find the pieces After about a year of listening to the VTL S 400 in different configura tions and comparing it with some very good amplifiers I m still at a loss to assign to it any specific overt sound It might sound a little polite and it might lack a little weight in the upper bass and lower midrange but then again it might not Its col orations were so minor that I have no frame of reference within which to isolate and identify them All I can do is describe what the S 400 did not add to the sound which is really just a description of what other amps I compared it with and the other com ponents in my system As superb as those other amplifiers and compo nents are the VTL neutrality clarity and transparency set it apart The VTL S 400 may be expensive but it is the product of an intense five year development effort and the mar riage of several engineering disci plines Whether you measure it against VIL s ambitious design goals or against its peers the S 400 is a stun ning achievement a_ technological tour de force and a home run in terms of both usability and sonic perfor mance Congratulations to Luke Man ley Bea Lam and the VTL design team Twenty years from now we ll all remember the S 400 as a benchmark i
23. the other was to set a new standard in sonic performance When I called Luke Manley to walk through the circuit s details the conversation quickly moved from what to the much more interesting questions of why and how The business has changed he explained There s a new generation of amplifiers that sound fast and very neutral and although we could achieve that to an extent with older designs such as the Wotan it was very intimidating and our target cus tomer doesn t want to fiddle with their equipment Our first choice was to use tubes They re inherently linear voltage amplifiers and require a very small amount of feedback Then to get the neutrality we wanted the tubes had to be kept at their optimum operating point hence the autobias However to keep the signal path simple the auto bias circuitry had to be completely removed from the audio circuitry dur ing operation so we needed to come up with a way to do that All these things and the need to manage current inrush during startup required the use of microprocessors And then we were quickly in a posi tion that required totally new design expertise but also enabled us to do a lot of things that we couldn t have oth erwise We were able to manage everything that we needed to to ensure optimum operation and maximum tube life Then we added all of the diagnostics and indicators to monitor conditions and satisfy the customer s curiosi
24. ty I asked Luke what made the S 400 sound the way it did what s the magic There are really three things that contribute The first is the collection of everything we learned and devel oped during the evolution of our other amps There are the wide band width output transformers with sin gle tap outputs We had a good start ing point for parts selection from before based on matching and exten sive listening and knew how to opti mize feedback The second piece was developing a true balanced differential input with 60dB of common mode noise rejection But the biggest thing is the preci sion regulated power supply Along with maintaining correct bias the way to keep tubes at their optimum oper ating point is by regulating the high tension power supply Normally the voltage begins to sag as power is pulled out of the supply and B voltage might drop by as much as 10 The S 400 s supply never varies by more than 0 1 or 0 2 The S 400 s supply is essentially a power amp in itself The regulation requires six 3 300uF capacitors and five 800V 30A MOSFETs They ve got tremendous transconductance so when the caps dump the regulation keeps the voltage propped up The screen grids are particularly sensitive so they re actually double regulated Their supply is taken off of the regulat ed plate supply stepped down and fur ther regulated at the lower level When I asked him to contrast this
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