The Vacuum State RTP3 and the Audio Research SP-11

The protagonists RTP3 and SP-11 in my system

In the mid 1980s the SP-11 was one of the Minnesota-based Audio Research Corporation's two top preamplifiers, the other being the much-loved all-valve SP-10. Released in 1985, the SP-11 was the first hybrid preamplifier from ARC, using a clever combination of JFETs, triode valves and MOSFETs in each active stage, and had a substantial outboard power supply as well as cascaded solid-state regulators for each stage. It gained very positive reviews, particularly in Stereophile, and a couple of years after initial release a Mk. II appeared with significant revisions. It sold for between $4,000 and $5,000 in the US (and about the same number of UK pounds over here) and there was a consensus, among both reviewers and audiophiles, that the SP-11 was close to the state of the art in preamplifiers, even though some preferred the SP-10 for its warmer sound and more intensely rendered tonal colours.

The Realtime Preamplifier is Allen Wright's differential preamplifier (there is a single-ended version called the SVP), and makes use of design concepts from 1960s Hewlett Packard and Tektronix instrumentation to reduce distortion and increase bandwidth without using global feedback. It uses a dozen 6922 double triode valves, and a closely matched pair of bipolar transistors in the phono input, all working in differential cascode mode, and each stage includes a JFET current source to stabilise the operating point and increase linearity. It too has an outboard power supply but, instead of the SP-11's op-amp-based series regulators, uses a shunt regulator for each channel's HT rail, fed by a current source; Allen considers this arrangement to lead to far superior sound than the more usual series regulation. The RTP as a concept has been in almost continuous development over the last twenty years, but now seems to be close to being a stable product. The RTP3, like the SP-11, has gathered some fulsomely positive reviews and, although Vacuum State isn't in the same business league as Audio Research, it is regarded by many to be close to the top of the current batch of commercial preamplifiers.

The latest version (the RTP3D) is available from Vacuum State Electronics in Switzerland either as a pre-built item or at a lower price for a kit with full assembly instructions. I couldn't afford either, but used schematics directly from Allen along with photographs from the Web to build one of my own, ending up with a reasonable facsimile of the units sold by Vacuum State. The main difference (apart from the rather amateur cosmetics of my own RTP3 and even more so its power supply) is that I started off with good-quality standard parts instead of the more exotic ones in the commercial release, though I plan to upgrade some of these. In particular, the RTP3 preamplifier I describe here used LCR polypropylene capacitors instead of the Mundorf M-cap ZN in Allen's units, and Sfernice potentiometers in place of the ELMA stepped attenuators.

For a while before I built the RTP3, though, I was fascinated by the SP-11. The way that solid-state devices were intimately entwined with valves, in such a way (it was claimed) to benefit from the appropriate advantages of each, appealed to me strongly. I wrote a lengthy article on this site about its circuits, without ever actually having seen one in the flesh or listened to one (although I had heard and enjoyed the conceptually similar, but less complex and much cheaper, SP-14). I got as far as making detailed plans for a copy of the senior ARC preamplifier based on the copious supply of information on Manfred Persson's terrific site. In the end, however, I decided to build the RTP3 instead: partly because its working principles seemed clearer, and partly because Allen's writings had started to convince me that the RTP3 had the potential to out-perform the ARC.

I have been enjoying the RTP3 for about nine months now, and I am particularly taken by its transparency, natural tonal palette and lively dynamics. In all this time I never expected an SP-11 to come into my possession. This was until I received an e-mail from a music-lover in Scotland who had read my articles about both preamps and had just located a second-hand SP-11 Mk. II. He asked me if I would consider listening to the two preamps in my own system before he took delivery of the ARC unit, so that he would get some idea of how well the twenty-year-old veteran would stand up against the modern competition, the Vacuum State preamp in particular. I thought that this would be a great opportunity to compare my RTP3 against a known benchmark, as well as a nice way of rounding off my interest in the SP-11, so I jumped at the chance.

Expectations and cautions

The reviews I have read of these two preamps suggest that both have strengths in neutrality, clarity and soundstaging, so I expected them to sound superficially quite similar. I have never read of any direct comparisons between them, so I didn't have a strong feeling for which would sound better in my system, even though my gut feeling a year or two ago was that a fully-tricked-out RTP3 ought to outperform an SP-11 - that is why I built one, after all! My own RTP3 uses much cheaper parts than the commercial RTP3D, with its exotic capacitors and resistors, silver foil connections, and ELMA switches, so it might not sound as refined and transparent as the commercial item. I haven't done any measurements on the frequency response of the phono stage, so I don't know how accurate the RIAA correction network is in mine (although it sounds fine to me!). The SP-11 comes with plenty of audiophile parts (including RELCaps and WonderCaps), but was twenty years old, so I didn't know what condition the power supply capacitors were in, nor how much use the valves had had, so it may not have been performing at its best.

I was also unsure whether the rest of my system would be transparent enough to reveal the subtle differences I expected between these two renowned and expensive preamps. Although I had been able to hear quite clearly the superiority of the RTP3 over my Croft Super Micro A and my previous DIY solid-state preamp through it, I am aware that my system has some deficiencies. For a start, the active crossover in between the preamp and power amps still uses a relatively unsophisticated op-amp circuit, which I suspect has a significant effect on the transparency of my system (put it this way - its architecture is similar to that of my old DIY solid-state preamp, which was certainly outclassed by the RTP3). The positioning of my speakers only ten centimetres or so from the back wall also means that the soundstaging capabilities of the system are necessarily limited.

First Impressions

This was my first hands-on encounter with a piece of Audio Research equipment, and a brief look inside the SP-11 confirmed all I had read about the legendary build quality of ARC products. For a twenty-year-old piece of consumer electronics, this unit was in excellent condition, and had a reassuring solidity to it; the internal construction was strikingly tidy, with a nice neat and firm wiring loom. For reference, the serial numbers of the SP-11 and SP-11 PS were 27414022 and 57435001, respectively, and the label on the back identified it as a "Mk. II". The valves were Sovtek 6922s, with a 2000 date code, so were certainly not the originals. This choice of valves did promise reliability, as the SP-11 circuit runs some of its valves at over 130V, which is the maximum specified for the standard 6DJ8 while the otherwise similar 6922 has a rather higher specification for its maximum anode-cathode voltage.

On switch-on the auto-mute LED blinked calmly for about a minute, and then the SP-11 was ready for action. I listened to a few tracks from a sampler cover CD from Gramophone Magazine, and was greeted by a confident, well-anchored sound with good bass definition and extension. Solo piano was dynamic and well focused, while the finale of Beethoven's Ninth had excellent orchestral presence and a nice smooth choral sound with good detail. Noise levels were very low with the volume control set for any normal listening levels.

Comparisons

I listened almost exclusively to vinyl LPs during the three or four weeks when I was comparing the two preamps: partly because my record deck is of a much higher calibre than my ageing Marantz CD player, partly because I have most of the music I love on LP rather than on CD, but mostly because in my experience there is, more often than not, much more musical communication through the analogue vinyl medium.

Before each session I let both preamps warm up for a couple of hours so each was at least reasonably close to its optimum operating point (although both companies recommend their preamps be left on for longer than this). When the ARC preamp was connected I listened entirely in "Direct" mode, as this sounded consistently clearer without the balance and the more exotic source election controls inline. I also preferred the results without the low-frequency filter in the phono stage, as I could just about hear the improvement in low-frequency extension with the filter switched off. Finally I was interested to find that my Lyra Lydian Beta generally sounded pleasantly airier loaded with 47K than with 100 ohms (the next highest impedance), although at 47K it could bring out some spittiness in the occasional recording. With the lowest impedance loads of 3 ohms and 10 ohms I found noticeable drops in level from the Lyra, and the sound became quite dull. The input load on my RTP3 is set to 1000 ohms.

My favourite disc for evaluating new equipment is Debussy's Nocturnes, with the Concertgebouw under Bernard Haitink; specifically the second piece, Fetes. The Philips analogue recording and the orchestral playing are superb, and Debussy's wonderfully varied orchestration really shows off all aspects of a system. With both preamps the brass fanfares near the beginning were thrilling, and the superficial amount of detail was similar with both, but I started feeling that there was something missing with the ARC: the orchestral palette seemed a little grey, and instruments were less filled out physically than with the RTP3. In the middle of the piece the music suddenly falls silent, and the harp and bass strings quietly play a rather Ravelian ostinato, while the rest of the orchestra gradually join in until a tutti climax. The sensation of a performing space consistent between both the quiet passage and the full orchestra was stronger with the RTP3, and instruments deep in the soundstage, like percussion and harp, were also generally more distinct. I don't want to over-emphasise any failings of the SP-11; in many ways it is a superb performer, and it only falls short next to the Vacuum State preamp.

I then followed up with some chamber music: the Octets by first Schubert (played by the Academy of St Martin in the Fields) and then Mendelssohn (BPO Octet). Although these are small-scale pieces, lacking in the thrills of the Debussy, the ability of the RTP3 preamp to differentiate instrumental textures and colours was superior to that of the SP-11. For instance, the clarinet, bassoon and horn were easily to distinguish in the Schubert, where with the ARC, even though they were spatially well resolved, they sounded a little homogenized tonally. Both preamps produced a lush and full sound from the strings in the Mendelssohn, with the RTP3 again characterising the instruments more than the SP-11.

Taking a sharp turn into the late twentieth Century, I put on Tippett's Triple Concerto (Pauk/Imai/Kirschbaum/Davis). Where Beethoven's Triple Concerto puts a piano trio in front of the orchestra, Tippett has a string trio, and blends the tonal colours of the violin, viola and cello so that when they play in overlapping registers only a high-resolution system clearly distinguishes them. The RTP3 passed this test easily where the ARC struggled a little. Similarly, the almost Boulezian slabs of woodwind colour in this challenging piece came across more vividly through the RTP3.

The piano is my own instrument, so I put on Chopin's Sonata in B flat minor, played by Murray Perahia, which contains some full-blooded romantic music. The Scherzo, with the ARC going first, sounded passionate and exciting, The size of the piano (as far as my speakers in my room can reproduce it!) came across well, and I could clearly hear that Perahia was sitting on the left of the stage, with the body of the piano towards the right. The bass came over as very firm and dynamic, while the passagework was clear and sharp. Through the RTP3 the impression was similar, although there was more impression of the depth of the hall, and the instruments were better defined. If anything, I would say that the RTP3 had perhaps less impact and power in the bass than the SP-11, although the difference was marginal. Next I played Prokofiev's Third Piano Concerto (Ashkenazy/RPO/Previn). This is a fine Seventies recording of one of my favourite concertos. Again the ARC preamp proved revealing and solid. By contrast, the bass from the RTP again didn't have the same sense of mass and power in orchestral tuttis, even though the cello and bass sections were well defined and detailed. This is the one aspect in which the SP-11 might be felt to be superior, although to decide conclusively whether it is a colouration or a real strength would possibly need a system with better bass reproduction than mine.

Moving from twentieth-century classical music to classic prog rock, I put on The Yes Album. In many ways this is the most naturally recorded of all the "main-sequence" Yes albums, even though it isn't musically as ambitious - some might unkindly say over-ambitious! - as subsequent albums like Close to the Edge or Tales from Topographic Oceans. The Clap is a very immediate live recording of Steve Howe's improvisations on his solo steel-strung guitar (better recorded than anything on the live album Yessongs, in my opinion), with plenty of ambience from the hall and the excitable audience. This is a virtuoso performance and sounded more vital and colourful through the RTP3, with the SP-11 sounding slightly restrained, even though all the notes were clearly there. The sound of Howe's foot tapping was evident through both preamps, where I hadn't heard this through my solid-state preamp, but the depth of the hall was again more obvious with the RTP3, with the audience response more differentiated from the guitar. The more complex arrangements of Perpetual Change, from the same album, came across well through both preamps, but with more instrumental colour with the RTP3. The second side of Pink Floyd's studio-recorded Meddle seemed more spacious through the RTP3, while the instruments and voices themselves had more character and presence.

One evening, after I had had the SP-11 for about a week, I asked my wife to have a listen to the two preamps. She has no interest whatsoever in the fiddly technical aspects of audiophilia, but appreciates good music and - as do many women - has a much more intuitive feel for what she likes and dislikes about aural presentation. I didn't tell her which one was connected (both had been fired up for over two hours), but within twenty seconds of Mountains of Things from Tracy Chapman's first album she said "This is the other one, isn't it? It doesn't sound as good as yours". Changing to the RTP3, the contrast was striking: the guitar had a more immediate attack and Chapman's voice was more palpable. The delicate percussion, especially the cymbal strike that ends this song, sounded rather more real and present than with the SP-11.

As a final test, I played a couple of CDs through the line stages of the two preamps. To start with, I put on Ry Cooder and V. M. Bhatt's Down by the River, which has Cooder's slide guitar on the left of the stage, and Bhatt's "Mohan Vina" (an instrument of his own devising, based on a slide guitar but with a very Indian colour) on the right. This is a very relaxing album, with nothing happening in a hurry, but the interplay of the two musicians from very different cultural backgrounds is mesmerising. Funnily enough, this is the one piece of music that showed up the differences more than any other. The ARC had a vivid dynamic sound which was compelling, but when I changed to the RTP3 the tonal differences between the two instruments were instantly obvious: I could hear far more of Cooder's percussive picking and sliding and the character of Bhatt's instrument with its Asian overtones was beautifully rendered. I could also hear the tabla player at the back of the soundstage very clearly, where he was far less audible through the ARC preamp. Overall, I felt the RTP3 gave a rather more lively and enthralling reproduction. I finished off with Diana Krall's One Night in Paris, a fine live album with plenty of energy. This was very enjoyable through both preamps, although again the performers had more presence and life with the RTP3.

Conclusions

I was surprised to hear such clear and consistent differences between my own homemade Realtime Preamplifier and the classic SP-11. If I hadn't heard the RTP3, the Audio Research preamp would have been very acceptable in my system, being superior in many respects to either my Croft Super Micro A or my DIY solid-state preamp. It is dynamically excellent, and its bass control and power makes orchestral showpieces like the Debussy Fetes quite thrilling. At the same time, though, the RTP3 excelled in revealing subtle tonal colours, and brought a sense of life to music that was a little missing with the ARC preamp. The extra detail I heard with the RTP3, particularly in the treble, is definitely not a result of frequency response errors in the RIAA equalisation circuits, as the differences were consistent between LP and CD listening. With my current system I couldn't say with certainty whether the subjectively powerful bass of the SP-11 is a colouration, or whether it really is superior in this respect, but after listing to a wide range of recordings I am starting to get a feeling that the RTP3 is more transparent at low frequencies and indeed sounds more natural to me.

In the longer term the RTP3 is definitely more satisfying to listen to. With both of the preamps in my system for the three or four weeks I had the SP-11 I could have connected either of them up to listen to music at any time, but outside the few days when I wasn't actively comparing them I always chose the RTP3. The more music I listened to, the more the RTP3 revealed differences between various recordings, and the more enjoyable I found the music. Despite the cheap Maplin capacitors, resistors and switches, the speed, dynamics and tonal resolution of this design shone through and it was certainly not outclassed in this case by the high-end commercial product.

The one caveat I have in writing this review is with the doubts I expressed at the start about the transparency of my system. I am open to the possibility that I am missing some of the qualities of the SP-11 - for instance, it may have a deeper soundstage than the RTP3, and I am just not hearing this, or it might be better at recovering ambient information (though I would be surprised if either really were the case). It could also be true that the apparent extra resolution through the RTP3 is an artifact of an anomaly in the frequency response, although again I think this unlikely. Allen told me that the RTP3 needs to be switched on for much longer than a couple of hours to sound its best (he leaves his own on twenty-four hours per day), and J. Gordon Holt says as much about the SP-11 in his Stereophile review, so it could be the case that I am missing hidden qualities in both preamps. All the same, I feel I gave both a decent chance. I still found the RTP3 more interesting and informative musically, and so, even though the two preamps are likely to sound rather different in another system, I have confidence in my overall conclusions about their relative qualities.

Anyway, these days a direct comparison of the value of the two preamps is a little irrelevant, given that it is possible to get a second-hand SP-11 in excellent condition for less than £1,500. I would love to hear an Audio Research SP-10 (with a new set of valves) against my RTP3: I get the strong impression from reading articles on the Web that this remedies some of the slight lack of tonal variety of the SP-11, and that it has some of the lively and colourful character of the RTP3, even if it is reckoned to have a bias towards a warm tonal palette.

My System

Analogue source: Pink Triangle Anniversary Vector / SME Series IV / Lyra Lydian Beta

Digital source: Marantz CD60SE

Preamplifier: Vacuum State RTP3 differential valve preamplifier

Amplifiers: Croft Series 5 25W EL84 valve amplifier (mid/treble); Giovanni Stochino designed 100W solid-state amplifier (bass)

Speakers: My own semi-active three-way design (Scan-speak D2905-9300 tweeter; Audax HDA 10cm mid and 21cm bass); passive crossover between midrange and treble, and solid-state active crossover between bass and mid.

Alex Megann, October 2007