CAMBRIDGE AUDIO Solo & Duo phono pre-amplifiers
With vinyl signals so teeny-tiny, phono preamps can hugely influence turntable sound.
It’s the most delicate signal in hi-fi, and handling it is one of the greatest challenges in audio engineering. Often forgotten is that vinyl reproduction is an unpowered system. We plug a turntable into the mains power to drive the motor which spins the platter, and in ‘automated’ decks the arm may be powered to lift and move onto the record. But the parts which actually play the music — the stylus, cartridge, tonearm and signal cables — these receive no power at all. The signal which sets forth from a turntable’s output cables is produced only by the tiny movements of a magnet attached to the stylus, movements which induce current in a surrounding coil of wire. (Or, instead of moving the magnet, a coil is attached to the top of the needle and surrounded with a magnet instead — a moving coil, instead of a moving magnet.)
With no power applied, and no internal amplification, the result is a tiny electrical signal, its potential measured down in the single millivolts, compared to the two full volts or more that come from a digital source. Moving-coil signals are lower still, often below a single millivolt.
So two things are evident. First is that such a low signal will be a thousand times more prone to interference than a line-level signal — and that’s a very good argument for keeping the initial treatment in a separate phono stage, physically apart from the higher-powered functions of a big fat integrated amplifier filled with stuff. Secondly the quality of that initial treatment can have an enormous effect on the final sound. The application of that thousand-fold amplification prior to that of your main amplifier means that any added noise or distortion is magnified, as will be other characteristics such as the quality of channel separation.
Curve ball
But there’s more. The phono stage does not only amplify. It also applies EQ — indeed a massive dose of EQ. Vinyl fans often go on about purity, but the vinyl path to playback is squeezed through two brutal EQ curves. The first is applied prior to cutting the disc, tilting the frequencies upwards — reducing bass frequencies and raising treble. Then in the phono stage a reverse curve is applied, boosting the bass and curbing all that extra treble.
Why do this? Because a downward treble adjustment on replay helps reduce hiss and clicks while bringing the music content back to its original level. Meanwhile the reduced bass content makes records easier to cut and safer to play, since large bass fluctuations create such deviations in the groove that the needle’s tracking ability can be compromised.
So the use of ‘emphasis’ and then reverse ‘de-emphasis’ allows safer and easier mastering, along with some other advantages to playback. Meanwhile the lower groove deviation from lesser bass means that grooves can be cut closer together, and that allows longer playing times to fit on a record — indeed record companies found this part of the equation so fiscally attractive that some of them introduced additional bass cuts over and above the EQ curve, just to extend playing time. (When these masters were carried through to the early days of CD and replayed digitally without the use of emphasis/ de-emphasis, they could sound thin and weedy, one factor in the early ‘vinyl versus CD’ debate.)
How do we know that the EQ curve applied by any given phono stage is the exact reverse of that applied by the record company? Well back in the 1940s, we didn’t — different record companies favoured different curves. So for each LP (or 78) you could look up the turnover frequency below which the particular record manufacturer diminished the bass, and the level of roll-off required for the treble, then you could set your variable phono stage to match. For each record! This became rather complicated once there were upwards of 100 variations in use by the end of the 1940s, so a consensus was eventually reached, adopting what was originally RCA Victor’s ‘New Orthophonic’ curve. This became known as RIAA equalisation and was eventually adopted very widely, if not quite universally, although the IEC later confused things by offering an amended version in 1976 (known now as IEC RIAA).
So phono stages must not only amplify a hyper-sensitive signal, they must perform RIAA de-emphasis as well. And it’s not even a simple curve — there are three transition points where the curve is redefined.
All of this is why phono stages can make such a difference to the sound of a turntable, and why they can sound so different to each other. They mould the signal at a very early and delicate stage, and even small differences become amplified into large ones. So a phono stage is not only useful for those who have an amplifier which doesn’t include one, a good phono stage can easily outperform those within amplifiers by merits of signal isolation and more dedicated design. As with headphone sockets, many built-in amplifier phono stages are basic built-to-a-price circuits. A dedicated phono stage can transform the sound of a turntable.
Easy upgrade
So that’s what we tried first, using the first of Cambridge Audio’s new phono stages, the Solo. This is almost black-box simplicity in terms of use — there’s simply the phono input with its grounding terminal (not required for some turntables, as you can read in our review of Rega’s Planar 6), and an output to your amplifier. We do like Cambridge’s habit of printing connection labels both the right way up and upside down above the sockets, so they’re easier to see when you’re leaning over.
One addition to the back panel is a dial which can correct for any left-right channel imbalance from your cartridge (though much more than 1dB and we’d be querying the cartridge performance). The mains socket feeds a “state-of-the-art switch mode power supply”, which along with surface-mount design aims to reduce unwanted noise, says Cambridge.
We tried it with Audio-Technica’s AT-LP3, an attractive automatic deck a step up from entry level; we love it for its press-to-play simplicity, and also its versatility in having its own
internal phono stage so you can plug it at line level into any amplifier. We expected a significant upgrade, then, replacing this with the Cambridge Solo handling the A-T’s phono-level output. In fact the A-T competed remarkably well, with no notable tonal changes as we flicked between the two options, just a slight improvement in clarity, almost as if underlying dirt was being wiped away. The Cambridge did improve one thing dramatically, and that was the low-frequency grind audible from the AT-LP3’s bearings between tracks; the Solo just stripped the deep stuff out, leaving a far cleaner noise-floor for the music. That’s because it has a rumble filter built in, where the AT-LP3’s own circuit clearly doesn’t. Despite fears of bass loss or phase distortion from a filter you can’t switch off (see below), the Cambridge implementation here gave no audible ill effects to counter the clear advantage in cleaning up the grind.
Two is better than One
We moved on to the Duo, using two higher levels of turntable in the Thorens TD 203 and Rega’s Planar 6 (reviewed elsewhere this issue), both with moving-magnet cartridges installed.
On the Planar 6 we cranked side two of a thrillingly clean copy of ELO’s ‘Face The Music’ rescued from a $5 record fair bin, and the Duo delivered a rock-solid interpretation of Poker, its intro crunchy and strong, the Moog keyboard arpeggios and patches providing the only stereo content to this mix, everything else (including strings) channelled dead centre. This restriction makes the wide beauty of the next track Strange Magic all the more beautiful, as its intro of strings softens into a hard cut to Lynne’s sliding guitar and Tandy’s soft Fender piano part. Using the Duo’s headphone socket we could A-B between this and the digital file of a 2006 remaster played through the headphone socket of a Musical Fidelity DAC. Unsurprisingly the digital sound was cleaner and more dynamic, with more air and zing to the flanged guitar jangle on the close. But it took only moments to settle back into the vinyl performance, plus all the joys of 12-inch cover art and nice legible inner-sleeve notes!
Indeed we rather grew to enjoy using the Duo’s headphone output — private vinyl listening is not something in which we often indulge. This circuit complemented the Duo’s smooth sound with a high-quality presentation that offered a close-listening window on the occasional clicks and grind and hiss of tables and discs, but a highly enjoyable musical delivery that could be enjoyed for long listening sessions, headphones permitting, certainly so on our open AKG K 242HDs, even if it might have offered a couple more notches of level for these 55-ohm 91dB ’phones — we were almost always up at its full rotation (but we do listen up towards reference levels, so your desires and your headphones may differ!).
Back on the speakers we loaded the latest Strypes album ‘Spitting Image’ onto the Rega Planar 6. In its digital incarnation this album can sound a little underdynamic, the guitarto-sax solo climax of Great Expectations, for example, getting a little overly dense and edgy at full tilt, though it opens up beautifully in the brief pause before the final verse. The vinyl pressing gets this fullness right without adding the edginess, and we found it far easier to enjoy (and to turn up).
In direct comparison to our usual Musical Fidelity preamp’s phono stage (rather older but price comparable), the Cambridge performed extremely well, its equal in general tonality but able to more accurately tighten the central vocal on Jeff Lynne’s ELO’s The Sun Will Shine On You, indeed to more accurate place and define instruments in general, a series of right-left splash cymbals after the drum entry being both more individually distinct and more clearly positioned.
Making the same comparison with the more sparse arrangement of Keith Jarrett’s Flying Pt 1, it became evident this control was again partly through a lower noise floor, and even a tendency of the Musical Fidelity to make loud piano notes sound overmodded, where the Cambridge kept them in check. The corollary of this control was that the Musical Fidelity allowed through more air, so that Jack DeJohnette’s taps on ride and China cymbals had a more open acoustic and greater sense of presence, where the Cambridge rolled them off a little. No big chips, this, though; we’d happily live with either, and the Duo’s controlled delivery definitely had its advantages as material grew in complexity.
Rumble filtering
That RIAA EQ curve is not without side effects. By boosting up the previously reduced bass frequencies, the EQ also emphasises any low frequency mechanical noise, such as rumbling from the bearings, or from warped vinyl. These can be further emphasised by resonance between the arm and cartridge at frequencies down around the limits of hearing. So some phono stages — including these two from Cambridge — include a rumble filter to sharply attenuate very low frequencies. Some also bring the stereo image into mono at lower frequencies.
Unsure which approach Cambridge uses, we spun up the LP of Giles Martin’s remix of ‘The Beatles Live at Hollywood Bowl’, in which, for reasons perhaps of attempted crowd cancellation, McCartney’s bass is panned hard left, in addition to being vacant and soft, sometimes wildly resonant (the fundamental of Ticket To Ride). This hard panning of bass is a no-no in vinyl mixes, but it got through the mastering
process here and we found it clearly delivered by both Cambridge phono stages, far more clearly panned than through our resident Musical Fidelity phono stage indeed. So we doubt such a mono-ing circuit is used here.
We note that on Cambridge’s previous phono products, the CP1 and CP2, this rumble filter circuit could be switched in or out. So why no selection switch now? Many argue against the use of rumble filtering unless strictly required, as it can add to other bass ‘cuts’ mastered into older recordings, and the high pass filter slope might affect phase response within the audio band. But sometimes it can be near-essential — we remember Scott Krix peering nervously at the cones of his prototype Heretix speakers in bassflap mode after no subsonic filter could be found to tame the table on which we played John Paul Young’s own vinyl collection a few years back at the Australian Hi-fi & AV Show in Melbourne.
Cambridge has set these products to power down automatically after a certain period of inaction, which is good for the environment, but a bit annoying here for the simple reason that it’s hard to divine which state the Solo is in — the power light is white whether the product is on or off; it’s just a bit brighter when it’s on. Many times we thought the Solo was on when off, and missed the first few bars of music while it booted up. On the Duo you can tell, because the second light (indicating moving magnet or moving coil) is only lit when it’s powered on.
Incidentally, should owners of the previous Cambridge CP1 and CP2 phono stages be wondering what’s changed, the new models quote a slightly higher signal-to-noise ratio for moving magnet cartridges (slightly lower for moving coil, but with a higher gain and significantly improved channel separation), plus of course you get the front headphone socket and volume knob.
Conclusion
Cambridge has kept things user-friendly in terms of simplicity here — it doesn’t allow anything in the way of cartridge gain matching, no ability to switch curves between ‘Standard’ and ‘IEC’ RIAA equalisation curves or any archaic curve of the past (were you to be aware which of your LPs were mastered that way). We think that’s sensible at this level of phono stage. The compulsory rumble filter didn’t phase us, though some vinyl fans may wish it were switchable. But as a no-nonsense reliable and high quality phono stage at the price, the Solo succeeds admirably, while the Duo brings moving coil compatibility and an enjoyable headphone stage. As it turns 50, Cambridge clearly has its finger on the pulse of those delicate vinyl signals. Jez Ford