POSITIVE FEEDBACK ONLINE - ISSUE 27
Tip Top TAP - the Transformer Volume Control preamplifier
as reviewed by Bruce Kinch
I hate preamplifiers. The stupid things are the only links in the audio chain that are designed (italic) from the get-go to restrict, impede, and otherwise diminish the audio signal. Audiophiles can and will spend all sorts of money on AC conditioners, source components, specialist recordings and reissues, precious-metal cabling, amplification, isolation devices, loudspeakers, room treatments—in each case in an effort to increase information retrieval. More detail, deeper bass, better high-frequency extension, less interference from noise and vibration, lower wow/flutter/jitter/resonance, wider/deeper/higher soundstage. If you are reading this, you know the drill, and have the Visa statements to prove it. And then you stick a fancy electronic bottleneck in the pipeline to throw away much of what has been "gained".
What's so hard about "Turn that damn thing down!" anyway? Really, faced with designing (or buying) a preamp, the fundamental issues are just (1) to attenuate the signal while doing the least damage to it in both frequency and time domains, and (2) provide optimal impedance matching between source and amplifier. Control flexibility options, brand-conscious bragging rights, and Spousal Acceptance Factor aesthetics aside - that's all a pre-amp is asked to do. And there seem to be two ways to do this properly: spend lots of money, or really keep it simple (stupid). The first option leads to complex circuitry, massive outboard power supplies, premium tube-rolling, and "voicing". The second path leads to passive preamps and "integrated" amplifiers.
Part of the problem, I think, is that the role of the preamp has changed since the classic designs of the 1950s. As the word itself implies, time was when the signals from phono cartridges and tape heads needed additional amplification and equalization prior to being passed to the power amps. Given the loudspeakers of the day, tone controls were considered essential, requiring additional gain capabilities. Today, even modest loudspeakers provide smooth and extended response, and most audiophiles employ outboard phonostages/digital players/tuners with essentially standardized output levels that can drive an amplifier and reasonably efficient speakers without the additional gain active preamps provide.
In other words, the 10-20dB pre-amplification provided by nearly all active preamplifiers is literally thrown away when the volume knob is at, say, 2 o'clock or lower. The signal has essentially been first amplified and then attenuated back to below the original level. Three steps forward and two steps back. In most cases (and the exceptions are where the real $$$ goes), the result of this active-circuitry "boost and cut" is audibly inferior to the direct feed.
That is also why passive preamplifiers and "integrated" amplifiers, typically with simple passive attenuators, have attracted the interest of audiophiles for years. They solve the level issue by definition, assuming the potentiometer or stepped attenuator is of sufficient quality: passive preamps are famously transparent and detailed in their presentation. Unfortunately, simple resistive attenuation also often compromises the impedance matching between source and amplification. While this is not a universal situation, traditional passive preamps do have a reputation for reduced dynamics, rolled-off frequency extremes, and distortion when some source components "see" less than the optimal impedance load provided by the variable resistor. The best conventional "passive" preamplifier, in my experience, is the "buffered passive" Supraphon Revelation III from Stan Warren, designed specifically to eliminate the impedance problems passive preamps oft fall prey to. But even with the remote volume option, the Rev III is still pretty much bare bones functionality by current standards – not even a balance control.
So how about something completely different and potentially transformative? Turns out you can attenuate or increase a signal without active electronics or even exotic metal film resistors. Happens all the time, actually. Welcome to Transformerworld, where what goes up can also come down, step by step. True, we usually think of transformers as fixed one-way devices, like 20dB moving coil step-ups or 220v to 110v converters. But one can configure a transformer that allows different degrees of attenuation or boost by having multiple secondary "taps" with different winding ratios to the primary. While a standard potentiometer (or pricey multi-resistor stepped attenuator) dissipates excess signal by essentially converting it to heat, a transformer attenuates by exchanging a high voltage/ low current input to lower voltage/higher current output, and amplifier drive capability is not compromised by impedance issues. In the past couple of years, high quality "preamps" using transformer attenuation have bubbled to the surface of audiophile consciousness, but such "autoformers" have long had application in the pro-audio world. To distinguish them from active or resistive units, there seems to be growing agreement to call them Transformer Volume Controls, or TVCs.
You may have seen some very positive reports on TVCs from Audio-Consulting, AudioZone, Music First Audio, Sonic Euphoria, and perhaps a few other household names in the mags and e-zines. Several of these products utilize the same model TX-102 transformers sourced from Stevens and Billington in the UK (Music First Audio, also in the UK, is apparently the "finished product" marketing arm of Stevens and Billington). While there is a wide range of chassis designs from frumpy to outré audio sculpture, they are all overtly no-frills minimalist devices. Well, knobs are fine as far as they go (and the TX-102 does go to 11, sort of, as we shall see), but in a world where you can get a remote for your iPod....
Seriously, exactly what functionality, if not tone controls, a phono stage, or active gain, should a modern passive preamp offer? Opinions will vary, but I'd ask for pretty much what active high-end preamps already offer. Remote control of balance as well as volume, please. Input switching too. A mute button, and a mono option would be nice. Balanced as well as single ended (RCA) connectors. Even (Hear me, Clark Johnsen!) absolute phase inversion. And let's face it, the option of multi-channel control. SACD and DVD-A have it today, and the hi-rez formats of tomorrow will too. Stereo is so-o-o-o 20th Century. Knobs? Weren't they needed to play grandpa's old Beatles records?
Enter Bent Audio. Based in Vancouver, Canada, Bent has been the primary North American supplier for Stevens and Billington transformers to the DIY community for some time. John Chapman (Mr. Bent) has also direct-marketed a no-compromise MC Step-up transformer (innards by S&B), the Noh preamplifier (which, though no longer available, retains a certain cult status), and a variety of kit-build units and parts.
And now he comes up with a passive Transformer Attenuating Preamp you have to plug in. What? Naw, the juice ain't needed for signal processing—the TAP is indeed passive, and based on the S&B TX-102, although Chapman specifies a special OCC (oxygen-free continuous cast) copper wire for his 102s. As suggested by the Spinal Tap reference above, the TX-102 is configured not only to attenuate, but also to provide up to 6dB of gain if needed. Mirabile dictu, such a transformer is in fact "Bent" wire with gain! The 9v outboard supply is used solely for control functions (via relays) and the dual output LED displays and other indicators. Not your basic RatShack wall-wart this, but a hefty beast from Tamamura (Chapman knows his iron). The feature list? Six inputs (RCA and balanced), four outputs (likewise) plus Tape Out. Volume, balance, phase invert, input selection and mute are available from remote or front panel. LED defeat, home theatre bypass, ground terminal, hot/floating ground loop nixer and balanced/single-ended switches on the backside, along with, of all things, expansion ports for additional channels! Yep, about everything this guy could want—except a mono button (hard to implement in something essentially dual-mono itself). This thing is so not no-frills that Music First Audio has arranged to take on the distribution responsibilities of the Bent unit as an alternative to their own well-regarded minimalist design. I was fortunate to receive an early production unit direct from Bent, but apparently little if anything will differ in the MFA-badged product.
When they change the name from Bent to whatever else, the chassis design will seem even a bit odder. The TAP is smaller and heavier (over 10lb.) than it looks in pictures, and has a distinct tilt to the (non-political) right. The lean is mirrored in the slanting ¾" LED characters, and also in the way the in/out jacks are offset on the rear panel. The metal chassis sits atop a 1" slab of clear acrylic (at least on the early sample I had for review; black may be an option), notched in front of the central level control. The plexi is routed out to securely mount the transformers, and a sheet of Sorbothane is used between the plastic and metal bottom plate. Small rubber feet are supplied, but the smooth and massive plexi base encourages the use of puck, cone, or rollerball footers, unlike some lightweight passives that can be cantilevered into the air by a stiff interconnect. Overall fit and finish is quite good, but then, it does not pretend to be chiseled from a solid billet of titanium. The one knob and touch buttons are silky smooth in operation. The remote, while plastic, is compact, comfortable in hand, and intuitive in operation. Inside one finds the TX-102s and not much else, other than the front and rear panel electronics. The signal path is short and sweet, and the assembly quality high, with none of the rats' nest of point-to-point wiring common to switch-based TVC or stepped attenuator designs. The RCAs are custom, the XLRs gold contact Neutriks.
And just what does this baby sound like? Like nothing, baby. Keep in mind there is no actual electrical connection of source to amp - just the transformer gap. Given a quiet amp (such as the Bel Canto 330M monoblocks that worked beautifully with the TAP in my system), you can put your ear right up to the speaker at full gain: no hum, buzz, hiss, anything. Avant-Garde should give these things away with their ultra-high sensitivity horns. I never had need for the floating ground switch or grounding post. You can play a quiet CD through the TAP in bypass mode, then engage the transformers and match the level, and go back and forth if you like trying to hear a significant difference: I couldn't. Yes, you are listening through a couple of pretty long wires all wound up in there, and that means burn-in time before maximum performance. Chapman suggests 100 hrs, but there is a catch. The TX-102 has 34 2 dB steps of attenuation, and most of the burn-in can be done at level 31—unity gain. But if you are obsessive (me?), you can exercise each step separately. I simply determined that my sources all worked best in the 20-30 range as indicated by the LED display. Over the course of a month or so, I left a CD running on repeat overnight and while I was out at level 25 on the 5th, 15th, and 25th—and so on through the range. The amps don't need to be on, just connected. Probably did wonders for the interconnects, too…
I won't wax eloquent as to what the TAP does to the beginning and trailing edges of individual notes, or how it outlines the shape of Norah Jones' lips, or any other ultraphile obsessions. The point of a passive device is that additive errors are eliminated, and subtractive errors can be minimized. What I can say is that, even before optimal burn-in, the TAP is the most transparent preamp I've had in my system, easily bettering my silver-wired, Vishay resistor - based Reference Line 3000 passive. Plus, it works beautifully with my tubed Herron phono stage, which the Ref Line does not—the passive preamp impedance bugaboo solved! Some might feel the TAP suffers from Mikey's TMI (Too Much Information) syndrome, but the impressive detail retrieval is not the result of edgy transients or etched emphasis as sometimes seems the case with active devices. It is largely a matter of superb textural definition on voices and instruments, across both frequency and dynamic ranges. Indeed, one of the pleasures of the TAP is the way resolution is maintained as the level goes down. That, in turn, translates into stable soundstage presentation. Images are precise, hall sound and stage width, depth, and height are exemplary on recordings that incorporate the appropriate spatial cues. On some ambiance-saturated recordings (like many of the SQ encoded LPs on the Absolute sound Super Disc list), the soundfield can seem downright hemispheric. Timing is also excellent, but as the TAP is passive, dynamics are pretty much determined by the source's output stage capabilities—not that I had the slightest problem getting it to boogie. Similarly, understand that you will not get the "bloom" of a tube preamp here, nor the incisive crispness of a solid state unit. Of course, such flavorings can be had elsewhere in the chain. What you do get is a remote controlled volume control and input switcher that leaves such tonal characteristics alone. The Bent Audio web site opens with the motto "Let the Music Flow", and the TAP is indeed more sluice gate than bottleneck when it comes to information transfer, and worthy of the best cabling you can conjure up (I used both expensive Nordost and the affordable and over-achieving Reality cables to good effect). And the XLR connectors are not just window dressing: if the ancillary gear is compatible, balanced operation would seem the way to go, the presentation being just that much more "natural" in my system running the power amps via the Neutriks.
What's not to like? No mono button, damn it. Damn it. The 2dB steps of attenuation are a bit unsubtle at times, but you adapt. The logic of the balance control is odd: one channel steps up, the other steps down as you hit the button. For small corrections, that keeps the overall "volume" about the same while shifting the image, but it is a nuisance if you want to hear one channel only. The Right channel (Red) connectors are on top—everything else in my system puts Right/Red on the bottom. The compact chassis makes stacking components awkward (not that you'd ever do that, of course). And it does look a bit wacko—if Frank Gehry buildings make you break out in hives, look elsewhere. On the other hand, it sounds very fine, indeed, and the missus here thinks it's cute.
With a MSRP of $2995, the TAP is slotted solidly in the middle range of the high-end audio price scale, but I betcha it will handily embarrass a lot of upper tier traditional active preamps. If $3K is above your price point, Bent also has resistor-based versions, same control flexibility, no XLRs, for less than half that price. There is even a kit for the solder brigade—somewhere, David Hafler is smiling…
And then there is the multi-channel option. What the hell good are 5.1 SACD/DVD-A discs if the "AV Controller" mucks up your everyday stereo? Chapman has promised to send me an expansion module so I can better assess the potential of the TAP in a surround configuration, so I'll be following up this review with some further thoughts and impressions. I know I'm not the only guy out there without the room (or the funds) for separate high-end stereo and home theater systems. But if stereo is your main thing, and you have any reason to suspect your present preamp of withholding information, the verdict is already clear: hair-shirt purist audio has just become a whole lot more user-friendly. Isn't this fun? Bruce Kinch