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Reviewer: Srajan Ebaen
Source: Zanden Audio Model 2000P/5000S; Opera Audio Consonance CDP 5.0 Droplet
Preamp/Integrated: ModWright SWL 9.0SE; AudioZone PRE-T1 [on review]; Music First Audio [on review]
Amp: 2 x Audiosector Patek SE run one channel each, the other shorted out
Speakers: Zu Cable Definition Mk 1.5
Cables: Zanden Audio proprietary I²S cable, Stealth Audio Indra (x2), Zu Cable Ibis, Zu Cable Birth on Definitions; Crystal Cable Reference power cords; ZCable Hurricane power cords on both conditioners
Stands: 1 x Grand Prix Audio Monaco four-tier
Powerline conditioning: 2 x Walker Audio Velocitor S
Sundry accessories: GPA Formula Carbon/Kevlar shelf for transport; GPA Apex footers underneath stand, DAC and amp; Walker Audio SST on all connections; Walker Audio Vivid CD cleaner; Furutech RD-2 CD demagnetizer; WorldPower cryo'd Hubbell wall sockets
Room size: 30' w x 18' d x 10' h [sloping ceiling] in long-wall setup in one half, with open adjoining living room for a total of ca.1000 squ.ft floor plan
Review Component Retail: $2,495 introductory offer for US market

What the hey is a passive aggressive preamp anyways? Er, passive magnetic? Passive of course means no active circuitry, i.e. no connection to AC wall power or batteries. In short, no power supply. Passive. Magnetic refers to the use of multi-tapped attenuation transformers instead of resistive volume control devices. The most well-known in this breed of transformers are from Stevens & Billington Ltd. And that's precisely what makes today's unit tick: two 80% nickel Permalloy core S&B balanced attenuation transformers. Housed inside Mu-metal canisters, they're strapped to a 24-position stereo ELMA switch for volume and another 6-position equivalent for source selection.

Two of the inputs are balanced (as is one of the two selectable outputs). A ground lift/output selector toggle and 6dB passive gain switch round out the feature set. Being a UK product, the British press naturally got tipped off to the PMP well ahead of audiophiles in the States. And what a harmonious chorus line they've formed. There's an "Editor's Choice 2004" and "Best Preamp of 2005" with HiFi News and major raves by HiFi World and HiFi Choice.

The English aren't the only ones hip to the concept. Numerous US and Canadian manufacturers have embraced TVCs - or transformer volume controls. AudioZone, AVTAC, Bent Audio, DIYHiFi and Sonic Euphoria come to mind. From Switzerland hails the capo di capi of the genre, Audio Consulting's Silver Rock. But this flavor of passives doesn't exactly suffer queues. Neither do conventional passives for that matter. As two of the commercially most successful ones in recent memory, the McCormack and First Sound units have already faded into the mustier pages of the HiFi annals.

Here's the $64,000 question: Are passives really passive as in 'flawlessly neutral' like the purest of diamonds? Or are they subtly subtractive? For 'neutral', common sense can predict various copasetic scenarios in audio systems when source, amplification and speakers form a perfect triumvirate. All that's required then is naked functionality: source selection and volume control via signal reduction only. Anything even a smidgen more would be already considered detractive. The concept of passives -- sonically invisible functionality -- then fits like a bespoke deerskin glove. But what if such devices really diminished the signal somehow instead? Ultimate usefulness would be rendered dubious. After all, why throw away portions of the signal no matter how minuscule? Why thin out the sound or curtail dynamics?

Unlike resistive passives which suffer impedance mismatches and concomitant frequency response aberrations depending on setting and cable load, Stevens & Billington claim 5Hz -100kHz bandwidth regardless of selected attenuation for their custom transformers. Passive without penalties. That's how supporters would view this miracle of fat-free functionality. Detractors meanwhile feel compelled to point at a predictable loss of body, drive and tone which -- in their opinion and experience at least -- can only be had from active circuits. Rather than neutral, they believe passives are really active in how they steal from dynamics and tone. Confused? It's the audiophile law of relativity.

The only way to solve that dilemma? The ol' bypass test: run a source with integral volume control amp direct, then insert our passive and report on the differences - if any. As luck would have it, I have one such source in my arsenal. The Consonance Droplet CDP 5.0 sports a butch valve-powered output stage with a digitally actuated analog volume control accessible by remote only. This would be the decisive test. Utter neutrality whereby insertion of the PMP wouldn't betray its presence by even a whispered peep? Better performance in any particular parameter to suggest active benefits without active circuitry? Or any indications of subtractive effects as would please the naysayers?

On one possible side of the fence, we thus have mean but lean, transparent but sparse, open but cold, honest but sterile - the bad and boring stuff. On the other side, we have unmitigated or even accelerated speed; heightened resolution via lowered noise floors (passive are quiet, period); reputedly improved dynamics at low volume due to zero signal losses (which are said to plague regular volume pots and ladder-type attenuators that throw away unwanted signal voltage as heat); and undiluted purity that doesn't morph and manipulate the signal. Which way do we turn, Josephine?

Time to query Harry O'Sullivan of S&B for more background on their concept and execution. It turns out that Music First Audio Limited is a wholly owned subsidiary of Stevens & Billington Limited which then manufactures the Passive Magnetic Preamplifier under the Music First Audio brand name. The transformers in the MFA unit are very similar to those made available to other companies for OEM purposes yet "subtle but definite differences in the way these transformers are wound" remain. Hey, Dynaudio and JMlabs too reserve their finest drive units for internal use. "More importantly, we regard the way in which we use these transformers as critical. We manufacture everything by hand and take a great deal of care and pride in the job we do. We make sure that each solder joint and connection is as close to perfect as possible. While this approach is very labor-intensive, we feel that 'every little bit helps' to get the most out of the components we use. We only use the best quality parts available at this price and we rigorously test and listen to every preamp we make."

This passive preamp is fully transformer-balanced input to output. Conversion from one format to the other is possible in both directions (pin 2 is hot). Internal hookup wiring is silver, the standard transformer windings are copper but silver versions are available. Ditto for highly polished silver-finish knobs for those who don't fancy the gold ones. Now for the tech stuff - such as the role of capacitance in the interconnect cables as determined by length. Which really depends on the source's cojones to drive. "A
definitive answer is that any source component should be able to drive the IHF Standard Load of 10K/1nF (some pre 1980's tube gear, old tuners and tape decks may be unable to operate into the IHF load), in which case the permitted load into the passive preamp is also the IHF load. As normal interconnects tend to be in the region of 100pf/m, up to 10m of average cable (foil types excepted) will be fine under almost all condition. Cables with above average capacitance may still work perfectly fine in 10m lengths but really should be proportionally shorter. Cables with below average capacitance can be proportionally longer.

If the source has a drive capacity better than what is required to handle the IHF test load (some tube equipment may have problems there but all solid-state gear should be able to drive much more severe loads than the IHF test load), the cable capacitance and thus cable length can be increased. With most modern transistor sources, 30m or longer runs of average interconnects will be no issue. We have yet to come across a situation where cable length has been a problem."

"In fact, a transformer simply takes the drive ability of the source and passes it through with either a slight decrease over the source (when turned all the way up) or a notable improvement when any significant attenuation level is used. By comparison, traditional passive preamplifiers impose large resistance values between source and cable, with additional resistance shunted to ground to accomplish attenuation. This severely limits their ability to drive cable capacitance loads. Worse, the source must provide the power to drive the volume control's resistance as it gets converted to heat.

Transformer-based attenuation with its electromagnetic coupling does not rely on the addition of resistance. It works by changes in primary/secondary ratios. The transformer itself has very high impedance across the audio band. Accordingly, very little of the source's output power is lost and only the very low winding resistance appears in series with the source prior to the preamplifier's output. Thus the source only needs to drive the load and cables connecting to and from the passive preamplifier. However, as the Passive Magnetic Preamplifier's attenuation is increased (preamplifiers in most systems end up cutting the signal by 20dB or more at all times), the output impedance rapidly falls to very low values. This in turn translates into much improved ability to drive a load or cable.

Here are practical examples.
With the volume control set below the 3:00 o'clock position, a source with around a 1K Z-out will result in output impedance below 300 ohms. This amounts to a threefold improvement in output impedance. With further increased attenuation, the output impedance falls progressively. If attenuation is set to -20dB or 12:00 (rather more common in many systems), the output impedance becomes around 36 ohms for a 1K source - an improvement of nearly thirty times.

The use of transformers not only dramatically improves on the ability of the passive preamplifier to drive cables (and loads) compared to traditional designs, in most systems the use of transformers will actually improve materially on the ability of the source to drive its cables as well. It is the latter part which explains the experiences of some of our customers who found the use of the Passive Magnetic Preamplifier to be a substantial improvement over using no preamp at all."

Prelude: With two AudioZone TVCs already in house -- the PRE-T1 in copper and silver transformer guise -- I've come to the conclusion that such impedance-matching passives can be the ultimate solution for tube amp lovers. The reason is simple. Tube amps -- especially low-power ones -- live and die on a copasetic match with just the right speakers (think lack of damping factor, speaker sensitivity and load behavior). Premium examples of such matching are like finely aged wines. They should only be preceded by pure table water so as to not dilute or otherwise alter the carefully wrought sonic 'in vino veritas' flavor. That's where TVCs shine by definition and personal experience. They let you hear your amplifier without any shadings. Another benefit is the passives' lack of gain. It removes a very real zone of potential noise, something valved components in general need help with. You could be surprised to discover that your cherished 2A3, 45, PX25 or 300B amp is far quieter than you thought once that active preamp with its redundant gain makes way for a passive.

Needless to say, there are no hard and fast rules. However, if your source can drive your tube amp to full output for volumes in excess of any practical use? Now TVC passives could be the ultimate solution. Of course their employ places the full brunt of the sonic burden squarely on the amplifier. Alas, when things are just so, that's exactly all you want. Now why did I leave out solid-state amps? To practice prejudice? Not really. When it comes to tone, most transistor amps need help one way or the other. For that, they rely on the right preamp to inject a bit of color. And color is not why anyone would go passive. Or is it?

Where the Music First piece goes beyond the Canadian PRE-T1 is connectivity - more inputs and a fully balanced in'n'out path. The latter could be the deal maker in favor of the British pre even if it otherwise sounded identical to the Canadian's S&B implenetation. The AudioZone enclosure is more substantial and its cosmetics more sculpted and upscale. The Music First Audio box adds functionality for a $500 'surcharge'. Either way, both pieces vie for the very same customer, with the British piece whispering promises of secretive parts tweaks or tighter tolerances. Audiopax, Harbeth and Lavardin importer Walter Swanbon of Fidelis AV in New Hampshire just became US attaché in all matters Music First. Because the Euro's ascendancy continues to strain the good will of Yankee trade relations, Mr. Swanbon has kindly decided to overlook actual math. He extends a friendly introductory pricing offer for this fresh import until his accountant forbids it.