A couple of issues back I went to town describing the workings of Audio Zone’s Pre T-1 passive preamplifier and the way it uses transformer tappings to give different levels of attenuation rather than the usual resistor ladder or potentiometers. Apparently this is an approach first proposed by Western Electric in the ’30s. The transformers used by Audio Zone are wound by Stevens & Billington of Hastings, who make their own transformer coupled passive device under the brand name Music First.
Unlike the Audio Zone with its choice of two inputs and two outputs the Music First is rather better equipped and has two balanced and four single ended inputs; it also has both flavours of output and an earth isolating switch. It uses Elma switches to change both level and volume and is hand-wired internally with in the case of this Silver version, pure silver cable. The reason why the Silver Music First is so much dearer than its copper stablemate is that this silver wire continues within the transformers themselves. Those transformers use an 80% nickel permalloy core and are shielded by Mumetal cans to keep out external magnetic fields.
The theoretical advantage that using transformer tappings confers on a passive controller is that it avoids the impedance lottery suffered by simple potentiometers. As soon as CD hit the scene, people started experimenting with 10k pots, often mounted in something as prosaic as a tea caddy, until Modsquad came up with a commercially available unit that offered source switching too. Hi-fi buffs just love the idea of getting something for nothing and the rest is, as they say, history – despite the fact that every time you change volume with a passive pot you change the impedance seen by both the source and the power amplifier, which can have dramatic effects on the ability of the one to drive the other. This means that both tonal balance and energy/dynamics tend to vary with level and choice and length of interconnect.
By incorporating transformers you can ameliorate this problem. In the Music First each tapping from the transformer gives a different level of attenuation, eliminating the need for resistors in the signal path, and while input impedance still varies it remains high through 99% of its operating range. In fact it only drops to a level that might have a noticeable effect on the signal when it is at maximum output, a situation that is unlikely to arise under normal circumstances. Put on a classical CD cut at an exceptionally low-level and couple it with a ‘deaf’ power amplifier and you might just get there if your speakers are insensitive enough. In reality, such chronic mismatches are rare – and should certainly be addressed by doing something other than buying a passive controller. Gain can be increased by 6dB via a step-up load should you find the volume control creeping too close to its end stop but this is only recommended with relatively high impedance loads such as you find with some valve amps.
Visually, the Music First Silver is differentiated from its Copper counterpart by control knobs finished in chrome rather than gold, and these look a whole lot more attractive against the brushed aluminium casework. The unit is quite hefty as a result of the internal transformers despite being compact, as is the nature of passive devices. The singleended socketry is silver plated which is a nice touch, albeit one that will require a bit of cleaning from time to time – and the aforementioned 6dB gain switch is rear mounted. Another little switch controls the grounding of the transformers for SE and balanced outputs separately (it’s probably unwise to try and use both simultaneously). If left in the ‘lift’ position this can result in nasty bangs through the system when changing interconnects and is thus a detail worth paying attention to!
The Zanden Model 300 Passive line stage is a rather more attractively finished but also more conventional passive controller. Like all things Zanden it isn’t cheap, but then exquisite rarely is and the Model 300 is nothing if not a joy to behold. On the rear panel it describes itself with commendable accuracy as a Stereo Passive Line Volume Controller, the lettering positioned underneath a bank of rather fancy Canare RCA phono sockets. These provide four inputs and two outputs, but inevitably you get none of the grounding or gain options available from a unit that uses transformers. The pot in this box is a 10k DACT attenuator from Denmark, the same device that you can specify in Border Patrol valve power amps should you be so inclined. The box itself is stainless steel with a champagne anodised, aluminium front panel. Fit and finish are superb, making one almost ridiculously covetous of what is such a simple device.
Sonically these two products produced a fascinating contrast, so alike in some ways, so different in others. The Music First has been garnering accolades at a healthy rate and it’s not hard to hear why when you use it in place of even quite decent active pre-amplifiers. It delivers a transparency that is rare at any price, but without the limitations usually found with passive designs. The bass for instance is as powerful and articulate as all but the best active devices while the highs are fully extended, which creates an openness and clarity that is hard to surpass.