LUNDAHL MOVING-COIL TRANSFORMER
Swedish company Lundhal’s coils and transformers are found in many of the world’s best-performing high-end products. Now the company has started to sell them under its own name.
Lundahl isn’t exactly a household name amongst audiophiles, yet you’ll find the company’s transformers in many of the world’s best-performing high-end audiophile products. Yet despite having been in the transformer and coil winding business since 1958, when the company was established in Sweden by Lars Lundahl, the company only started building consumer products in 2009. Until then, it had built products specifically for other manufacturers, which used its coils and transformers in industrial, electronics, aerospace and professional audio applications.
THE EQUIPMENT
We initially thought that the sample we were provided for review was a prototype, since all the circuitry was housed in a small (120×80×100mm – HWD) wooden box with a hinged lid that’s kept in place when it’s closed by two magnetic catches. But no, it turned out that we’d been loaned a fullfledged production sample—this is exactly what you’ll receive when you buy one. Although it’s very practical (as you’ll discover later), it’s rather left-field, so it’s likely to be something you love or hate. Many people would prefer a classic case, such as those used by Rothwell for its MCL step-up transformer (which, incidentally, uses Lundahl transformers!), which has a conventional chassis and faceplate, and the input, output and ground connections on the rear panel.
Obviously, it’s a step-up transformer, which steps-up the voltage from a moving-coil cartridge so it can drive a standard moving-magnet phono stage, but it’s a stepup transformer with a difference, because buyers can customise every aspect of it.
For example, our review sample was supplied with a pair of Lundahl LL1931 transformers (one for each channel). These use Lundahl’s uncut amorphous cobalt core and dual-coil structure and the coils are wound with high-purity copper wire made by Cardas. According to Lundahl, the dual-coil structure improves a transformer’s immunity to external magnetic fields, but to be on the safe side, the transformers are also encapsulated in a mu-metal shield/case.
However you can specify these transformers to be wound with pure silver wire instead, in which case the transformers in it will be LL1931Ag types. You can also specify the transformer core material, so if you’d prefer your step-up transformer with a mu-metal laminated core, rather than a strip-wound amorphous cobalt core, you can. You can also specify whether you’d like the mu-metal core wound with copper or silver wire. In this case, the transformer designations become LL1933 and LL1933Ag respectively.
All four step-up transformers are intended for medium-to-high output moving coil cartridges and so have selectable coil winding ratios of 1:8 and 1:16. If you need even-higher turns ratios, Lundahl can oblige, with transformers wound with 1:32 and 1:64 ratios. Once again, you can specify core and wire types, with the transformers being types LL1941, LL1941Ag and LL1943 and LL1943Ag.
Once you have established which configuration best suits your needs, you still have configurable options, because you can choose between two gain settings (18dB or 24dB in the case of our review unit, but 24dB/30dB with different transformers) via micro-switches and, via links on the circuit board, different grounding arrangements as well. You can also ensure the Lundahl is providing the exact load for the phono stage you’re using, with the particular cartridge you’re using, because there are tubular posts on the PCB into which you can insert resistors (one for each channel). You can use these posts as ‘temporary’ connections while you ascertain the correct value of resistance, then, once you’ve determined that, you can solder the resistors permanently to the PCB using the thru-holes alongside the posts. (Or, if you’d prefer not to do any soldering, you could just leave the resistors press-fitted into the posts.)
Our review transformer had a gold-plated ground terminal, but did not have gold-plated RCA terminals for its inputs and outputs. We would have preferred these, but presumably they’re available as an option, or you could easily change them yourself.
Why Use A TrAnsformer?
You’re probably asking yourself why you’d use a transformer to step up the output from a moving coil cartridge when most people use active, powered devices. It would be a good question, and it can be answered with just a single word: noise. Any powered device needs power (dah!) and that power usually comes from the 240V mains—a voltage source that is a perfect conduit for picking up electrical noise from other sources and channelling it directly to the power supply of the device that’s stepping up the voltage from the moving-coil cartridge.
The available power solutions—battery power, super-quiet filtered power supplies, and so on—are usually expensive, sometimes extraordinarily so. And if you go with one of the less expensive battery-powered solutions, the lack of power (you’re usually looking at around nine volts) and the requirement for minimal current drain (to ensure adequate battery life) usually ends up compromising the quality of the audio circuitry, so you get limited bandwidth, low overload margins, reduced channel separation, higher distortion (both THD and IMD) and increased noise.
A transformer, on the other hand, has none of these problems, mostly because it does not require any power at all. The voltage in one coil of wire (the primary winding) causes magnetic flux that induces a voltage in another coil of wire (the secondary winding). There is no electrical connection between the two coils, and no other components: just two coils of wire. If the coils each have an equal number of turns, the voltage induced in the secondary coil will be equal to that of the voltage in the primary coil. If the secondary has twice as many turns as the primary it will double the voltage, so for example if there’s one volt introduced to the input of the primary winding, there’d be two volts produced at the output of the secondary winding. If the secondary coil has half the number of turns, the voltage would be halved—so in our example, you’d end up with just 0.5 volts at the output of the secondary coil. When the voltage is increased, the transformer is called a ‘step-up’ transformer (which is the case with the transformer in this review). Conversely, when the voltage is reduced, the transformer is called a ‘step-down’ transformer.
Although step-up transformers are completely noiseless and have an extremely flat frequency response, they do have the problem that the impedance of the primary coil is much lower than the impedance of the secondary coil, because in order to have double the number of turns on the secondary side, you obviously need twice the length of wire that was used on the primary side. But this impedance change is not just double, as you’d expect. Increasing the voltage in the secondary by a factor of 10 (an increase of 20dB) sees impedance increase by a factor of 100. In other words, the increase in impedance is equal to the square of the increase in voltage. You also get a reduction in current in the secondary coil, compared to the primary.
But there’s yet another advantage of using a transformer rather than an active step-up device, which involves the type of distortion produced by both (and all step-up devices will introduce distortion to the audio signal. Whereas an active device has a fairly constant resistive input impedance, the input impedance of a step-up transformer is frequency dependent. This has the effect that any harmonic distortion produced by a step-up transformer is highest at the lowest frequencies and drops as the frequency rises, whereas in most active step-up devices distortion increases as the frequency rises. (There are, of course, exceptions… but they’re usually very expensive exceptions!)
TrAnsformers Ain’T TrAnsformers
Just as Sol here in Australia used to say ‘oils ain’t oils’ when promoting a particular brand of engine oil, Per Lundahl in Sweden, if presenting a similar advertisement, would probably say ‘transformers ain’t transformers’. There are many cheap step-up transformers available (most of them made in China) and they’re cheap not only because the materials used are not of a high standard, but because of the way they are wound. To see the difference you could use a piece of string and a piece of dowel. Rapidly wind the string around the dowel within two pencil marks 2cm apart. Have a look at the result. That’s how cheap transformers are wound. Now unwind the string, and wind again, slowly, making sure that each wind is tight up against the other, before winding a second layer.
You’re probably asking yourself why you’d use a transformer to step up the output from a moving coil cartridge...