Focusrite and the Lundahl LL1538 Transformer

Focusrite grew out of a 1985 request from Beatles producer George Martin to recording console designer Rupert Neve (after he'd sold his eponymous company), to build a no-compromise mic pre and EQ for the Forte mixer — destined for Air Studios on the island of Montserrat — and subsequently ravaged by a volcano! The modules, prefixed with the letters ISA (Input Signal Amplifier), featured a mic preamplifier with the Lundahl LL1538 transformer, used in every Focusrite ISA mic pre since. Why did Rupert Neve choose Lundahl, rather than a Marinair transformer as used in the Neve 1073 module, and what makes this transformer from the northern part of the Stockholm archipelago so special?

To understand why, we need to transport ourselves back to the '80s. Nowadays, transformers are touted by some manufacturers as talismanic 'vibe bringers'. However, in 1985 audio signals passed not only through mic input transformers, but were conveyed to console outputs, into tape recorders, out of tape recorders and back into mixers again via transformers at each stage. While the gain and electrical isolation provided by this circuitry was mainly necessary, the 'funky' harmonic spectrum imparted by this group of galvanics was not always appreciated by production pros. Back in the day, engineers and producers were looking for a cleaner and more 'transparent' sound. Neve turned to the LL1538 because of the open and expansive character of the audio it produced when amplifying mic signals. To understand how this sound is made possible, it's worth considering how transformers are made, and understanding how the Norrtälje-based Lundahl factory makes devices with a difference.

  • Focusrite and the Lundahl LL1538 Transformer

If you've seen a modern electronics production line, then a visit to a transformer manufacturer can be a bit of an eye-opener, with distinctly mid 20th-century equipment, a lot of manual work and a decidedly home-spun atmosphere. The spinning metaphor is apt, as there are plenty of bobbins and wire, and a very low-precision environment when compared to PCB production. This is where the Lundahl operation differs: apart from the raw wire and mu-metal (a nickel-iron soft ferromagnetic alloy), everything is custom-made onsite.

The Lundahl transformer configuration was developed from first principles by Lars Lundahl, who along with his wife Gunnel founded Lundahl Transformers in 1958. It's a family business: son Per took over as MD in 1994.

A transformer is constructed from magnetic core and coils of wire. Lars re-imagined the way transformers were usually made, as Per notes. “I have no idea if Rupert Neve ever tested the LL1528 and eventually challenged Lars to improve it, or if the LL1538 was the first of our transformers Rupert Neve tested. As far as I know, already at that time Rupert Neve designed most the transformers in his circuit designs himself. Anyway, for some reason Lars redesigned the LL1528 from a 2+2 section design to a 3+3 section design [this refers to the winding configuration of the transformer]. Each coil now consisted of two low impedance primary sections and one high impedance secondary section. Between each section are Faraday shields. If by luck or by calculation it is impossible to say, but the new transformer, the LL1538, was a success."

Not only did Lars Lundahl develop his own way of making a transformer, but he also designed all of the manufacturing equipment! A full-time machinist remains part of the Lundahl team, to assemble new equipment and maintain existing machines.

  • Stick wound coils

Lundahl transformers use 'stick wound' coils: the wire is wound in layers on a coil PET former. The use of stick winding with interlayer insulation provides a coil that is uniform in geometry from end-to-end and layer-to-layer, resulting in better (and more consistent unit-to-unit) electrical performance than the more common bobbin-wound coils.

The winding operation is overseen by skilled workers, but the winding itself is computer-controlled with design-specific software. Every coil is the same as every other and performance matching between transformers with the same part number is excellent. This is one of the key reasons Focusrite have used the same Lundahl component for every device in the ISA range for over 30 years: the assurance that the user will receive great sound every time.

  • Lundahl Transformers

The flat winding process has several benefits, one of which is that Lundahl can include a Faraday shield which extends beyond the edge of the windings, to better block electromagnetic interference. Lundahl have a stellar reputation in the audiophile world, where signal levels can be far lower in level and more vulnerable than those from professional microphones. When Rupert Neve chose the LL1538 he was making a conscious 'step up' from the type of artisanal construction used for transformers in the vacuum tube/valve era.