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High-quality Audio Transformers from Sweden Lundahl of Stockholm, Sweden have a reputation for quality in both the Pro Audio and “Audiophile” communities. Their unique manufacturing processes produce power, audio coupling, valve output, and audio/video isolation transformers with a high degree of consistency. I n the Pro Audio world, Lundahl make transformers for ground isolation, signal splitting, balanced/unbalanced conversion, microphones, mic preamps, line input and output, digital audio impedance matching/isolation/splitting, 100V PA work and even telephone systems. Their pro audio transformers are designed with a wide bandwidth and for minimal impact on audio quality. In an interview, owner Per Lundahl told us that Lundahl isolation transformers are often provided in pro audio gear as an option for those serious about sound quality. For equipment that comes with Lundahl transformers as standard, this is usually a sign that it was designed with ultimate audio quality in mind. It’s also quite common for musicians to buy gear with cheaper transformers and This cross-sectional view of a Lundahl signal transformer shows how the neat winding layers are each separated with insulating plastic. This is one of their wound amorphous cores. One of Lundahls’s bobbin-less winding jigs, producing six sets of windings simultaneously. After each coil layer is wound, another insulating layer is placed on top and then the winding continues. 46  Silicon Chip siliconchip.com.au Two of the finished winding assemblies have been attached to a baseboard and the windings are being terminated to it. There are many steps required to produce these transformers; some are automated or semi-automated. C-cores are inserted thorugh the windings of the finished assemblies. Strip-wound amorphous cores as shown opposite are generally considered to give lower audio distortion but C-cores are also a common option. swap them for a Lundahl equivalent as an easy upgrade. According to Per, Lundahl transformers have a reputation for audible “transparency” – which we take to mean that they introduce very little distortion to the signal being passed through. He said that one reason for this may be the fact that they are generally designed to pass a much wider range of frequencies than just the audio band, to minimise roll-off and distortion of audible signals near the upper or lower ends of the spectrum. The consistency and performance of their pro audio transformers is in part due to the unique bobbin-less method of construction, a cross-sectional view shown below left. The second photo shows the winding rig which creates multiple sets simultaneously. After the tight-packed windings are wound on each insulation layer, a new insulating layer is placed on top and the next layer started. This results in low inter-winding capacitance and a high breakdown voltage. The photo above left shows two of these now separated windings being wired to a series of pins on the baseplate after which the core is inserted. Two types of cores are used for these small transformers, either a pair of C-core halves as shown in the following photo or a single amorphous core as shown in the cut-away. This type of construction seems physically impossible but the core is actually wound from a metal strip of amorphous material which passes through both already completed windings. This is the preferred type of core for low-distortion audio applications. Whichever type of core is used, the assembly is then placed in a shielding can and impregnated with a plastic compound to resist moisture ingress. ther 105-125VAC or 210-250VAC mains. There are thirteen different types, providing a range of different HT voltages (from 110VAC up to 530-0-530VAC) and filament or low voltage supplies; typically this includes two or more ~6V windings and in some cases an extra 48VAC winding (eg, for microphone phantom power). Lundahl offer a large range of output transformers – more than fifty types, to suit various different output impedances ranging from 600Ω up to 23kΩ and secondary (speaker) impedances of four, eight or 16 ohms. Power ratings range from 5W up to 320W! (30Hz, push-pull.) For a complete list of Lundahl output transformers, see the URL at the end of this article. We’re particularly happy to note that full PDF data for all their transformers is available at their main site (www. lundahl.se). This includes physical dimensions (including numbered pins), along with a diagram showing all the windings, their pin connections and orientations, turns ratios, winding resistances and impedances, maximum recommended DC current, maximum signal voltage and power, inductance etc. In short, they provide a very comprehensive set of data, exactly what you need to design a project around the transformer and ensure that it’s the best one for the job. They even provide suggestions for the various different configurations in which the transformer could be used in the relevant data sheet. Browsing the various data sheets is an excellent way to shop for the appropriate transformer for your next project. Valve amplifier transformers Lundahl also have an extensive range of transformer for valve amps and vinyl preamps including power supply transformers, output transformers, filter chokes, anode supply chokes and moving coil pickup input transformers. As with their pro audio transformers, the valve audio transformers have a reputation for clarity. All their mains transformers are designed to work from eisiliconchip.com.au Australian Distributors The Australia and New Zealand distributors for Lundahl transformers are CDA Pro Audio, a “sister company” to Control Devices. They have offices in Sydney, Perth and Auckland. For more information contact their Audio Products Manager, Peter Orehov on (02) 9330 1750; (e-mail info<at> cda-proaudio.com). A list of available Lundahl transformers can be viewed at www.cda-proaudio.com/Subcategorie/ Transformers_products.html SC December 2015  47