The Audio Research LS5 MK2 is one of the most interesting balanced line preamplifiers in the historic production of the American brand. Designed for high-end two-channel systems, it works entirely in a balanced domain and uses a bank of small-signal tubes to deliver great dynamics, micro-detail, and exceptional soundstage reconstruction. It is an instrument aimed at those seeking uncompromising performance, but like many components originally built for the US market it is often imported in its 120volt version and therefore requires a proper 230volt conversion to be used safely on the European mains.

In this project I carried out a full overhaul and the conversion from 120volt to 230volt of an Audio Research LS5 MK2 imported from the United States. The goal was not simply to “make it work,” but to restore the unit to full long-term reliability, respecting the original design and carefully addressing power supplies, wiring, and components so that the owner could enjoy the preamplifier in a European system without adapters, external transformers, or compromises.

This Audio Research LS5 MK2 was delivered to me for a complete overhaul and for conversion to 230volt, since it came from the USA and was equipped with the original 120volt power transformer. Using an external step-down transformer would have allowed the unit to power up, but with compromises in terms of practicality, safety, and long-term reliability. The cleanest and most professional solution is always to replace the transformer with one designed specifically for the European mains, maintaining the same powering philosophy and the correct voltages intended by the manufacturer.

In the first photo you can see the original Audio Research transformer, kept intact and preserved, and in the second photo the new 230volt replacement transformer. This new transformer is built to provide identical filament and high-voltage supplies required by the LS5 MK2 circuit, with suitable current reserves and particular attention to insulation and magnetic quietness, both crucial aspects in a preamplifier of this calibre.

Replacing the transformer requires careful wiring and verification work: primary and secondary connections must be checked, voltages measured both unloaded and under load, and ground and reference points verified to avoid hum or unwanted loops. In a fully balanced preamplifier like the LS5 MK2, the quality of the power supply has a direct impact on the signal-to-noise ratio and the perceived black background, which is why every wiring detail is handled with precision.

Once the new transformer is installed and wired, the next steps include controlled first power-ups, voltage checks across the various stages, and a preliminary test of both channels and their balanced inputs to ensure everything operates correctly.

During the measurement and listening tests I later identified a fault on one channel. One of the tubes showed a completely burnt grid and abnormal bias, a clear indication that something upstream was malfunctioning. In such cases the correct approach is not to simply replace the tube, but to locate the underlying cause, checking grid resistors, bias network components, and the surrounding wiring.

The troubleshooting continued with detailed point-by-point measurements, comparing the healthy channel to the defective one and checking the actual values of key components, often stressed by heat and age. This level of verification is essential to prevent the fault from reappearing shortly after a superficial repair.

The fault was traced to a burnt resistor in the bias circuit, which caused the tube to run out of spec until the grid was damaged. After replacing the resistor with a properly rated component and verifying all surrounding values, I carried out a new series of measurements and listening tests to ensure both channels were perfectly matched.

At the end of the intervention the Audio Research LS5 MK2 was fully operational, correctly converted to 230volt, and ready to return to service in a European audio system. The owner can now use the preamplifier directly from the household mains with improved safety, long-term reliability, and the level of performance expected from a component of this class.