Unison Research S2 is one of the most appreciated Italian tube integrated amplifiers among audiophiles. Produced during the 1990s and early 2000s, it represents the brand’s sonic philosophy: warm musicality, artisanal craftsmanship, and an elegant design that combines wood and metal in a distinctive style. The circuit employs a pair of EL34 tubes in single-ended configuration, driven by ECC82 signal tubes. When properly maintained, it can still deliver outstanding performance today, although over time it may require inspection or bias adjustment.

This particular Unison Research S2 arrived in my workshop because it no longer powered on: the internal fuse had blown. Upon initial inspection, one of the two EL34 output tubes was completely worn out, while the other still showed around 80% of its original efficiency. I searched through my stock to find a similar tube to replace the faulty one and keep repair costs reasonable.

After replacing the fuse and powering up the unit, I began checking all operating voltages. I quickly noticed that one of the cathode resistors of the output stage was heating more than the other — the cathode voltages were indeed quite different. One tube was drawing much more current than the other, and that channel exhibited audible distortion. This was puzzling, considering that both EL34s had been tested on the uTracer and appeared perfectly matched.

Fault identification

I proceeded to measure the control grid (G1) voltage of the output tubes, which should theoretically be at 0 volts. Instead, one tube showed +5 V and the other +12 V. The first suspicion fell on the coupling capacitors, which can sometimes become leaky with age. I disconnected and tested them with an insulation meter, but they were perfectly fine. The PCB also showed no signs of carbonization or leakage paths.

Attention then turned to two bias resistors in the circuit whose values were incorrect, causing an error in the grid bias voltage. After replacing them with properly rated components, the grid voltage returned to 0 V as expected, and the EL34 bias stabilized at the correct operating point.

After replacing the defective resistors and verifying all critical voltages, the Unison Research S2 was restored to perfect working order, once again delivering the warm, dynamic, and musical sound that has made it a favorite among enthusiasts. Below are some instrumental plots taken during testing, provided as examples of the observed behavior.

Frequency spectrum on resistive load

And on reactive load

If your Unison Research S2 or any other tube amplifier is showing signs of malfunction, contact me for professional diagnosis and repair. Every intervention is carried out with specialized measurement tools and utmost care to preserve the integrity and originality of the equipment.