Integrated Amplifiers

Unison Research, in common with other progressive brands like Audio Research [HFN May ’21], and PrimaLuna [HFN Mar ’21], offers switchable pentode or ultralinear and triode modes on its integrated and power amps. The KT77 beam power pentodes fitted to the Triode 25 Black Edition are well suited to triode adaption while offering greater headroom than the more common EL34 ‘equivalent’. Flick the toggle switch and the KT77’s control and suppressor grids are shorted to the cathode, effectively converting it into a triode. Triode operation typically confers lower gain and lower (50%) power output but with the prospect of improved linearity/lower distortion. Although the latter does not always hold true [HFN May ’25], it’s this difference in the harmonic ‘fingerprint’ of triode vs. pentode modes that has the former finding favour with many audiophiles, and especially those with higher sensitivity loudspeakers.

In this instance, the difference in overall gain between pentode and triode modes is +38.2dB vs. +37.8dB, and power 48W vs. 29W/8ohm, respectively, while the S/N ratio falls by ~3dB in triode mode. However, distortion is lower in triode mode, at least through the midrange [see grey and blue traces, Graph 2, Lab Report], for example 0.095% vs. 0.085%, respectively, at 1kHz/10W/8ohm. Amplifier output impedance is often lower in triode mode, but that’s not the case here as the Triode 25 Black Edition offers a steady ~1ohm (20Hz-20kHz) from its 6ohm transformer tap in both modes. The frequency response, and the changes in response with loudspeaker impedance, are the same in both modes. PM