Headphones

The Absolute's key areas of performance are entirely dominated by the choice of push-pull or single-ended tube configuration, by the Low (L), Medium (M) and High (H) gain/headphone matching setting, and by the position of the 'negative feedback' control. The L, M and H settings (notionally matching L, M and H impedance 'phones) determine the gain of 9.9dB, 15.9dB and 19.2dB; the maximum voltage output of 8.9V, 14.8V and 24.4V; and the amplifier's output impedance of 6.9ohm, 27ohm and 54ohm [see Graph 2], respectively, all in push-pull mode with max. feedback applied. The power output into 25ohm is 1.14W, 1.8W and 1.9W at <3% THD with H, M and L settings, increasing still further to 2.3W into a low 8ohm load [see Graph 1, below]. So, with more than sufficient volts available to drive high impedance (600ohm) 'phones but with a minimum source impedance of ~7ohm, this indicates the Absolute is best used in 'L' mode regardless of your choice in headphone and/or eargear, thus minimising unpredictable swings in response caused by the impedance trend of the headphones themselves.

Into a 'flat' 25ohm load there's a uniform loss of some 2dB and a response that reaches out from 10Hz-70kHz (re. ±0.4dB) before falling to –1.7dB/100kHz (max feedback). With minimum feedback applied, gain is boosted by a further +9dB but response rolls away in the low bass from –0.3dB/20Hz to –6dB/4Hz, and at HF from –0.7dB/20kHz to –6dB/80kHz. Changes in frequency response are further exaggerated into low impedance loads with minimum feedback applied. Bearing in mind this is a multi-stage tube amp with limited feedback applied, the 88.2dB A-wtd S/N ration is impressive and hum/noise low enough at –87dBV. PM