Pre/Power Amplifiers

PrimaLuna devotes considerable webspace detailing the quality of its PSU and output transformers, its 'AutoBias' regime, passive component selection, dual-mono layout and other engineering niceties – but the upshot is a preamp and, particularly, a power amp of exceptional quality. Noise, as claimed, is very low indeed with A-wtd S/N figures of 98dB for the preamp and a full 94dB for the power amp (re. 0dBV and 0dBW, respectively). The latter is 10dB better than that achieved by many solid-state amplifiers! Distortion increases with output in both amplifiers, from 0.03% at 100mV to 0.38%/1V and 0.8%/2V with the preamp and 0.19% at 1W to 0.8%/10W and 0.9% at the rated 70W/8ohm for the EVO 400 power amp (UL mode). I discuss the differences between triode and ultralinear/pentode modes in my boxout but in both cases THD increases at low frequency through transformer core saturation, reaching ~2% at 10W/8ohm and ~1.3%/20Hz at 0dBV from the preamp [see Graph 2, below].

This is perfectly typical, as is the increasing source impedance of the preamp at LF from its moderate 270ohm midband figure to 1.1kohm at 20Hz. The power amp has a high but consistent ~5.5ohm output impedance that will influence the overall system response according to the impedance trend of the attached speaker. Into a 'flat' 8ohm load it offers an extended bass (–0.3dB/ 20Hz) allied to a treble that rises slightly above 20kHz/+0.5dB to 'peak' at 40kHz/+1.55dB. This will be 'magnified' in practice because the EVO 400 pre also has a +3.1dB lift at 42kHz. Power is plentiful too, those double pairs of EL34s achieving 2x73W/ 8ohm and 2x85W/4ohm with 112W, 110W, 105W and 82W into 8, 4, 2 and 1ohm under dynamic conditions [see Graph 1]. PM