Making a rapid return to these pages after some minor running changes to the power amp (which Russ Andrews was keen for us to hear) these two little boxes make up an unusual pre/power system. The power amp is the more conventional of the two. a 50 watts per channel unit in about the simplest form imaginable, but the HP-l preamp is in itself a miniature power amp intended to drive high quality dynamic headphones (hence the jack/XLR socket on the front panel).
The HP-1 only has two inputs, selected with a simple toggle switch, but it sports a further switch for gain and another, at the rear, for a filter. Marked simply 'A' and 'B'. these settings are basically flat (A) and rolled off in the very high treble (B). A subtle tone control, in other words, which is useful in taming over-fierce recordings and also in compensating for the bright nature of nearby speakers as in the case of desktop systems. In our normal domestic hi-fi we stuck resolutely to A to avoid complicating matters.
Very neatly designed and assembled, these amplifiers are in fact made in Korea by Stello, then subtly tweaked by Russ Andrews. Circuit design of each is fairly straight forward, with simple mechanical sWitching and volume control no remote control plus good quality components throughout. The small case puts limits on the size of mains transformer in the PA-1, but it's perfectly capable of supporting the rated power output. while the one in the HP-1 is smaller but more generous.
Manufacturers submit previously well-liked products for reassessment at their peril. What if a different panel of listeners, playing different tracks should mark them down? Luckily, no such reversal of fortune affects RA on this occasion and, in fact. many of the most positive comments of the session refer to these amps. More than anything, it was the full bodied character of the sound that impressed our panel. Something that goes well beyond simple bass fullness (not in itself markedly more than with the other amps in the group) and gives a highly plausible, almost tangible, impression of live performers in a real space.
Low frequencies certainly play an important part in that. and there was praise for the duo's tunefulness and precision in the lowest octaves. Sudden bass notes appear startlingly, just as they should, and decay without undue overhang or blurring. As a result, the interplay between instruments and lines is made clearer, the harmonic structure of a piece of music likewise. The only warning note and a gentle one sounded by our listeners regarded quiet music. which doesn't quite match the grip and solidity of louder passages.
In terms of tonality, these amps seem to be pretty much blameless, while their grasp of detail is good and never maintained at the expense of emotion. Imaging is good laterally and certainly the best of the group in terms of depth, while rhythm is also fine, not quite as pronounced as some perhaps, but that's very much a question of taste. All in all, a happy blend of virtues. The relatively high distortion of these amps would appear to support the accusation that some distortion can subjectively enhance sound. We're not sure that's correct. For a start. that hypothesis usually invokes low order distortion and there's quite a lot of high harmonic output from these amps (mostly from the PA-1), characteristic of mild crossover distortion.
And then, in the worst case, it is still only about 0.15%. It just so happens that 1kHz at 2/3 power (our usual numeric specification condition) is worst case. In most other areas performance matches the other amps. Output impedance is high, but not by enough to tweak the overall frequency response more than a few tenths of a dB.
The power amp now has a flat frequency response. in contrast with the earlier sample we measured which was 3dB down at 17kHz. and just about meets RA's power specification.