Part 6: Listening Impressions
As received, the AU-555A did sound quite nice, it had a non-fatiguing/laid back presentation (one that many have termed "musical"). The bass range was quite full sounding, this characteristic was particularly noticeable with the phono input; some have also described this, in less generous terms, as "bloated" or "muddy" bass. The midrange response was smooth, with with vocals a bit further back in the soundstage. Similarly, the treble range could be characterized as smooth or soft, and did not seem particularly extended. Also, there was a noticeable background white noise/hiss between musical tracks, especially if in close proximity to the speakers or on headphones. Overall, the AU-555A exhibited what can be termed as "sins of omission" rather than "sins of commission": there is a lack of detail and dynamics in comparison to other restored/upgraded solid state units on hand.
After each stage was restored/upgraded, there were short (~30 minutes) listening sessions to ensure all was fine and to see if there was any change to the sound from the original assessment. From my notes, there were noticeable sonic changes with every subsequent listening session, with the most dramatic/noticeable improvements being from the implementation of the Baxandall diode (~50% reduction in distortion), the Vbe Mulitiplier by-pass capacitor (improved high frequency performance) and the RIAA equalization curve modification (accuracy improved to ±0.2dB).
Extended listening sessions with the restored/upgraded 555A indicate that the sound issues that were initially noticed have been largely corrected. The bass range (<240Hz) which was initially thought to be "bloated/slow" was now more neutral and seemed to be more extended and "tight" (the combined electric/double bass and bass drum in Lou Reed's "Walk On the Wild Side" provided an excellent demonstration of such). Another example was heard with "Aja", where there were gains in the clarity of the drums and bass. Improvements in the sound of the treble range were also noted. Whereas before the restoration/upgrade it was somewhat "soft and diffuse", it was now more present and more accurate sounding (e.g., different percussion timbres are easily discerned in Steely Dan's "Aja" and Brubeck's "Take Five"). In the midrange, the sound of brass, violins/violas and acoustic guitars had more "bite", making them sound much more "real" following the restoration (e.g., DMP Big Band/Carved In Stone, Beethoven/Violin Sonatas and Clapton/Unplugged, respectively). Vocals were also rendered with improved clarity allowing lyrics to be more easily understood (e.g., Steely Dan, "Black Cow") or just more detailed (e.g., Charlie Watts Quintet/Warm & Tender; Julie London/Julie Is Her Name). In terms of stereo/spatial presentation, the restored unit produced a wider soundstage and with improved image depth/height as compared to the stock version (e.g., Beethoven Symphonies; Bach Brandenburg Ctos.). There was also a noticeable reduction in the background white noise/hiss, while it has not been completely eliminated, it is no longer intrusive during quieter musical passages and is typically masked by the background noise/sound present in the source recording.
Some of the recordings (CD and LP) used during the listening sessions (in no particular order): Charlie Watts Quintet/Warm & Tender; Steely Dan/Aja; Miles Davis/Kind of Blue; Eagles/Hell Freezes Over; DMP Big Band/Carved In Stone; Julie London/Julie Is Her Name, Vol. 1 & 2; Jacques Loussier/Plays Bach; Dire Straits/Love Over Gold; Eric Clapton/Unplugged; Beatles/Abbey Road; Sarah McLachlan/Surfacing; Beethoven (Nishizaki and Jando)/Violin Sonatas No.5 and No. 9; Dave Brubeck/Time Out; ECO (Leppard)/Bach Brandenburg Ctos; Duke Ellington Orchestra/Digital Duke; VPO (Böhm, 1972) Beethoven Symphonies no. 6 & no. 9; NYPO (Bernstein, 1961), Beethoven Symphony no. 5; Lou Reed/Transformer.