Sound Impressions of Restoration/Upgrade
Before starting the restoration/upgrade, the AU9500 was auditioned for a couple of days; it replaced a Hafler DH-110 preamp and harman/kardon hk770 power amp in one of my setups. Selected tracks from recordings (listed below) that I'm very familiar with were used to evaluate the AU-9500. Some notes were kept during these initial listening sessions (to avoid relying on just my sonic memory) for comparison with the restored/upgraded unit.
As received, the AU-9500 gave the impression of prodigious power, able to play loudly without any noticeable strain. The bass was particularly "full" sounding, with a lot of weight but it also had a somewhat "loose/underdamped" quality. The midrange had a non-fatiguing, smooth presentation, with vocals further back in the soundstage as compared to the Halfer and harman/kardon set. Similarly, the treble range could be characterized as smooth or soft, but it did not seem particularly extended (i.e., sounded rolled off). Overall, the unrestored AU-9500 was a powerful "warm" sounding amplifier, with a laidback presentation.
Following the restoration/upgrade, I was able to evaluate the Sansui for a couple of days before it was returned to its owner. The sonic difference from the stock condition was readily noticeable; the most obvious changes were in the sound of the upper midrange and treble range. Whereas before the treble range was somewhat "soft" and diffuse, it was now subjectively more extended and natural sounding (e.g., the different percussion timbres are easily discerned in Brubeck, "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., in Ellington, "Cottontail", Pinnock/ECO, "Summer"; Clapton, "Hey Hey", respectively). Vocals were also rendered with improved clarity, some lyrics that were previously difficult to decipher were now more intelligible (e.g., Doors, "L.A. Woman). The change in the sound of the bass range (<240Hz) was more subtle than that observed in the mid and treble ranges: it too was now subjectively more extended and "tighter" sounding (the 2009 remaster of "Come Together" sounded particularly good). Another example of such was observed with "Aja", where there were gains in the clarity of the drums and bass. Overall, the sonic presentation of the restored/upgraded AU-9500 was more lively/dynamic than before.
A few more related comments:
In my experience, improving the preamp stage typically has a large impact on the sound of an integrated amplifier and that was the case here. Possibly due to the circuit topology requirements for larger signal path capacitors, the AU-9500 preamplifier stage reverted to the use of polarized electrolytic capacitors (as compared to the preamplifier stage of the earlier AU-999 which used primarily film capacitors). Fortunately, due to the miniaturization of film capacitors, during the restoration, all of the signal path electrolytic capacitors in the AU-9500 preamplifier stage were replaced with better film types and the single original polyester film capacitor was upgraded to a polypropylene type. Similarly, the main amplifier stage of the AU-9500 utilized polarized electrolytic capacitors in the input and feedback loop whereas the AU-999 had a film capacitor and a bi-polar capacitor, respectively, in these positions. The restoration of the AU-9500 main amplifier stage used a film capacitor and bi-polar capacitor in the same manner as in the main amplifier stage of the AU-999. Of course there were many other changes made during the restoration of the AU-9500, but the aforementioned are worth highlighting as they are probably responsible for the majority of the sonic improvements.
Recordings used in listening sessions:
Steely Dan/Aja; Miles Davis/Kind of Blue; Eric Clapton/Unplugged; Beatles/Abbey Road; Dave Brubeck/Time Out; ECO/Bach Brandenburg Ctos; ECO (Pinnock)/Vivaldi Four Seasons; Duke Ellington Orchestra/Digital Duke; Bohm, 1972 Beethoven Symphonies no. 6 & no. 9; Doors/L.A. Woman)