Leestereo's Restoration/Upgrade of a Sansui AU-555A

Discussion in 'Exclusively Sansui' started by Leestereo, Mar 30, 2018.

  1. Leestereo

    Leestereo Super Member

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    A very nice Sansui AU-555A was on the workbench for a complete restoration and upgrade.

    Part 1: Power Supply

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    The stock C001 capacitor (2200µF/63V) was replaced with a 6800µF/80V low ESR type; note that the voltage rating of the replacement was increased to 80V since this capacitor is subject to ~60V. Similarly, the capacitors at C002 (470µF/50V), C003 (1000µF/50V) and C004 (470µF/50V) were replaced with low ESR types rated at 1200µF/50V, 2200µF/50V and 1200µF/50, respectively. The stock diodes, D001 and D002, were replaced with 2A(200PIV) soft recovery types. The voltage dropping resistors, R001-R003 were replaced with 1% metal film types rated at 2W.

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  2. Zekeman

    Zekeman Super Member

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    Nice. Can’t wait for more on this.
     
    Last edited: Mar 30, 2018
  3. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    I agree!
     
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  4. BLAH BLAH

    BLAH BLAH AK Subscriber Subscriber

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    Nice old venerable machine...only ever had good words for the ol' 555A...:hug:
     
  5. LBPete

    LBPete Rolling Along Subscriber

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    :lurk:

    - Pete
     
  6. Leestereo

    Leestereo Super Member

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    Part 2: Tone Control Block (F-1272-1)

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    Signal Path Components
    The signal first passes through the input high-pass filter capacitors (C701/C702), originally 0.22µF polyester film types; these were upgraded to 0.47µF Panasonic polypropylene film types. The 10µF low leakage electrolytic coupling capacitors at C707/C708 and C723/C724 were replaced with Nichicon ES bipolar types (at the same capacity as the originals). The stock 0.047µF polyester film bypass capacitors (C725/C726) were replaced with 0.33µF stacked polyester film types. The 1µF low leakage electrolytic capacitors at C717/C718 were replaced with WIMA polyester film types (at the same capacity as the originals). The stock polyester capacitors in the tone circuit proper (C709-C716, inclusive) were upgraded to polypropylene or C0G types (at the same capacity as the originals). The transistors (TR701-TR704) were 2SC63A types (rather than 2SC871R as shown in the schematic). Since TR703 was found to be noisy, all transistors were subsequently replaced with hFE matched pairs of Fairchild KSC1845s: hFE=~380 for TR701/TR702 and hFE=~450 for TR703/TR704.

    Non-signal Path
    The emitter bypass capacitors (C705/C706 and C721/C722) were replaced with Nichicon FG polarized types. The replacements for the C705/C706 capacitors were increased to 100µF from the original 47µF to extend their effectiveness to below 20Hz. A 33µF/35V low ESR capacitor was was added to the solder side of the board as a local DC filtering capacitor for this stage.

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  7. Leestereo

    Leestereo Super Member

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    Part 3: Main Amplifier Block (F-1266-1)

    This particular AU-555A is a late production version with the F-1266-1 board (also used in the AU-505). The circuit is essentially the same as with the F-1266, except with the use of 2SA726, 2SC875, 2SA532 transistors instead of the so-called "flying saucer" transistors (i.e., XA-495, 8002-1 and 9002-1, respectively); there are also minor changes to the copper trace layout.

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    Signal Path Components
    The ceramic capacitors (C801/C802) in the input low pass filter were replaced with 10pF C0G types. In the high pass input filter, the 0.47µF polyester capacitors (C803/C804) were upgraded to 1µF polypropylene types. The polarized electrolytic capacitors (C807/C808) which specify the low end cut-off of the feedback loop were replaced with bi-polar types of the same capacity. The output capacitors (C819/C820) were replaced with capacitors rated at 6800µF/42V. The capacitors (C809/C810) in the secondary feedback loop (taken after the output capacitors) were replaced with capacitors of the same capacity. The feedback resistors (R811-R816) were upgraded to 1% metal film types. The input transistors (TR801/TR802) were the failure-prone 2SA726 types and these were replaced with KSA992 transistors.

    Non-signal Path Components
    The replacement capacitors for C813/C814 (located behind the C819/C820 output capacitors) were low ESR types rated at 120µF/50V. The replacements for the emitter bypass capacitors (C815/C816) were increased to 470µF from the original 220µF to extend their action to below 20Hz. The ceramic capacitors (C821, C823) in the regulated power supply for the main amplifier were replaced with 0.47µF/100V polyester film types. The electrolytic capacitor (C822) at the base of the pass transistor was replaced with a low ESR type rated at 330µF 80V. The local electrolytic local decoupling capacitors (C805/C806) were replaced with stacked film types. On the solder side of the board, the 2.2µF/100V local decoupling capacitors (C824/C825) were replaced with low ESR types rated at 47µF/80V. The trimmers at VR811-814 were replaced with cermet multi-turn types.

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    Last edited: Apr 6, 2018
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  8. stereofun

    stereofun Super Member

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    Thanks for the details ! Your restorations are like a recipe from a Michelin chef.

    It must sound stunning within it's class ?
     
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  9. restorer-john

    restorer-john Addicted Member

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    I've said it before and I'll say it again. Your marking-up of the photographs is truly excellent. :)

    It takes a lot of time to do that, arguably more time than the actual work- I know.
     
  10. Leestereo

    Leestereo Super Member

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    Glad that you're enjoying the "menu", and yes it sounds quite nice...more to come...
     
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  11. Leestereo

    Leestereo Super Member

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    Thanks for the kind words, much appreciated.

    Yes, these restoration posts are relatively time consuming: its much faster/easier to replace/upgrade a board than to take "before/after" pictures, label, and describe the process in a coherent fashion.
     

     

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  12. BLAH BLAH

    BLAH BLAH AK Subscriber Subscriber

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    Beautiful work...much appreciated...even from a Tech novice like myself!
    Pleasure to watch a pro at the top of their game!
    (Even more so when it's Sansui related!)
    :thumbsup: :lurk:
     
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  13. Leestereo

    Leestereo Super Member

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    Part 4: Power Amplifier Modifications

    As with many capacitor-coupled amplifier designs, the AU-555A employs a quasi-complementary output stage (TR811-TR814). Since the driver stage is complementary, the load on the emitter of the NPN drivers (TR807/TR808) is different than that on the PNP drivers (TR809/TR810). Why this even matters: this asymmetry results in significantly more crossover distortion than a fully complementary design. Fortunately, this situation can be effectively corrected by the installation of a Baxandall diode (i.e., diode across the emitter resistor of the PNP driver). Although the concept of the Baxandall diode and its effectiveness is well covered in D. Self's Audio Power Amplifier Design Handbook, there isn't much information on a specific resistor value nor the type of diode to use. I am indebted to @ConradH with whom I was able to discuss the implementation of this modification in the AU-555A: the 27ohm resistors at R835 and R836 were each replaced with a 150ohm (metal film type) paralleled with a 1N4002 diode (note the cathode is towards the emitter).

    An additional modification was the installation of film capacitors (0.33µF/100V) across the emitter and collector of TR805 and TR806. This Vbe multiplier by-pass capacitor ensures good high frequency performance (by improving the voltage regulation across the Vbe transistor at high frequencies). Further information regarding this by-pass can be found here: Capacitor type for the bias circuit (polarized or bipolar). Since this by-pass capacitor will also affect the usable range of the bias trimmers (VR813/VR814), replacing the stock 1.2kohm series resistors (R823/R824) with 825ohm ones will restore it.

    Also worth mentioning is ConradH's excellent web page which documents the improved performance of the power amplifier stage of a Sansui 2000X (same topology as the AU-555A) following the installation of a Baxandall diode and a Vbe Multiplier by-pass capacitor: http://conradhoffman.com/Sansui2000x_mods.htm.

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    Last edited: Apr 7, 2018
  14. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Very interesting ! Beautiful work
     
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  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    +1

    I also read Conrad's comprehensive and interesting write up in the link - excellent!
     
    Last edited: Apr 7, 2018
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  16. Zekeman

    Zekeman Super Member

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    You know - I have two AU-555A's here and I am going to completely go through one - along the lines of Mr. Leestereo (this I will attempt anyway) and will leave one stock. I will then do a direct A-B comparison.
     
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  17. Leestereo

    Leestereo Super Member

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    Should be an interesting comparison; looking forward to reading about it.
     
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  18. Leestereo

    Leestereo Super Member

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    Part 5: Equalizer (Phono) Block F-1267

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    Signal Path Components:
    The stock capacitors (C601/C602) in the high-pass filter for the phono input were 0.33µF polyester types; these were upgraded to 0.47µF polypropylene types (note that R601/R602 were also replaced since these were glued to the the original capacitors). The stock output capacitors (C611/C612) were originally 1.0µF/50V electrolytic types, with 0.047µF polyester film by-passes (C617/C618); each pair was replaced with a 1.0µF stacked (polyester) film type.

    In the RIAA equalization feedback loop, the resistors R619/R20 and R623/R624 were replaced with 332kohm and 27.4kohm metal film types, respectively. The capacitors in the RIAA feedback loop, C613/C614 and C615/C616 were replaced with 10nF and 2.7nF C0G types, respectively. The resistors at R621/R622 were replaced with jumpers. This last change modifies the RIAA equalization circuit to a set of parallel RC pairs, this is the topolgy that is employed in the phono stages of the AU-666, AU-777 models (and in later series Sansui phono stages). The modified AU-555A RIAA equalization is accurate ±0.2dB from 20Hz-20kHz.

    Non-signal Path Components
    The emitter bypass capacitors (C607/C608 and C609/C610) were replaced with Nichicon FG polarized types; the value of C607/C608 capacitors were increased to 22µF to extend their function to 20Hz. A 33µF/35V low ESR capacitor was was added to the underside of the F-1267 board socket as a local DC filtering capacitor for this stage.

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  19. slimecity

    slimecity Super Member

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    It will be great to see how these extensive mods improve the sound Lee. I've had a couple of 555a's and ultimately got rid of them as they sounded great, but still quite "fuzzy/grainy" when compared to my 999 (unfair comparison I know).

    But I still do miss that nice fat and warm sounding amp quite often.
     
    Last edited: Apr 17, 2018
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  20. Leestereo

    Leestereo Super Member

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    Part 6: Listening Impressions

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    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.
     

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