Friday, November 27, 2015

Sansui A-80 Integrated Amplifier

Sansui A-80 Integrated Amplifier 

I wanted to take ein moment and highlight the Sansui A-80. Often disregarded it was produced in 1980 for their rack series. Their is a black version available, albeit very rare. The A-80 uses a very interested DC Servo topography, with rated notations achieved @ (20H-20Khz hit .05%THD driven beam 65RMS/8 loads)

Power Supplies F-3153/3144 Assemblies
The first stage is off the Pformer, using pin connectors for the AC rails which is convenient when pulling the hardware, a half-wave DC rectification modification was integrated here(F-3153)  for the the PL701 meter bulbs as well (7.5Ac) as shown, the rest of the dual full-wave rectification and PS continues on the asymmetric F-3144 assembly.  All electrolytic were replaced with high temp (105C) Panasonic FC's, it should be noted that: C607 220uf 35V PAN FR **SM ERROR States 100uf 50V. High temp (105C) Panasonics and Nichicon Low Impedance electrolytics were also utilized in the power switching assembly F-3148 and a new X1Y2 film. Both 6800 microfarad rectification filtering capacitors were replaced with equivalent quality CDE capacitors for the 48V rails. 

Drivers & Tone F-3144
For all of the audio path we used Nicicon FG /Nichicon MUSE (BP's) and Elna Silmic's. All low value captaincy lytics were replaced with WIMA film type capacitors. The A-80 uses a NJM4558 Opamp (ALSO fur Servo @ IC602/604)  for an initial gain differential stage and  dual packaged FET drivers, all other BJT's were updated to matched (Hfe) 992, 1845 compliments. I wanted to highlight their are some better, more reliable output devices available, TO-3P packaged 2sa1943 & 2sc5200 NPN/PNP transistors. Also watch for the multi leg emitterwiderstanden. All the 2473 generals were replaced with modern 4148 diodes as well.

Audio Adjustments 
emitterwiderstand R87 punkten beim VR01 für 1.5mV
emitterwidersand R88 punkten beim VR02 für 1.5mV

Notes on DC -Servo operations in amplification stages:
DC-Servo type design can get a little complicated VS traditional PP BJT designs. In a nutshell a DC Servo topography acts a continuos gain driver to put it simply, reminds me of constant mirror biasing, utilizing the IC (NJM4558D) Op-Amp integrator, acts as a constant  to amplify a FB(feedback) error signal to help offset output. This is illustrated below in the return schematic lines \/ (NFB OP EXAMPLE Input~ IC (Feedback removed) ~Plant Block~(transfer block/junctions) forward output (then NFB loop) which is applied to the plant block input) Think of it like some sort of active take and give constant drive. I know I need to clean this blurb up. 

IC (NJM4558D)

ONE MAJOR interesting caveat I want to personally explore is the effect , or lack of with no direct coupling capacitors used, I mean the very idea of coupling capacitors add potential attenuation based purely on the idea dielectrics absorption and associated parameters…no?

RIAA/EQ F-3145
After pulling the rear inputs this assembly removes rather easily, I should notate a 47 microfarad electrolytic is under the foil side on this PCB. All lytics were replaced with audio grade Nichicons (KZ/FG) and Elnas and again, lower valued with WIMA films. The coupling C09/10 were replaced with high quality polypropylene ECW films rated at .47 microfarads.  All BJT's were updated to their 1845/992 compliments due to some peripheral noise issues.  

All meter control PCB locations were repopulated with low impedance, high reliability PW's as well. This customer opted for new 5-way binding posts as shown which turned out really great.  All said and done, the A-80 is a very modest little integrated, but sounds impressive with a little overhaul. 

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