Second generation of Sansui's most long-lived series : the 07.
07s got perfected until the late 1990s through various shapes, technologies and internal layouts which made the ultimate AU-A907NRA (1998) something completely different than the original AU-707 (1976) and its later AU-D907 version...
In between those two extremes were, in rapid succession :
the AU-D907 Limited, the AU-D907F, the F Extra and F Extra II, the AU-D907G Extra, AU-D907X, AU-D907X Decade (built by AZUMI), AU-A907 (A for Alpha), A907i, A907i MOS Limited, A907L Extra, A907DR, A907KX, A907XR, A907 Limited, A907MR and AU-07 Anniversary.
Almost one version per year !
The majority of these were Japan-only "editions", making all together a jungle of sorts - a very Luxman-like jungle - which I will in part bring online on TVK.
Being a 1978/1979 design, the D907's engineering revolved around TIM reduction and current capability of all stages, combining hardware size and circuit design to do so.
The AU-D907 is powered by two transformers in Penta-Power fashion : one toroidal separately feeds the Class B driver/output stages (after L and R independent rectifying), while one EI feeds separately and in Class A the L pre-driver, R pre-driver, L MC stage, R MC stage and flat amp.
The circuits are all of the DC kind with a patented item added for the phono EQ and output stage, DD/DC (Diamond Differential DC) which allows high slew-rates and fast rise-times over a very wide bandwidth of 500Khz.
Inputs start with thermally-matched dual-FETs arranged in differential and output ends first with a current differential push-pull driver, then with a three-stage Darlington parallel triple push-pull using Sansui-tagged NMC-1012 / NMA-1012 bipolars (made by NEC ? Sanken ? Toshiba ?).
Capacitors are as original as those Sansui-tagged output transistors : they are oval and were said to "help improve transients in the supply system [...] by keeping internal impedance very low over the widest possible frequency range".
Each oval cap amounts to 15,000uF, for a total of 60,000uF.
The phono stage allows two 'tables to be hooked, either MM/MC or MM only.
The EQ section is naturally all-DC with 16 low-noise transistors per channel ; input is dual-Fets differential with cascode-connected bootstrapping and dual current source. The second stage uses the DD/DC circuit while the third is Darlington-connected with a low output impedance of 200 Ohm. Current overload is at a high 350mV (MM) and RIAA accuracy at ± 0,2dB.
The flat amp section is also, yes, all-DC, has its input as dual-Fet, coscode-connected with a current source for stability ; the second stage is a current-mirror, current-differential p-p driver (patent pending...) and the final stage an inverted Darligton p-p. This makes for "excllent open-loop characteristics and low distortion".
Since all this is DC and DC only, protection runs through two systems : DC voltage detection which opens a relay to separate speakers from the output terminals and overload current detection which protects the output transistors by turning off the speakers' output.
The AU-D907 was an AU-919 outside Japan and if everything looks exactly equivalent inside and outside, it seems the power board wasn't ; there may be differences there as well between the USA and EU models.
If the AU-919 is the only one of the series to have been exported in significant numbers, it sold in Japan less than some of its later versions (close or distant) such as the beautiful AU-D907 Limited (1979, LAPT transistors, Nichicon oval caps, Sanken transistors, copper plating all-over) or AU-D907F Extra (1981, eight non-oval caps plus a large... Heat-Pipe !).
Market-wise, it was the end of the growth era for Sansui.
An AU-D907 at the indispensable amp8.com.