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This item has been sold but we have left this content in place for anyone who may have an interest in the device

Background:
I acquired a Sanyo Plus N55 signal processor, which I intended to resell. After receiving a very low offer for it, I did some research and discovered it is a very sophisticated device. I have always liked the DBX approach to signal processing and the Super D method is quite similar.
Here is some discussion about the device I found at http://www.stevehoffman.tv/forums/archive/index.php/t-107963-p-2.html

From the text from the Instruction Manual:
"Super D - Theory of Operation
Cassette tape decks are popularly used today among audiophiles for sound recording and reproduction. Despite their many excellent performance characteristics, there are some problems which have remained unsolved. Of these, the most notable one derives from the slow tape speed and the narrow sound tracks, which make it difficult to tape-record and play back record music. FM broadcasts, live performance and sound existing in the natural world with a superb SN ratio and without sound distortion over a wide dynamic range by cutting noise at the small sound level and by eliminating distortion due to saturation at the large sound end.
By the combined application of the level compression-expansion techniques and the newly developed complementary band divider, the SUPER D system of SANYO compresses (encoded) sound signals in the recording process and restores them to their original condition by expanding (decoding) them in the playback process. This is, accordingly, a kind of compander system...
The SANYO SUPER D (model PLUS N55) unit, when adopted, expands the dynamic range of a cassette tape deck approximately 100 times (by about +40dB) wider than when it is not in use. At the same time, noise can be reduced to about 1/100 (by about -40dB). Thus, clear high-fidelity sound can be reproduced over a very wide dynamic range. Meanwhile, the breathing phenomena (i.e. noise fluctuations due to changes in signal level) in the actual application of the SUPER D system has been reduced to an imperceptible level in incorporating into it a complementary band divider.
Effects obtained by expending dynamic range
The dynamic range of a tape deck is determined by its noise and saturation levels. The SUPER D unit, when adopted, raises the saturation level and lowers the noise level over the entire band width. Hence, the dynamic range of the typical cassette tape recorder is expanded to about 100dB, when the SUPER D unit is adopted, from about 60dB, which is normal when it is not in use. This means an improvement of approximately 40dB (=100 times) in performance, giving the cassette tape deck a dynamic range even wider than those of expensive reel-to-reel tape decks.
Noise reduction
The SUPER D unit achieves an improvement of 35 ~ 40dB in SN ratio, which is regarded as epoch-making. Consequently, hissing noise is eliminated to an almost imperceptible level.
Reduction of breathing phenomenon
The complementary band divider incorporated in the SUPER D unit makes it possible to effect compression and expansion of signals under optimum conditions within respective band widths. Thus, the breathing phenomenon itself is reduced and noise within respective band widths is lessened to an inaudible level as it is covered by signal sound."

I ordered a scan of the service manual. Pages 2-4 are presented HERE for more background behind the unit's design.

I did some testing with the unit to provide a better description of its condition.
I had a vintage reel-to-reel deck handy so the plan was to digitally record a portion of a track from a good vinyl recording in 3 modes: direct from the N55 with no encoding; the second test was to digitally record the output from a tape recording made through the N55 without encoding; last, I digitally recorded the output from a tape recording made/played back through the N55 with encoding/decoding.
Tape Deck: Sony TC-558
Tape: well-used standard quality
Turntable: Kenwood KD-500 direct drive with a good Audio Technica white cartridge
Computer: Dell E520 with an integrated sound system on the motherboard
Software: Goldwave audio recording/editing version 5.20. I used this only to make samples of the same section of music- no filtering was used
Recording: 'Rossini Overtures' by Orpheus Chamber Orchestra. Deustche Grammophon 415 363-1. A digital recording with tremendous dynamic range
Although this is not a glamorous audiophile system, the components match my ear, which I do not claim to be of high audiophile quality, but it's not 'tin' either.
tests:
Connections: Receiver Record Out to Encode In
     Computer Line In from Decode Out
     Tape Deck Line In from Line In
     Tape Deck Line Out to Line Out
Test 1
 Played a vinyl disc with Plus N55 switch settings:
  MPX Filter Off
  Super D Off
  Monitor Source
  Record Level Max
 Recorded Decode Output directly
 
Test 2
 Played a vinyl disc with Plus N55 switch settings:
  MPX Filter Off
  Super D Off
  Monitor Tape
  Record Level Max
 Recorded on tape
 Played tape back
 Recorded Decode Output directly

Test 3
 Played a vinyl disc with Plus N55 switch settings:
  MPX Filter Off
  Super D On
  Monitor Tape
  Record Level Max
 Recorded on tape
 Played tape back
 Recorded Decode Output directly
 
Results:
To my ear, all recordings sound good but the sample made through the Plus N55 has a more open, pleasant presentation than the others.
MP3 samples of the three tests are below.

Raw from Monitor
Raw from Tape
Super D Encoded/Decoded