When digital was the magic word... but not yet a pressing obligation.
The DAP-5500 is a preamplier + digital converter combination : three analogue and three digital inputs, two analogue and one digital tape loops, balanced and s-e analogue outputs plus a DAC out direct (analogue) out of the digital section which bypasses all the (analogue) preamp functions and switches.
The d/a conversion is done with four BurrBrown PCM56P-K 16bit converters in push-pull, two per channel ; filtering is done at 4x oversampling, low-pass filters are 7th order.
Opto-couplers from Hewlett-Packard link everything in the DAP-5500 : two between the digital filter's outputs and the d/a converters, two more between the filters' sync'ing (digital side) and the error correction circuits (analogue side).
Analogue inputs benefit from a quadruple push-pull of PNP/NPN FETs and so does the DAC out analogue output ; the preamplifier's attenuator is a 4-gang from (probably) ALPS.
Unlike many later pre+d/a components like the ultra-successful Sony TA-E1000ESD or TA-E2000ESD, analogue sources can come out of the DAP-5500 as analogue outputs, sans digital conversion and re-conversion to analogue.
No phono input was however deemed interesting or necessary : in 1987, that part of the future was pressing.
The chassis is... two chassis and four boxes : the digital and analogue sections are completely separated from each other, each with its own toroidal transformer, line filtering and power-supply regulation.
Each box holds two horizontal boards, separated by thick and compound-damped steel plates, the lower ones resting on a supplementary bottom plate which is a sandwich of steel-compound-steel. Six heavy alloy feet make it stand up - bullet-proof big box.
The typical "11" tag is in all local and export serial numbers so the 5500s were built at Denon and not in (one of-) the subsidiary outlets which were used by Denon and Columbia : #7031100471, #7091100418 or #8081101622, where "471", "418" and "1622" are the actual serial numbers.
Shortly after the DAP-5500 and Sony TA-E77ESD were introduced, the rage in recording studios would however soon be all sorts of valve emulators or the use of old Pultec compressors in hope to find that elusive "warmth" again.
Denon didn't make a separate d/a component before the DA-S1 in 1992 and as the "bit-trek" was going fast, having to switch preamps only to switch DACs was perhaps a little optimistic, market-wise.
Any low end surround receiver is nowadays adorned what that many digital plugs, but in '87, this was future-proof High-End.
Lots of nudies and detailed descriptions at kameson.com ; images of a NOS/NIB at audioscope.net.