Denon POA-S1

X-rare power amplifiers launched when Japan's economy was steadily going down - and even lower.
Vanishingly low production run for those 79kg (each !) beasts, each outputting 1,4kW output at 1 Ohm. Received a COTY Award in 1994.

Inside is (was) some of the old and some of the new : old is the circuit structure and simplified power stage, new is (was) the UHC-MOS MOS-Fet power devices which were sourced from Hitachi, alongwith the related pre-driver circuit design and, apparently, an evolution of a Hitachi-patented circuit for the output section.

All about the POA-S1 and the PRA-S1 can be read here - in french. Do use the Google translation tools for the ultimate meat on the circuits of these ultimate Denon monsters.

But in a nutshell -
Old : cascode bootstrap in the power stage, as first revived from the tube era by Sony in the 1977 TA-N7B masterpiece (masterpiece as in "all-time Hall of Fame"), and revived again by Nelson Pass and Marshall Leach later on to much audiophile acclaim.

Old : as seen in many 1960s and 1970s bigga powerhouses, one pair of power output devices, no more - if a good (dedicated) circuit is preceding them, sixty pairs of hot bingos aren't necessarily... necessary.

New : UHC-MOS (Ultra-High Current) output transistors which can take a lot of current but nevertheless dissipate little heat.
Each can output peaks of 160A (40A continuous) but heat up at only 100W (2SK1297) and 160W (2SJ217) respectively.

The cascode bootstrap scheme pairs ten Sanken bipolar transistors with each of the two UHC-MOS to even out the initial load and keep the UHC-MOS therefore working in their maximum current region only.

According to Denon, one UHC-MOS (in such a cascode bootstrap circuit) is the equivalent of 35 regular MOS-FETs or 3 bipolars - without the inherent problems of powering and using multiple output devices.
The two transformers are hooked in parallel to feed the output stage with lower impedance and doubled instantaneous current capability. A third transformer is delegated to the first (input) stages.
The two main caps are custom-ordered 30,000uF / 63V low-ESR ELNA for Audio (LPO series), both paralleled by a huge 2000uF / 50V ELNA of the same kind and also custom-ordered.
The protection circuits remain outside the signal path.


There are cascode circuits all over inside the POA-S1 (and even more in the PRA-S1 preamplifier) and it is symmetrical from top to bottom. Passive and active componentry can be summed up as "no limit" : ELNA Silmic Alpha, Super Gold, LPO (huge size / high current / low value) and Duorex II, Nippon Chemicon, Annaka and Riken RMG resistors. And solid copper rails, solid copper feet, diecast aluminium and steel frame.
 

Better yet : again, all about the POA-S1 and the PRA-S1 can be read here - in french. Do use the Google translation tools for the ultimate meat on the circuits of these ultimate Denon monsters.
This review is complete with measured specs, tests, all circuit descriptions and diagrams aplenty - read it !


Also present online is Denon's own JP Museum page, alas with tiny tiny images.