The NAD 1300 was originally produced in the late 1980’s. The design is based on a set of IC’s creating blocks of functionality with a discrete phono stage for MM and MC cartridges using discrete FET’s. In some respects the design is reminiscent of the APT preamp approach, however the original IC’s are NJM2043DD which a BJT rather than FET’s.
The power supply, as with so many designs of this period, although adequate from the perspective of the overall current draw of the preamp when operating, which is around 100ma, certainly benefits from a new higher rated bridge rectifier and larger main power supply capacitors. The rest of the power supply is then upgraded with low impedance electrolytic caps and new C0G ceramic caps. The original IC’s had +/-20v supply which was reduced to +/-18v for the new replacement IC’s.
All the ICs are replaced with modern audio types, LM4562, which are all socketed. All the ceramic caps that are used in the feedback loop of the IC’s were replaced with WIMA polypropylene film types. Additional decoupling capacitors were added to the circuit at a couple of places. A number of Mylar (polyester) caps were replaced with polypropylene film caps. The main relay was also replaced.
The result was a significant reduction in THD+N and an associated increase in detail, transparency and dynamics. When compared to my reference APT, the NAD had a slightly “warmer” overall tone, but the detail and dynamics were very similar.