The NAD 370 is a 120w per channel integrated amp from around 2001.
The design is very similar to the NAD C270/C272 power amps coupled with a C160 preamp.
NAD developed a Line-in (MD1B)/Line-out module(MD1C) which was effectively an” IC” type device using Class A topology employing discrete components on a small PCB mounted inside a metal case, such that the regulators for the power supply for the module were mounted on the case. This involved the use of very small components, and from the perspective of replacing/upgrading those components, severely limits the range of components that can be used.
The C370 uses 4 of these modules, two for the signal input stage (MD1B) and two for the tone/volume and balance stage (MD1C)
The upgrades started with the power supply, where the original 10k uf 80v caps were replaced with 15k 80v. All the electroltyic caps on the power supply, which supplies a range of voltages, +/- 68v for the main outputs, +/- 64v for the power amp, +/- 34v for the modules as well as 12v, 5v and 64v, were replaced and in many cases uprated.
The preamp section had all electrolytic caps replaced, those in the audio path were replaced by Nichicon BiPolar MUSE caps or polypropylene depending on value. A number of ceramic caps were replaced by WIMA FKP/MKP polypropylene.
The power amp boards are separate from the main PCB, and the case effectively needs to be disassembled to get at the main board. The power amp boards have two metal cages around the input circuitry providing a heat sink function as well. All the electrolytic caps were replaced, with either Bipolar MUSE for other audio specific caps for audio, low impedance. Suitable value ceramics were replaced by WIMA FKP/MKP or Panasonic polypropylene, or C0G ceramics. A number of the electrolytic caps were uprated.
The amp uses a pair of TL082 IC’s as a comparator and as with the protection circuitry all electrolytic caps were replaced.
The results were especially pleasing, the amp measure incredibly well with THD an order of magnitude less than the published specifications and THD+N equally less. The uprating of the various capacitors increased the dynamics giving the amp an effortless quality.
The NAD series of C270/C272 power amplifiers, C370/C372 integrated and the 160/162 preamps are excellent candidates for refurbishing and upgrading. I have purchased another C370 to see if I can optimize this design even more.
