The NAD 2400 was a THX certified 100w per channel amp that came out around 1989. It shares some commonality with the NAD 2200/2600/2700 from a circuit perspective, though it, along with the smaller 2100 was built to a price. That said the 2400 can be upgraded to create an excellent amplifier. Like most NAD power amps of that generation there were a number of cost driven compromises made so that the amps could be sold at the appropriate price point.
Generally the 2400 uses a “stripped down” version of the 2200 circuit, with a smaller mains transformer coupled to the same power supply capacitor arrangement (2x 10,000uf 100v and 2 10,000uf 80v). The 2400 uses the same dual rail class G circuit topology as the 2200/2600/2700. Another aspect is the single finned heat sink, whereas the 2200/2600/2700 all use two.
Another major difference is in the output transistors, the 2200 et al use 2SA1302/2SC2381from Toshiba, whereas the 2400 uses 2SC3907/2SC1516. The 2400 also uses different transistors for the high voltage rail, whereas on all the 2200/2600/2700 (but the very early 2200’s) use the same output transistors. Whether these choices were from cost or supply basis I do not know.
The 2400 has 5.7db of headroom, that is the same as the 2200/2600/2700, enabling an effective quadrupling of the output power for short periods, (i.e.musical transients) due to the class G topology.
The basis for the upgrades is outlined below. I have done these to a number of 2400’s all with very pleasing outcomes, including an order of magnitude reduction in THD. Like the 2600/2700 the 2400 has an impedance switch on the rear which uses differing transformer taps for 4 and 8 ohm speakers.
Power supply-Replace 10k 100v and 10k 80v with new electro caps, the size and availability will determine which to choose. The 80v 10k units can be increased to 12k. The low voltage (+/-18v) rail can have all the capacitors around the regulators increased, that is C810/C811 and C812/C813 and C814/C815, I have doubled the value of each, for example C811/C812 from 1000uf to 2200uf all at 50v.
Input section-The input board uses a NJM 2043DD IC and this was replaced with a socket and LM4562. The input caps, both film and electrolytic should be replaced, C302/C302/C303 with polypropylene film and C304 with a 47uf Nichicon MUSE UES or equivalent Bipolar. All the capacitor designations are from the circuit which shows one channel,duplicate for the other channel. Also replace R306 with 1% low noise resistor (Vishay CMF or similar).
The input pots on the daughter board can be removed from the circuit if you don’t need them.
Also note the LAB in has a different path to the input transistor pair of the main amp.
Main amp-Generally the power side of the circuit can be beefed up. C404/405/C416/C704/ can all be increased to 100uf from their original 10uf, with their respective voltages (50v or 100v). Similarly C706/C708 replace with 100uf, C412/C416 with 100uf and C501/C502 with 22uf, all using low impedance- (Nichicon UPW/UHE or similar). C403 can be a 330uf low impedance. All electo caps should be the same or higher voltage rating.
There is only one cap in the audio path that is electrolytic so replace with a 47uf Nichicon MUSE UES Bipolar (C401). For C409 use 47uf 25v and C702 use 100uf both Nichicon UFG Audio cap.
D703/D708 are the switching diodes and these can be replaced with MBL100100 or other 10A Schkotty diodes
Protection– replace all electro caps C818,C817 with low impedance types, replace C819 with the same value film cap.
The speaker relays can be replaced if they are troublesome.
These replacements will deliver the major benefits, however nearly all the ceramics can be replaced with WIMA FKP/MKP polypropylene, though the values remain the same.
The result is a significant reduction in THD+N, with increased dynamics, detail and transparency.As with other NAD power amps the 2400 can be bridged in mono capable of delivering 300w with 4db of headroom.