I recently upgraded a pair of NAD 2155 power amps. These are stereo 50w per channel units, and as such are an excellent small power amp for efficient speakers. However when switched to mono, they produce 100w per channel, and this is where they represent extraordinary performance.
The circuit differs from their bigger brothers, the 2200/2600/2700 series in that the input circuit uses discrete components rather than the IC used by 22xx series. When compared to a 2700, which had also been upgraded there was a perceptible increase in the detail at the same volume level. The 2700 delivers 150w per channel in stereo and over 400 in mono, but with efficient speakers, the 2155 performs as well if not a little better.
The upgraded 2155 internals
As with most NAD upgrades the power supply was improved, with new main reservoir capacitors and faster rectifier diodes. There was considerable improvement in the low end distortion through this. All the electrolytic caps were replaced with low impedance, audio or bipolar caps, and some values were increased where appropriate. All of the small ceramics were replaced, generally with WIMA FKP, polypropylene, capacitors as were the bias trimmers. A new relay was also fitted, which was attached to the internal frame.
The 2155’s in the owners home set up, the 2700 and 1130 have also been upgraded.-as the customer stated” I am very impressed, even considering the 2155 is about half the rated power as the 2700, I think it sounds as good if not better. Detail and clarity is very good.thanks for a great job.”