IIRC, The 250 and 135 share the same PSU (transformer size etc) but as you say, the 135 PSU is just powering one channel rather than being shared between the two in the 250. The fan cooling allowed the 135 to drive harder and for longer, into difficult loads (typically Linn speakers of that era, namely Sara and Isobarik).
If you have a spare pair of 250s then it’s worth a try to compare to a single 250, but if you want the best (and your speakers are a more demanding load) then 135s are the way to go (serviced of course).
Nap 135 (higher voltage) and give 75W continuous over 70W and a transient handling of 500VA vs 400VA.
The nap 135 also have better signal earthing.
But the Nap 250 can sound a little warmer than the Nap 135´s.
I would assume 135 or 350 mono are matched and aging in the same way. Buying two 250 stereo and use as mono must be challenging in getting left/right match perfectly. I would never do such at least.
If you study the insides of the 250 and 135 you’ll see identical amplifier and regulator modules, and the same transformer / rectifier/ smoothing capacitors. So you might wonder why 135s sound different- even at low volume levels where the sharing of one primary power supply would have little negative effect. Look closely and you’ll see the grounding systems of both are very different though. In the 135 the main ground point is between the two large capacitors, while in the 250 it’s between the ‘speaker negative terminals. This is why they sound so different IMO
I’m not sure that it’s safe to run just one channel each on two NAP250s. The amp “expects” to be driving one speaker per channel, so you may make it unstable if you run just a single channel over any length of time.
Perhaps someone knowledgable such as @NeilS can confirm.
As it happen I have two Nap 100 which I ran in such L + R configuration only.
No issues leaving other channel unused, and sounded great in my office setup.
Yes, no problem with only running one channel, but I would recommend using the dedicated L+R XLR cables - that way, the unused channel won’t be loading the input signal.
Good answer i think. The grounding is intresting as i did see sonewhere else others taking about that exact difference, with one being considered better than the other. Wonder why naim did not change to the " better" grounding arrangement ?
Anyway, i need to get off my ar$e and get the 250 into my main system and try that. And a better topic of discussion than " my naim internet is not working/is working/ is not working" !
The main reason for the difference was likely due to things being so crammed in the 250 that something had to be a bit compromised, and that ended up resulting in less optimal ground wiring layout. In the context of your original question, running 2x 250s in mono actually partially* reduces the grounding compromise so may actually work very nicely.
*Further discussion probably best left for elsewhere due to forum rules.
The 250 and 135 power supplies are the same except that 135 uses a single channel. Earthing a single channel is simpler on 135 and is done directly on the 0V ground reference.
On 250, the location of the grounds is more suitable between the negative HP terminals, in the immediate vicinity of the 0V ground reference of the capacitors. No real impact.
With the nap 300 and 500, there is no loss of energy despite the length of the cable linking the 0V reference capacitors on the 300 PS and 500 PS with the supply distances of the nap 300 and 500 circuits.
