I have bought an old NAP 250 (Olive series) from '91 to pair with my Naim Nova. This is a good improvement over the bare Uniti Nova increasing PRaT and cohesivety.
I am actually running a pair of Dynaudio Contour 2.8 with double OCOS cable. As I have already read, older Naim amp are sensitive to cable and can oscillate with inappropriate cable.
There was no problem with the Nova and this cable. Actually the NAP is a little warm (powered for 12hours), about 40°C on the top of the case, is it ok ? Can I use the Ocos wire with the old NAP 250 with no problem ?
I wouldn’t presume to guess whether a 27 year old amp is working optimally or even safely. Unless it has been serviced recently I’d get it checked/serviced.
In terms of the cable, you need it to provide the right amount of inductance for stability of the amp, so you’ll need to know the specs of the cable you’re using. Richard describes the requirements here Speaker Cable for Naim power amplifiers .
Why shouldn’t it ? The question about the condition, safety and need for a service of an Olive amp is the same as for a younger amp. (a 27 year old amp is a baby… ). Any second hand amp should be checked/ serviced.
As for cable, the OP, if not sure, should consider Naim’s NACA5 cable or an alternative from Chord (I use Odyssey2, but don’t know if they still produce it). Both work extremely well with Olive.
I can’t find any spec’s for OCOS cable & as its been out of production for some time, finding anything might prove impossible.
Looking at the www pictures it appears to be a coax, something we never see these days, probably one of those marketing fads that went out of fashion.
I also found a bunch of tweaks with OCOS cable that added a zobel circuit, that is the traditional way of suppressing the oscillation you mention, so that does not bode well. Maybe a good idea to start over with the right spec cable.
@Mike-B : I can’t find any specifications on this cable too. It seems to be a coaxial construction but the insulator is made of graphite, it’s not pure coaxial.
I also have the specific ocos adapter, with and with our label circuit. I was using it with Nova with the HFC Filter. The match with Dynaudio/Naim was clearly better than Naca5.
I will check/service the amplifier in few weeks. I have opened the case, some trace of hot points on resistance, soldering problem, etc as every 27 years old amp have ! The good new are the Holt&Fisher toroidal and the big red tank capacitor (LMT). Even if the amp is not in perfect working conditions, the sound is already fine (it may be have lost some dynamics).
A NAP250 is a regulated power amp, and as such the performance really goes off once components start to drift off spec. This is why an 8-10 year service interval on the 250 is pretty much mandatory.
The speaker cable is effectively seen as part of the amp circuit, and provides the necessary inductance for stability, so anything overly capacitive or of too low an inductance should definitely be avoided. If in doubt, NACA5 is ideal but it does require a few things in order to fully reach its potential; firstly, it needs to be properly soldered, preferably to Naim SA8 speaker connectors at the amp end. Quality of the soldering can make or break the performance here; secondly, it needs to be reasonably long - at least 3.5m per channel, more is better here, ideally around 5-10m per channel; thirdly, and perhaps most importantly, NACA5 takes ages and ages to come on song from new. I’m convinced that most who dismiss NACA5 have never heard a properly run-in pair of cables. Indeed, there have been times when even I have had my doubts while running in a new set. And then one day you’ll put on an LP and, wow! The system has not just great rhythm and timing, but also texture, width, depth and a tangible sense of “being there” - and you think, “yes, that’s what its all about!”. Quite why this is so, I have no idea, but it’s real and something that has been well documented in the past (I seem to recall a reviewer - was it Martin Colloms? - writing about this very phenomenon a few years back in one of the hifi rags).
I would say that the NAP250 should remain fairly cool at idle, and no hotter than moderately warm when running at normal levels under normal environmental temperatures. If the casework gets hot at moderate listening levels that could be cause for concern.
Either way, if it hasn’t been serviced within the last 10 years all bets are off. While unregulated power amps can go for many years without servicing - performance just gradually falls off - regulated amps are not so graceful when they age. Make it a priority.
I guess its down to the design. You effectively have two amps - one acts as a supply regulator for the other. It means that when things begin to fall out of spec then performance drops off quite quickly. Certainly that has been my experience. I’m no electronics tech though, so while I understand the general concept, if you want more detail than that you’d have to ask one of the Naim engineers.
The regulator depends on the voltage on the rectified ‘mains’ side of it being always higher than the ‘Amp’ side, so for Naim it is typically 50v (average with mains ripple) down to 40v clean DC volts for the Amp.
When the big rectifier Caps on the mains side age they work less well and the ‘ripple’ of voltage on the mains-side gets larger and eventually is not sufficient for the Regulator to work - you hear a 10Hz Buzz sound as it cuts on and off - needs a service!
Without the Regulator you don’t have that effect and the Amp rejects the ripple a lot more and the sound goes soft-focus as it ages.
The old 250 needs about 5m of NAC A5 to sound happy - and it needs to be placed in a position of good air-flow as the case is the heatsink. 40C is too hot IMO and heating can be caused by the Amp doing more than you realise - if it has the wrong speaker leads it can oscillate at VHF - you can’t hear it but the Amp is running hard and hot doing that, usually because it was designed not to have a short speaker lead and does not like that load.
). Any second hand amp should be checked/ serviced.