Help with Naim NAP250

Hi there, someone I know suggested I ask here:

I'm basically doing a very simple Mod to an old NAP250 which is to power the left and right hand sides of the amp seperately. I have Naim coils, 8 x BHC Aerovoxes and have wired everything up back to the DC side of the bridge rectifiers *except* the transformer!

The Naim ones have several secondaries, and differing colourings so I'm getting really confused as to wht gets wired to what. I'm not using a Thermal Trip Switch as the amp case I use has two enormous heatsinks, but thats the only thing I have left out... can anyone help me?

I've worked out that Pink is system ground and connects to the IEC Earth pin... That two of the wires are joined for 240V use... but err... thats about it.

Where I am getting stuck is the transformers have several secondaries and both have differing coloured windings, which is confusing the hell out of

The transformers are Naim ones and of a different vintage, but it still looks like a rats nest in there and I want to do it right...

I hope you may be able to help as you seem very knowledgable...

Right Transformer:


Red/Blue striped


Orange (rectifier AC?)
Red (rectifier AC?)

Pink (System Gnd)
Green (- on Cap?)

White (+ on Cap)

Green+Yellow striped
Blue+Red striped/Brown (break for 110V?)

Left Transformer
Green (it appears black but is mostly heatshrink!)
Pink (System Gnd)
Red/Blue (striped)
Red+Brown (striped)/Blue (break for 110V?)

[Edited by colin betts on 11-21-2001 at 05:19 AM]
Dear Colin,

I do not really understand how you can build a 135clone with two different transformers !?

There is a thread in this forum also about this subject:
Naim Clones:

Be sure to have understood the power supply technique of Naim amps before you go on building... There are no bridges, etc.

In my Nap250 (from the late 80th) the color code is like that:

Light green___All grounds, star ground point at the speaker outs

red______+ from + half bridge
black______- from - half bridge

and so on...

Secondaries from the Transformer:

There are two pairs of power secondaries and two half bridges and two psu caps.

dark green______ground (connectecd to the - of the +cap )
white______ground (connectecd to the + of the -cap )
orange______to negative half bridge
red______to negative half bridge
yellow______pink (thin) not used
pink (thick)______electrostatic screen (I suppose)
blue/red striped______primary in (check out the voltage setting)
brown______primary in (check out the voltage setting)
blue______primary in (check out the voltage setting)

The easiest thing is to measure the transformer with an ohmmeter and - powered - with a volt meter !

Hope that helps !
Its not building a 135 clone but independently powering the left and right hand sides of a NAP250 (dual mono - 2 reg boards - 2 Amp boards) rather than sharing left and right between 2 x caps and one transformer.

Thanks for the the details however. Events have since overtaken themselves and I've got it all sorted out now thanks.
Dear Colin,

sorry that I overlooked your first post and gave an answer too late.

You have to consider that doubling the psu in a 250 is exactly following the 135 topology, so this is what your trying to make.

The most important thing is to follow strictly the psu techniques and the star ground layout. Here are a few details more, which I forgot to explain. Pretty obvious, but still.

Transformer secondaries and primaries have different outlet positions.

Secondaries color code (add.):

The center taps should not be mixed:

dark green (middle tap) / grey / yellow: positive half bridge and plus cap

white (middle tap) / red / orange: negative half bridge and minus cap

Additional to ground connections of the amp (all light green) to the star eath point between the minus outpot poles:

input (from amp board input only)
zobel (from amp board)
ground (from regulator board output)
from psu cap (neg from plus cap)
from psu cap (pos from minus cap)

There are eight different ground connections in a NAP250. In a 135 there will be five. In your 250/135 crossbreed then there will be ten.

best whishes