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gilwe 7th November 2012 10:41 AM

Replacement transistors, NAIM NAP90
Hi all,

I have a great sounding NAP90 power amplifier. However, one channel has been shorted and a few transistors are burnt and need replacement. The one on the NAP90 are kind of obsolete, so I thought I might upgrade them in this occasion and make it better...

The ones which has burnt out are:
J13009-2 (power transistor which has been used instead of the stock ones)

Also I saw that one ZTX348C has it case broken.

Can anyone point me to any replacements that will fit, and sound at lease as good ??

Thanks a lot !

jaycee 7th November 2012 12:17 PM

The ZTX652/752 are obsolete but ZTX653/753 are not - these just have higher Vceo, otherwise identical specs. Zetex became part of Diodes Inc.

The J13009-2 is probably FJP13009 (H2 grade) from Fairchild. Still in production.

The ZTX384C is long obsolete. Based on specs, the BC550C would be appropriate. From a quick look at various Naim schematics it looks like these would have been part of the input LTP - if so, both should be replaced.

You'll find all those parts on Mouser and probably Digikey too.

Ian Finch 7th November 2012 12:52 PM

For that list of parts to have failed, there must have been some awful accident(s). NAIM sound is a parts issue and swapping types like the input stage and VAS transistors is not a good idea if you want to keep the performance close to original.
There are always opinions about but the term "improvements" as in "make it better" can just as easily mean ruining what audiphiles prize as "sound quality". Fix it first with the right or closest available parts. If you want to try other parts options that's fine but they won't make anything better other than change the distortion to something different - perhaps OK, perhaps not but not likely better overall.

The ZTX 753/653 are the commonly preferred range as they have a 20V margin over the ZTX752/652 range. They are easily obtained at mouser etc.

ZTX 348? I don't think so. Probably you mean ZTX384C, similar to BC384C which equates to BC550C now.

J13009-2 appears to be a house code. I'm sure NAIM addicts know the basic part spec. but they won't know the particular selection/matching required for these output transistors.
BTW, the driver transistors are likely suspect too, check them out.

gilwe 7th November 2012 03:15 PM

Thanks all,

The NAP has been shorted out, probably by having a short between the + and - speaker outs.... One channel has been severely damaged, having a few resistors totally burnt out, as well as one ZTX652, two ZTX752 and one J13009-2, all of the same channel.

The ZTX384C (not 348, my mistake) is just physically broken so it needs replacement anyway.

From what I understand from the previous owner, he replaced the stock power transistors with four J13009-2, so they are not original. The tantalum cap has been also replaced, and I replaced the 4 electrolytic caps, with cheap brand though.

I would very much like to get the NAIM character back to it actually. I assume having the new tantalums there is a good idea (they short over time). If you say the BC550C can replace the ZTX384C then I will probably need to change it on both channels ? I have a few in hand.

ZTX753 and ZTX653 are readily available, but should I swap the same positions in the "heathy" channel as well ?

As for the J13009-2, I managed to find MJE13009 as replacement for the burnt J13009-2, should it be ok ? I have no idea what it used there before the swap, but looking here I guess they were NAIM NA005 transistors?

gilwe 7th November 2012 04:19 PM

Is NA005 a BD743 actually ?

Ian Finch 7th November 2012 05:55 PM

MJE 13009 and FJP13009 are likely very similar and they are 400V TO220 types with a high current rating too.

The part equivalent BD743 on Pitbull's schematic is also TO220 and probably closer to the house code NA005. You won't find that spec. as it is NAIM's IP. The issue is the Hfe and matching for consistency which you don't see on the schematic but I would prefer that type over the ones fitted in your amplifier. If the Hfe is not sufficient, the drivers will get too much of a work out. I would be hoping for Hfe >25 @ 2 amp collector current.
Note the required version is probably BD743C, if available, which would be a 100V part.

The link below is to a forum for total NAIM freaks and modders. The discussion seems to confirm NAIM's use of the part since they nominated the replacement. Some of the other equivalents are not so smart but it's not a technical forum and many views are quite ill-informed.

Even this is informative

gilwe 11th November 2012 01:01 PM

Ian, does the set on the ebay auction you linked to should be a good choice ?

Also, I wonder what is the better substitute to the BD743 -
BD711 or BD907 ?

Thanks again !!

djk 12th November 2012 06:39 PM

An interesting current limiter, sensing the driver current.

Ian Finch 12th November 2012 07:43 PM


Originally Posted by gilwe (
?... I wonder what is the better substitute to the BD743
BD711 or BD907 ?....

I've actually looked at a few datasheets for interest over the weekend and I note ST Micro's BD911 (Mouser) is about right. Cheap and plentiful too! BD911 | Mouser Electronics, Inc.. I think this is a best and latest version and I like ST products - they exceed their specs in the right areas consistently. ~$1.32 for a 100V/15A part.

I guess we all are sus. about Ebay parts from unknown vendors. Those appear old enough to be genuine because shiny new parts that are over 10 years old is always suspicious to me. I can't really say but at the price, why would anyone bother to fake a small power transistor? With due respect, it's not like there are many NAP 90 or Nait 2's in need of what are pretty ordinary power transistors.
I think you'll be happy with BD911 so get some spares from the same batch in case you like to balance things up too.

Ian Finch 12th November 2012 07:55 PM


Originally Posted by djk (
An interesting current limiter, sensing the driver current.

Yes, a departure for Naim but when you look at the stingy E-line driver, I guess that's what needs the protection most :o
No, wait....EF driver dissipation is near constant so what can it do with only the CFP half in play - short of disaster management?

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