The first option usually means very cheap, generic parts from China with no guarantee that they will have the same quality as original specification parts. or from bona fide distributors like Farnell, RS components, TLE, Digi-key, Mouser etc? The question is what is your preferred buying option? Is it from platform sellers at Ebay, Ali Express, Amazon etc. Perhaps I can help with a question first and then some possibilities for you to consider. rensli advised that he had to leave the forum. the semis have had the VAS linearity thing wrong all these years? Given the task to design a voltage/transimpedance amplifier and its current sink or source for low audio distortion, what transistor characteristics would you select for the roles? Would it be the conventional low capacitance types or a fat capacitance type? Alternatively, would it not matter at all and the guys who design the amps and spec. Other types used less often by Naim were ZTX 452/552 which had only half that capacitance and appear to be in a similar class to BC639/640. Since then, I've checked this a few more times but not as thoroughly as there's been no need. It may not prove the source of the effect but it's reasonable to conclude that there's a connection between the effects and the types of semi. I think that's a uniquely hign Cob for the application.
You can also try other transistor pairs better suited to VAS and I think you'll find all the obvious sound effects more or less track with the capacitance (it's a staggering 30pF in the case of ZTX 652/3, 752,3. There were also splashy, low level sounds from the striking of tiny bells, cymbals, even breaking glass for example. Before tinkering though, I actually heard more effects than just the PRaT there.
The amplifier had similar to original voltage gain at 20kHz and a sinewave under load still looked very clean. Really? In the course of trying a few modifications requested by the owner of an old NAP110, I replaced the ZTX types with, as I recall, 2SC2911/2SA2902 (just a few pF Cob). Unless you are already adept at electronic design and have wide experience with top quality audio, it will always help to have agreement or not and comments to consider with your own findings. Meanwhile, lets get others, friends maybe, to listen too and see if they agree with your findings. I'm sure that it points to a bigger problem that will probably affect all of us at some time. Circuits of mine and designs that I thought sounded great ten years ago have changed or rather, I have changed and I don't like them now. That often leads nowhere but around in circles. When experimenting, one has to think and ask why something that concerns you is so before just trying new and different ideas to change it. Nothing in audio electronics is ever that simple but I think it's the most significant one. However, I don't believe that is the only mechanism in play. The kits all came with "better" VAS transistors, like 2N5551/5401, D667/B647, A1145/C2705 but alas, none of them sounded great and had very little PRaT. Every wonder why?Īs a comment, I've listened to many cheap clone kits assembled by kids, audio enthusiasts and guys like me who have the time and interest to DIY their audio.
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The proof that Naim have long known about this particular "error", is that these types of transistors were still used exclusively in original NAP and Nait series models for as long as they were in production. In my view, this was a very fortunate mistake by JV because the products proved to sound very nice to many people who auditioned them at the time. If a designer is not clear about the principles involved in transistor amplifiers, they are likely to make mistakes by choosing semis from only their their basic specifications. For example, have you never wondered what Naim's "PRaT" is all about? More to the point what distortions do you seek in your own experiments and how far from Naim sound are you already? As you have no reference, you can't know that and neither can anyone else. Wrong! It happens that many people like to hear certain distortions and will pay more money to hear them in their "audiophile" types of amplifier. If an engineer were to design the generic Naim amplifier circuit back in 1970 or even now, they would most likely use another type of transistor with low capacitance to gain a lower distortion figure. It's been clear for many years that the ZTX series transistor types always specified in NAP and Nait series models, have unusually high Cob (output capacitance) for a "transimpedance" or voltage" amplifier and its current sink.
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