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NAP250 clone
NAP250 clone
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Old 4th January 2019, 01:46 PM   #1
JeffYoung is online now JeffYoung  Ireland
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NAP250 clone
Default NAP250 clone

I'm looking at building a NAP250 clone. Since this will mostly be a nostalgia project, I'm trying to be fairly faithful to the original.

However, I won't have the volumes Naim did (for parts matching), and some parts production has since been moved to China (so I'm probably looking at greater variability to start with). The knife-edge stability of the original therefore has me a bit worried.

Here's something pretty dang faithful, but with the standard Miller compensation replaced by transitional Miller compensation (which seems appropriate as it was first proposed by Baxandall, who also has his diode in the quasi-complementary circuit):

NAP250 clone-nostalgianap-jpg

It serves its primary purpose of improving the stability without sacrificing the slew rate, although I couldn't get the stability up much without introducing a lot of distortion. So the phase margin is still, well, marginal. On the other hand, the slew rate has doubled, and the frequency response has improved considerably.

Any thoughts on this, including what it might do to the sound signature?

Thanks,
Jeff.
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File Type: jpg nostalgiaNAP.jpg (116.6 KB, 2038 views)
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Old 4th January 2019, 03:20 PM   #2
lineup is offline lineup  Sweden
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Nice amplifier.
It is very conventional op amp style as a whole.
But what is different is in all the small details.
And there is where the performance comes.
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Old 4th January 2019, 03:32 PM   #3
Ian Finch is offline Ian Finch  Australia
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It's good to take an interest in ideas from Naim and other popular manufacturers of the seventies etc but I'm sure you're aware that it's not a straightforward, textbook design. Perhaps changing the compensation scheme this much would be too radical a change to what is already a heavily tweaked design - and also has a few weaknesses that are actually essential to its distinctive sound.

No doubt, a certain amount of instability arose from those modifications that might otherwise have been eliminated by "proper" design. You might also consider what TMC would do to Naim's Pace-Rhythm-Timing effect. That would certainly be interesting to investigate

Current versions such as the 250DR model, don't have quite the degree of sound character of the earlier models so maybe that's where you should look now for a reference point to Naim's sound quality.
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Old 4th January 2019, 08:12 PM   #4
JeffYoung is online now JeffYoung  Ireland
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NAP250 clone
Quote:
Originally Posted by Ian Finch View Post
.... You might also consider what TMC would do to Naim's Pace-Rhythm-Timing effect. That would certainly be interesting to investigate
That's an interesting idea. The designs are similar enough that I could do a single PCB design on which either could be built....
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Old 4th January 2019, 08:45 PM   #5
JeffYoung is online now JeffYoung  Ireland
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Hmmm... I did a bit more digging and my "original" I had cobbled together from the McBride, HackerNAP and NC200 schematics appears to have been a bit off.

This one:

NAP250 clone-nap250-schematic-jpg

shows slightly lower values for the Miller integrator and the phase lead cap.

Plugged into SPICE it doesn't do all that bad:

Code:
                original     TMC version
Phase margin      24           40
Gain margin       11dB          13dB
Slew rate         9V/uS         16V/uS
THD @1watt        0.011%        0.015%
Only one number jumps out: how scared should I be of a 24 phase margin?

Thanks,
Jeff.
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File Type: jpg NAP250 schematic.jpg (92.0 KB, 1925 views)
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Old 4th January 2019, 11:10 PM   #6
JeffYoung is online now JeffYoung  Ireland
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Someone on pinkfishmedia pointed out that it's going to take a lot of finesse in the board layout to keep a 5p capacitor at 5p. Definitely gives one pause for thought.

Here's a 3rd version with 39p in the TMC. Even better stability, but you can see that the THD suffers:

Code:
                original     TMC/3pF    TMC/39pF
Phase margin      24         40        50
Gain margin       11dB        13dB       15dB
Slew rate         9V/uS       16V/uS     12V/uS
THD @1watt        0.011%      0.015%     0.028%
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Old 5th January 2019, 04:02 AM   #7
Ian Finch is offline Ian Finch  Australia
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Don't be commited to reducing distortion with Naim designs. The main concern for users is the actual sound quality you hear. I realise you are trying to achieve some technical improvements through simulation but the significant levels of low order harmonics that all NAP models have to some degree is the desirable sound quality. Lose or mishandle that and it reverts to a mediocre sounding quasi-comp, probably as found in RCA's cookbook designs of the period.

Unless you are just interested in analysis, could I suggest you get yourself a few cheap but good enough 140/250 clone kits (some out there have omissions or may suit pinouts for Japanese transistors) and assemble one as original alongside a modified one and listen, comparing over a a week or so to lessen expectancy and other biases we can't avoid.

In direct comparisons, you can learn a few more things about audio and the importance of distortion to sound and spatial perception that you'll never find in simulation or texts of electronic theory. It isn't formal psycho-acoustics but it occupies many us working on perfecting our clone builds. Naim sound may have stood for what was possible and made audio headlines almost 50 years ago but it's still well worth pursuing, if just for the educational aspects of turning a handful of junk parts into a respectable amp with a very entertaining sound quality. Keep at it
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Old 5th January 2019, 08:46 AM   #8
huggygood is offline huggygood  France
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Old 5th January 2019, 09:50 AM   #9
huggygood is offline huggygood  France
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in fact Jeff, there has been a great job done here(on "nap140 clone thread") for years on this schematic.
I compare the NAP to jlh, we love it for what it is and every time we want to improve it, we lose our identity in favor of something different.
as the jlh is quite flexible in therm components, as the nap is very sensitive and even prisoner of some transitors.
there are only the output transistors that are "not too much"
critical.
Rensli has begun an interesting work on resistances and their quality, I will follow his story with interest.
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Old 5th January 2019, 10:46 AM   #10
JeffYoung is online now JeffYoung  Ireland
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@huggygood, indeed it was partly that thread that got me started on the "minimalist intervention" idea. (By minimalist I mean just enough to increase my odds of keeping the smoke in, rather than trying to "improve" the sound.)

Which also talks to @Ian's suggestion of building one of the clone kits: they're sort of in the opposite direction: most of them try to improve the sound (eg. LTP degeneration) while by and large not much improving the stability (aside from adding inductance to the output).

I'm probably answering my own question, though: just go build the original spec and see what happens. If nothing else, smoke can be a good learning tool.
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