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MPSA18 VS BC239C
MPSA18 VS BC239C
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Old 16th September 2021, 12:31 AM   #31
Ian Finch is offline Ian Finch  Australia
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MPSA18 VS BC239C
Yes, it does look easy. However the Nait2 amplifier is quite small, closed and was also pre-conditioned by applying a sine wave with a fixed load to stabilise its temperature prior to testing. That makes it look easy to get repeatable test results but it isn't what happens when you casually listen to music. I simply turn an amplifier on, select the program and play but it takes time for Naim amps to settle at a stable bias level, particularly at modest volume levels which are necessary in my home. The sound isn't quite "right" until it does settle, with some fanatics claiming at least half an hour is necessary or that you should leave the amplifier on 24/7 which is high-risk For DIY and other simple designs.

I think the recommended bias setting for NAP models is actually 6.5mV or 13mV across both resistors but as the author shows, it's not really critical if the requirement is just lowest measured THD.

Last edited by Ian Finch; 16th September 2021 at 12:38 AM.
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Old 18th September 2021, 09:41 PM   #32
Kelion is offline Kelion  Italy
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I notice that 6.5 is exactly half of 13 mV is there a reason for such values ​​or is it a range within which to adjust the value between the two 0.22 Ohm resistors?
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Old 18th September 2021, 09:47 PM   #33
mdardeniz is online now mdardeniz  Turkey
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You can find here bias adjustment measurements; Naim, NAPs, Naits, distortion and bias-current settings
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Old 19th September 2021, 12:12 AM   #34
Ian Finch is offline Ian Finch  Australia
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MPSA18 VS BC239C
The bias current that passes through both output transistors and their emitter resistors, is all the same. Therefore the voltage drop across each emitter resistor will be virtually the same. This means that you can just as easily (and more precisely with basic digital voltmeters ) measure the total voltage drop of 13mV across both resistors. There are options for how you measure the bias current, but applying Ohm's law to the total 0.44 ohm resistance will be slightly better.
@mdardeniz - yes, see post #29. That is the same article discussed here.
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Old 19th September 2021, 11:00 AM   #35
Kelion is offline Kelion  Italy
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Quote:
Originally Posted by Ian Finch View Post
The bias current that passes through both output transistors and their emitter resistors, is all the same. Therefore the voltage drop across each emitter resistor will be virtually the same. This means that you can just as easily (and more precisely with basic digital voltmeters ) measure the total voltage drop of 13mV across both resistors. There are options for how you measure the bias current, but applying Ohm's law to the total 0.44 ohm resistance will be slightly better.
@mdardeniz - yes, see post #29. That is the same article discussed here.
Perfect. Thank you for the clarification. I just have to get to work and cross my fingers.
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Old 19th September 2021, 12:42 PM   #36
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Ian Finch View Post
P.S. to #6
BC549C or BC550C from Taiwan semi or Onsemi are just fine as the input long-tail pair and they have better safety margins in that VCEO is 30V or 45V respectively, rather than the 25V of the BC239C.
BC546A/556A do actually a better job despite the smaller Hfe that do not matter much anyway, their higher VCE (smaller parasistic collector/emitter and collector/base resistances) reduce the distorsion of the input stage.

Otherwise there s japanese items that cumulate all advantages, high VCE and very high Hfe, notably the 2SA872(A)/2SC1775(A) that are only 300mW though, and the 2SA992/2SC1845.
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Old 20th September 2021, 01:06 AM   #37
Ian Finch is offline Ian Finch  Australia
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MPSA18 VS BC239C
In principle I would agree with your suggestion though perhaps with Naim product, we misunderstand the design and assume it is intended to be just another low distortion design from the 1970s. However there is clearly an imbalance of the LTP currents to encourage even harmonic distortion and there are a few more sound effect "tricks" in there too. All early Naim product follows this line of thinking and design topology, which was the basis of their early popularity and success.
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