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Measuring bias voltage in NAPs

Andrea Pignanelli said:
I even replaced the 47 mF caps and the zener diodes on the regulator board.
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I have a comunication from Naim to my dealer from 1989 that states: it is very important that these are checked when the amp is cold, i.e. has been switched off for at least 3 hours and left at room temperature. It will take about a minute after switch on for everything to settle down.
Set the power supply before doing the bias, as the voltages have to be spot on.
Now this is what I have noticed: if I set the voltages (-+ 39 volt) cold and then set the bias at 7 mV just after, the -+ voltages slightly rise over time (roughfly 0,6 volt).
If I turn the amp on (after 3 hours swithed off) the -+ voltages are not spot on (at -39,9, +39,3) but circa 0,6 volt less, this means that I have to dial 7 mV in these conditions?
 
You have to set the bias at 7.0 mV with the amplifier cold as described in your first paragraph above. It will, and is supposed to, rise as the amplifier warms up. This rise is not the cause of the symptoms you described.

The absolute values of the rail voltages are not that important. What is important is the 0.6 V difference between the magnitudes of the positive and negative rails.
 
NB the only reason for that 0.6v difference is so the amp clips symmetrically (!)

i.e. it's not much worth worrying about, and doesn't affect its dynamic performance within the power envelope... (my guess would be the NAP250 and then 135s was set-up that way so it didn't get remarked-on in magazine tests)
 
misterc6 said:
You have to set the bias at 7.0 mV with the amplifier cold as described in your first paragraph above. It will, and is supposed to, rise as the amplifier warms up. This rise is not the cause of the symptoms you described.

The absolute values of the rail voltages are not that important. What is important is the 0.6 V difference between the magnitudes of the positive and negative rails.
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Please, setting the bias value at 7 mV cold, what bias reading should I measure when the amp is on for 1-2 hours?
Thank you
 
You’re really overthinking this. It doesn’t matter what it measures after a period of time, it will change again when you put the case back on.

At the risk of repeating myself, set the bias to 7.0 mV with the amplifier cold and look forward to listening to some music!
 
misterc6 said:
You’re really overthinking this. It doesn’t matter what it measures after a period of time, it will change again when you put the case back on.

At the risk of repeating myself, set the bias to 7.0 mV with the amplifier cold and look forward to listening to some music!
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Ok, got it.
Thank you very much.
I will follow your advice.
 
Andrea Pignanelli said:
Ok, got it.
Thank you very much.
I will follow your advice.
Click to expand...
Thank you Malcolm, bias setting ok, new parts burning in, sounds fine.
Please, as you have tested the Avondale ncc 200, ncc Kudos boards, did you ever test the new ncc 300?
I have the TPR4 in my Hicap for my Nac 62 and sounds great and was thinking to upgrade my nap 250, any advice please?
Maybe the voltage regulator first, or building a new amp with Avondale parts?
Thank you
 
Andrea Pignanelli said:
Thank you Malcolm, bias setting ok, new parts burning in, sounds fine.
Please, as you have tested the Avondale ncc 200, ncc Kudos boards, did you ever test the new ncc 300?
I have the TPR4 in my Hicap for my Nac 62 and sounds great and was thinking to upgrade my nap 250, any advice please?
Maybe the voltage regulator first, or building a new amp with Avondale parts?
Thank you
Click to expand...

With apologogies, I haven't being following the development of the NCC300 closely and suggest that you post on that thread:

https://www.pinkfishmedia.net/forum/threads/avondale-ncc300-amp-project.212730/
 
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