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Feedback resistor values in bolt-down NAP250

R

RonR

pfm Member
I was just carrying out some acoustica mods to my bolt-down NAP250, and noticed that the feedback resistors have different values to all the NAP, NCC or HackerNAP schematics I've ever seen. Using the acoustica schematic as reference (http://www.acoustica.org.uk/t/naim/poweramp_pix/NAP250 schematic.jpg):

Schematic:
R3=1K
R4=27K
R5=1.8K

My Board:
R3=1.8K
R4=27K
R5=2.7K

I've checked online images of both 250 and 160 bolt-down boards, and they appear to have the same values as mine.

So now to my question - how would these values affect the sound of the amplifier, and should I change them to the "correct" ones?
 
No, do not change them. Early amps had slightly lower gain, hence 1k8 resistor. I've seen 2k here too. Changing those could affect the stability of the amplifier that you won't be aware of without the right test equipment.
 
On the face of it lower gain = higher feedback = less stability. In reality leave them be...

Paul
 
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