NAP140 bias current pls

[laverda]

Just need to know the correct bias current

TIA

Graham

[zener]

anything between 36 and 38.5 mA should be Ok , the only difference is if you run it higher it runs hotter , but 38.5 is fine , its the same for all Naim power amp boards.

[Robert]

Agree with Jon, I set my 135s at 36ma but I'd recommend checking the final value after an hour or so rather than the often stated 20 minutes.

I can't tell any difference down as low as 28ma on my amps.

[laverda]

thought is was but just needed to make sure..........

Many thanks..........Job Done

Graham

[markt]

Interesting, prefer mine leaned out at 22ma, used 38 then 32 for a while, just sounded too fat to me, horses for courses?

[zener]

I've always found Naim amps to have a bloated bass(NAP250/200/180/160) , could your suggestion of setting the bias to 22mA be the answer. Mark , is the rest of the sound unaltered??????? , just less bloated in the bass

[martin clark]

Altering the bias won't affect the bass per se, but will alter the distortion harmonic structure. Underbias increases %thd at crossover, esp odd harmonics radiacally - so done subtly I suppose it might bring a slight presence emphasis that seems like the tighter bass. But it's not correct.

Can someone measure the voltages you get across the 0R22 emitter resistors at the specced current draw? That's actually the key figure.

[markt]

Dunno, this was before the regulated supplies were considerably stiffened with modification, haven't played with that since, just checked it on the odd occasion, the bass was as you say a bit bloated, sounds cleaner on mine at 22 throughout, not exactly a standard 250 though, so a case of suck it and see, runs nice and cool too.

[markt]

Originally Posted by martin clark

Altering the bias won't affect the bass per se, but will alter the distortion harmonic structure. Underbias increases %thd at crossover, esp odd harmonics radiacally - so done subtly I suppose it might bring a slight presence emphasis that seems like the tighter bass. But it's not correct.

FWIW, the standard 250s I've measured (only three different ones so far) all had a bias around 20-26ma, so 22ma might just be correct, can't say for sure.

Can someone measure the voltages you get across the 0R22 emitter resistors at the specced current draw? That's actually the key figure.

Couple of weeks ago I would have obliged, my hifi is now in a new position neatly tucked away underneath the furniture out of sight, convieniently unobtrusive but a pig to get to.

Interested to hear others thoughts on bias.

[martin clark]

Quite - I should make clear I'm not dissing your choice Mark, quite the opposite in fact.

What I''m thinking is that measuring the total current draw of the amp is actually a pretty imprecise way of setting bias - for which the only thing matters is the current at the output transistor and specifically the voltage across the emitter resistors - Doug Self has published some pretty exhaustive analysis of this for many differnt output configurations.

If you change the operating voltages, the PSU 'stiffness', regulation of the front end or better definition of its operating current, the temperature at which all the BJTs operate... you change the balance of the total amp current draw to the output stage in untested ways, probably quite radically*. So it's entirely possible that with a heavily modded amp, a different bias set by total current draw is correct - because that's what gets this particular output stage in its sweet spot. Go on, measure the voltage across the 0R22s...


*Naim amps do not have thermal compensation for the bias setting, which is why it's critical to 1) set them warm and 2) not to run too high a bias - they can run away thermally.

[markt]

Go on, measure the voltage across the 0R22s...

OK Martin, for you and knowledge in general I'll haul it out, give me an hour though, I don't fancy it yet, need a cup of tea :P

[zener]

What have you started. This thread is exactly why I love this forum

[Robert]

Actually the figure I gave above was wrong and mine are set at 31.8ma - call it 32 for cash

I remember backing off the bias slightly in the end. Voltage across the 0.22R is 7mv and stable after a couple of hours. My amps hit 6 mv after about 20 minutes then very slowly climb and flatten out at 7mv.

[martin clark]

That's about the values I thought Rob, thanks.

Think I'll lash-up a rig later and attempt to post some pics on distortion vs bias setting, probably tomorrow.

[Robert]

Originally Posted by martin clark

What I''m thinking is that measuring the total current draw of the amp is actually a pretty imprecise way of setting bias - for which the only thing matters is the current at the output transistor and specifically the voltage across the emitter resistors - Doug Self has published some pretty exhaustive analysis of this for many differnt output configurations.

Just finished reading the Self stuff and very interesting it is too, as are his view on OP amps and subjectivism

Putting my DVM in series with the + line on a NAP135 board gives 55ma when the actual output stage standing current is around 32ma. This threw me off the track initially (as Martin knows from my many PMs ) so I'd definitely recommend measuring voltage across 0.22R.