Help Needed - Nait 1 Owners please respond

Hi to all you Nait 1 owners out there, I may have a problem, but am not sure since it is along time since I ran a Nait.

Bought a dud Nait 1 off Ebay (knew before I bought it), and have managed to repair same (bad fault though, one channel completely goosed from the power trannies back to the protection circuitry, even had an open circuit 0.22 Ohm output resistor). Anyway, I have got it back on running status, although I had to substitute the O/P trannies with an alternative to Naim ones obviously.

All this aside, I am a little worried about the heat it generates. After only about 30 minutes, the case is almost too hot to touch, although no sound degredation can be heard, I have run it for more than three hours with no problems other than the heat. So, question 1 is: - is this normal ????

Second question, with all the manhandling of the board in and out of the chassis, I may have knocked the two presets off setting, and am also unsure of what they are for (quiescent current for the output stage ??). So, anybody got a schematic for a Nait 1 (green LED variety), or advice either way ??.

Thanks to all

And a Happy New Year.

Mike

The pots set the quiescent current in the output stage, and this is most likely the reason for the overheating.

Without looking in mine I cannot remember specifics, but you need to find the emitter resistors, measure the voltage drop across them and then work out the quiescent from that, using Ohms law.

I(quiescent)=V / R.

It's normal to set this after maybe 15mins or so, re-checking it after a period of idle at this current.

Naim normally set this quite low, try 10mA for starters that should at least prevent overheating, if I get a second I'll check the idle current in mine. Most Naim power amps have very low bias, due to the attention to detail in matching the output stage, I'm not so sure how this applies to the complementary stage of a Nait, which may be less well matched than the Quasi-comp stage of the big amps.

Andy.

Andy

Are you sure that the NAIT1 had a true complimentary output pair (ie. npn + pnp)? The NAIT2 is quasi-comp, and I had assumed that all Naim power amps would be the same, using 14mA (3mV across the .22R resistor as measured in a NAP140) as a bias current standard.

My reason for being surprised is that Julian V had written somewhere that npn output transistors behave very differently to pnp's --- hence the reason Naim use quasi-comp pairs.

Mike

What were the model numbers of the output transistors in the NAIT1 BTW? And what did you change them to? My old NAIT2 contains 4 x NA005 power transistors (two of them are blown) and I read that Naim recommended BD743's as a replacement. Does anyone know if this is true?

Andy

Bob,

Yes, Nait1 was a complementary o/p stage. I thought Nait 2 was the same, but obviously not.

JV was right though, a quasi comp has it's advantages, in terms of matching, it's less of an issue at lower powers though, due to greater device choice. Maybe the Nait 1 was different for this reason.

14mA sounds familiar too.

Andy.

Originally posted by BobMaximus
Andy

Are you sure that the NAIT1 had a true complimentary output pair (ie. npn + pnp)? The NAIT2 is quasi-comp, and I had assumed that all Naim power amps would be the same, using 14mA (3mV across the .22R resistor as measured in a NAP140) as a bias current standard.

My reason for being surprised is that Julian V had written somewhere that npn output transistors behave very differently to pnp's --- hence the reason Naim use quasi-comp pairs.

Mike

What were the model numbers of the output transistors in the NAIT1 BTW? And what did you change them to? My old NAIT2 contains 4 x NA005 power transistors (two of them are blown) and I read that Naim recommended BD743's as a replacement. Does anyone know if this is true?

Andy

Click to expand...

Andy/Bob ??

I contacted Naim about this too since it was not worth the money for Naim to repair it, they also quoted the BD743 (and 744 for the complimentary transistor), which I have put in as recommended. As we sit, it works OK so far as I can see, with only the heat problem so far.

As AndyWeekes stated, the Nait 1 is certainly a proper complimentary output pair, and I must say that when I checked the HFE on the trannies prior to insertion, they were vastly different from PNP to NPN (IIRC the NPN's were about 40-50, and the PNP's were in the area of 130 !!, or vice versa), so I can see why JV might have had his reservations regarding the COP arrangement.

HTH
Mike

Postscript

I will adjust the bias to 14mA then (or 3mV across the 0.22 Ohm resistor), but I would still like to have a copy of the Schematic if anyone has one !!!!!

Thanks guys
Mike

Andy/Bob,

Thanks for the sterling advice on the quiescent current setting for my Nait 1, it's now running at normal temperatures, and sounds better too.

Just a note, when I measured the voltage across the 0.22 Ohm emitter resistors, they were 26mV (left channel) and 86mV !!!(right channel), so no wonder it was running hot.

It's worth noting here that there is a 0.22 Ohm emitter resistor for each transistor, one NPN and one PNP type, adjusting the small presets adjusts the bias for both transistors reasonably equally, although one one channel in mine (the one with all the damage, and a couple of substitute transistors), I could only achieve 3.2 and 3.7mV respectively, but it sounds fine.

Thanks again
Mike

MJSM - I've just taken a survey of two Nait2s and measured between 4.7mVand 7.0mV across the 0.22ohm emitter resistors. This indicates a standing bias in the range 21-31mA, which looks about right given the layout uses a complimentary pair. My Nait definitely sounds better around the 25- 28ma mark (6mV).

Martin

Thanks Martin

Although you need to be aware that the Nait 1 and 2 are different in that the Nait 2 does not use a true Complimentary Pair arrangement (see post from Andy Weekes and Bob here), but a 'quasi-complimentary pair' using only NPN type transistors.

Mike

MJSM

I would be inclined to use Martin's suggested settings. Since he has tested two NAIT2's, it is likely that his information is reliable. The fact that your NAIT1 is a bit different does not mean that it will require a lower setting (probably the opposite is true).

In theory, the way to set the bias current is to start with a minimum setting (which will give a distorted sound) and then gradually increase the current until distortion reaches a minimum. Going any further will, as you have observed, cause overheating and will waste PSU resources (although there may be other issues that I'm not aware of). The only problem is having to wait for temperatures to stabilise after making each change. Martin has already identified the setting giving the best sound.

Remember that the text book bias would be about 50mA. Naim uses an incredibly low setting because of careful matching. Your NAIT1 may not be as well matched because of its complementary OP transistors.

BobMax

In theory, the way to set the bias current is to start with a minimum setting (which will give a distorted sound) and then gradually increase the current until distortion reaches a minimum

Click to expand...

This is a method I have used previously, I just attached it to my FFT analyser and set the knob for min distortion.

Andy.

Andy

Where did you get your FFT application from? Is it on the Net? Is it free? Can I have one please?