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Old 15th March 2015, 04:00 AM   #31
AKSA is offline AKSA  Australia
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Hi Sjef,

This is much better news! Thank you for telling us the story.

I generally send the bias a bit higher than 85mA, around 100mA for the N70, and it does have an influence.

You raise the transformer heat. This might indicate one of the two transformers is a bit higher flux; please check the voltage on the rails are at +/-42, and both identical. A difference in the two rails might be indicating there is an unbalance in the transformer secondaries, and I know this affects any audio amp.........

Cheers,

Hugh
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Old 16th March 2015, 02:12 AM   #32
LPM is offline LPM  Australia
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Quote:
Originally Posted by Sjef View Post
Anyways, I'll stop posting about it over here , just wanted to let you know, that's all
Actually don't stop. At least one final update would be good way to let us all know the results after any final troubleshooting and resetting the values to those Hugh has advised. A great reference to anyone unlucky enough to have similar issues in the future.
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Old 16th March 2015, 06:49 AM   #33
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Quote:
Originally Posted by AKSA View Post
Hi Sjef,

This is much better news! Thank you for telling us the story.

I generally send the bias a bit higher than 85mA, around 100mA for the N70, and it does have an influence.

You raise the transformer heat. This might indicate one of the two transformers is a bit higher flux; please check the voltage on the rails are at +/-42, and both identical. A difference in the two rails might be indicating there is an unbalance in the transformer secondaries, and I know this affects any audio amp.........

Cheers,

Hugh
Um, dumb but all too common - check the transformer secondaries are correctly connected. Transformer working flat out doing nothing.......
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Old 16th March 2015, 11:28 PM   #34
Sjef is offline Sjef  Netherlands
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Yes maybe you are right and could it be helpfull to others if they ever face the same problem.

Hi Hugh,

I have read in the instructions that the N70 is supposed to have 85mA +/- 10mA of bias current. Are the later production versions set at 100mA or was this a setting for personal use ? Does this have an influence on long term stability (I guess not) I also thought that the bias was self limiting at 150mA, is that right ? The reason that I haven't set the bias allready is becuase it seems to be varying continue. At the moment that I think it's stable for minutes or even 20 minutes it start varying again. The variations are getting smaller after some time but I let the amp running idle with a voltmeter attached for two hours and it still varies after this time. That makes it hard to adjust the bias, what to measure ? The peak bias ? this hardly ever exceed 75mA on this one. Left and right channel are not varying in the same manner so it's not a matter of temperature only. This is an amp that could benefit from being on power 24/7

I might try to raise it to 100mA. This way the amp will be operating in class A most of the time. Since I only intend to use them in an active setup for mids and tweets with medium high sensitivity.
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Old 16th March 2015, 11:55 PM   #35
AKSA is offline AKSA  Australia
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Hi Sjef,

Thank you for coming back to me!

Yes, 100mA seems to be better than 85mA, and yes, it's a preference! The bias is not self-limited at 150mA; as long as the amp is cooled well there is no problem with this.

Bias control on all AB amplifiers is problematic, and I improved this on the N80, which has rock solid quiescent in all situations. The N70 has a tendency to move to higher quiescent in 4R loads and in high temperatures with insufficient cooling. Cooling is important of course in all amp. The variations of quiescent tend to minimise in operation but you'd be hard pressed to find a difference in the sound. It's a small 200R trimpot, blue, near the four 4148 glass diodes. Turning it CCW increases the quiescent. I'd suggest that 75mA is a bit low; 100mA is better....... and I would not bother leaving it on all the time, BUT, it takes 20 minutes to give the best sound.

You might try, as the previous poster suggested, reverse ONE of the secondary transformer windings on ONE of the rails, that is, either top side of the pcb, or the bottom side. Just remove the two spades, reverse them and refit.

Cheers,

Hugh
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