Hello everyone, I'm a long time lurker, and am in need of some help.
I have recently purchased a NAD 7600, and have been cleaning it up, and trying to restore it. However while checking the idle current, I noticed that the left channel was running at 180mV, while the service manual states that the idle current should be just 7.5mV!!!
The strange thing is that I have been using this amp on headphones only (headphone socket is powered by the amps) before cleaning and testing it for about 2 weeks, and have noted no issues with the sound, but one thing I have noticed is that the left channel amp's heat sink runs cold, while the right channel runs fairly warm.
So as you can imagine, I was really shocked to see such a high idle current, on an amp with a stone cold heat sink attached to it! I have set the idle current as low as it will adjust to, which is about 55mV. The right channel was running at about 15mV, but has now been corrected easily to the specified 7.5mV, and it's heat sink runs lukewarm.
Do any of you have some ideas as to what is going on? And maybe some general pointers on what I can do to get this amp in shape?
I have recently purchased a NAD 7600, and have been cleaning it up, and trying to restore it. However while checking the idle current, I noticed that the left channel was running at 180mV, while the service manual states that the idle current should be just 7.5mV!!!
The strange thing is that I have been using this amp on headphones only (headphone socket is powered by the amps) before cleaning and testing it for about 2 weeks, and have noted no issues with the sound, but one thing I have noticed is that the left channel amp's heat sink runs cold, while the right channel runs fairly warm.
So as you can imagine, I was really shocked to see such a high idle current, on an amp with a stone cold heat sink attached to it! I have set the idle current as low as it will adjust to, which is about 55mV. The right channel was running at about 15mV, but has now been corrected easily to the specified 7.5mV, and it's heat sink runs lukewarm.
Do any of you have some ideas as to what is going on? And maybe some general pointers on what I can do to get this amp in shape?
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Sounds like the left channel may have a bad emitter resistor that's gone up into the dozens of ohms (or bad solder joint in that vicinity). It's nothing you'd notice right away with the light loading provided by headphones, but obviously trying to gauge idle current via Ohm's Law is going to produce bogus results if R is totally off.
So that's what I'd check. These usually are cement resistors, 0.22-0.47 ohms at 3-5 W up.
So that's what I'd check. These usually are cement resistors, 0.22-0.47 ohms at 3-5 W up.
Thanks for the idea, I'm away from home right now, and should be able to check the amp in a week or so.
I can't discount the possibility of there being bad solder joints, as I have heard that NAD of this era have a reputation for this. I think I'm building up the courage to do a complete re-solder job on it.
My speakers are away in storage, and I hope to get access to those in a few weeks. I did however get hold of an 8r dummy load resistor, and tried to calibrate the amp using that, but as yet I have been unable to connect a real speaker to the amp.
Do you think that this possible fault could also be the reason that the amp runs cold on this channel (at idle)? When this channel is driving the dummy load, it gets nice and warm.
I will update as soon as I can.
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Hi, I have used both test points, as these are the test points for checking the idle current.
DC offset is tested by using the speaker terminals. DC offset is something that I have had luck with, it's at +-0.3mV on both channels after warmup.
One other quick question regarding idle current... Is there a benefit to setting this current higher than specified in the manual?
I have heard some people say that setting a higher current can have a positive effect on sound quality, as this makes the output transistors switch at a higher rate.
I do not have the knowledge to know if this would have any kind of audible effect.
I have heard some people say that setting a higher current can have a positive effect on sound quality, as this makes the output transistors switch at a higher rate.
I do not have the knowledge to know if this would have any kind of audible effect.
I'm afraid either those people have no clue or you misunderstood, as the amplifier can only switch or "cross over" at the rate of the signal applied. What higher bias currents can do is delay switching such that both devices will remain on together for longer periods before switching occurs. The crossover region then becomes an increasingly larger area of class A operation as bias current increases.
In typical, low level domestic use, this can often result in better perceived sound quality. If using headphones, you would probably always be listening in class A. Yes, there is much more heat to dissipate as the amount of bias current increases but it can be more satisfying to some folks in designs that can also tolerate operation that way with safety.
However, there is a big problem in that few amplifiers are actually rugged or "overbuilt" sufficently to operate safely at levels much above those specified and more importantly, they may become unstable in maintaining current correctly, according with temperature. Thermal runaway and destruction becomes a real threat to people tinkering without full knowledge of what thermal stability means and how its control system is an important part of solid state audio design.
I have had to fix many recent disasters due to unwise tinkering that you don't see on forums but you do see them in dumpsters or resold on Ebay as "working" amplifiers 😡
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