You're right, there are too many variations in the story of this faint right channel hum to be consistent and credible evidence of a fault in the amplifiers. I think you should plan your test methodology better and use that oscilloscope correctly so that others can follow what you may report here. A typical 20-50 MHz bench oscilloscope should be be able to outperform your hearing through speakers many times over, though you'll need something like at least 10W, 4-10 Ohm resistors as dummy loads, for serious testing. The resistors could well become very hot even with only a few watts output but amplifiers driving nothing don't make for realistic test results so you need something in a linear resistance form that can shed the heat there. You can buy high power, metalclad resistors cheaply but they also require heatsinks - the expensive part.
Whatever beliefs people have about trusting their ears above all else, nobody's are calibrated to any standard other than that imagined in our highly individual and often flawed sensory perception. We normally have problems with perceiving steady state sounds like test tones too, so it's wise to always refer to standards and specifications before pronouncing something is good/bad. I also think most audio devotees would be aware that NAD made entry level, lowest possible price gear so there's a limit to how quiet or balanced the sound from this 50 year old design should be anyway.
I have 2 NAD 3020s and a 3120 here, all with a feint 100 Hz hum on the right channel. The 3020's restored and the 3120 untouched but in truly excellent original condition. Not even a ‘welcome hum’. I don’t have a scope right now so I’ve been using what methods I can.
The hum is so feint and, given it's not affected by the volume control, I doubt many users have even noticed it. It only becomes marginally noticeable with the bass control at max. That being said, it's an interesting anomaly to me and I would like my amplifiers to be without it.
The left channel is totally clean, and surely I shouldn't expect different performance from each channel? I appreciate what you're saying regarding the limitations of cheaper NAD designs, but they're not in the department of fidelity with the 3020. The sound quality produced by them when in good condition is excellent.
It seems apparent that the minor issue is a design flaw and it’s not within my capabilities to figure out where that is at this time. So, that being said, the issue certainly represents a limitation of this design.
I am just saying what I can to help others understand. I suppose the ‘variations’ and confusion have arisen from my on the fly articulation of my findings. That being said, I think I’ve made what the issue is beyond clear in my last few or so posts. If it is still unclear then please allow this comment to tell all.
I think the points you have made are clear enough but there are at least 2 problems getting to your goal, as explained. First, you need to define this barely noticeable hum by magnitude and waveform appearance. Then check this against the amplifier specifications and see if there really is a problem or you've just been been expecting too much from these pretty basic NAD products? If you can hear hum, you definitely will be able to see it on a decent 'scope and be able to estimate the magnitude.
Personally, I think the hum appearing only in the right channel of all 3 amps is against the odds unless they all have a common design or component flaw which is only in the one and same channel of each. I suggest it shows the signs of a hum problem in a common input source to all 3 amps instead. Apparently, that's not the case but have you actually tried swapping the links (use short RCA leads if you have nothing else suitable) from pre to power output stages to narrow down the location?
Then if you can't shift the problem from one channel to the other by any other means, the problem must therefore be in the same channel output power amplifier section of every amp. That should lead to focusing on reasons why just one power amp. should be noisier than the other.
Personally, I think the hum appearing only in the right channel of all 3 amps is against the odds unless they all have a common design or component flaw which is only in the one and same channel of each. I suggest it shows the signs of a hum problem in a common input source to all 3 amps instead. Apparently, that's not the case but have you actually tried swapping the links (use short RCA leads if you have nothing else suitable) from pre to power output stages to narrow down the location?
Then if you can't shift the problem from one channel to the other by any other means, the problem must therefore be in the same channel output power amplifier section of every amp. That should lead to focusing on reasons why just one power amp. should be noisier than the other.
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Have you tested for the hum with shorting plugs in all input sockets?
Also try installing a pair of RCA cables on the back instead of jumpers, but swap the channels.
Does the hum stay in the right channel?
Also try installing a pair of RCA cables on the back instead of jumpers, but swap the channels.
Does the hum stay in the right channel?
What I can tell you is that I have had almost all the NAD 4-digit series, including the tuner amps, that they have all been connected to several sources and several speakers and that, except for breakdown or wear due to their age, I've never had a buzz or hum, I just have an aluminum front 3020B that buzzes slightly on both channels if I turn the volume knob too far down on its zero stop.
I also think that you have a problem elsewhere and that you will have to proceed methodically and take notes of each test to be sure.
I also think that you have a problem elsewhere and that you will have to proceed methodically and take notes of each test to be sure.