Sorry. I know what you're thinking. Not another NAD hum thread ( : But please bear with me for a second.
And apologies. 1. Taking pictures of an ancient scope is not something I do very well. 2. Amp was bought like this hoping it would be a quick fix. Not entirely so unfortunately.
Here's what I see on the speaker terminal. Scope is set to 50mV, so there abouts that on the output:
Anyway - onwards and upwards: Standard issue hum from both channel. Fixed level so I could just turn up the volume and then I wouldn't notice. No change depending on input selection or volume.
Started by replacing the main caps in the power supply for the power amp. No change. Then replaced all caps in pre and power amp and pre-amps power supply. Same same.
So pulled out the old scope and started measuring. Ripple on the power amp PSU: (scope set to 0.5v). Not super great but ok'ish for an old NAD: (sorry for the offset, scope wasn't zero'ed in)
At this point, I didn't suspect the pre-amp PSU, so I disconnected the signal cables between the pre and power amp (no pre-out/main-in on this). No result - still humming along nicely.
Then I started looking more at the schematics and noticed that +41.2v and -27v from the pre-amp PSU actually goes to the power amp and I'm definitely not smart enough to come up with a good explanation as to why NAD chose this solution:
Pre-amp PSU:
and power-amp schematic:
Since I'm not smart enough to figure out why they would do so, I figured I'd try to remove the fuses for the pre-amp PSU. Now I'm not sure if this completely shuts down the power-amp part, but it certainly killed the hum.
So the hum is definitely coming from the pre-amp PSU (in my head at least), so put the scope probe on that one: (Scope still set to .5v)
Considering there's about .10v ripple on the power amp PSU, this seems quite excessive - like something is pulling much more juice than it's supposed to. To pressure test that theory, I re-added the original 330uF/63v across the new ones:
Well. It's a result, but a band-aid on an axe wound (and the amp is still humming 😉 )
So I guess my theory is (somewhat) correct. There's a much larger power draw on the pre-amp PSU that the designers factored in. Could be the power-amp since we still have +41.2v and -27v going from the pre-amp PSU to the power-amp. So removed the fuses from the power-amp PSU hopefully shutting it down - the ripple on the pre-amp PSU stays the same so I guess the power-amp section is ok.
So either the circuit in the pre-amp PSU is gone haywire('ish) or the pre-amp section has. However, sound is absolutely fine. Sinus curves looks pretty much spot on too.
Any pointers as to where I should start to poke with the probe from the old oscilloscope (which I sorta borrowed when I was an apprentice - which I finished in 1996. I love old stuff that still works today) or what I can check next?.
Thank you - and apologies again for another "NAD, now with extra added hum" thread.
And apologies. 1. Taking pictures of an ancient scope is not something I do very well. 2. Amp was bought like this hoping it would be a quick fix. Not entirely so unfortunately.
Here's what I see on the speaker terminal. Scope is set to 50mV, so there abouts that on the output:
Anyway - onwards and upwards: Standard issue hum from both channel. Fixed level so I could just turn up the volume and then I wouldn't notice. No change depending on input selection or volume.
Started by replacing the main caps in the power supply for the power amp. No change. Then replaced all caps in pre and power amp and pre-amps power supply. Same same.
So pulled out the old scope and started measuring. Ripple on the power amp PSU: (scope set to 0.5v). Not super great but ok'ish for an old NAD: (sorry for the offset, scope wasn't zero'ed in)
At this point, I didn't suspect the pre-amp PSU, so I disconnected the signal cables between the pre and power amp (no pre-out/main-in on this). No result - still humming along nicely.
Then I started looking more at the schematics and noticed that +41.2v and -27v from the pre-amp PSU actually goes to the power amp and I'm definitely not smart enough to come up with a good explanation as to why NAD chose this solution:
Pre-amp PSU:
and power-amp schematic:
Since I'm not smart enough to figure out why they would do so, I figured I'd try to remove the fuses for the pre-amp PSU. Now I'm not sure if this completely shuts down the power-amp part, but it certainly killed the hum.
So the hum is definitely coming from the pre-amp PSU (in my head at least), so put the scope probe on that one: (Scope still set to .5v)
Considering there's about .10v ripple on the power amp PSU, this seems quite excessive - like something is pulling much more juice than it's supposed to. To pressure test that theory, I re-added the original 330uF/63v across the new ones:
Well. It's a result, but a band-aid on an axe wound (and the amp is still humming 😉 )
So I guess my theory is (somewhat) correct. There's a much larger power draw on the pre-amp PSU that the designers factored in. Could be the power-amp since we still have +41.2v and -27v going from the pre-amp PSU to the power-amp. So removed the fuses from the power-amp PSU hopefully shutting it down - the ripple on the pre-amp PSU stays the same so I guess the power-amp section is ok.
So either the circuit in the pre-amp PSU is gone haywire('ish) or the pre-amp section has. However, sound is absolutely fine. Sinus curves looks pretty much spot on too.
Any pointers as to where I should start to poke with the probe from the old oscilloscope (which I sorta borrowed when I was an apprentice - which I finished in 1996. I love old stuff that still works today) or what I can check next?.
Thank you - and apologies again for another "NAD, now with extra added hum" thread.
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Trace at spkrs=not good.
Ripple traces look ok-ish to me
Using the spkr trace to guide you try and trackdown the source of this waveform, ok it might not be as simple as that, but you might get lucky. Check audio path and pre/power boundary and go from there.
Since both channels affected, yeah probably a power supply issue/tired regulator. I'd test each +/- supply looking for something approacing the spkr trace rather than normal ripple. wave
Ripple traces look ok-ish to me
Using the spkr trace to guide you try and trackdown the source of this waveform, ok it might not be as simple as that, but you might get lucky. Check audio path and pre/power boundary and go from there.
Since both channels affected, yeah probably a power supply issue/tired regulator. I'd test each +/- supply looking for something approacing the spkr trace rather than normal ripple. wave