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NAD 3020i Hum
NAD 3020i Hum
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Old 30th May 2019, 09:00 AM   #21
renevoorburg is offline renevoorburg  Europe
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Originally Posted by ubergeeknz View Post
Before you start changing things, I would look to see if anything is drawing excessive current off that rail. Maybe a component downstream of it is bad causing excess draw, and therefore higher than normal ripple. Just a thought. I would start with any tantalums or electrolytics going from that rail to ground.
Thanks. A sound advice, I guess. I don't seem to be the only one who has a Q508 that runs very hot, coloring the circuit board around it, but that doesn't mean nothing is wrong there, perhaps something is straining it too much, with that ripple as a side effect.

I am not that experienced yet in fixing amps. Do you have any tips in how to proceed with that? Is the easiest way by checking the dc resistance of those caps connection the -28V rail and the ground? I don't have a meter to check caps.

Thanks, René
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Old 30th May 2019, 09:08 AM   #22
ubergeeknz is offline ubergeeknz  New Zealand
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Just remove one at a time and see what happens. You'll need to replace it but they are only bypass caps so the circuit should function with any one missing and you should see the ripple go away or reduce if you find the culprit. Happy hunting!
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Old 3rd June 2019, 08:45 PM   #23
renevoorburg is offline renevoorburg  Europe
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I just replaced the regulator filter caps C516 and C517. Here is what I measured at the -28V (? -29 V) output:

330uF -> 38 mV ripple (before)
680uF -> 20 mV ripple (first attempt, not yet satisfied)
1000uF -> 15 mV ripple (a bit of a push to get the wires through ...)

I did expect the bigger caps to have more effect. Listening to the result, the level of hum is more or less half of what is was (with the 330uF caps). Nearly negligible, but what is itching is that I don't get what causes it. Indeed, I will try to hunt that down a next time.

Cheers, René

Last edited by renevoorburg; 3rd June 2019 at 09:05 PM.
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Old 4th June 2019, 07:41 AM   #24
Mooly is offline Mooly  United Kingdom
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NAD 3020i Hum
You may be trying to improve the performance beyond what the basic design is capable of. Many commercial amps have slight hum/buzz due to constraints in the layout and design. 'Nearly negligible' is probably normal tbh.

I just wonder now whether the ripple you are seeing isn't in fact ripple due to a problem with the regulators but simply an effect of the layout and wiring and circulating currents.
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Old 4th June 2019, 12:07 PM   #25
huggygood is offline huggygood  France
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I had all models of 3020 and several times each model.
what i can tell you is that i had several amps with buzz and others not.
I have recapped a dozen with or without effects on the buzz.
I tried to search on several devices and I ended up dropping.
I still have a 3020 "serie 20" full of origin that is silent as a grave and a 3020 A fully recapped that is noisy.
I would be happy if you found the "why of how"
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Old 12th June 2019, 09:58 AM   #26
renevoorburg is offline renevoorburg  Europe
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After tripling the capacity of the filter caps of the regulator (330 uF to 1000 uF, see a previous post) I doubled the capacity of all other electrolytic capacitors in the regulator. The ripple is now reduced to just 5 mV peak to peak. I think I'll consider it fixed, more or less, since probably I just lessened the symptoms, not tackled the root cause (?).

To sum things up: the light 100 Hz hum I could hear over the speakers was traceable in the -28 V output of the regulator, visible on the scope as a sine wave.

original situation -> 38mV peak to peak ripple
filter caps to 680 uF -> 20 mV peak to peak
filter caps to 1000 uF -> 15 mV ripple
doubled other regulator caps -> 5 mV ripple.

Cheers, René
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Old 12th June 2019, 10:17 AM   #27
huggygood is offline huggygood  France
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interesting.
I still have to get two 3020 for maintenance in the next few days, if one of them is noisy, I'll try.
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Old 12th June 2019, 10:52 AM   #28
renevoorburg is offline renevoorburg  Europe
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Minor update: I am not sure if this is a new thing for this 3020i but now I can hear a transformer hum coming from the device. It might be new, it might have become audible now since there is no hum coming from the speakers anymore.
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Old 12th June 2019, 02:28 PM   #29
Mooly is offline Mooly  United Kingdom
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NAD 3020i Hum
Pleased to hear you have lessened the hum.

I doubt the transformer buzz is anything new. You'll probably find it a bit variable and better at some times than others due to purity of the mains (no harmonics) and perhaps even if the mains is a little low in value (less chance of saturation).
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Old 12th June 2019, 02:57 PM   #30
academia50 is offline academia50  Argentina
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My 3020 has a buzz coming from the PSU transformer, it is heard approaching the ear to it, but it is not amplified by the circuit that remains silent as the first day.
By the way, the only problem I had was a continuous and very high hum after a "plop" very pronounced when turned on, so much so that I thought it would ruin a speaker.
The change of the diode bridge and the filter capacitors solved everything.
But the transformer's physical buzz is still there.


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