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NAD 3020i Hum
NAD 3020i Hum
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Old 13th June 2019, 09:31 PM   #31
renevoorburg is offline renevoorburg  Europe
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Thinking about the possible root cause of the hum I am beginning to suspect the mute-on-startup circuit. That is comnnected to the -29V part of the regulator, the part that has the ripple on it. Further, I've learned that adding capacity / capacitors towards the input of the regulator seems to have more impact and that indeed is where the mute circuit connects. Moreover, the circuit board and components in that area look like they have experienced some stress.

In the schematic I used there is a Q516 tranistor which I don't understand. It has it base connected over a 47K resistor to the -50V lead of the regulator:

Click the image to open in full size.


However, I just found a different schematic, supposedly also of the 3020i. There Q516 is also connected to a transistor Q517 and a thermistor, which does seem to make some sense:

Click the image to open in full size.

So as a next thing I'll check which schematic my 3020i follows. I will check the state of Q516 and D601 (latter schematic). Further, I will just disconnect the leads from the regulator to the mute circuit (should not be a problem I gather) and test if that effect the ripple. I think I am getting close now.

Last edited by renevoorburg; 13th June 2019 at 09:33 PM.
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Old 14th June 2019, 07:43 AM   #32
Mooly is offline Mooly  United Kingdom
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NAD 3020i Hum
Could be...

I remember a thread on DC offset issues with the mute circuit and at the time I even got around to simulating it.

DC offset on NAD 3120 preamp output

If you suspect the mute then just link the FET out as a test.
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Old 14th June 2019, 07:49 PM   #33
northpaw is offline northpaw  United States
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In the face of a persistent hum that is weakened but not eliminated with new filter caps and increased filter capacitance, academia50's suggestion that the diode bridge may be going bad seems as though it might have some merit.

Quote:
Originally Posted by academia50 View Post
...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...

NAD 3020 amp upgrades?
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Old 14th June 2019, 09:28 PM   #34
renevoorburg is offline renevoorburg  Europe
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Quote:
Originally Posted by northpaw View Post
In the face of a persistent hum that is weakened but not eliminated with new filter caps and increased filter capacitance, academia50's suggestion that the diode bridge may be going bad seems as though it might have some merit.
Might be indeed. I just checked Q516, Q517 and D601 and they were all fine. I fully disconnected the mute-on-startup circuit (except for the FETS) and it had no effect whatsoever on the ripple.

To be continued...
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Old 18th June 2019, 08:40 PM   #35
renevoorburg is offline renevoorburg  Europe
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Quote:
Originally Posted by northpaw View Post
In the face of a persistent hum that is weakened but not eliminated with new filter caps and increased filter capacitance, academia50's suggestion that the diode bridge may be going bad seems as though it might have some merit.
Regrettably, that didn't help.

If it would have helped, I would be wondering now how a broken rectifier bridge could result in a 100 Hz noise, when I think 50 Hz is to be expected. Or am I mistaken?

Think I'll move over to a next project. Thanks and see you later.
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Old 18th June 2019, 11:59 PM   #36
academia50 is offline academia50  Argentina
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Quote:
Originally Posted by renevoorburg View Post
Regrettably, that didn't help.

If it would have helped, I would be wondering now how a broken rectifier bridge could result in a 100 Hz noise, when I think 50 Hz is to be expected. Or am I mistaken?

Think I'll move over to a next project. Thanks and see you later.


I remember that I measured the original diode bridge and one of the four was wrong. It drove in both directions - with the simple test of the tester - although the leak was very slight, it was not shorted or fully open. But it was a leak! I oversized the amperage and voltage of the new bridge, but not the Uf value or the voltage of the new electrolytic filter capacitors.
(They lower the ripple, until they reach pure DC and from that point they should be like a chemical battery, but it seems that it never becomes totally like that, I found out about it here (I think it was DF96) It is not necessary to oversize the capacitors of the PSU, the circuits of the PCB and some components are not prepared for that over current availability.
A teacher many years ago told us that the electrolytic capacitors should be replaced by the same maximum voltage, not over-dimensioning them, because this modifies the capacity too. And he had written several electronic books, so I took it literally since then.
The first time a colleague introduced me to the NAD 3020, after listening carefully to the low solids of the amplifier, he introduced me to the amplifier "without the clothes" and when I saw the size of the filter capacitors, I could not believe it. Obviously, engineers take many factors into account when designing a circuit.
The buzz that persists in my amplifier is originated by the PSU transformer, probably it was always, and it will continue there, since it is not amplified.
Good luck and I hope there is a happy ending.
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Old 19th June 2019, 12:10 AM   #37
academia50 is offline academia50  Argentina
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Quote:
Originally Posted by Mooly View Post
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.
I think it can be like this .....
Do not forget that the great performance of the first Nad was characterized by an excellent development in the circuitīs , and the accessibility of its components, being very easy to get / replaceable their transistors. (Line Phillips BC /BD) But its components were not up to par, which on the other hand would have increased its price much. Nothing of Alps potentiometers here!
There are no free lunches once again.

Last edited by academia50; 19th June 2019 at 12:13 AM.
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