Hi everyone,
I'd love some help diagnosing a persistent hum issue in a NAD 7020E (amplifier section is nearly identical to the NAD 3020E and other 3020 variants).
The symptom:
Despite all of the above, I'm still getting the loud hum and seeing ~5.5 V AC ripple at 50 Hz.
Any ideas or suggestions welcome. Can attach photos if that will help.
Thanks in advance!
I'd love some help diagnosing a persistent hum issue in a NAD 7020E (amplifier section is nearly identical to the NAD 3020E and other 3020 variants).
The symptom:
- Loud, constant hum through both speakers
- Present even with all inputs disconnected and volume at zero
- Hum is at 50 Hz (I'm in Australia)
- Ripple measured on power rails is ~5.5 V AC
- Both transformer output fuses (F503, F504) pass continuity
- ~34 V AC measured across F503 and F504 outputs
- ~17 V AC measured from each fuse output to chassis ground - transformer appears balanced
- ~5.5 V AC ripple (50 Hz) measured:
- F503 output to cathode of D505 (+V rail)
- F504 output to anode of D507 (-V rail)
- Replaced both main power filter caps (C508, C509)
- Upgraded from 4700 µF 35V to 4700 µF 50V
- Replaced all four bridge rectifier diodes (D505–D508)
- Upgraded from 1N5401 (100v) rated to 1N5408 (1000 V rated
Despite all of the above, I'm still getting the loud hum and seeing ~5.5 V AC ripple at 50 Hz.
Any ideas or suggestions welcome. Can attach photos if that will help.
Thanks in advance!
so did you buy this amp non-working, or has it "died" since you have owned it? Did it play fine and recently developed a "hum" ?
What have you tested so far? Did you diode check all the diodes in the PSU (shows at least 8 of them for the High and Low rails "power envelope") Has there been any previous repair attempts that you know of?
I would start with the red marked junction points and write down your voltages. Right side of Diodes will be AC, left side of diodes will be in DC. Because it's in both speakers, sounds like you're on the right trck with the PSU as teh culprit. I would check all diode/zener or capacitors as a guess, if you're gtting leakage all the way through the amplifier.
Im sure some more qualified folks will step in shortly and offer more detailed assistance to help you get this sorted.
What have you tested so far? Did you diode check all the diodes in the PSU (shows at least 8 of them for the High and Low rails "power envelope") Has there been any previous repair attempts that you know of?
I would start with the red marked junction points and write down your voltages. Right side of Diodes will be AC, left side of diodes will be in DC. Because it's in both speakers, sounds like you're on the right trck with the PSU as teh culprit. I would check all diode/zener or capacitors as a guess, if you're gtting leakage all the way through the amplifier.
Im sure some more qualified folks will step in shortly and offer more detailed assistance to help you get this sorted.
Besides the excellent questions asked above, can you tell us what you replaced the capacitors with? Are they reputable parts that you know to work properly, and be of the advertised specifications?
Excellent point, did it always have this problem since you got it, or did the problem happen recently?
Thanks for all the feedback — really appreciate the input. I'll try to answer your questions as best I can:
Q: Did it always have this problem, or did it develop recently?
A: I bought it in this condition at a garage sale, so unfortunately I don't know when or how the hum started.
Q: Any known previous repair attempts?
A: Not that I’m aware of, but again, I don’t know its full history.
Q: Did you diode-check all the PSU diodes?
A: Yes. Mine is the 7020E, which uses 4 diodes in the main bridge (D505–D508). The service manual also covers the 7220E, which uses 8 diodes for the Power Envelope circuit — but that doesn’t apply to this model.
Q: What have you tested so far?
A: Here’s a quick summary of the key measurements (using a DMM):
Q: What did you replace the capacitors with? Are they good quality?
A: I replaced the original 4700 µF 35V caps with 4700 µF 50V ones. They’re generic brands (similar to what was originally in the amp), sourced from a local electronics supplier I’ve used many times. I've had good experience with their parts in the past, so I have no reason to doubt their specs.
Let me know if you’d like me to post additional measurements or photos — happy to dig deeper.
Thanks again!
Q: Did it always have this problem, or did it develop recently?
A: I bought it in this condition at a garage sale, so unfortunately I don't know when or how the hum started.
Q: Any known previous repair attempts?
A: Not that I’m aware of, but again, I don’t know its full history.
Q: Did you diode-check all the PSU diodes?
A: Yes. Mine is the 7020E, which uses 4 diodes in the main bridge (D505–D508). The service manual also covers the 7220E, which uses 8 diodes for the Power Envelope circuit — but that doesn’t apply to this model.
Q: What have you tested so far?
A: Here’s a quick summary of the key measurements (using a DMM):
- Both transformer output fuses (F503, F504) pass continuity test
- ~34 V AC across F503 and F504 outputs
- ~17 V AC from each fuse output to chassis ground, so transformer appears balanced
- ~5.5 V AC ripple at 50 Hz, measured at:
- F503 output to cathode of D505 (+V rail)
- F504 output to anode of D507 (–V rail)
Q: What did you replace the capacitors with? Are they good quality?
A: I replaced the original 4700 µF 35V caps with 4700 µF 50V ones. They’re generic brands (similar to what was originally in the amp), sourced from a local electronics supplier I’ve used many times. I've had good experience with their parts in the past, so I have no reason to doubt their specs.
Let me know if you’d like me to post additional measurements or photos — happy to dig deeper.
Thanks again!
Attachments
Pictures form multiple angles with good lighting would probably be best at this point.
also, check with your DMM at these locations for DC, using speaker ground for your black probe. Curious to see if you are getting the ~40Vdc and is it consistent through the PSU. If you can test the +/- rails at the amp boards, do that too. Again, you should see +/-40-42Vdc if the schematic is accurate.
also, check with your DMM at these locations for DC, using speaker ground for your black probe. Curious to see if you are getting the ~40Vdc and is it consistent through the PSU. If you can test the +/- rails at the amp boards, do that too. Again, you should see +/-40-42Vdc if the schematic is accurate.
Is anything getting hot? I had a strange hum issue a while ago that turned out to be a bad output transistor that was bogging down one of the rails.
You could have a ground problem. See if there is any ac voltage between the input signal ground and any other known ground points. A floating input signal ground could cause a hum problem. I do wonder if the amplifier works normally except for the 50 HZ hum that you have from your speakers. Are you sure that the hum is in both speakers or just one. 5.5 V ac does seem like a large ripple voltage with no load. See if there is a DC voltage across either of your speakers. njswede made a good point about having a bad output transistor. Also, maybe the current bias in the output stage/s has become high and is causing high load current on the power supply which would cause high voltage ripple.
Last edited:
You mention about 5.5V ripple on the 4,700uF caps. Is this 5.5V pk-pk measured with your scope, or is it a measurement with your voltmeter?
At the 100Hz ripple frequency, a 4,700uF cap has reactance of about 3 Ohms. Assuming the caps have low ESR, it appears that a lot of current must be flowing, probably over an Amp, maybe more depending upon measurement method. An Amp out of both supplies is a lot of heat. Is anything getting hot?
At the 100Hz ripple frequency, a 4,700uF cap has reactance of about 3 Ohms. Assuming the caps have low ESR, it appears that a lot of current must be flowing, probably over an Amp, maybe more depending upon measurement method. An Amp out of both supplies is a lot of heat. Is anything getting hot?
Yup. It's easy to estimate if we assume that the voltage drops almost linearly during the troughs (which is typically does)
Q=V*C, so 5.5V on a 4700uF capacitor, the charge removed during the trough is 5.5*4.7e-3=25.8mC. And since Q=I*t, and the trough between the half waves is 1/100=0.01s, we get I=25.8e-3/0.01=2.58A. (I hope I got that right. 🙂 )
Conclusion: Something is shorting the supply. Disconnect both speakers and measure the ripple again. If it drops significantly, you have DC on one of the speakers. Measure the DC voltage on the outputs. The offset should ideally be <20mV or so. Anything over 100mV is bad.
Think of changing out the amps with a new plate amp circuit, if you cannot find the fault.
Hopefully, you got top quality parts from a reputed supplier.
NAD is seen as cheap junk by many people, that was the reason for advice to replace.
Check the DC volts at the speaker terminals, should be less than 100mV.
That will tell if the outputs are bad, if there a a series capacitor, that could be faulty.
I did not check the circuit schematic, some designs use a series capacitor between output device and speaker terminals for various reasons, among them DC block and short circuit protection, IIRC.
Hopefully, you got top quality parts from a reputed supplier.
NAD is seen as cheap junk by many people, that was the reason for advice to replace.
Check the DC volts at the speaker terminals, should be less than 100mV.
That will tell if the outputs are bad, if there a a series capacitor, that could be faulty.
I did not check the circuit schematic, some designs use a series capacitor between output device and speaker terminals for various reasons, among them DC block and short circuit protection, IIRC.