I've been playing around with my 2 NAD 3020 Series 20 amps today. They've both been recapped and serviced and sound excellent, but both have a low volume hum on the right channel of the preamp that is not present on the power amp output. It's a bass frequency around 100 Hz that does not respond to the volume control but increases in volume with the bass control, as you might expect given the frequency.
This is a bit annoying to say the least despite the low volume of the hum. I do have sensitive hearing and now I've noticed it, I can't stop. I can make it go away by turning the balance dial all the way to the left channel or removing the pre-power links for the right channel. It would be nice if both channels of the preamp were as clean as each other. BOTH of my units are going this. I hope it isn't normal!
Any ideas what could be causing this? I replaced the 4 1N4002 rectifier diodes in one of the units without change.
This is a bit annoying to say the least despite the low volume of the hum. I do have sensitive hearing and now I've noticed it, I can't stop. I can make it go away by turning the balance dial all the way to the left channel or removing the pre-power links for the right channel. It would be nice if both channels of the preamp were as clean as each other. BOTH of my units are going this. I hope it isn't normal!
Any ideas what could be causing this? I replaced the 4 1N4002 rectifier diodes in one of the units without change.
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Have the main filter capacitors been replaced?
Try powering the amp from a lab DC supply instead of the AC line. Connect the supply just AFTER the rectifiers
so that it is not shorted out by the secondary winding. Does the hum go away?
Try powering the amp from a lab DC supply instead of the AC line. Connect the supply just AFTER the rectifiers
so that it is not shorted out by the secondary winding. Does the hum go away?
The main filter capacitors have indeed been replaced.
It turns out that the hum is affected by the volume control, so I was wrong to say that it does not respond to the volume control in my original post. I'll try to edit that.
I'm currently void of the 2 instruments that might make this repair easy - a DC power supply and an oscilloscope. All I have to hand is a high quality multimeter and a couple of other more superfluous instruments. That being said, the hum may not be loud enough for the oscilloscope to pick it up. I am definitely hearing a 100 Hz sine wave.
All this being said, the power transformer is right next to the left channel of the power amplifier section and the hum is present only on the right channel of the preamp, which is on the total opposite side of the unit. Removing the links between the pre out and amp in connectors for the right channel causes the issue to disappear completely.
I'm baffled by how my 2 near-identical units (different transistors in power and driver sections) are doing the same thing. My gut is telling me that it has something to do with the power supply of the preamp but that is merely superstition on my part.
Since the power supplies are shared, and there is hum in only one channel,
there could be another bad electrolytic capacitor, or a bad connection.
Tracing out the problem will be easier when you get back the test equipment.
Other than the phono stage, before the volume control are only RCA jacks, RC filters, switches,and pcb traces.
Try shorting plugs in all the RCA input sockets and see what happens to the hum.
there could be another bad electrolytic capacitor, or a bad connection.
Tracing out the problem will be easier when you get back the test equipment.
Other than the phono stage, before the volume control are only RCA jacks, RC filters, switches,and pcb traces.
Try shorting plugs in all the RCA input sockets and see what happens to the hum.
Actually, one unit has a hum that responds slightly to the volume control where the other does not. This may be due to the fact that I have partially disassembled the unit where the volume control has a slight effect. For argument's sake, let's say the hum does not respond to the volume control.
Touching the pre-main jumper for the right channel increases the volume of the hum considerably whereas touching the one for the left induces no interference. This is the case for both units.
Sorry for the confusion.
Touching the pre-main jumper for the right channel increases the volume of the hum considerably whereas touching the one for the left induces no interference. This is the case for both units.
Sorry for the confusion.
There certainly can be more than one source of hum, but after the volume control is much more complicated.
Instead of the back panel jumpers, try a pair of RCA audio cables. Does this help with the hum?
The body of the volume control must be bolted to the front panel, and the front panel must be bolted to the chassis,
to be properly grounded, and hum-free.
Instead of the back panel jumpers, try a pair of RCA audio cables. Does this help with the hum?
The body of the volume control must be bolted to the front panel, and the front panel must be bolted to the chassis,
to be properly grounded, and hum-free.
If two amplifiers of same model are experiencing the same behaviour then it may be a design problem - there are long signal tracks from back panel to source switches in front that could act as antennas in certain cases.
In order to locate the source of picking up the hum I would redirect the signals to opposite channels at volume pot input by cross-soldering R501 and R502.
If the hum stays in the right channel then the amplifier stage is the source of it.
If it moves to the other channel then source switches, signal tracks and cables should be inspected (next thing to do would be to cut off the tape recording signal path).
Since the amplifiers have been serviced then the capacitors (47uF/6V3) in the amplifier stage have already been replaced for new ones?
In order to locate the source of picking up the hum I would redirect the signals to opposite channels at volume pot input by cross-soldering R501 and R502.
If the hum stays in the right channel then the amplifier stage is the source of it.
If it moves to the other channel then source switches, signal tracks and cables should be inspected (next thing to do would be to cut off the tape recording signal path).
Since the amplifiers have been serviced then the capacitors (47uF/6V3) in the amplifier stage have already been replaced for new ones?
...
You should return to square one - assemble fully both units, close all covers and start the search for the hum source then. 😉
I think I'm going to have to dispel the idea of looking at both amplifiers simultaneously because it turns out both have different symptoms despite showing exactly the same one at the surface. The hum in one is responding to the volume control where the other is not.
On the unit where the hum does not respond to the volume control, I've narrowed down the issue to the area around Q502, possibly Q502 itself, unless what I'm seeing in this area is a symptom of another problem. Touching components in that area with a wooden stick causes the hum to increase dramatically, particularly R516, C508 and C506, with a slight increase when Q502 is touched. I'll start by updating Q502 and its sibling in the left channel with a new ONSemi device before replacing R516 and R512 and so on. It seems something is noisy rather than open or short and it could well be a resistor. A solder problem is unlikely as every single joint in that area was redone with total removal of the old solder, given the old was in a very strange condition with most joints having a web-like appearance.
These amplifiers have a plastic faceplate which fits over a metal chassis, so no worries about potentiometers not being grounded or any breaks in the grounding on the partially disassembled unit.
I'll report back tomorrow once I've replaced the suspect components.
On the unit where the hum does not respond to the volume control, I've narrowed down the issue to the area around Q502, possibly Q502 itself, unless what I'm seeing in this area is a symptom of another problem. Touching components in that area with a wooden stick causes the hum to increase dramatically, particularly R516, C508 and C506, with a slight increase when Q502 is touched. I'll start by updating Q502 and its sibling in the left channel with a new ONSemi device before replacing R516 and R512 and so on. It seems something is noisy rather than open or short and it could well be a resistor. A solder problem is unlikely as every single joint in that area was redone with total removal of the old solder, given the old was in a very strange condition with most joints having a web-like appearance.
These amplifiers have a plastic faceplate which fits over a metal chassis, so no worries about potentiometers not being grounded or any breaks in the grounding on the partially disassembled unit.
I'll report back tomorrow once I've replaced the suspect components.
Watch out for radiated noise also. In one instance I had a CFL lamp inducing noise in an amp, and another had a 'deathdapter' USB charger which was putting heaps of junk into the mains close to the amp.
Try plug the amp in at another location and turn off any fluorescent lamps nearby to rule that out.
Try plug the amp in at another location and turn off any fluorescent lamps nearby to rule that out.
Mine are the earlier model with the 4 main filter caps behind the headphone socket and not in the centre of the board.
No dice at all replacing the components in the area around Q502 including Q502 itself. Complete waste of time doing so as it's doing exactly the same thing.
Any ideas as to why the hum would be increasing when I poke the base of Q502, both ends of R516 and C508? They cause the most interference but I also get smaller interference touching R512, R514 and C506.
I'm starting to think it could be the volume control itself. Ever seen a seemingly perfectly functional volume pot cause a hum?
Any ideas as to why the hum would be increasing when I poke the base of Q502, both ends of R516 and C508? They cause the most interference but I also get smaller interference touching R512, R514 and C506.
I'm starting to think it could be the volume control itself. Ever seen a seemingly perfectly functional volume pot cause a hum?
Have you closed the covers and started fault diagnosis from that point? If yes then what do you find?
Yes. The cover being on doesn’t matter because that does not interfere with the grounding, and the fault does not change.
Poking the area around Q502 with a wooden stick causes the hum to increase. Touching the pre-power jumpers does the same. Nothing of the sort happens on the left channel, which is dead silent without a signal.
Ok, in the state of closed covers - if you use RCA cables instead of the rear panel connectors (between preamp and poweramp) and redirect the signalds between channels then does the hum stay in its initial channel or does it move to the other channel?
You won't believe this. I've just been out to pick up a very, very nice NAD 3120, a 3020B but without tone controls, in totally stock form down to the original electrolytics, and it is doing EXACTLY the same thing.
This very low level hum on the right channel absolutely must be normal, which is sad because that means I'm now not as much of a fan of these amps as I was.
When I get my scope I will be sure to try to pinpoint the source of this issue. That being said, I fear the hum is of too low dB for the scope to pick it up...
This very low level hum on the right channel absolutely must be normal, which is sad because that means I'm now not as much of a fan of these amps as I was.
When I get my scope I will be sure to try to pinpoint the source of this issue. That being said, I fear the hum is of too low dB for the scope to pick it up...