Assistance in diagnosing Hum/Buzz/Noise

[xenithon]

Hi all. I hope this is the right section to post this question.

I recently purchased a North Star Model 192 MkI DAC from an official distributor in the US. Where I live we use 220V like Europe, so the distributor performed the voltage modification as per North Star instructions (simple rewiring of transformer and changing of fuse). He said he checked it with a 220V upconverter and it worked perfectly and quietly. I received it and it is fully operational. However, there is an audible hum/buzz/noise. I have tried different power cables, interconnects, AC receptacles etc. but the hum/noise remains. The hum/noise goes up in volume as the volume knob of the amp is turned up. Also, the noise/hum seems to be only in the left channel.

In order to troubleshoot I have tried the following: I disconnected the earth wire on the AC plug of the power cable. When I used this unearthed power cable, the noise completely disappears. When I reconnected the earth, the noise returns.

I thought this may indicate a ground loop issue? However I have read that ground loop noise does not usually increase in volume when an amp is turned up, so I am not sure if that is the cause. I also know that running the DAC without an earth is dangerous and I would not want to put either myself or the equipment in danger of course (safety is most important). I tried connecting a different source (an old CD player) and it is dead quiet with no noise/hum. When I check the power cable of that CD player, I can see that there is no earth connected.

Unlike that CD player though, the North Star DAC is designed to be earthed using a normal 3-wire power cable. On the back of the DAC though it states "this device should be earthed". (The CD player has a hard-wired cable with only the two wires)

I tried connecting just one RCA interconnect cable at a time. With the right cable only, there is no noise; with the left cable only, the noise comes back. When I swap the cables around, the noise moves to the right side. To try eliminate it being an issue with the amp I tried the following configurations (with two different sets of interconnect cables to make sure it wasn't a problem with the cables):
- both channels connected, hum/noise in left channel
- right channel connected only (to right input of amp), no hum/noise
- left channel connected only (to left input of amp), hum/noise in left channel
- left channel connected only (to right input of amp), hum/noise in right channel
- right channel connected only (to left input of amp), no hum/noise

Does this possibly indicate an issue with the grounding of the left RCA jack of the DAC? Or, could it be that the ground is only connected via the left channel RCA interconnect? Does the above "exonerate" the amp as being the culprit? This is stricly a headphone system, using the Woo Audio WA2 headphone amp / preamp.

Also one thing to mention - I was trying out another DAC about a month ago and it had the same type of issue: noise/hum in the left channel, increased with volume increase on amp. I did not try that without an unearthed power cable though as I thought it was a problematic DAC.

Can anyone assist in troubleshooting what the problem could be, and how it can be permanently and safely resolved?

Cheers
X

[jitter]

Sounds complicated. Your experiments seem to point to a system problem and not just the DAC or the amp.

Which channel has the hum if you connect both channels but L/R swapped on one side?

i.e.
DAC L -> AMP R
DAC R -> AMP L

[xenithon]

With both channels connect but swapped the hum is in the right.

Last night was very confusing - to confirm 100% that lifting ground/earth eliminated the hum, I disconnected the earth wire again on the power cable and hooked everything up. Noise/hum was back! Tried a regular power cable again (earthed) - same hum/noise. Connected the old CD player - silent background and clean.

[jitter]

Originally posted by xenithon
With both channels connect but swapped the hum is in the right.

That rules out the amp as the cause. It still may be that the DAC in another system works fine. I had a similar unhappy marriage between a Micromega DAC1 and my DIY headphone amp.

Did you try the DAC on another amp?

Last night was very confusing - to confirm 100% that lifting ground/earth eliminated the hum, I disconnected the earth wire again on the power cable and hooked everything up. Noise/hum was back! Tried a regular power cable again (earthed) - same hum/noise. Connected the old CD player - silent background and clean.

That rules out the earthing as the cause. If the DAC's L channel also hums in another system, the fault lies there. Trying the DAC in another system would be my next step.

In the meantime one thing you could try is turn the toroidal transformers in the DAC a little, see if that has any influence.

BTW, can you plug the power supply in SA two ways like in the part of mainland Europe that use Schuko plugs and sockets? Sometimes turning a plug 180 degrees (swapping L and N) can be a cure for hum.

[xenithon]

I took the DAC to a hi-fi technician today. We opened it up and checked it using a voltmeter and oscilliscope (not that easy as we do not have a schematic).

We found the following:
- in one area of the power supply section, on the PCB, there are three components (look like the component in the attached image; I am not sure what they exactly are ) in a row.
- one of these (labeled IC1) measures roughly 3.3V and is flat on the 'scope
- the second one (labeled IC2) measures roughly 5V and shows a 0.2V point-to-point ripple on the 'scope
- the third one (labeled IC3) measures roughly 9V and shows a large 1.4V point-to-point ripple on the 'scope

He also measured the outputs on the scope. The left RCA jack exhibits a 0.4V ripple (right output is flat/silent, as it should be).

As to what the above actually means and how it can be fixed, I do not know. I have sent the info to the manufacturer and am awaiting a reply. I don't suppose you could shed some light on this?

[wakibaki]

It's impossible to say from your picture, but the DAC is RTB as a manufacturers return via your distributor. It's faulty.

w

[xenithon]

It was a demo unit from an official distributor in the US (I am in South Africa). I have been in contact with the distributor and he is refusing to accept a return.

[jitter]

Originally posted by xenithon
[B]I took the DAC to a hi-fi technician today. We opened it up and checked it using a voltmeter and oscilliscope (not that easy as we do not have a schematic).

We found the following:
- in one area of the power supply section, on the PCB, there are three components (look like the component in the attached image; I am not sure what they exactly are ) in a row.
- one of these (labeled IC1) measures roughly 3.3V and is flat on the 'scope
- the second one (labeled IC2) measures roughly 5V and shows a 0.2V point-to-point ripple on the 'scope
- the third one (labeled IC3) measures roughly 9V and shows a large 1.4V point-to-point ripple on the 'scope

In the picture left to the component I can see "C1" which is cut off. That would be the "IC1" you mention. I also see a 0.1uF cap and an elco in the vicinity. For me they are signs that these components are most likely stabilisers.
They should not have more than some mVpp ripple when fully loaded (which they are not, there is no heatsink on them).

He also measured the outputs on the scope. The left RCA jack exhibits a 0.4V ripple (right output is flat/silent, as it should be).

There's the source of the hum! Since the R channel has no hum, it doesn't share the power supply with the L channel and must have its own (useful for comparison when measuring!).

As to what the above actually means and how it can be fixed, I do not know. I have sent the info to the manufacturer and am awaiting a reply. I don't suppose you could shed some light on this?

It's either an overload of the power supply (in which case the stabilisers get very hot) or the stabilisers don't get sufficient voltage to work correctly (bad smoothing cap or defective rectifier).
Let the technician measure the input voltages of the stabilisers with the scope and compare them to the R channel. I bet there's a significant difference.

[xenithon]

In the picture left to the component I can see "C1" which is cut off. That would probably be "IC1". I also see a 0.1uF cap and an elco in the vicinity. For me they are signs that these components are most likely stabilisers. They should not have more than some mVpp ripple when fully loaded (which they are not, there is no heatsink on them).

I unfortunately did not take a photo of the entire PCB, and the DAC is with my technician at the moment. I found a low-res image in a review and have attached an image with lines pointing to where these are located. There are three in a row (~3.3V, ~5V, ~9V). Looks like the path is from transformers to PCB through some caps and then to these, so perhaps they are indeed voltage stabilizers.

There's the source of the hum! Since the R channel has no hum, it doesn't share the power supply with the L channel and must have its own (useful for comparison when measuring!).

Hmmm, one thing I recall the technician saying (working from memory here) when he first look at the PCB and checked with the voltmeter, is that I believe there is a single resistor on the PCB which is, what he called, a "common ground" across all the RCA jack (there are 4 in total - 2 digital RCA inputs, and the 2 analog outputs).

It's either an overload of the power supply (in which case the stabilisers get very hot) or the stabilisers don't get sufficient voltage to work correctly (bad smoothing cap or defective rectifier. Let the technician measure the input voltages of the stabilisers with the scope and compare them to the R channel. I bet there's a significant difference.

Will do, thanks. But are you refering to the 3 stabilizers (2 of which have the ripple) or components nearer the analog output stage?

[jitter]

Originally posted by xenithon
I unfortunately did not take a photo of the entire PCB, and the DAC is with my technician at the moment. I found a low-res image in a review and have attached an image with lines pointing to where these are located. There are three in a row (~3.3V, ~5V, ~9V). Looks like the path is from transformers to PCB through some caps and then to these, so perhaps they are indeed voltage stabilizers.

My work is in electronics and that makes recogising certain circuits a little easier. In your picture from right to left (top transfo): transfo, rectifier, smoothing caps, stabilisers, caps, circuits to be fed.

Hmmm, one thing I recall the technician saying (working from memory here) when he first look at the PCB and checked with the voltmeter, is that I believe there is a single resistor on the PCB which is, what he called, a "common ground" across all the RCA jack (there are 4 in total - 2 digital RCA inputs, and the 2 analog outputs).

Yes, that sounds logical. In most designs everything shares the same ground (common). But they are kept separate except at the connection point.
But that still means that a faulty power supply of one channel will have no effect on the other.

Will do, thanks. But are you refering to the 3 stabilizers (2 of which have the ripple) or components nearer the analog output stage?

I'm referring to the stabilisers.