As mentioned above, I have tried 2 DC blockers, both with no improvement of the hum. I have not tried a balanced transformer. Does this have to be “audio grade”?
You tried a different DAC?
I used to use a different DAC and it had no hum - but then it had a different type of power supply.
Thank you for your insight.
I think that unsound transformer (or position, fixing, etc.) ought to be changed - or whatever what hums. May be it was constructed for different frequency of network voltage?
Last week I have returned apparently very good amplifier - was gently purring to me… It could increase in the future - mechanical vibrations!
Agreed, if there’s no hum with a different DAC in the chain take it back and insist on a refund.
As @evand suggested, try another DAC and if there’s no hum, get your humming DAC replaced with a silent one 
or insist on getting your money back.
I don’t know if you’re in the mood for a silly joke about humming audio components, but if you are, click here.
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Thanks for the ongoing insights here.
I have 4 further small findings that might help in the quest to remove the hum;
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I took the case off (again) to see if the rod that holds the transformer needs tightening or loosening. What I found before getting to that however, was that the casing seems to be contributing to the noise. With the case removed, the noise is definitely less. It is still audible, though perhaps heading towards the level that would not be noticeable from the listening spot. I had tried removing the case before and don’t remember a particular reduction in noise, but there you go. So I got some thin rubber pads and tried to insert them in-between the front panel and side casing. Once the screws were again tightened, this did not seem to improve the noise. Flexing or moving the top casing also seems to affect the noise somewhat, though inserting anything in-between this and the base is quite tricky. Either way I did not have the impression that inserting further bits of rubber would likely affect the noise, given my findings with the rubber at the front.
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Trying again to isolate the area that the hum is coming from, I think it is coming from a different component to what I had expected. I had thought that the noise was coming from the main transformer (a bit round thing which I have labelled ‘A’), though it appears to be coming from somewhere nearby. The exact culprit is difficult to determine, though I’m wondering if it is coming from the component highlighted in my photo below (which I have labelled ‘B’). I’m not sure what this component is but am wondering if it is also a kind of transformer?
- I noticed that one of the bolts/pillars holding the PCB to the casing appears to have current running through it (if that is the right term) - when I touched it, I felt a small shock. I can’t remember if my other hand was touching the casing or not. I also don’t know if this is normal/expected or not. The unit was powered on at the time. Maybe I was stupid to be prodding around inside with the power on. I don’t know if the other 4 bolts are the same (I didn’t fancy testing)! I have highlighted this bolt in the picture below also.
- I took the DAC to my workplace and plugged it in there. Same hum. It is however only about 3 miles away as the crow flies - don’t know if this is relevant or not.
Thanks again for the help here. Hwz - the Rowan Atkinson sketch made me laugh.
OK, I had it checked by a local electrical engineer - he seemed to think that the level of transformer hum was within manufacturers tolerances.
Luckily you didn’t electrocute yourself. I’m not an EE, but I can’t imagine there should be current flowing through the mounting bolts, and no properly functioning DAC I’ve heard has a mechanical hum. The transformer may be within manufacturer spec, but no DAC should have an audible hum. Take it back and insist on a refund.
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The engineer did not find any stray current in the system. As it happens, I am sending it back for them to look at it again and update one of the transformers.
You say that no properly functioning DAC has mechanical hum - I actually went into a few hifi shops and noticed other items with transformers emitting mechanical him. Naim stuff especially.
Edit - and yes if the problem cannot be fixed I would look for a refund. But it has been a few months since purchase - would I still be covered for this?
Just realised that I hadn’t put an update here. I returned the DAC to the manufacturer, they replaced the tranformer with a newer model and hey presto, the hum is gone!! So all’s well that ends well. Hopefully my journey here might help others who have a similar problem.
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