Looking for advice on mechanical hum

[extracampine]

Conducted a further experiment. I plugged the DAC in in a music shop some 35 minutes drive away to see if it would still hum whilst on a different part of the grid - and it did. So it would seem to be less likely that it is something else in my house, or the power supply to my house, which is causing the problem.

[extracampine]

OK, I had it checked by a local electrical engineer - he seemed to think that the level of transformer hum was within manufacturers tolerances. 

[extracampine]

A final update for anyone that is interested or experiencing a similar problem. I ended up returning the DAC to the manufacturer (Lampizator) a second time. This time they replaced the transformer with the one they are using in their newer DACs. I had thought that the noise was coming from the choke (what I had initially thought was a EI transformer), so was somewhat skeptical that replacing the toroidal transformer would make a difference. 

 

So on unpacking, inserting the tubes and making the various connections at the rear, with trepidation I pressed the power switch. And....BOOM the DAC blew up!! Just kidding...THERE WAS NO HUM!!! Hallelujah! Or, more accurately, there was a very low level of hum which is in keeping with the other audio components and not audible once a foot or two away from the DAC.

 

So after a (fairly) long road, this problem has been solved. Thanks to all for the advice given  

[Ryan Berry]

Sorry I caught this one late, but glad you got it sorted.   We've ran into a few instances where someone would say a transformer was humming but could not replicate it at Ayre.  The problem is that most companies have a lot more ambient noise between lighting, equipment, people, etc. than you do in your house, which can easily mask the sound.  I've had to stay after everyone else has left on a few occasions to shut everything else in the building off to hear what the customer was talking about.

 

Almost every transformer has a little variance and it in some cases can cause otherwise unexplainable audible hum.  Geometry of the transformer (particularly with EI core bobbins) is one of the biggest contributors to how quietly it will perform, though there's other factors as well.  We've tried various experiments including having the wires individually coated in epoxy as they were wrapped to try and get the noise down, but in the end the only real solution was to get the shape right for the current we needed.  Even then, you run into one every so often that doesn't behave as well as the others and there's not much you can do except replace it.  Of course, as  you noted, the transformer is still "within tolerance", so figuring out what to do with them is the next problem.

[extracampine]

Yes, you really needed a fairly silent environment to pick up the hum (with no ambient noise in the background). I had worried about there being too much noise at the Lampizator testing lab and had mentioned this to them in my first email.

 

But that is how I (and likely others) like the listening room - silent! From some of the comments, I'm surprised that this isn't a more recognised issue. It's not that I have super-sensitive hearing or that my room is an anechoic chamber or anything!   

[One and a half]

@extracampine, glad this problem is put to rest and the manufacturer found a quiet transformer. In all good engineering practice, a larger than necessary transformer was spec'd for the DAC, and transformers being what they are like to have a minimum load to keep them quiet. The construction though is the main issue, two transformers built on the same day can have different tolerances, but it's up to the designer to factor in those tolerances. 

 

In some respects SMPS are very quiet acoustically, except perhaps for a low level high audio frequency pitch from the switching transistors. That's about their only advantage!