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Transformer Hum

cctaylor

cctaylor

pfm Member
Just had an interesting experience. Sitting quietly in the study the transformer hum level on my amps suddenly dropped. I have a Naim system with 3 bolt down 250s. The oldest one (by about 6 months) is the noisiest and one is silent.

This would suggest that there is an appliance producing a DC offset. Most likely candidates are the Samsung freezer or the Panasonic washing machine.

Looks like a DC blocker experiment might be worthwhile.
 
The timing could also suggest less strain on the grid as folks return to work, kids go back to school?
 
I used to experience exactly this, mad buzzing hum starting up out of nowhere, and stopping too, not obviously related to the time of day. I built a sjostrom DCT03 and now I just have gentle hum all the time. I didn't notice it until I went active, then the combo of 2xHicap and 2x250 seemed to make it worse or perhaps just more noticeable.
 
Heckyman said:
Why not just temporarily unplug all your appliances and see if that alters anything?
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Other than proving the source is in the house, there would be nothing I could do with that information. It still may be on the incoming mains line. Our houses are increasingly being filled with gadgets with switched mode power supplies and often they are poorly designed. One on it's own may not be too bad but add them all up and you start to see problems.
 
My Croft power amp has a mains transformer which gets louder and quieter every 10 seconds. It gradually gets louder, then subsides. It’s not very loud overall though. No idea why it does this.
 
I recently tried an ifi DC blocker that you plug into the iec socket of the amp to see if it would reduce a mild bit of hum on a power amp, made no difference at all. Some amps just hum regardless I guess.
 
Nagraboy said:
My Croft power amp has a mains transformer which gets louder and quieter every 10 seconds. It gradually gets louder, then subsides. It’s not very loud overall though. No idea why it does this.
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My Rega Elex r does exactly this too, kind of oscillating.
 
roach said:
My Rega Elex r does exactly this too, kind of oscillating.
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Funny thing is, I’ve never noticed this sort of thing happening with ‘cheap’ equipment. Very curious.

My Croft preamp was buzzing quite a lot when new but when I opened it to put a NOS cathode follower valve in it (courtesy of @The Bish) it was silent. Turns out I’d not done the screws up tightly enough when I initially opened it to put a pair of JAN 5751 in the phono stage, and the loose screws enabled the case to vibrate. As I say, it’s silent now.
 
Some transformers hum more than others, if you are just getting constant levels of hum I suggest it unlikely it is dc on the mains, that seems to come and go a fair bit more. If you are experiencing loud humming buzzing sometimes that comes and goes over a period of tens of minutes, or hours, then that, to me, was my sign I had dc issues. Then not all dc blockers are born equal, I don't know about the ifi but I was warned off the sjostrom DCT02 as potentially blocking insufficient dc voltage to fix the problem, the DCT03 was recommended, and worked.
 
There were/are quite a few reports of Rega and newer Audiolab (6000a) amplifiers with transformer hum issues.

I believe Audiolab did supply a DC blocker FOC for a time to affected owners but probably not anymore.

I have been lucky and only ever had the issue with a NAD receiver back in the early nineties. The thin casing was resonating in tune and it sounded like a giant bumble bee. I remember changing the receiver 3 times through RS Southampton until I gave up and bought an Arcam.
 
My Carat amp's transformer also hum. I have been thinking about trying the DC Blocker from IFI, but I would have to extend the wire somehow, since there isn't room behind the amp as is.
 
david ellwood said:
Given all houses live on the secondary side of a substation transformer, I dont know where a DC offset would be coming from.
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A number of sources. Just putting a diode in series with a heating element thats often done to give a 'half power' switch (especially with older devices) will put DC onto the mains.

It should be noted that this affects toroidal transformers through a process of magnetostriction (the size of the core changes/vibrates) and then usually only for transformers greater than around 300VA as the larger power devices have lower primary DC resistance where even a small DC offset will cause noise due to asymmetric saturation. Surprisingly this is most likely to be heard under low load as this is when transformers have maximum flux density. So the toroid in an amp may be noisier at idle than when under use.

Fun no?

DV
 
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