Buzzing mains Transformers

[Darren D]

Still on the quest for minimum hum...

What's the boards consensus of Mains transformers with mechanical hum?

Mine buzz away merrily, but I don't think that affects or is connected to any hum in the circuit?
or does it,

without another pair of trans with the same voltages it's difficult to test.

I would like to buy another pair, 325-350V, but the fear of ending up with the same issue with new transformers puts me off. Does anyone make transformer these days that don't hum on todays higher mains voltages. My mains is 248V.

[andrew ivimey]

248V might well be the devil in the detail (mine's 237v). Would a mains conditioner of some sorts be the answer instead of buuying individual new transformers?

[Global]

DC on the mains will contribute to the hum problem as well. One of the many blockers is shown here.

http://www.diyparadise.com/dablok.html

I know Andrew tried this and it turned out DC was not his problem but it certainly helped mine!

Don't touch this with a barge-pole unless you are knowledgeable and comfortable working with mains voltages. I did not go with the '6.3v caps are OK' theory. I had some 100v caps in the box.

Cheers

Mark

[Darren D]

Decided to measure my mains this morning.

246VAC
1.835VDC

is 1.8 VDC enough to give us problems? I know 246VAC is high, it can sometimes go to 251V.

I tried a couple of line conditioners that brought the voltage down to around 230VAC, didn't measure the DC, perhaps I'll try again.

They didn't make one spot of difference though. In fact it buzzed merrily along as well.

[Global]

1.8v DC is a serious offset!

If you use the circuit I linked to, you should use three diodes in series as a replacement for the single diodes shown.

This should reduce hum and transformer heat.

Mark

[Mike G]

Hi,

Having a KAT6550 with a main's transformer that run's hot and buzzes to some degree, i'm very interested in trying this mod, and wondered if it would be worth a go.

Sourcing the capacitors should not be a problem, I think I may have a couple tucked away somewhere, but what about the diodes? Can anyone recommend which type, or maybe an RS number or alike? I did have a quick look today in the catalogue, but there are pages and pages to choose from!

Also I was considering building it like an external mini mains conditioner, so it can be easily removed if it doesn't work, or would it be most effective, built into the amp as close to the mains tx as possible.

Your thought's would be appreciated.

Many thanks.

Mike.

[Global]

I made mine with RS 380-4849 which are 3 amp, 200v devices. This is well over the top but I had them to hand.

The theory, as I understand it is that the capacitors conduct the AC with low reactance but block the DC. The diodes' forward voltage drop eliminates the DC. So in the case mentioned earlier the diodes will see a forward voltage of 1.8 volts. If they have a typical drop of say 1.2v, this will leave about .6v remaining offset, therefore use two or three in parallel to eliminate more DC.

I built mine into a plastic case with hard-wired mains leads in and out. I did not want anything plugged into the device which might try to draw a heavy current.

Again, please don't try unless you are really comfortable with mains power and REMEMBER - THERE SHOULD BE AN APPROPRIATELY RATED FUSE ON THE MAINS SIDE OF THIS DEVICE!

Cheers

Mark

[Mike G]

Thanks for that reply Mark, i'll have a look in RS tomorrow.

I have an idea for a container, and suitable components to use as a fuse holder, so i'll have a go at building an external device, and post so feedback once i've tried it.

Mike.

[Stratmangler]

I built the DC Blocker on http://www.diyparadise.com/dablok.html
and lifted the performance of my equipment by 6 or 7%.

And all of my stuff is Solid State.

I fitted the filter into a plastic project box with a mains socket on the front, and then plugged my mains distribution board into the filter.

Didn't kill much of the (mechanical) noise from my amp's torroidal though.

Chris

[Primalsea]

How did you measure the DC offset?? You didn't just use a DMM did you??

You really need to use a scope and possibly as resistive divider if the scope can't handle mains voltages. A DMM set to read DC volts won't give a correct reading unless it has a special function for DC offset.

Trannies seem to hum for a few reasons. DC offset, construction & overload. You have to find out why the trany hums or buzzes first.