Toroid ~ grill interaction

[kriss99]

I built a amplifier with 2 toroids 200VA each. As I remember I had had some buzzing problems but they dissapeared after a while,I dont know how and when. Today I fired up my lovely amp and all of a suden the buzzing came back from his planet ) OMG! what now?.. But I remember of a article here where a diyer said that some appliances interact through the mains and cause the toroids to buzz.

Well, I went to the kitchen full of doubts of this explanation,returned to my room and yes! the buzz disappeard!)..I made this go and return from kithen to my room several times to be sure,so not to waste your valuable time. The grill is causing buzz.That person said that hair dryers made the toroids to buzz too.

Do you know of some protection circuit to build for this problem?
Thans a lot.Kris

[Conrad Hoffman]

I haven't had the problem, but apparently harmonics or even DC offset on the line can cause it. IMO, the circuitry to filter it isn't practical, and the best solution is to rubber mount the transformers so the sound isn't magnified by the chassis. Usually toroids come with mounting disks that are suitable.

CH

[Speedskater]

See my post #31 from just 1 hour ago:

DC noise come from live or neutral?

Also see:

http://www.idc-online.com/technical_...D_BY_POWER.pdf

[kriss99]

Thank for the reply.

I found this comment (Nelson Pass) and several others.
"If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +. "

I'm going to try this first.

[AndrewT]

you must fit a voltage limiter across these DC blocking capacitors.
They are in the mains feed. If something goes wrong you could get 250Vac across these caps. You must put in something to prevent a catastrophe.
A pair of inverse parallel diodes can do the job but thwey must be rated for fault current.
I use a 25A or 35A rectifier bridge that allow hundreds of amps to pass and still generate only a few volts of Vf in either direction.

[kriss99]

Can they explode?Its the only thing I'm affraid of.It occured to me sometimes in the past and its not funny at all.

[Conrad Hoffman]

Thus my comment about the circuits not being practical. Wrongly implemented, caps attached to the line can be a significant hazard. I don't think you'll find that solution in very many, if any, commercially offered products. My guess, and its only a guess, is that getting it through UL or CE safety evaluation would be problematic. Personally, I'd just turn off the grill when listening. To paraphrase Clint Eastwood, If you're gonna listen, listen, don't grill.

CH

[AndrewT]

Hi,
both Pass and Bryston use DC blocking in the mains feed.
I suspect there are quite a few others.

[Conrad Hoffman]

I'm amazed, but that's not unusual ;-) Aside from the bypass, the constructor should fully isolate the caps, taking into account creep and clearance distances, since the outer insulation most likely isn't rated for line service. There's no legal requirement for UL approval on things sold in the US, but I wonder how they'd view it. Ditto on CE.

CH

[kriss99]

Ok,I put 2 electrolytics from a failed computer PS, 500uf/200V back to back with (+) connected just for testing.The buzzing sound from this appliance dissapeared totaly.Maybe, I'll try something more safe in a separate isolated box with some diodes as in the schematics.Or I'll buy a new grill, either way I use it once in a month )