Humming toroid transformers

[johnheath]

I have tried to  build a couple of things with two toroid transformers to reduce hum and noise and it works …most of the times.

In my latest LA2A clone, using two toroids, the piece is quiet until I am about to fasten the lid.

Now, for some reason, I get a quite loud hum again??? I move them around… find a new sweet quiet spot, but when the lid is placed on top the hum is too loud for my acceptance.

I do not have equipment to measure noise floor and similar stuff… I go strictly by ear.

Can anyone explain why this happens and also preferably how to get rid of it?

Thanks

/John

 

[ruffrecords]

Is it a steel enclosure?  Is the top near to the toroid? If it is. try using a brass screw to mount the toroid.

Cheers

Ian

 

[CJ]

a lid is metal so it can act as a lamination and pick up stray fields,

this can pull flux in closer to the circuit,

just a guess, : the lid  can complete a magnetic path with the bottom and sides?
in other words, taking the lid off might be like adding a air gap to a transformer, it lowers the perm of the chassis by breaking the magnetic circuit,

 

[johnheath]

Well, the entire chassi is made of aluminum and even so the lid.

I have tried to mount them standing or laying down and there is always a sweet quiet spot but the lid is bringing the hum back.

"Brass screw" … what effect would that add?

Thanks

/John

 

[ruffrecords]

If the chassis was steel and you used a steel screw then when you attach the lid you make a nice magnetic circuit as  CJ said. You could even get a shorted tirn if the steel screw touch top and bottom which will definitely give you a hum. A brass screw is non-magnetic and so breaks the magnetic circuit.

But you have an aluminium chassis so I am as puzzled as you are.

Cheers

Ian.

 

[johnheath]

Thanks Ian

I started to feel stupid as hell here =)

I post i picture of the build and hopefully you, or somebody else, can see the "problem".

Thanks

/John

 

[ruffrecords]

Did you not get a mounting kit with the toroid? Usually consists of a 6mm screw and nut/washer and a couple of rubber pads and a round metal clamp.

Cheers

Ian

 

[johnheath]

Oh… well, yes I have it… the photo is just taken while trying to place them in the chassi.

Thanks

/John

 

[gyraf]

John,

I hope you are aware that when mounting a toroid, the holding/center screw must only have connection to chassis in one side. If screw touches or otherwise connects to top plate, it forms a shorted "winding" on the toroid at low voltage and VERY high current.

So make sure that the mounting bolt only touches chassis in one end.

Jakob E.

 

[johnheath]

Hi Jakob

Yes, I am aware of that.

I know that you have used two toroids in your G9… and I have used that idea with good results but have you experienced this problem as I now have?

/John

 

[johnheath]

This thing with double transformers works great in preamps but it seems to be tricky in the LA2A circuit…. or am I wrong here?

I was thinking if it might be a mistake to add two transformers inside the LA2A but what do I know?

Or… would there be a difference if I used toroids rated for higher effect? Mine are 50VA so would it be better with let's say 80VA for less "hard-working" transformer and therefore less EMC?

I know that there are a lot of smart guys here so hopefully somebody can give me some ideas.

Thanks

/John