Good evening.

I recently signed up and would like to ask you for advice.

I'm building a transformer for the P******t Phono Box II SE.

This phono preamp is powered with an external 230vac - 16vac transformer, so the rectifier components are inside the phono preamp, and the wallwart is just a small 230/16v 1A transformer.

I purchased a 16v 2A toroidal transformer that I would like to put inside an aluminum box.

The bolt that fixes the toroidal (made of non-magnetic steel) is glued into the aluminum, so there is no electrical continuity between the aluminum case and the fixing bolt. Can this create electromagnetic problems?

Thanks for your help,

Best regards,

Luca

The fixing bolt doesn't cause any interference problem I've ever heard about, but I'm sure there will be many imagined ones.

I'm assuming you understand about the single shorted turn caused by grounding the top of the bolt to the case? Destruction is usually instantaneous, if the fuse doesn't blow in time.

There is no need to isolate the bolt from the case (except for the above warning). And there is no need to use a stainless steel bolt - a plated steel bolt is just as good.

I assume it has a dimpled top washer? And you'll use the rubber washers top and bottom to reduce the crush on the windings?

Thank you Mr. Slee for your kind response.

Clearly I am not an electrical engineer; I know how to use a soldering iron, do small electrical jobs, but obviously I need help understanding some things..

The previous transformer actually works very well (small, with laminations) but I had some free time and I just wanted to do some experiments to see what changes the new transformer could make to the sound :-)

Yes, I had read up on the short circuit of the bolt that touches the case in the upper part; Precisely for this reason I was wondering if completely isolating the bolt could create problems.

I will have to glue the bolt inside the box with epoxy resin for "practical" reasons: I can't have the head of the bolt outside the case. I need both ends of the case to be flat.

So I wondered whether this solution could create electromagnetic problems.

I thought of non-magnetic steel bolts for a reason: if they are immersed in the magnetic field they shouldn't vibrate... (or not?). Since they only cost 10 cents more than iron bolts, I figured they were worth it...

I then thought that I could use also a microphone cable to connect the low voltage power from the transformer to the phono preamp.

I saw that there is the Cordial CMK250, which is shielded cable, flexible, and has two 0.50mm2 conductors and should be more than enough to carry 16v 2A (other microphone cables have 0.22mm2 conductors on average).

I would like to try connecting the shielding of this cable to the aluminum case, and the aluminum case to the electrical earth; in this way I would offer an effective shielding to the cable.. or not?

I apologize if many of my thoughts may seem like blasphemies to a professional, remember that I am just an enthusiast..

From Doug Self's book on amplifier design

Thanks Graham for your response.

In my case, the toroid bolt is electric insulated both top and bottom.

However, figure B is not clear to me: in the upper part the bolt is close to the case cover, but does not touch it... does this cause problems anyway? I was convinced that there had to be contact with the top bolt and the case..

Anyway, I send a drawing of my situation..

Figure B is Self's illustration of an accident waiting to happen. Doug is good at dry humour - some call it sarcasm - but he makes the point.

Your diagram shows you've covered that base even if the top got squashed in, as the bottom of the bolt is not electrically connected to the case.

Ok, thanks for the explanation, I understand now.

I suspected (figure "b") that a minimum proximity between the bolt and the top case was enough to create some magnetic short circuit, but if the problem is an accident that hasn't happened yet, the italian irony is even more "scratching": it would be enough to put a plastic sheet between bolt and top case :-)

Ok, as soon as I get all the material, I'll get to work and try to evaluate any differences in the sound..