Cambridge Audio A500 Amplifier problems!

[Mark Hennessy]

It's simply because they are more "ideal" components than IE transformers. Better magnetic circuits, less copper for a given power rating, lower I2R losses, etc. Now, if only I could remember where I found the article that gave the real detail on all that...

I've had my fair share of failed ones. I wonder if it's mechanical? The wire is put on at a high tension, and by the time it's all been clamped together, the stresses on the wire, especially at the corners, must be significant. The primary windings are made with fairly thin wire.

[Skywave]

Yes Mark, 'mechanical' could be highly relevant, along with thermal considerations, especially if the primary is wound first (as in IE transformers).
A few thoughts: if the secondary is supplying a substantial current, the primary winding will obviously heat up & the copper will try to expand. That heat cannot escape easily. All that expansion and subsequent shrinking (as it cools down after power off), could lead to wire fracture, especially if the wire is thin and initially wound tightly around the small radius curves of the core. The secondary winding is on the outside, usually thicker copper wire, greater radius of curvature: so far less chances of similar breakage.

Does some of that sound feasible to you? Or is the cause far more obvious: some toroid-wound transformers are simply cheap 'n' nasty.

Al.

[Old Sparky]

In the case of the A500 I've recently repaired, it's 'so far - so good'. The amplifier has been run for 2 or more hours at a time, albeit at modest volume levels, switched off, left for several hours, then switched on again, when it continues to work O.K. This procedure has been repeated for three days now.

[camallison]

You're probably aware that toroidal transformers can have a large inrush current on power up.
Colin

No, that fact I didn't know. Presumably, you're comparing that to the in-rush current for a conventional transformer of the same VA rating. Why is that current so much larger for a toroid - and do you have any idea of the ratio of those two currents?

Just curious - and keen to learn.
Thank you.

Al. / June 5 //

Yes, compared to a conventional transformer. Why? I don't know, but it was something I learned the hard way, with a product recall on industrial control equipment of the 30 to 50 VA rating. I will consult one of my learned engineering buddies who was involved at the time and ask the question.

Colin

PS - while waiting for his reply, I found this:

"When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork."

[camallison]

Caught him reading his emails over breakfast! His response is:

" Because of the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher in rush current, compared to laminated E-I types."

He reckons anything up to double or treble has been witnessed, hence the slow blow fuse. He himself has seen 50 to 60% and more. A solution can be to add an NTC in the circuit to limit the current although these tend to heat up (as expected) and need a good 60 seconds cooling after switch off before attempting a restart otherwise their efficacy is reduced.

Colin

[Nick]

...I found this:

"When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork."

This is why toroids are supplied with rubber washers and normally a warning note - a bolt to the chassis connected at both ends would be a heavyweight shorted turn - the xfrmr will get toasty and die.

There are many plus points for toroids - the main ones from an audio standpoint are less stray flux (contained in the core), compact size/weight, better efficiency (higher flux densities are possible), lower power consumption when off-load, less magneto-constrictive hum, etc.

[camallison]

...I found this:

"When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork."

This is why toroids are supplied with rubber washers and normally a warning note - a bolt to the chassis connected at both ends would be a heavyweight shorted turn - the xfrmr will get toasty and die.

There are many plus points for toroids - the main ones from an audio standpoint are less stray flux (contained in the core), compact size/weight, better efficiency (higher flux densities are possible), lower power consumption when off-load, less magneto-constrictive hum, etc.

>> from an audio standpoint are less stray flux (contained in the core),

I agree Nick - my main use of them is in audio equipment and PC-based sound systems - lovely job they do.

Colin

[Old Sparky]

The toroidal Transformer in the A500 is fixed, like most, AFAIK, with a single bolt through the center hole. Insulating sheets are fitted both above and below the transformer, which has a metal disc on top of it, then the fixing nut above that(see the photo in my earlier post) This, presumably, makes a capacitor out of the fixings, with the chassis as one 'plate' and the top disc as the other, and the transformer as the dielectric. No idea what the capacitance would be, though!