audio-talk

Before I got into forum based DIY hi-fi I relied on magazine articles and things like the excellent Russ Andrews "cookbooks". His Quad 11 one was of great help when I rebuilt my pair with modern components.

One thing that stands out from Russ's literature was his belief that no transformer could be too big, and bigger is much better. In my projects I've always tried to over specify transformer ratings (within cost and space reasoning) hoping that there would be some benefit.

Seeing how Richard now has a phono stage with four 1000va transformers in the power supply (with impressive results he say) what is the current view on here re transformer ratings ?

The only downside I can think of with larger than required transformers is inrush on startup and potentially the charging current for the first cap in the power supply.

Nick wrote: ↑Wed Jul 12, 2017 12:14 pm The only downside I can think of with larger than required transformers is inrush on startup and potentially the charging current for the first cap in the power supply.

Understood Nick.

I used an inrush limiter on my breadboard BMU and I had to uprate to a 9A version as the original one died of heat exposure.

So Nick, what is your preferred overrating factor ? In practice, is 2X the required rating enough, or would 3X be better ?

I don't. I get the winder to use the next size up core, but wind for the specified rating. I forgot to add that you also will fall into the problem of a larger transformer on a smaller load will probably be proving a larger that expected voltage. Using the larger core size reduces the chance of saturation and makes them physically quieter.

Personally I spec the transformer for the job, otherwise I am paying for something I don't need.

Are we talking about over-sizing transformers for passive SE amp power supplies ?
In which case I suppose the aim is to get improved voltage regulation ( often, as Herb Reichert suggested, using a significant bleed resistor to take up some of the current capacity in a stable way ).
This all becomes less relevant if you move on to regulating the supply properly so the demand to the Mains Tx is more constant. Then you only need a moderate Tx over-speccing to keep the temperatures down.

(duplicate ..)

What about increased electromagnetic stray field with bigger transformers ?

Stu

Well in theory it should be no greater given competent winding, its less loaded so the field spray should be no more. However all things being equal, it will be bigger and closer to things around it. Of course, all things are rarely equal and a absolute answer to such a poorly specified question will always be close to a guess.

I made an enquiry to Airlink re a toroid wound on a bigger core, and this was the reply.

"Thank you for your enquiry.

Please let me know the reason for wanting your transformers wound on an oversize core; Is it a)To reduce the flux density in the core or b)To reduce the temperature rise of the transformer? The answer to this question will determine how we design the transformer."


What should my reply be ?

He he 1
Now they've put you on the spot !
I suspect both would decrease with a bigger core.
I still have no idea what sort of amp this is. What sort of audio improvement do you think you'd get for asking for a bigger transformer or a bigger core. Is it to reduce buzz in the windings, or to improve voltage regulation ( hence bass quality if SE amp ) or what ?

The tricks I found to get physical hum down was:

1, Larger core so lower density
2. Hum band
3. Care how they are turned on to avoid creating a offset in the core that can become locked in. There is one good do I have found out there that talks about this and not switching at zero crossing.

http://www.te.com/commerce/DocumentDeli ... DocLang=EN

I had several transformers fail in the ref phono before I discovered this.

*Cue Fat Kid at the back*

Whats a 'hum band' Nick?

Dave the bass wrote: ↑Mon Jul 17, 2017 7:27 pm *Cue Fat Kid at the back*

Whats a 'hum band' Nick?

Gong ?

@Phil, shurrups you's!

@Nick, oh, thats what they are! Thanks, I didn't know that was a 'thing'. Seen them lots of times in the Scientific PSU's we used to repair. I didn't know that was their purpose.