Advantages of using a toroidial mains transformer in a GSSL?

[MatthewF]

Hi there, I'm about to start building a GSSL compressor based on Gustav's PCBs. While I could take the easy option and buy a mains transformer to mount on the main PCB, I'd be willing to go to the extra effort of mounting a toroidial unit elsewhere if the reduction in interference and ability to drive stuff like a GR meter backlight were significant.

Any advice on this would be very much appreciated.

P.S. If I do go for a toroidial, would the first one on this page below be suitable? (I live in the UK)

http://uk.farnell.com/jsp/endecaSearch/searchPage2.jsp?Ntk=&comSearch=true&select2=210313&select3=191229&paramSelectCount=7&showImages=true&st=parametricSelection&N=1004685+401&Ntt=&comSearch=true&paramSearch=true&st=parametricSelection

 

[gyraf]

A 2x15V toroid would probably be the best choice.

Make it 10W or larger, then you have some juice for the backlight as well.

Jakob E.

 

[MatthewF]

Thanks for your quick reply. I realise that a 2x15v transformer is recommended by many people including Gustav (its written on the PCB).

This confuses me slightly as this 2x15v will surely drop by 1.4v per rail due to the drop over the diodes in the rectifier?

The +/- 15v regulators would not then be able to supply a full +/- 15v?

(I've put question marks after everything as I realise I must have something wrong somewhere!)

I'd be really grateful if someone could aid my confusion here.

P.S. Am I right in thinking that the only advantage of a 2x15v transformer over a 2x18v one is that the +/- 12v regulators will run cooler?

Thanks again for any help.

 

[radiance]

A 2 X 15v transformer will usually give you something like 2 X 21v. I have not meassured this and I don't know the exact figures but I'm very sure a 2 X 15 will do and yes, things will run cooler this way......

 

[alk509]

[quote author="MatthewF"]This confuses me slightly as this 2x15v will surely drop by 1.4v per rail due to the drop over the diodes in the rectifier?

The +/- 15v regulators would not then be able to supply a full +/- 15v?[/quote]

The transformer puts out AC... 15VRMS is 21Vpeak.

Peace,
Al.

 

[MatthewF]

Ah, of course, I really should have realised that!

Thanks for keeping me right,

Matthew

 

[Svart]

to further Al's post, AC rectified to DC is 1.4 times higher. so 15vac will be ~21vdc.

remember that the VA of the trafo will determine if the trafo can put out enough amperage to support that voltage..

in layman's terms that means that if your trafo is rated at a rectified 15vdc and your max current is 1 amp that once you pull upwards of 1 amp that your 15vdc will start to go down and the trafo will likely melt (of course you used a fuse and that won't happen..) but pulling an average current near the maximum rating will not only heat the trafo excessively, your power rails might flucuate.

Lets not forget that using a much higher current rated trafo will also lower certain types of noise and buying the next higher rated trafo will not likely be much more money.

 

[smallbutfine]

I think the recommended 2x15 4,5VA on the board should remind you of a minimum you should fit. When i looked at my readily etched pcb for the first time, i thought: yeah, this one is psu 'scalable' - one option for a compact and easy to wire unit and another for a slightly more expensive low noise off the board trafo.
(After all i realized that this is an early project with easy diy and reliably good results in mind. High fun factor, easily built (low parts count, easy to find components, transformerless / highly affordable, no complex measurements needed), outstanding features (full stereo performance, auto release, balanced in/out,...) and very good sounding - a really cool beginners unit possible to be built on one weekend, very rewarding.
Thanks again, gyraf.)

Happy DIY

Martin