Hi all!
Some words about transformers, rectifiers and an important "NULL"...
What transformer do i need to build the amp and what is "centre tapped" or "dual secondary"?? What is the "ground" and how do i build it?? At least - what the heck is "NULL" (or simply 0)?? OK - here it is.
At first:
To build the amp like on the project page, you will need a "dual secondary" transformer with 2 x 18 Volt AC. A dual secondary transformer has 2 seperated windings with 18Volt AC (alternating current) per each. Usually, such a transformer is labled with something like: 2 x 18VAC - 120VA or 18VAC//18VAC - 160VA.
It is recommended to use this dual secondary transformer. As said: 2 x 18VAC - 160VA. This transformer is a very usual type and you can get it nearly everywhere. If you already call other transformers your own, you also could use them. It is not(!) especially needed to use the 18VAC. One 15VAC / 12VAC, found in your "scrapbox", will also do the job. But, the output power will be less with every volt that goes below 18VAC. I suggest to choose a transformer with at least 120VA. This VA rate can deliver 3,3 Amperes per rail @ 18VAC. VA? VA! VA: is the apparent power. Just divide the AC output voltage with the VA factor and the result is the current capability per rail / winding. = 18VAC / 120VA = 3,3 Amperes per rail (6,6 in total).
Please don't go above the 18VAC suggestion with the transformer. The max. IC input is 25VDC. That is reached by using the 18VAC transformer. Every Volt above 18VAC will result in senseless voltage regulators and efforts, that are not worth the time and more grey hairs. Centre tapped?!:
A "centre tapped" transformer has one (1) single winding that is tapped at the centre of this winding. This types are always labeled like "18 - 0 - 18 VAC" or "18VCT". Please read the transformers manual, or ask your dealer, if you are not sure with the transformer descriptions, or if you don't know if the offered type is a dual or CT.
Transformers are always rated with AC (Alternating current) and never in DC (Direct current)!.
I bought a 2 x 18VDC transformer. What's to do now?
Simple! Just refer to the project page and build the power supply that is shown on the "supply section".
The 2 x 18VAC guy is hooked up by using that way:
Attachment:
dual.GIF
Again - you will surely see the positive rail (+), the negative rail (-) and the ground rail. The ground rail is the rail, that is connected in the middle between the 2 capacitors. This rail is also called "NULL" or "0". This "null" rail provides a point where both potentials (+/-) are equalized. That's important for the electron flow and to define the rail potentials as positive and negative. Without the null, you wouldn't have a negative potential. Please take care of the rectifier bridge setup. The upper bridge is "null to negative" and the lower is "positive to null".
I bought a 18-0-18 VAC or a 18VCT (Volt AC - centre tapped) transformer. Is that wrong? What can i do now??
Now - that's not a mess and you surely can use it. How?
Attachment:
centre_tapped.GIF
Just change the supply input a little bit for use with one (1) single rectifier bridge, because you don't need to build a "centered" PSU like the one above. The PSU is centered by the transformer because the transformer is (now guess what) - centre tapped

.
Whoops - why don't we use it as standard? Because the performance is slightly lower and centre tapped transformers (VCT) are not usual sold types. I want to say, that it's way easier to get a dual secondary type.
Which rectifiers do i need? Just use a pair 25A to 35A silicone bridge rectifier. This rectifiers are standard and available everywhere. There is no need to use "ultra fast forward / recovery diodes". The bridges are cooler anyway and "softer".
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Please regard, that the 2 pictures above, only show the basic layout and how to connect the transformers to the rectifiers. The complete and accurate scheme for the PSU (with ALL components) is still on the project page.
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Please regard that my English still isn't the best... If you find some errors - feel free to do the corrections!
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I hope, that the PSU section is complete now. Have fun!