Some noobish questions about power supply transformers

[newbiediy]

Having built low wattage amps (Ruby & Little Rebel) with some degree of success, I am now looking to build bigger watt amps. I've downloaded Marshall MG15 schematic and intend to build the power amp section. The problem is that I can't figure the needed transformer's rating.

So here are the noobish questions:
What is the proper transformer's rating for this amp?
LM1875 datasheet says P out is 25W at +/-25V. Is it ok to use +/-25V? Will this amp actually deliver 25W instead of 15W if I use +/-25V?
I have a transformer with "1A" mark on it. It has dual 12V, 15V, and 18V outputs. Can I use it?
What happens if transformer's rating is below needed voltage and amperage? What happends if it is over/beyond?
How to decide the proper amperage rating from a chip's datasheet? I'm guessing I should not exceed max supply voltage, but cannot determine the needed amperage.

Sorry for the annoyingly confusing questions. I don't speak English very well and I AM a newbie in electronics. Any help would be VERY APPRECIATED.

Note: In my country, transformers are not rated in VA. They only marked 1A, 2A, 3A, 4A, 5A, and so on.

[J M Fahey]

1) You *can* get 25W using +/- 25W, but I'd play it safer and build it as original, 15W.
Not that big sound difference anyway and you will have more of a dissipation margin.
2) *IF* your transformer has 2 18V windings, able to supply 1A each, it's rated (18+18)x1= 36 VA.
In general: multiply the voltage by the current rating to find the VA.
I'd use the 15V windings (which will give around +/-20V rails).
3) if voltage is below needed, nothing bad happens, only your amp will not put out as much power.
If above, it may be dangerous.
If rated higher current, will work cooler, fine.
If lower, will overheat and eventually fail. Bad.
So in a nutshell, if you get a transformer rated more than 1A , say, 1.5A or 2A, it will be better.
Probably price won't change that much.
What Country are you writing from?
PS: Rather than "one big PCB holds everything" I'd build a Power amp + PSU PCB first, and test it.
Worst case, you feed your MP3 player or Computer audio out straight into it and listen to Music..
Build a separate preamp, so you can experiment with more freedom.
You can also start with very simple ones, and progress to others more complex.
Go step by step.

[newbiediy]

Wow, thanks. That clears my hesitations right away. :-)
I'm from Indonesia. On the world map you'll find it just above Phill's continent. (I think he's Australian. Am I correct?)

I am planning to build 3 separate boards: preamp, power amp, and psu; hoping that I can reuse the boards in future designs. Will I get problem with this? Particularly with noise and ground loops. I've read about ground loops but still don't quite understand.

[J M Fahey]

Fine.
I personally prefer to split designs in a couple functional blocks, unless it's something simple as a pedal.
Helps both troubleshooting and future upgrades.
Good luck.
PS: and see Indonesia quite often, specially in "Travel and Living Channel" on cable TV .... would *love* to visit it in person.

[newbiediy]

Thanks for the quick reply.

One more general question. You said more current capability is better for transformer. How to determine the MINIMUM required current from the datasheet? Or simply (for example) I=P/V=25W/25V=1A ?
I'm taking the numbers from LM1875's power output vs supply voltage graph. Just for example, not that I will use 25V instead of 20V that you recommend.

Awaiting you to visit Indonesia. ;-)

[el nino]

@newbiediy: slam knal bung, ane juga orang indonesia raya bung. Ane d Surabaya

[newbiediy]

Salam kenal juga. Saya dari markasnya Pak Jokowi: Solo, Jawa Tengah. :-)

Translations:
Nice to meet you too. I'm from Solo, Central Java. :-)

[el nino]

Salam kenal juga. Saya dari markasnya Pak Jokowi: Solo, Jawa Tengah. :-)

Translations:
Nice to meet you too. I'm from Solo, Central Java. :-)

okay masbro,good luck with the project... & i hope Solo found someone like Mr. Jokowi as a major, cause i believe he will success as jakarta's governor:-)

[newbiediy]

Thank you.
Anyone can explain regarding my last question (minimum current needed for a chipamp)?

[J M Fahey]

As a rough rule of thumb: take RMS power , say 25W .
Add 50% because a regular SS class AB amp wastes approximately that, so we now need 37.5W from the PSU.
Always round up , never down, so we need 40 W which in a transformer is measured in VA.
So we need a 40VA transformer for our 25W RMS amp.
Divide 40 by the end to end winding voltage .
If 16+16VAC we have 32V end to end.
So 40VA/32V=1.25A.
So if you had made a 15W amp, the 1A rating would have been ample enough, but if you now want 25W, I guess 1.5A will be necessary, because I doubt you will get a 1.25A one.
There's a more exact calculation if you wish, but this "practical" one is good enough.
In Tube amps it's more complex, because they also must feed filaments, the preamp too, and power tubes are biased to a high idle current , plus they are a lot less efficient, that's why tube amp transformers are much bigger than for a similar SS amp.

[newbiediy]

Thanks alot, Fahey. That was great. Too bad chip point is limited to only 1 point per hour. :-)

Does "power waste" in your explanation mean "power dissipation" in datasheet? If yes, I'm reading
@Vs=+/-20V & Rload=8ohm
Po=15W
Pdissipation=11W

Is it correct?

[J M Fahey]

Basically yes.
All analog power amps allow some of the power they draw from the PSU as useful Audio; the difference is lost as heat.
In theory a perfect class B SS amp is 75% efficient , but to begin with, it passes a little current even at idle, to avoid crossover; power gets lost in emitter ballast resistors; the drivers eat something too; the predriver(s) works in class A (surprise) ; it has either a couple resistors as load or an active current source (all waste); the input transistor(s) also dissipate something; the power transformer has its own magnetic and resistive losses when idle, even if unconnected to any load; PSU diodes lose 1W for every Ampere through them; filter caps have lossy ESR (basically, internal resistance)  ..... even the power LED and its series resistor has losses!!!
But in practical terms, that 50% extra I suggested usually covers it all.

We are quite conservative in Electronics; in Civil Engineering, Buildings, Bridges, etc. are calculated ... and a 300% safety margin is added.
I'm not kidding, it's usual practice there, time tested from the time of Romans (*many* of whose bridges, aqueducts, roads and buildings are working perfectly over 2000 years later !!!!!)

[newbiediy]

Wow.. Thanks alot again, Fahey. Your replies worth to read for noobs like me who try to optimize their budgets without risking quality and long term performance. I would suggest pinning this thread for future (newbie) members.
+1 again for you. :dbthumbsup