Power supply with different voltage output?

[OliG]

Hello, Im looking forward to build a gainclone, using 2 parallels lm3875 per channel, based on the pa100 parallel amplifier available in National Semiconductor Application Note 1192 (http://www.national.com/an/AN/AN-1192.pdf).
I am also considering to add input buffers based on the Pedja Rogic Jfet discrete buffer circuit.

I want to build a split regulated power supply that deliver +30V/-30V (or +25V/-25V?) to feed the 4 lm3875 chips, but i will also need to feed the 2 buffers, who needs +15V/-15V. What is the best option?
1. buy 2 transformers, one for the amps and one for the buffers, and build 2 separates power supply.
2. buy 1 transformer and build 1 power supply and parallel outputs with added resistors to get the proper voltage, as I saw on project 115 of Elliot Sound page. (Project 115 - GainClone) I know this one is not regulated but I would regulate both outputs.

finally, If I choose second option, what would be the appropriate max load for the transformer? I read on decibel dungeon than 80 watts per amp is enough but I read here on the forum that 150 will give best results. Also I dont know how much power is needed for the buffers...
will a 500VA 25 + 25 be enough or should I choose 625VA 25 + 25?

Thanks for your time !
Oli

[pacificblue]

Option 1 has the bigger potential for a good sounding amplifier, but also needs more space and money.

If you use Rod Elliott's circuit according to Figure 6 you actually have a regulated power supply for the buffer. The Zener diodes do the regulation. It is worth a try, because it is inexpensive and may sound better, than many people think.

The buffers need next to nothing and you don't need to worry about their impact on the the transformer rating.

±30 V will give you ~40 W into an 8 Ohm load, ~80 W into a 4 Ohm load and ~160 W into a 2 Ohm load.

±25 V will give you ~30 W into an 8 Ohm load, ~60 W into a 4 Ohm load and ~120 W into a 2 Ohm load.

The transformer rating should be bigger than or equal to the combined output of the amplifiers. So, e. g. if you use ±25 V and two 4 Ohm speakers, the transformer should have more than 120 VA, but more than 360 VA is a waste. Somewhere in between is the best compromise for you. Where exactly that is, depends on the power supply capacitance, whether you use the amplifier at full throttle or in a neigbor-friendly way and on your assessment of how much money a small increase in sonic quality is worth it. In your case, you may tend towards a higher power rating, because you also have to make up for the losses in the voltage regulators. 500 VA is certainly overkill.

If you want ±30 V from your regulators, a 2x25 V transformer won't do.

[OliG]

Thank you for taking the time to answer with all thoses details.

I will go for the first option, two transformers and 2 power supply. Im not looking to terrorize my neighborhood with this amp, so I will choose the transformer accordingly...

Can you please explain why I cannot get +30V/-30V out of a 2 x 25V please? I tought than you have to multiply the secondary AC voltage by 1.4, wich give 35 and an adjustable regulator like the lt1083 will dropout only 1-2 volts?

thanks again !
Oli

[OliG]

Hooo, forget about the lt1083, I just saw that the max voltage output is 28.5...