output voltage/current

[aypo]

Hello good people, I am thinking about some new amplifier and was wondering about one thing, if you were about to design an amp of about 50W to 8ohms what would you prefer to achieve that, a higher voltage at rails and lowering the number of coupled output transistors (i know 50W is possible to achieve only with one pair) or lower voltage at rails with two or three pairs at the output. Do you think there would be a significant change in overall sound quality? (i know that more active parts introduce more distortion and so on) all I'd like to ask is how you see it only from the point of view of using higher voltages or higher currents. (and maybe i am totally wrong with this question :-)
thnx

[nigelwright7557]

Given you are only using 8 ohms that limits what minimum voltage rail you can use.

I always leave a bit of headroom for transient response.
Its amazing how quickly an amplifier can run out of headroom on transients.

On the other hand you dont want the speakers over driven by someone with a heavy hand.

I would go for one pair with a few volts headroom.

[wakibaki]

You've got this a bit wrong. You need a given voltage to achieve a given power output (in a transformerless transistor amplifier that isn't bridged).

The power is (V^2)/R. That's V squared upon R, where the R in your case is 8.

So 50 = (V*V)/8 or V*V = 400. The squareroot of 400 is 20. That's 20 volts RMS, so the voltage needs to be 2*20*sqrt(2). So you need a single rail of 56.6 volts (above zero or ground) or dual rails of +28.3 and -28.3 volts referenced to zero or ground.

These are the voltages across the nominal load of 8 ohms, there will be losses in the output transistors, so the actual voltages will need to be a bit (couple of volts) higher.

w

[wahab]

A 2X30V AC / 220VA transfo perfectly fits the job for a stereo amp,
330VA are necessary if you are to use 4 ohm speakers occasionnaly..

Even if you use only 8 ohm speakers it would be better to mount
two pairs of power devices as it wont change the total cost while
providing lots of advantages.

[aypo]

nigelwright thnx, i finally know what the headroom is (yes, i am not native eng :-)

wakibaki i know that pretty well, but i am familiar with only a few designs i had to analyze recently and i'd like to know some info you'll never told in school.

wahab, could you please provide some more info about those advantages? or any materials to study where i could find info like this?