power supply for an amplifier

[happybenguy]

hi everyone, I was just pondering somthing the other day...
How come all the amps i've seen have a transformer in their power supply? I have a schematic of a powersupply i think would work below, but I dont wanna try it until i know why they use transformers instead of just making a +120 and -120 volt rails strait from the outlet. Is the voltage level the only deciding factor for using a transformer? I was just wounderin because high powered transformers seem to be big and expensive so i figured maybe this would work instead....
let me know what you guys think!

p.s. I know the capacitors would have to be sized differently, but i used those just for example.

 

[Nigel Goodwin]

hi everyone, I was just pondering somthing the other day...
How come all the amps i've seen have a transformer in their power supply? I have a schematic of a powersupply i think would work below, but I dont wanna try it until i know why they use transformers instead of just making a +120 and -120 volt rails strait from the outlet. Is the voltage level the only deciding factor for using a transformer? I was just wounderin because high powered transformers seem to be big and expensive so i figured maybe this would work instead....
let me know what you guys think!

p.s. I know the capacitors would have to be sized differently, but i used those just for example.

If you wanted + and - 180V (not 120V), you could use a crude PSU like that. But it has a number of massive disadvantages.

The entire circuit would be live - a transformer provides isolation from the mains.

It would only work for 120V mains, 230V mains would blow it to pieces, a transformer is easily tapped for either mains voltage.

It wouldn't feed any normal impedance speakers, you would need special high impedance ones - a 4 ohm speaker could be driven to 4000W from a supply that high (except the mains won't supply that amount of power).

The components required to do it would also be very large and very expensive, including the amplifier components as well - which would need to be totally redesigned to use far more expensive high voltage components.

The mains transformer is there for very good reasons, although you can replace it with a suitable switchmode PSU - it's probably no cheaper (and most likely more expensive) - but it will be smaller and lighter. There are quite a few PA and guitar amplifiers using switchmode supplies these days.

 

[ante]

I don’t think this will work at all! The input at V1 is +120VDC.

Ante :roll:

 

[John Sorensen]

Stay away from the outlets (the "mains" for the UK folk), happybenguy, until you get a better idea of electronics, or you'll be smoulderingbenguy :wink:

Start with batteries (but not the one in your car :shock: )

j.

 

[Oznog]

The ground of the circuit is now the power supply neutral. Any signal you feed will end up with 60Hz ground loops.

Suppose you have a 4 ohm speaker driven a loud 4 watts. It uses 1 amp and 4 volts. Starting with a 170 volt rail, you will drop 166 volts in the amp's transistors, since it's 1 amp it creates 166 watts of heat. That's an enormous amount of power to sink! And that's only a modest amount of power output.

Transistors capable of switching this much voltage are also rather expensive and/or a bit poorer in other properties.

 

[ante]

Hey, where is the AC here???

Ante :roll:

 

[plot]

Hey, where is the AC here???

Ante :roll:

Well, the original poster specifies "+120 and -120 volt rails strait from the outlet", to which we can assume it's AC (as i've yet to see a 120VDC power grid anywhere), and since he says 120 volts, we can also assume he's based in America and therefore it's at 60Hz.

 

[happybenguy]

hmm

yea, maybe its not the bestest idea in the world BUT, i just have two questions...ok first, wouldnt the 60 herts ground noise be eliminated by the capacitors? and second, could i use a triac to control the amount of voltage the capacitors are charged to, maybe cut it down to somthing like 60V and -60V. Oh and BTW, i have a cheap(free) circuit simulator on this computer, and i couldnt find an ac source symoble anyware so I used the dc.....incase anyone was confused by my schematic.... :lol:

Ben

 

[Nigel Goodwin]

Re: hmm

yea, maybe its not the bestest idea in the world BUT, i just have two questions...ok first, wouldnt the 60 herts ground noise be eliminated by the capacitors? and second, could i use a triac to control the amount of voltage the capacitors are charged to, maybe cut it down to somthing like 60V and -60V. Oh and BTW, i have a cheap(free) circuit simulator on this computer, and i couldnt find an ac source symoble anyware so I used the dc.....incase anyone was confused by my schematic.... :lol:

You would need very large and expensive capacitors, and it wouldn't really be a good ideas to try and do it - it's fraught with difficulties and likely to be unreliable and perform badly.

There's also the MAJOR disadvantage that it will all be live to the mains, so would be highly dangerous and make it difficult to connect to anything.

It's a really bad idea, forget it!.

 

[ante]

OK, so “V1 120V +V” equals AC then?

Ante :roll:

 

[plot]

OK, so “V1 120V +V” equals AC then?

Ante :roll:

when it's "from the outlet" it does.

 

[ante]

I have never seen the “+” sign used for AC but it’s 30 years since I finished school! What do I know, it might have changed? Maybe it has been too long and I should go back to school again. :lol:

Ante :roll: