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snakebite 27th September 2012 01:37 PM

my first amp based on lme49830
 
hello,

this is my first post here so i introduce myself. i'm study EE in uni. i'm now on my last year and maybe this will be my graduate project!
before i'm starting my project, i wanted to ask a question about the transformer.
i know it's better to use a toroide type transformer because of the low noise high effictioncy and the small profile but i'd like to know which one i need.

i'm planning to build 100w x 5 amp @ 8ohm . so based on ohm's power law:
P=I * V = V^2 / R = I^2 * R
so if i need P=100W and R=8ohm
V=28.28VDC (after the rectifing , smothing and regulation)
A= 3.5A.
is it correct? or i need a safe margine?

based on this i need a transformer rated on about 500VA (or more??) ??

thanks
Ido

AndrewT 27th September 2012 02:26 PM

if you build one channel (two channels won't get you any higher grade) then expect a 100VA to 200VA transformer to successfully power a 100W amplifier.

If you want that 100W into 8ohms speaker the use a 35+35Vac transformer to get the PSU voltage right.

But here's me giving you the answers.

snakebite 27th September 2012 11:28 PM

so if i want 500W amp i need 500VA transformer ? what about the current calc. that i made, is it correct? or do i need to take higher current rating for spikes or whatever (i calculated about 3.5A per channel so if i take 4A per channel is it enough or i need more?)

AJT 27th September 2012 11:43 PM

^if you are just playing music, then that should do.....

snakebite 27th September 2012 11:54 PM

what you mean by "just playing music" and if i watch DIVX movies it won't be enough?? can you explain yourself...?

Boscoe 28th September 2012 10:57 AM

How do you not know this stuff if your at uni? It's very basic, just do a little reading. If you knew basically how an amplifier works its very easy to make a psu for it.

Boscoe 28th September 2012 10:57 AM

Quote:

Originally Posted by snakebite (Post 3181782)
what you mean by "just playing music" and if i watch DIVX movies it won't be enough?? can you explain yourself...?

Doesn't matter.

Headbanger 28th September 2012 02:52 PM

I wonder if anyone have used LME49830 at unity gain, inverting, compensated.

sreten 28th September 2012 02:53 PM

Hi,

Unfortunately EE courses don't teach most of the basics of sensible
cost effective analogue audio design, or teach you any common
sense regarding properly selecting and properly dimensioning
components and hardware for higher power audio amplifiers,
and that ends up being a matter of opinion as well as design.

The decision process for a power supply is far more complicated
than outlined in the initial post but with experience designers settle
on "rules of thumb" that simply won't do in a project write-up.

Different designers can have different "rules of thumb". Different
rules can apply if cost is the major issue compared to performance
(at higher cost) being the major issue. There are no simple answers.

Good design is balancing these factors.

rgds, sreten.

Generally I don't like Mosfet outputs with simple supplies
as you lose too much voltage swing at the output for me.

hochopeper 28th September 2012 03:03 PM

Quote:

Originally Posted by Headbanger (Post 3182446)
I wonder if anyone have used LME49830 at unity gain, inverting, compensated.

From memory LME49830 stipulates min gain of 26dB for stability.


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