Amp power and supply?

[Vitrolin]

hey
I have been wondering when building an amp how do i choose the correct transformer?
I guess first  i should start with choosing output power, lets say 120W_RMS, then amplifier class since it says something about how effective it is, i think that AB is about 70%, not sure.
If so, then 120/0,7= 171VA supply?
How do you determine supply voltages, output power and speaker loads?

Voltages should bee high enough to put out 120W in 8R:
V=SQR(P*R)
V=SQR(120*8 )
V= 31V
but that's only RMS, The peak voltages are then:
V_p=V_RMS*SQR(2)
V_p=31*SQR(2)
V_p=44V

I believe these are important considerations when designing an amp, im not sure if im on the right path here

[phatt]

Hi Vitrolin,
Yep gets confusing but a look inside gear already made can give you an idea.
In truth you don't need to go overboard with exactness unless you are building a bass amp you can get away with less.

Sounds like you have a better handle on the maths than myself,, but try this page it will answer most of what you ask.  :tu:
Have fun.
Phil.

http://sound.westhost.com/project04.htm

[J M Fahey]

Yes, you are basically right; I'd add a little "extra safety margin" and choose a transformer with *at least* 50% more VA than the rated RMS power; with 2X being an even better  choice.
As of supply rails, most published schematics already suggest a value, they have been calculated by the amp designer.
If not, first calculate peak voltage (you already did that) , add 10% for loss in the typical emitter resistor ballasts, 4 to 6V for drop across the transistors themselves , and then consider that a typical unregulated PSU "drops" 20% to 30% voltage under full load.
I think Phatt's link says something similar.

[phatt]

You amaze me Jaun,,
That Stuff I would spend forever trying to work out and you have it all in your head just sitting there.

If I ever win a million bucks mate, I'll jump on an Airplane and come over to Argentina just to meet the Argentinian Falcon in person.  I'd likely learn more far more from You than a tourist visit to Marshall and Fender factories
Phil.

[J M Fahey]

Thanks, you'll make me blush 

As of the cars, they just found a cache of 43 Military Falcons, unused for 30 years !!!

Just slightly used between 1976/1982 to kidnap Government opposition.
They are soon going on public auction.

You only need to replace tires, wash the dust off and dispose of the mummified bodies in the trunks, nothing too special.
Blood washes away easily.

http://tn.com.ar/politica/secuestraron-43-falcon-de-la-dictadura-en-bahia-blanca_087231

If you read the comments , nobody cares about that, everybody wants to buy one. 

[Vitrolin]

thank you both very much

i knew the ESP page but never saw the articles only the projects...i have now change bookmark to main page.

i have also been wondering how do you know if what minimum load your output is capable of?

[phatt]

As a rule of thumb, if you see 4 power transistors on a heat sink you can run 4 Ohms.
But if only 2 are present then it may struggle.

I believe Mosfets are tougher but do check the specs if you can.
I think Rod at (ESP) goes into that stuff on several of his pages.

There is a mass of wealth on ESP web site and does take time to find stuff but it's very well written so if you are keen it's worth the effort to read it.  :tu:
Phil.

[phatt]

Thanks, you'll make me blush 

As of the cars, they just found a cache of 43 Military Falcons, unused for 30 years !!!

Just slightly used between 1976/1982 to kidnap Government opposition.
They are soon going on public auction.

You only need to replace tires, wash the dust off and dispose of the mummified bodies in the trunks, nothing too special.
Blood washes away easily.

http://tn.com.ar/politica/secuestraron-43-falcon-de-la-dictadura-en-bahia-blanca_087231

If you read the comments , nobody cares about that, everybody wants to buy one. 

Can I get one with genuine bullet holes?  I can only get fake sticker bullet holes here
Phil.

[J M Fahey]

Probably, and I'm not kidding  :loco
An uncle of mine used to buy them at Government auctions for repainting and resale for a good profit.
Since they want "not one left" because they have to clear the warehouse or whatever, you can't bid on "just one" but on lots of 3: usually a *perfect* one, probably assigned to some bigwig Police Chief and always driven by a white gloved chauffeur, a "regular" one, and a crashed/burnt/bullet ridden one, which anyway is good to pull engine parts from, to repair some of the others, so you are (as usual) quite close to the bulls eye.