Lm3886 transformer

[jmar]

I'm waiting for one of those modules from Ebay 2 x Lm3886 and it calls for
a 28vac x 2 AC -- so obviously the power supply components are on board.

I really don't need to crank this amp up to full spec so can anyone suggest
a transformer (chassis mount) (and not torroidal) that will do the job?

Any cheapies out there in a vct transformer on Ebay or MCM etc?

And what kind of current do I really need ? This is a 2 x 68w but really I thought
I'd wire the amps in parallel and go for maybe 2 x 40w or so at most.

Thanks.

[Maxxtr0]

Here!
The link in the first post could also help you to find out that you should use a ~120VA trafo with 18V to maybe 24V on secondary side, depending on the impendance of your speakers!

Edit: The LM3886 datasheet, wich can be found here, can also be useful i think...

[jmar]

18 - 24 vct and 120va PER side?

I hate VA measurements can never remember the formula and sometimes you don't
know whether there talking about the primary fuse requirements or the secondary.

I like old fashioned current capability -- 18 - 24vct and what, 2 amps per leg? 4 amps total? Like I say, going after 2 x 40w -- only asking to save time, would like to order the part by the end of the day.

[Maxxtr0]

No, 120VA (= 120Watts) for both channels, secondary! The advantage of the VA measurement is that you´re able to calculate the amp-rating depending on the output voltage...
I recommend 120VA total, even if you only want to use 80VA (aka Watts) because this is told to improve sound quality!


So to calculate your 2 x 40 watts per channel: (As though it would be useful to know your speakers impendance!)


18V from trafo -> rectifier (18V*1,41) -> 25,38V
40W/25,38V -> 1,57A needed

24V from trafo -> rectifier (24V*1,41) -> 33,84V
40W/33,84V -> 1,18A needed


But as mentioned above you always should choose a trafo too big, for not to run it at its limits!

A 120VA trafo, for example, with 2 x 18V would be able to deliever:

120VA/2 = 60VA per chanel -> 60VA/18V -> 3,33A per channel - pretty good for your needs!


P.S. Sorry for the "," you know it shall be a "." for you

Edit: Missed that you wanted 2 x 40...

[jmar]

Ok.

They're calling for a 28vac transformer on this which is a weird value and too much
so I'll just try to find a common 24vct good for 1 to 1.5 amps per side -- that should be fine for what I want to do.

In fact, I think I have some 24v single secondary laying around -- I'll just use
2.

Thanks !

[jmar]

Seems to me, 24 vac HVAC transformers would be ideal at least as far as availability and price.

They're always pretty robust (1.5 - 2 amps) and plenty of them around.

?

[raypalmer]

Googling... HVAC's don't really seem all that much cheaper than the usual toroidals?

[jmar]

A zillion 24v hvac transformers on the surplus market -- CHEAP.

I don't get the toroidal thing -- isolate and shield the transformer well (many are already shielded somewhat) -- and that's good enough.

Toroidal transformers?

Kinda like monster cable if you ask me.

[raypalmer]

Originally Posted by jmar

A zillion 24v hvac transformers on the surplus market -- CHEAP.

I don't get the toroidal thing -- isolate and shield the transformer well (many are already shielded somewhat) -- and that's good enough.

Toroidal transformers?

Kinda like monster cable if you ask me.

Well they don't sound better. That's not the benefit. The benefit is that that the toroid is squat. So you can fit them into sexier cases, the EI's are too tall. I agree with you than anyone making the claim that a toroid sounds better than an EI sounds better than an R-core is cooking with snake oil. (Obviously the build quality of individual transformers has some effect.)




But yeah, the reason most people dig toroids is that they're smaller. Also they mount to the chassis much more easily, just a single bolt rather than the four usually needed for ei.

[Apex Jr]

Have some 2 X 28.4V 160Va Toroids at $22.00
2 X 24V 200Va Toroids at $22.00 not on website
Picture of the 28.4V listed 1/2 way down on this page
http://www.apexjr.com/miscellaneous.

Steve @ Apex Jr.