LM3875 Transformer

[hotscot]

Anyone give me an indication on what transformer to purchase for Gainclone?
(Do you calculate it from the desired rectified input voltage to the chip?)

Alan

I have a choice of three

Avel-Lindberg Inc
Input: 2 X 120Vac 50-60 Hz
3 3/4" Dia X 1 1/2" Tall
Output: 2 X 28.4V @ 2.67Amps over 150Va

Avel-Lindberg Inc
Input: 2 X 120Vac 50-60Hz 3 1/4" Dia X 1 1/2" Tall
Output: 2 X 25V @ 2 Amps 100Va


Chia Yu Co Ltd
Input: 2 X 120Vac 50-60Hz 3" Dia X 1 1/2" Tall
Output 42Vct 21V- 0 - 21V

[lineup]

I think you should use
2x22 -> 2x25 Volt AC transformer.
This will give
2x30 -> 2x35 Volt DC after rectifier.

One 100 VA maybe can supply two LM3875.
But some would want more, 150 - 250 VA rated transformer for this.

[Nordic]

The second one has the right voltage for LM3875, but its a bit low amperage... consider that 1 lm3875 can put out close to 5 amps at peak.

[hotscot]

So a better transformer to look for would be about 25V output before rectification and around 5 amps power?

[hotscot]

How about

Amveco Magnetics

100V-110V-120V input 120V
3.5Amp, 21V 19Amp
8.5V 10.5Amp, 37V 3Amp and 12V .2Amp

[Nordic]

You don't want to go past 40VDC, with these chips, and even then you would need a huge heatsink.

Rather aim for about 35VDC, as per the datasheet.
To get a rough estimate of DC voltage after the rectifictation multiply AC voltage with the sqaure root of 2.

The 21V one seem decent, even if it will produce less power than 35V rails, it should give some solid current handling ability...

[joecool85]

Bigger is better with transformers. I'm going with a 25+25 330va toroidal transformer for my lm3886 amp. One lm3886 chip mind you. And a huge heatsink large enough for 2-3 lm3886s. I talked to Brian (chipamp.com) and he's the one that suggested such a huge transformer. I was going to go with a 160va or 250va, but the 330va is only $10 more than the 160va, and it allows more headroom.

**edit**
This is the one I'm getting:
http://www.partsexpress.com/pe/showd...number=122-640

If you go to partsexpress.com search for toroidal and you will find all the transformers I was talking about.

[moutoulos]

Originally posted by hotscot
Anyone give me an indication on what transformer to purchase for Gainclone?
(Do you calculate it from the desired rectified input voltage to the chip?)

The better trafo for this amp (LM3875) to drive all load (4 - 8 ohm) is
+- 28Vmax (+- 30V can't drive 4 ohm load) , so you want one trafo per chip 2x20Vac 2.5A (100VA).
The trafo give +- 2.5A, so 2.5 + 2.5 = 5A, the output current of LM3875 is 4A max (limit).

[hotscot]

Thank you all. Very informative

[joecool85]

Keep in mind you normally only get 1/2 to 2/3 of the transformers power due to rectification and other things. So, if you think you need a 100va on paper, in practice you'll want anywhere from a 150-200va.