Which transformer for 3886 chipamp.com kit?

[tomchr]

2 x 25 VAC will give you ±35-36 V rectified. Under that condition, here are your transformer VA requirements:

For music/movie program you'll need 90 VA per channel for 4 Ω and 50 VA per channel for 8 Ω operation assuming a 14 dB crest factor.

For continuous sine wave operation you'll need 110 VA per channel (8 Ω) and 210 VA/channel (4 Ω).

You can find the math and explanation here: Taming the LM3886 Chip Amplifier

Tom

 

[torgeirs]

To FatCookies:
Yet another way of looking at it:
How many watts can your heatsink dissipate.
Total watts needed are: heatsink termal watts + load electical watts.
load electical watts = 1.5 x heatsink termal watts.

So Total watts from transformer are: 2.5 x heatsink termal watts.
Heatsink for 225 VA transformer = 225/2.5 = almost 100 watts.

So if your heatsinks can dissipate more than 100 watts continious your transformer may be upgraded. If not the heatsink will be the limiting factor and the chips will be to warm anyway.

 

[JMFahey]

Der Torgeirs, although the path you follow is correct, as in calculating needed transformer power handling based on speaker+thermal dissipation, it´s unnecessarily long and tortuous, similar to calculating a car engine energy fuel efficiency by measuring relative driver shoe pressure on the accelerator pedal.

Yes, the more he presses the faster the car will run, and you may eventually make a table relating shoe pressure to car seed to fuel consumption which related to distance can give you fuel efficiency so you might even draw a little graph by the driver´s foot where a needle glued to pedal shows that , but ... why do things that way?

Like in your earlier post when you tried to calcuate transformer size based on (acoustic) power and speaker efficiency.
Mind you, possible, but quite the Rube Goldberg way.

The suggested transformer VA calculation based on watts consumed or even amplifier RMS is the right and direct one.

For those who do not know the Rube Goldberg way of doing things,it´s basically doing them the most indirect and complicated way, but still functional:

The "Self-Operating Napkin" is activated when soup spoon (A) is raised to mouth, pulling string (B) and thereby jerking ladle (C), which throws cracker (D) past parrot (E). Parrot jumps after cracker and perch (F) tilts, upsetting seeds (G) into pail (H). Extra weight in pail pulls cord (I), which opens and ignites lighter (J), setting off skyrocket (K), which causes sickle (L) to cut string (M), allowing pendulum with attached napkin to swing back and forth, thereby wiping chin.

In 1931, the Merriam-Webster dictionary adopted the word "Rube Goldberg" as an adjective defined as accomplishing something simple through complicated means.[4]

for a modern Physics Lab example (yes they DO work):
This is how Ariel Llama opens his door... Rube Goldberg Machine
https://www.youtube.com/watch?v=ieQSiDnOhzY