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σ25 vs σ11

sigma25, sigma26, sigma24

σ25 vs σ11

Postby imstimpy » February 7th, 2014, 7:32 pm

I'm trying to gain a better understanding of the reasons why one would choose a σ25 over a σ11. Obviously cost is a big factor. The technical highlights lead me to believe the σ11 is better in every other way except cost.

Personally I intend to build a σ25 as a gateway PS for my DAC. I will likely build a σ11 later, but it seems odd to use such a PS for a low current pice of electronics like a DAC. With its small footprint, the σ25 doesn't seem like a likely candidate for headphone amplifiers. If I used it to power my class A amp, would it cook itself or just inject a lot of noise? A σ11 seems like the better choice there, but under the assumption that a σ25 is too small.

My apologies if this has been discussed elsewhere. I looked through both official pages and every thread in the two sub forums.

Thanks,
-Jon
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Re: σ25 vs σ11

Postby amb » February 8th, 2014, 1:14 am

σ25: smaller, easier, cheaper, but still a reasonable linear regulated supply.
σ11: higher performance, higher current, larger, more complex, more expensive, high-end solution

The ultra-low noise characteristic of the σ11 will allow the DAC to perform at its best.
The σ25's 78xx voltage regulator with default heatsink may cook itself when powering some headphone amps. You need to work out the junction termperature based on current draw.
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Re: σ25 vs σ11

Postby imstimpy » February 15th, 2014, 6:41 am

The σ25 lists a current cap of 300mA with the stock heatsink. The σ11 lists 1A with the stock heatsink.

How much additional current is safe for each PS once equipped with a larger heatsink?

My σ25 is up and running, but I've only verified its function while unloaded. I was dismayed to find out the Raspberry Pi draws around 700mA so I have to build out my γ1 before I have any low current components to use as a load. I also need a σ11 for the RPi, I'd guess.
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Re: σ25 vs σ11

Postby amb » February 15th, 2014, 11:20 am

Depends on how large a heatsink...

For the σ25, the maximum current rating of the 78xx regulator is 1A, so you can get bursts of current up to that and perhaps a little more. But you can't practically operate it anywhere near that on a sustained basis.

For the σ11, the hard limit is the ratings of the rectifier diodes and transformer, and it has no current limiting circuit, so it could provide bursts of current of several Amperes. But depending on heatsink size, the junction temperature of the MOSFETs will rule how much current could pass constantly. You should make some calculations as described in the σ11 website "Board & heatsinks" section.

The RPi is a "computer", so you could use a switch mode supply with far more efficiency.
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