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USB power and dual supplies

sigma25, sigma26, sigma24

USB power and dual supplies

Postby ShinyFalcon » December 27th, 2016, 9:02 am

I'm thinking of miniaturizing my current Opus DAC, currently stuffed in a 12" x 12" x 3" case. I have a Doodlebug coming, which is a USB power isolation device.

My idea is to have the clean 5V from the Doodlebug power the s25 and s26, which will then power the main components of the Opus DAC and the two alpha24s. My concern is with the alpha24, as it takes a dual power supply. I'm looking at the negative rail of the s26, and I see the issue with the swapping of 5V and G. Looking at the bridge however, since there's diodes inside, would there be any issue with the single 5V supply powering both the s25 and s26?

If so, my plan then is to wire up a rail splitter with the TLE2426 and a 220uF cap, which would be +/- 2.5V for the alpha24. The 5V components will be powered directly from the Doodlebug. The opamps at a quick glance are rated to at least +/- 2V.

Any potential issues with my plan?
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Re: USB power and dual supplies

Postby amb » December 27th, 2016, 10:13 am

Can you draw and post a schematic diagram of what you're trying to do? The description is complicated but a picture is worth a thousand words.
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Re: USB power and dual supplies

Postby MisterX » December 27th, 2016, 1:38 pm

How much current does a pair of Alpha 24s need?
I'm thinking an inverting charge pump would be a better solution then a TLE but did not check the current requirements for the Alpha.

http://cds.linear.com/docs/en/datasheet/3265fa.pdf ;)
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Re: USB power and dual supplies

Postby amb » December 27th, 2016, 3:13 pm

α24's quescent supply current is around 30mA per board (channel). If the outputs are being driven into a low impedance load then the supply current will increase.
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Re: USB power and dual supplies

Postby ShinyFalcon » December 28th, 2016, 8:14 am

The LTC3265 seems like an interesting solution. I'm reminded by the Carrie amplifier effort. I almost freaked out by the 3mm x 5mm form factor though, but 6 x 7 mm really isn't any better. Noise is a bit high at 100uV, but s78/79 to the rescue? :D I think I'm getting too close to minimum voltage tolerances though.

I'll try to clarify a bit. Opus components are the Cirrus mux, ASRC, and the WM8741 boards themselves. They all take 5V as the input and have local regulators similar to the gamma2/1. I still see the issue with virtual ground (half of 5V) and real ground (V-, Doodlebug's shell/ground, etc). I was hoping there is some sort of isolation on the s25/s26's AC bridge. I'll draw something up with Digikey's schematic maker. For reference I was looking at Tangentsoft's virtual ground article and borrowed the simple TLE schematic from there.

It would be cool to have a gamma2.5 though... a gamma3 in portable form. :D
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Re: USB power and dual supplies

Postby MisterX » December 28th, 2016, 12:11 pm

Yeah the package size of LTC3265 makes for a bit of a headache.
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