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I'm trying to connect my phone to my portable DAC which has an optical in. I am looking for a solid 24-bit (up to 32-bit) cable for signal transmission, pre-processing. I've researched TOSLINK TX architectures, but not with reference to phone USB power-handling or their expected milliamp/volt consumption.

(As an aside, if there's a cheap OTG cable for USB-C to Micro USB without the power leg on the Micro side, that would be a wonderful solution.)

The problem I'm trying to fix is that my phone's output via standard electrical cable automatically charges the amplifier, draining the phone battery. I'm well-aware I can make and constantly repair my own USB OTG cords and omit the power rail on the micro-USB side, but I'm looking for a lower-maintenance solution.

The reason I say mostly-passive is that the TOSLINK TX device has an internal IC to drive the LED.

It seems there ought not be an issue, but every related device I've seen on the market is $100+ due to its containing an internal DAC, which I specifically don't want...they can't drive my high-impedance headphones. Cries in audiophile

Christopher Karr
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You need at least a minimal USB implementation that tells the phone what formats are supported (because the optical link is unidirectional) and that unpacks the USB frame structure and adds the TOSLINK frame structure.

I'm not aware of any ready-made products, because that is kind of a niche that would not see sufficient sales to justify the engineering cost.

It's doable for an electronics hobbyist (provided the parts are available :( ), but not at a sensible price.

I'd expect around $50 cost for parts for ten units, with cost for PCB manufacture and shipping cost being the defining terms.

Simon Richter
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  • Thanks; I'm now scouring android dev. forums to learn host/peripheral topology. For something this small, who needs a PCB? Hot glue and heatshrink are old friends. I thought the TOSLINK internal chip might get recognized during the phone's default usb audio-routing step as being a compatible digital IO device. – Christopher Karr Sep 30 '21 at 19:46
  • If it implements USB, then yes. It absolutely makes sense to have USB and TOSLINK in the same IC, because the bitrate on the optical link needs to be derived from the USB clock so there are no buffer over-/underruns during long data streams. – Simon Richter Sep 30 '21 at 19:54
  • Thank you. I will bring that into my design consideration. Worst case I blow out a USB port on my computer during testing. Now to find out if UAC1 or UAC2 (USB Audio Class) is compatible with S/PDIF. – Christopher Karr Sep 30 '21 at 20:36