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The amplifier comes standard with USB, and RCA inputs. The USB is for a digital connection from a computer, because the amp also has a built-in DAC. With an phone or tablet, you'd very likely not use the USB, and instead just patch your analog headphone output into the RCA. If you need a link to such an adapter cable, I can send you one. They are common and easy to find.
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You can use an adapting cable such as this:
Also, if you are set on using the digital input, then you'd need to purchase a more expensive adapter, and you may need one type for your ipad, and a different for your iphone, depending on the model you have of each of these devices. With the analog headphone approach, you'd be able to use the same cable as shown above provided your device has a standard headphone output. Since this is a unique analog retro tube driven amp, it's very debatable as to which type of connection is better, although I'd recommend the analog in most cases. If you connect with digital , you may get a ground loop noise.
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Also, you mention not seeing the RCA inputs. They are on the back, right next to the digital USB input as shown here: The red/white (right/left) sockets.