Can I use airport extreme USB port to connect to a USB DAC to play iTunes music through my Hi Fi?

No.. it is not possible.

Apple would have to make it possible and they never did. You need an Express which I believe can be modified to output via USB..

https://hifiduino.wordpress.com/2013/06/15/apple-airport-express-usb-audio-output-mod/

This would require high grade DIY skills.. be warned!!

The Extreme has no internal sound components you can tap out. The Express does.

Since the Express already has optical out as well as standard 3.5mm audio out.. why reinvent the wheel??

USB DAC can work with a computer but there are no drivers for Extreme which is based on NetBSD and is well protected against fiddling.

The good news (I think) is that the USB DAC I've ordered has an optical audio in as well. Maybe that is worth considering?

If it has optical port then yes .. you can use Apple TV. And that will give good sound via the DAC.

Plus you avoid having to buy an Express at least for the time being.

if you just wish to play music from a computer to a hifi stereo then I just got a cheap Bluetooth Music Receiver | Belkin

would not use it for movie audio because bluetooth has no real way of keeping in sync with the pictures so if it proccessing give a 1/2 sec delay then you're gonna have a bad time.

another product which may be of intrest is Belkin planning AirPlay 2 and HomeKit audio adapter for retrofitting smart features - 9to5Mac

seems to be close to the replacement people have been looking for, to having airport expresses to get add audio output to apples audio ecosystem.

FWIW, I had a pre-AirPlay, pre-Bluetooth Denon amp & associated pre-amp system ... also "loaded" with RCA input/output ports. My solution, as others have mentioned, is to use an AirPort Express base station. The Express has a fairly good DAC built-in and I never needed to augment it (although you could) with a dedicated DAC external to it. Since the Express' audio port is a dual analog/digital-optical port, it was a simple matter to connect a 3.5mm stereo-to-RCA type audio cable between them for great sound reproduction through my Denon system.

The audio source for most of my music was originally LPs, then CDs, and now, mostly digitized versions of both. I still miss the "warm" sound of a good vinyl recording via a tubed amp ... be it Beethoven's 6th or Dave Brubeck's Take Five. Sigh!

Connect Apple TV to a home theater receiver that has an optical digital audio port, using an optical digital audio

That will work. Most later Home Theatre amps have HDMI passthrough which would also work.

The DAC does have an optical in. It is a Cambridge Dac Magic 100.

So the DAC is for an older hifi amp??

a) Wireless from my MacBook to my (old) Apple TV.

b) Optical audio cable from my Apple TV to a stand alone DAC.

c) RCA audio cables from my DAC to my stereo pre amp.

You can probably trial it today.. If you have a smart TV with RCA sound out.. or headphone out.. you can interlink the whole thing and test how it works. If you use headphone out you should keep the sound very low because it will be amplified. But a decent pre-amp should handle the signal no problem.. otherwise just make up a resistance load and tap the output across it. Of course you are using HDMI but that should not matter. It will be same basically as the optical.

but I'm working on Lotto

Hmmm.. might take a while. Your average time to a big win is around 1,000,000 years I think.

We could ask Morrison for a big increase in the pension.. Watch out.. Pigs are getting ready for take off. (Great to reach the age where the gov starts paying me instead of visa versa).

the appletvs with an usb port can only use it for restoring

usb is a universal serial port, it transmit raw data and to interpate the raw data into something a device driver is requied to be installed, that is only really a thing for windows, linux and macOS computers.

there are some device drivers which are generic which makes it possible for things like iphones and ipads to import photos from usb sticks and sd cards though.

Could I run a USB from my Apple TV to my DAC rather than to my TV?

No.. in fact the 4K model does not have a USB port. And earlier models it is used for Apple service and diagnostics.

i.e. it has no user functionality whatsoever. On the Gen1 you could boot from USB. But no sound out.

What model Apple TV do you own. Sorry but even if it has USB it is useless to you.

Did you mean HDMI? You don't have USB running to your TV from Apple TV.. you have HDMI.

HDMI can output digital audio for sure. But you need an Amp or DAC with HDMI input not USB.

I have actually ordered a USB DAC mainly because of cheaper cost. I failed to consider the cost and inconvenience of a 5 meter cable.

Cancel the order if possible. 5M is limit of USB signal without special adapters and it is thick and ugly. Plus for digital sound I think you will get other issues.

I figure you must be audiophile which I am long past since old age and hearing diminish horribly with age. But for computer sound you really don't need a DAC of that value.

Explain a bit more why you need the expensive DAC and why not use Express with Optical or 3.5mm analogue stereo?

What input on the amp were you plugging into? How many channels is it?

If you are really stuck with USB look at raspberry pi .. they are able to do all kinds of useful stuff.

just google

raspberry pi dac usb

Just to finish, can I play my iTunes music through my Hi Fi with a USB cable from my Macbook Pro to a DAC Then naturally RCA cables from my DAC to my pre amp? Does my MacBook have all the drivers or whatever to achieve my goal?

Yes!!

If the drivers are not built in you should be able to download them.

Express is out of production but you can pick them up cheaply enough.. The best sound is actually not the Gen2 according to the article I already listed.. people seem to disagree if the Gen1 or the earlier G model.. But you can get the Gen1 N wireless for $30 or so on eBay in Aus. The Gen2 you will be lucky to pay less than $60.

Power supply issues on Gen1 are easy to fix. And you can use it over ethernet or by wireless.. much the easier way to go IMHO..