[The Monkey]

Hi all,

Hoepfully, one of my next projects is a DAC. But first I want to make sure I can use it in the following way.

I want to get an Airport Express so I can stream tunes to my main stereo rig. However, I just realized that my NAD 372 doesn't have a TOSlink or other digital in. So I was thinking that I run the Airport Express --> DIY DAC --> NAD. I figure I'll have RCAs as output on the DAC. Does that make sense?

 

[Zorander]

As long as the NAD accepts RCA connections and the DAC you are building accepts the digital signal from the Airport Express (optical and/or electrical), everything looks in order.

 

[The Monkey]

Quote:

Originally Posted by Zorander As long as the NAD accepts RCA connections and the DAC you are building accepts the digital signal from the Airport Express (optical and/or electrical), everything looks in order.

Cool. Thanks. Now, any suggestions about which DAC for this application? I'm not looking to break the bank, so under $200 would be nice. Easy to build would be nice, too.

Maybe I should just get the Micro DAC?

 

[Francis_Vaughan]

The Airport Express only provides optical S/PDIF. You can of course get an after market optical to coax converter. The jitter charateristics of this may be, um, not the greatest. But it will work.

The Airpport Express does of course also have a PCM2704 DAC inside. The quality is, well, indifferent.

 

[Zorander]

the M-Audio SuperDAC I'm using falls within that price range, but it is unfortunately discontinued (i.e. you have to search for used units). And it comes pre-built already, so it won't cure your itch for DIYing.

 

[Twombly]

Quote:

Originally Posted by Francis_Vaughan The Airport Express only provides optical S/PDIF. You can of course get an after market optical to coax converter. The jitter charateristics of this may be, um, not the greatest. But it will work.

Could you rip an TOSLINK-to-electrical SPDIF pulse converter board outside of a cheap commercial one to put it inside the DAC and supply the correct voltage and current from the DAC's PSU, preferably with some kind of delay switch? That would be a suboptimal solution as well, but at least you would have one box. It'd need good noise isolation.

 

[Garbz]

Don't bother. Toslink and Coax have the same signal (S/PDIF) only the electrical characteristics are different. One is transmitted via light the other via a coax cable. Just buy a TORX179 part and build it into your DAC, or find a DAC design which excepts Toslink.

The best solution would be to modify the Airport Express. Remove the TOTX, use 2 resistors to convert the TTL signal to Coax S/PDIF specs (75ohm 0.5vp-p) and whack a BNC or RCA socket into it.

 

[The Monkey]

Lol, this isn't as easy as I had hoped.

 

[Francis_Vaughan]

The problem with optical to coax, is that you get all the jitter inherent in the awful TOTX TORX interface (which is quite dreadful) plus the problems in your coax transmitter, the cable, and the quality of the receiver. It will work, but the DAC will have a rotten time of getting back a clean clock (or rather won't get back a clean clock, and you will live with the sonic results.)

But the problem with modifying the Airport is that it is a very tightly built device, with essentially no room left inside, and a welded plastic case that needs essentially destroying in order to get inside.

 

[The Monkey]

Francis, do you have any suggestions about what to build for this application? I'd prefer not to open up the Airport Express.

 

[Garbz]

Only option otherwise is to build/buy a DAC with a Toslink interface.

 

[The Monkey]

Thanks Garbz. So to sum up, I'd want to build a DAC with a TOSlink interface (meaning a TOSlink jack in, yes?) and have RCA output jacks?

 

[Twombly]

I wasn't aware of any hand-solderable optical input boards or DACs that included them.

 

[The Monkey]

Quote:

Originally Posted by Twombly I wasn't aware of any hand-solderable optical input boards or DACs that included them.

I was afraid of that. Sounds like I won't be able to do this?

EDIT: In other words, I'll have to use a regular mini-to-RCA out of the Airport Express and rely on the AX's internal DAC.

 

[Francis_Vaughan]

There do exist optical to coax S/PDIF converters - they are pretty popular due to the needs of home theatre. It is just that you can only expect the (poor)performance inherent optical, at best. But it would probably beat the AX's internal DAC. Using an optical input with a S/PDIF coax based kit should not be too hard - the TORX device spits TTL level out, so it should not be hard to use one - just a pain from the mechanical point of view, both mounting and getting power to it. But I would want any DAC you tried to have serious attempts at jitter reduction inside.