Need help adding USB to my DAC
Nov 18, 2004 at 6:20 AM Thread Starter Post #1 of 3

Graz

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I want to gut my m-audio transit and connect the s/pdif output from that card directly to the input of my ciaudio vda-1 DAC via a very short (2-3 inch) direct connection.

I need some advice with the following questions...
- What about the short cable length, and it's direct impact on jitter? I've seen that long cables are supposedly better for s/pdif...what about very short connections...is this a bad idea?
- What about the 75 ohm termination? How do I ensure that the termination is properly implemented on this short connection?
- What about galvanic isolation? Is this a must have, or should it only be used on a case-by-case basis...i. e., use it if I have a problem with hum?

I've contacted Steve Nugent at empirical audio (www.empiricalaudio.com). He will perform this exact mod, but he wants $350-400 to do the work (which includes the cost of the USB audio solution...transit-based I believe...and the mechanical work to add the USB connector directly to the DAC enclosure). When I respectfully asked if he would consider dropping the price a bit, it was clear that he wasn't going to budge. Consequently, I've decided to try to do this myself. I can easily do the soldering, wiring, and enclosure work myself...and the total out-of-pocket expense for me would be negligible since I already have the USB sound card & the DAC. I just need a few key questions answered...

Thanks in advance for any advice with this!

Peace,

Graz
 
Nov 18, 2004 at 9:34 AM Post #2 of 3

Garbz

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I'm just wondering where you are getting the SP/DIF output from? The optical output from the Transit is TTL as far as i know. http://www.epanorama.net/documents/audio/spdif.html has some circuits to help convert TTL to SP/DIF.

Do let us know how you go I'm especially interested since I intent to add a coaxial SP/DIF output to my transit sometime soon.

Also from what I understand if you can terminate it at 75ohm Jitter becomes less of a problem.
 
Nov 18, 2004 at 10:36 AM Post #3 of 3

Graz

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Transit uses the Asahi AK4584...

Data sheet for this device can be found here:

http://www.asahi-kasei.co.jp/akm/en/...84/ak4584.html

On page 24 of the data sheet, it reads:
"The AK4584 inlcudes a TX output buffer. The output level is 0.5V, +/-20% using the external resistor network shown below...".

I was planning to implement the circuit shown in Figure 7 of the data sheet (which is shown just below the text I've quoted). The T1 device provides the galvanic isolation, and I have a suitable device for this application.

Anyway, I'm thinking it will fly but I still have concerns about the short connection length between the output of the Transit & the input of my DAC, as well as accurately implementing the desired 75 ohm termination.

Still hoping for some expert opions...
eggosmile.gif


Peace,

Graz
 

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