USB DAC - ground isolation

[pho_boi]

Hi everyone!

I currently own a Behringer UCA202 USB DAC.

http://www.behringer.com/UCA202/index.cfm?lang=ENG

It uses the Burr Brown PCM2902 chip which features SPDIF output. Currently it has an optical output and I am thinking of modifying it so that it would output through coax.

I'm not very experienced in electronics, but after looking at the datasheet for the PCM2902 it seems as though I can just cut the traces from DOUT and attach an RCA socket to that pin.

Some more googling reveals that its best to isolate the gnd on the digital. I then found a schem on hagtech for the hagusb, which uses a small transformer to do this.

http://www.hagtech.com/images/hagusbschem.gif

Does anyone in Australia know where to get a small transformer like that? I know you can get them from Digkey, but not sure if its worth paying shipping on a single item.

And what should I be looking for when trying to find a small transformer to do this?

Is the IS0150 from TI a good way to isolate gnd?

I'm not even sure if it's worth modding, but its something I'd like to do for fun

Thanks for reading!!

[macboy]

No, you cannot simply connect the RCA jack to the DOUT pin. The DOUT pin provides a logic-level signal (that's 3.3 V logic, not 5 V), with a maximum current of +-4 mA (Refer to "Electrical Characteristics"). The RCA S/PDIF output needs to drive +-1.0 V (i.e. 0.7 Vrms) into a 75 ohm impedance, with a 75 ohm source impedance. It requires 6 MHz or more of bandwidth. It is no coincidence that these are the same requirements for composite video, nor that the jacks and cables are the same (75 ohm cables with RCA plugs). You can use a video buffer or any opamp that can be used as a video buffer for this task. Many people try to use logic gates to try to provide this output amplifier functionality, but that is just plain silly, IMO.

[pho_boi]

Originally posted by macboy
No, you cannot simply connect the RCA jack to the DOUT pin. The DOUT pin provides a logic-level signal (that's 3.3 V logic, not 5 V), with a maximum current of +-4 mA (Refer to "Electrical Characteristics"). The RCA S/PDIF output needs to drive +-1.0 V (i.e. 0.7 Vrms) into a 75 ohm impedance, with a 75 ohm source impedance. It requires 6 MHz or more of bandwidth. It is no coincidence that these are the same requirements for composite video, nor that the jacks and cables are the same (75 ohm cables with RCA plugs). You can use a video buffer or any opamp that can be used as a video buffer for this task. Many people try to use logic gates to try to provide this output amplifier functionality, but that is just plain silly, IMO.

Thanks for the info

But the way that Jim Hag did it in the HagUSB, is it a bad way of doing it?

[alleycat]

A bit off topic, but I was wondering about the analogue output of the UCA202. I'm currently using an M-Audio internal soundcard but I would like to try a USB DAC. Would you consider the Behringer to be a worthy upgrade?

[pho_boi]

Originally posted by alleycat
A bit off topic, but I was wondering about the analogue output of the UCA202. I'm currently using an M-Audio internal soundcard but I would like to try a USB DAC. Would you consider the Behringer to be a worthy upgrade?

The Behringer's analog output is very mediocre in sound quality. I basically purcahsed it for the spdif out and native asio support.

[alleycat]

Thanks, you saved me from wasting some money! I might have a look at the Silverstone EB01, which costs twice as much, but there doesn't seem to be anything else out there.

[anbello]

I think this can help

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3122

http://focus.ti.com/docs/prod/folder...sn65hvd30.html

RS 485 is the same interface of spdif from electrical point of view,
the two link are two transceiver from 3.3 v logic to RS 485 one Maxim and on TI.

ciao
andrea

P.S. Is it possible to see some photo of the internal of the UCA202? Or even better some schematics?

[macboy]

Originally posted by anbello
I think this can help

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3122

http://focus.ti.com/docs/prod/folder...sn65hvd30.html

RS 485 is the same interface of spdif from electrical point of view,
...

That is not true. S/PDIF is a single-ended signal using a 75 ohm sheilded coaxial cable for transmission. You may be thinking of AES/EBU digital, which is differential and uses 110 ohm twisted pairs. Refer to this excellent tech note at Rane.com.

[anbello]

Originally posted by macboy



That is not true. S/PDIF is a single-ended signal using a 75 ohm sheilded coaxial cable for transmission. You may be thinking of AES/EBU digital, which is differential and uses 110 ohm twisted pairs. Refer to this excellent tech note at Rane.com.

Maybe not completely true, but i saw many circuits doing similar things based on the SN75179 (similar to my proposed solution but with 5V logic) and anyway you can use the differential out of the driver with the circuit in fig. 1 of your link to have spdif out.

ciao
andrea