How to Run Long Distances with 110-ohm I/O

Question:

I have a question about your AD2408-96. I am planning on using the version with AES/EBU outs. What cable length can the drivers in the unit handle? I think that the AES/EBU spec indicates 1500M, under ideal conditions.

I expect to drive around 400 feet of Gepco 110-ohm cable, terminated at Yamaha CD-8 AE interface cards at a 02R. I assume this won't be a problem but it never hurts to ask.

Answer:

AES-3 (110-ohm) and AES3-id are matched impedance transmission systems, and transmission distance is not a function of the transmitters. It is a function of the cable losses, and the characteristics of the receiver.

AES-3 (110 ohm twisted pair) will generally support transmission distances of up to 100 m. With 400 feet of Gepco 110 ohm cable, you will be uncomfortably close to this limit. You would probably need to apply cable EQ at the receive end in order to maintain adequate noise margins. I would suggest considering a 75 ohm coaxial interface. In general the 75 ohm interface will support cable lengths of 300 m without EQ, and 1000 m with EQ.

Keep in mind that these numbers are very dependent upon cable loss. For example, low-loss 7C2V cable can support connections that are twice as long as connections using 3C2V cable. The AES3-id document recommendations are based upon 5C2V cable. The maximum cable lengths with EQ are 650 m for 3C2V, 1000 m for 5C2V, and 1300 m for 7C2V cable. In general, cable losses should not exceed 6 dB at 1 MHz without cable EQ, or 14 dB at 1 MHz with cable EQ.

Question:

Using Gepco 5596GFC, according to their specs, the loss at 1 MHz over 400 ft would be 3.6db (.9db/ 100 ft). At 12Mhz, though, the attenuation would be 8.6db (2.15db/100ft). So it would seem that for 44.1Khz it would be OK, but 96Khz may be a bit dicey.

I would like to stick with AES format, as Yamaha does not make a 75 ohm interface for the 02R, and I would rather not add the complication of a format converter. So, I may need to consider EQ. Any suggestions?

Answer:

For 96 kHz keep the attenuation less than 6 dB at 2 MHz. I believe you will find that the attenuation of your Gepco cable is about 6.1 dB at 2 MHz. Like you say - "a bit dicey". You are correct to avoid 75 outputs driving 75-Ohm to 110-Ohm format converters. By itself, a 75 to 110-ohm transformer type format converters will introduce 10.4 to 12 dB of attenuation. Resistive matching networks introduce an attenuation of 12 dB. However, please note that 110 to 75 ohm conversion works very well. A transformer type 110 to 75-ohm converter will produce a 75-ohm signal that is 10.4 dB hotter than normal (if it does not have an internal resistive pad). A pair of complimentary transformers will introduce a net loss that is nearly 0 dB and will introduce very little loss at 2 MHz. Also, quite inexpensive!

Here is our suggestion: Simple, Inexpensive, and Reliable! Purchase our A/D converters with 110 ohm XLR outputs. At each digital output connect a Canare BCJ-XJ-TRA (transformer type XLR to BNC adapter). Run 75-Ohm 5C2V cable and connect it to a Canare BCJ-XP-TRA (transformer type BNC to XLR adapter) at the O2R inputs. Your cable loss will now be 1.6 dB at 2 MHz (or 1.13 dB at 1 MHz). The loss in the Canare transformers is small at 2 MHz. Caution, do not use the Canare BCJ-XJ-A10TR as this device incorporates a 10 dB attenuator!