Hi Folks. 
As between BNC and RCA, which is the recommended connection between the Off-Ramp Turbo3 and a DAC?
I have read the BNC is always favored over RCA becuase there is less chance of the Ohms dropping below 75. I have also read that this is nonesense. Fact or fiction or it depends? 
Mike
It really depends on the cable used and it's length. There is always an impedance discontinuity with an RCA connector, and less of a discontinuity with a BNC connector. Even though the BNC connector can be 75 ohms, the backside of the connector inside the chassis is usually not 75 ohms if it is a solder-type connector so even BNC does not eliminate a discontinuity. The short cable inside the chassis is also involved and should be 75 ohms. BTW, many manufacturers erroneously use a 50 ohm BNC.
The ideal situation is to have a 75 ohm bulkhead-mount BNC to BNC so that there is a BNC jack outside the chassis and a BNC jack inside the chassis. This is the ONLY way that you get 75 ohms without any discontinuity all the way to the circuit-board. Then when you reach the circuit-board with a short 75 ohm cable, the trace on the board needs to be 75 ohms also. No gear that I have ever seen does this. It is probably overkill for this application.
What Empirical Audio does is cable the Off-Ramp 3 and Pace-Car from the circuit board to the RCA or BNC connector with low-loss 75 ohm coax, but it is soldered to the backside of the connector. We choose to do it this way because the solder-in connectors are all poor quality compared to the bulkhead-mount solder-type connectors.
Fortunately, with S/PDIF it is possible to use a minimum cable length and avoid most of the reflections caused by these discontinuities. See this white-paper:
http://www.positive-feedback.com/Issue14/spdif.htmSo, if you use a 1.5m cable length with most consumer gear, you dont have to worry about the reflections due to these short discontinuities. Since the risetimes on Empirical Audio gear are faster, a 1m length is sufficient. As long as a low-loss RCA connector is used (Teflon/copper), it is not a problem.
It is still critically important that the cable be 75 ohms and low-loss to miminize jitter, but the effect of the RCA connectors is really minimal if the cable is sufficiently long and the connectors are good ones IME. This is why we use RCA connectors on our excellent Bitmeister USB cable and only offer 1-2m lengths. This makes it compatible with most stock gear without any performance impact. One reviewer for Positive-Feedback said that our Bitmeister with RCA's was the best commercially available digital cable. I have yet to hear a better one, and other manufacturers have even sent them to me for comparison.
Steve N.
