User manual

3.3. S/PDIF Coax Output
The S/PDIF coax output (optionally AES/EBU output) is the primary output that will
connect to most DACs. The output impedance is 75 ohms. The cable that connects
from this connector to your DAC should preferably be of highest quality and match the
75 ohm impedance. The S/PDIF output is transformer-coupled and isolated from earth
ground. The RCA jack is made of high-quality direct-gold plated copper with Teflon
insulator, so it is better than most BNC connectors. Optionally, a Canare BNC connector
can be supplied here, which is also excellent quality. The cable attached to this
connector should be at least 1.0m in length, but minimize the length beyond that. 1.5m
is usually perfect and will work best with other gear. The internal cable in the Off-Ramp
4 also has 75ohm characteristic impedance.
3.4. AES/EBU Output
The AES/EBU output is on a gold-plated XLR connector. Pin-out is standard pin1-Gnd,
pin2 plus or non-inverted signal, pin3 minus or inverted signal. The output impedance
between plus and minus conductors is 110 ohms. The output is transformer-coupled.
The cable attached to this connector should be at least 1.0m in length, but minimize the
length beyond that. 1.5m is usually perfect and will work best with other gear.
3.5. I2S output
I2S is a more direct D/A interface than S/PDIF. It was developed by Philips when Philips
and Sony created the CD format and the S/PDIF interface specification. I2S is a three or
four-signal interface that includes clocks. All four signals are provided at 3.3V logic levels
at the Off-Ramp 4 output, but 5V logic levels are available optionally. I2S is the native
interface for most D/A chips. Therefore if one can drive I2S directly and avoid S/PDIF
translation, there is the opportunity to reduce jitter even more. Empirical Audio standard
I2S interface uses 4 signals and 4 returns and the RJ-45 connector. It is compatible with
I2S on several other manufacturers DACs, including Stello, Perpetual Technologies and
Northstar, as well as our own Overdrive DAC. Other manufacturers DACs require an
adapter cable to make the connection since they don’t all use RJ-45 and the pinouts are
not compatible. Most of the other manufacturers DACs also require the 5V logic level
option. Make sure the I2S connectors are mated at both ends of the cable before
powering-on either the Pace-Car USB or the DAC. Likewise, power both down
before disconnecting the I2S cable.