Given an output signal level and an SNR specification, we can calculate the output noise level.
The XLR outputs on the Benchmark DAC3 D/A converters have an SNR of 127 dB at an output level +24 dBu. This +24 dBu
output level is a standard calibration used for balanced connections in many recording studios.
These values are already entered below. Click the calculate button and you will see that the output noise
voltage produced by the DAC3 is 5.48 uV.
The XLR outputs on the Benchmark DAC3 converters are equipped with passive 10 dB and 20 dB output attenuators that
attenuate both the output signal and the output noise. As a result, the DAC3 achieves the same 127 dB SNR at all
attenuator settings. The 10 dB pad is required when the DAC3 is driving most hi-fi products. With 10 dB pad
engaged, the output level is 14 dBu which is about 4 Vrms.
Enter the following in the calculator above:
Output Signal Level = 14 dBu
Signal to Noise Ratio (SNR) = 127 dB
Click the calculate button and you will see that the output noise voltage produced by the
DAC3 is 1.73 uV with the pad engaged. The passive attenuator reduces the signal level by 10 dB. It also
reduces the noise level by 10 dB.
Notice that, for a given SNR, the noise voltage can be more than 3 times as high in a professional balanced
system as compared to a 4 Vrms hi-fi balanced system. Stated differently, the professional balanced system
provides a 10 dB improvement in SNR for a given noise signal.
If we wish to achieve a 127 dB SNR using unbalanced RCA cables we are going to have problems! In hi-fi systems, RCA
inputs and outputs are calibrated for a maximum signal level of just 2 Vrms.
The following levels are entered in the calculator below:
Output Signal Level = 2 Vrms
Signal to Noise Ratio (SNR) =127dB
Click the calculate button and you will see that the output noise voltage would need to
be 0.89 uV to achieve a 127 dB SNR. This is difficult to achieve over an unbalanced interface. Most
unbalanced output drivers and input receivers produce more than 1 uV of noise, and this makes unbalanced
interfaces unsuitable for low-noise audio systems.