Given an output signal level and an SNR specification, we can calculate the output noise level.
Example 1:
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.
Enter 24 dBu and a SNR of 127 dB. The calculator will show that the output noise voltage produced by the DAC3 is 5.48 uV.
Example 2:
The XLR outputs on the Benchmark DAC3 converters are equipped with passive 10 dB and 20 dB output attenuators. These 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 into the calculator:
- Output Signal Level = 14 dBu
- Signal to Noise Ratio (SNR) = 127 dB
The calculator will now show 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. If you enter 1.73 uV and 5.48 uV into our gain calculator, you can confirm that difference is 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.
When comparing products, compare the SNR specifications and not the output noise specifications.
Example 3:
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. This is too low to achieve a good SNR. Lets do the math:
Enter the following into the SNR(Vrms) to Output Noise Calculator:
- Output Signal Voltage = 2 Vrms
- Signal to Noise Ratio (SNR) = 127 dB
The calculator will show that the output noise voltage would need to be 0.89 uV to achieve a 127 dB SNR. This noise voltage 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 completely unsuitable for low-noise audio systems. Unbalanced RCA interconnects have no place in a high-quality audio system.
Notice that if we were to compare an RCA output with 2 uV of noise to a professional XLR output with 5.48 uV of noise we could incorrectly assume that the RCA output was better. If you enter the following numbers into the calculators, you can see that this particular XLR output is 7 dB better than the RCA output.
Enter these values into the first calculator:
- Output Signal Level = 24 dBu
- Signal to Noise Ratio (SNR) = 127 dB
Enter these values into the second calculator:
- Output Signal Voltage = 2 Vrms
- Signal to Noise Ratio (SNR) = 120 dB
Note the output noise volage of both outputs. This XLR output has a noise voltage that is more than twice that of this RCA output. But notice that the SNR of this XLR output is 7 dB better than that of this RCA output, even though the noise voltage is more than twice as high on this XLR output. This demonstrates why your should compare SNR specifications and not noise voltage specifications in most applications.
