- Jul 17, 2018
- Syracuse, NY USA
This IMD (and THD) is a direct result of omitting a differential amplifier after the output of the chip. Like most high-quality D/A converters, the ESS chip outputs are balanced. Many designers think it is OK to connect these directly to the outputs on the back of the unit. Typically the balanced connection from the chip gets buffered and connected to pins 2 and 3 of the XLR jack. Often, the RCA output is directly connected to pin 2 of the XLR jack and consequently, the RCA output only sees one side of the converter's balanced output. If you take the XLR output and connect it to the balanced input on an Audio Precision test station, the IMD will not show up. But if you connect the RCA output to the same analyzer, the IMD is present.
What is happening??
All D/A converter chips produce significant common-mode distortion on the balanced outputs. This unwanted common-mode distortion is easily removed with a differential amplifier. If it is not removed, the result is IMD.
The AP test stations have transformer-coupled inputs that provide excellent common mode rejection. This is equivalent to the function of a differential amplifier. The transformer-coupled input on the analyzer removes the common-mode distortion at the output of the XLR and everything looks good, but it really isn't. Most XLR inputs do not have the common-mode rejection ratio that is provided by the inputs on the AP test station. More importantly, the RCA outputs on these boxes have high IMD and there is no way to remove it downstream.
How to eliminate the IMD:
The solution is to place a well-trimmed differential amplifier at the balanced outputs of the D/A chip. This removes the IMD while producing an unbalanced output that can be buffered and sent to the RCA outputs. It can also be re-balanced and sent to the XLR outputs. The is the correct way to provided balanced XLR outputs from a D/A converter. The transition from balanced to unbalanced and back to balanced is necessary. This is what removes the common-mode distortion.
We often provide a balanced pair of differential amplifiers so that the we have a balanced output from the differential amplifier pair. This technique is used in our new HPA4 line/headphone amplifier.