Channel Balance Adjustment
If there is a channel imbalance within the soundcard (Art Phono Plus), will it skew the data and cause inaccurate results?
Many have asked whether a Loopback cable should be used to measure and adjust channel imbalance. However, this method is flawed.
When using the soundcard to perform actual measurements, only the INPUT is used. But during loopback, both input and output are used, and the results will be magnified by 2x or 100%. Compensation using loopback will actually result in overcompensation, or inaccurate balancing because the Output from the same soundcard is being used. .
If loopback is used to measure the Art Phono Plus soundcard, most will forget to make the proper setting changes. Most will use "Monitor Output" to "Line In" with "Monitor Source" set to "Both". The measured amplitude difference between the two channels is about 0.4 dB and the measured phase difference is about 0.08 degree as shown. With 0.4dB’s amplitude difference, the mismatch of the two waveforms can be seen clearly.
However, for loopback test, “Monitor Source” should be set to “CPU” setting on the sound card. With this setting, the measured amplitude and phase difference became 0.12 dB and 0.17 degree respectively.
In the above loopback test, the mismatch between the two channels was contributed by both the input and output channel (the loopback cable used is an ordinary one, with output L -> Input L, and output R -> Input R).
For turntable adjustment, only the mismatch between the two input channels needs to be checked. We performed the test with an external signal generator to inject the same signal to the two input channels of USB Phono Plus. The measured amplitude and phase differences are only 0.02 dB and 0.19 degree respectively, which are negligible. The channel imbalance is so small, smaller than most cartridges and phono stages' inherent channel imbalances.
We have tested 20 samples of the ART Phono Plus, and nearly all of them have the same spec, they all gave very similar results.
Channel Balance adjustment on the Art Phono Plus is, therefore, unnecessary.
For the sake of argument, let's assume another soundcard is used, and there is significant channel imbalance, AnalogMagik will still allow you to achieve an accurate setup because we are concerned with "Incremental changes", or in the case of azimuth, finding the narrowest range between L and R channel. Inherent channel imbalance will cause the gap between L and R channel to widen, but you will still be able to find the narrowest point.
In reality, we found the majority of the cartridges on the market to have some channel imbalance, far greater than the 0.02% of the ART Phono Plus, therefore channel balance adjustment is unnecessary if you are to use the Art Phono Plus. However, we cannot say the same about other soundcards, which is why we recommend AnalogMagik to be used with Art Phono Plus only.