AnalogMagik Tutorial No. 6: Optimizing Load & Gain on:
your Phono Stage
ost Phono preamps for MC cartridges or Step-up Transformers will allow users to choose different loading choices to match with their cartridges. Cartridges manufacturers will also provide a suggested loading range. However, the actual loading requirements for each cartridge is affected by a number of different factors such as different tonearm cable lengths, which will affect capacitance and resistance. Most manufacturers will advise users to choose a loading which "sounds best" to their ears. We believe there is nothing wrong with going with your ears, but AnalogMagik attempts to provide for you a more scientific and repeatable approach to optimising loading.
Changing the resistive loading has the same effect as "Pushing the bars" on an equalizer up or down, it will affect the frequency response, and to a lesser degree the amplitude of the signal. If accurate signal retrieval is our goal, then the optimal Loading would be one which causes the least amount of change to the frequency response of what was originally recorded on the LP.
With the Analog Magik "Loading" Test track, AnalogMagik will display the flatness of the Frequency Response curve specific to your choice of loading. Simply change the loading and repeat the measurements to compare the flatness of the Frequency Response curve which is expressed in decibels. The optimal Loading will correspond to one which displays the lowest number in decibels.
Meaningful numbers are dependent upon cartridge quality as well as the accuracy of all the setup parameters. You should always perform the loading test after adjusting all other setup parameters.
When performing this test, the volume knob on the sound card should remain constant. The only variable should be the loading on your phono stage.
Some Phono stages will offer different GAIN levels to accommodate different cartridges, however, most manufacturers will simply ask you to choose a GAIN level best suited for your ear. There is nothing wrong with that, and we do that too. AnalogMagik attempts to come up with a more scientifically repeatable method of setting gain levels.
AnalogMagik believes the Optimal Gain level is one which yields the highest Signal to Noise Ratio. Play the OPTIMAL GAIN TEST TRACK on the 33 1/3 rpm TEST LP, and observe the behavior of the Signal to Noise Ratio. Repeat the measurement under different GAIN settings on your phono stage. If a correlation can be found, chose the GAIN level which yields the highest Signal to Noise Ratio, look for the highest set of numbers in decibels.
Some very high-quality phono stage and preamplifiers, the Signal to Noise Ratio can produce excellent readings across all gain settings, in such cases, there will be no observable differences in the S/N Ratio. We have also encountered scenarios where external factors such as tube noise, or grounding problems which will render unstable numbers. Under circumstances beyond our control, the numbers will be rendered meaningless.