One challenge of creating a phono stage that works for both MM and MC cartridges is to implement a amplifier stage with a wide gain range. For our comfortINA project we’re looking for an input stage with a gain between 30db and 60db – that’s a voltage gain between 30 and 1000!
There are variable gain low noise amplifiers available. One example is the PGA2505. Unfortunately this chip can’t be used for a MM stage, because the input impedance is only 9.2kOhm, which is too low for a MM system. Another option would be a THAT1570/5170. However, this chipset is only available in a QFN packages, which is hard to solder for the hobbyist.
Therefore we go back to our previous design using an instrumentation amplifier. If you use an instrumentation amplifier like the INA163 or an operational amplifier, the gain can be controlled by an external resistor. The value of the external resistor has to be between about 200 Ohm and 6 Ohm. On the low end, a low tolerance is needed. There are no digital potentiometers available in this range. Therefore we will use low-Ron analog switches and external resistors. This design is also shown in the CN0146 application note from Analog devices.
Note, that the circuit diagram only shows the implementation of the variable resistor.
The minimize tolerances, we’re using parallel switches for the low value resistors. Let’s do some calculations. If the tolerance of an analog switch is 0.5 Ohm, the maximum gain difference between 2 channels would be 0.25db. In reality much lower tolerances are expected. With a 0.1 Ohm tolerance, the maximum gain difference between two channels would be only 0.05db – looks good!