Tag Archives: preamp

A symmetrical variable gain amplifier for phono stages

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!

Link: Phono MM/MC Preamp

DC4300 Digital CameraSome phono enthusiasts prefer audio equipment with vacuum tubes. I’m also fascinated by these “old” equipment, but did not build a tube amplifier or preamplifier by myself. While the circuits usually are quiet simple, most of the performance of tube amplifiers come from the realisation. The grounding concept or the placement of transformers can make the difference between a great and a bad amplifier – even with exactly the same circuit.

If you still think, that you want to build your own phono preamplifier based on tubes and even want to use it for very low MC signals, check out this project: MC/MM phono preamp. Even if you can’t understand the german text, the pictures of the mechanical construction are worth a look

Another older project: the PhonoClone

phonocloneThis is a clone of the circuit used in the 47 Labs phono stage. The circuit was published by RJM audio.
It is simple and cheap and works well with many low-output MC systems. However the system itself (inductance and resistance) has a big impact on the sound. Therefore it might not work well with some MC systems.