Moving magnet/coil phono pre-amplifier

Rohit Balkishan Dubla

Description: The pre-amp consists of a balanced input and gain stage U1a (1/2 TL072) which provides a gain of 20db and the 75µS (2122Hz) equalisation, U1b (1/2 TL072) which provides a gain of 20db and the 3180µS (50Hz) equalisation and U2a (1/2 TL072) which provides the final 20db gain** (U2b is for the other channel, along with one more dual op-amp for the preceding stages). Thus the total LF boost imparted by the circuit is about 60db (actually slightly more since U1b is operating in non-inverting mode).The reason for using a balanced input stage is so that the pre-amp can be installed within the turntable housing along with the power supply and still be able to suppress any hum pick up. Rp & Cp form the load for the pick up coil and are typical for EEI Classic 500/700 cartridges. These may need to be chosen according to the data sheets for the pick-ups that you may be using. C7 must be a non-polar type if available. The 100Ω resistor at the output is needed to suppress any oscillations that may occur because of the interconnects or the cable capacitance. The simulated response is shown in the figure below.

As can be seen from the plot, the low frequency response extends upto well below 10Hz and hence the pre-amp will amplify sub-audio signals such as turntable rumble. So it is required that the turntable have as little rumble as possible. If required, a 2nd or higher order high-pass filter cutting off at about 15 to 20Hz can be added at the pre-amp output to eliminate any rumble. To keep hum & noise as low as possible, the supplies must be regulated and properly decoupled. If possible, the pre-amp must be housed in it's own metallic enclosure which must be grounded, so that any stray interference may be suppressed. The devices used are TL072 dual op-amps. Almost any other op-amp IC can be substituted for these, an example being NE5532.

** The final 20dB gain stage is needed for moving coil type pick ups. Though it will still work with moving magnet types, expect the output to be rather loud. If that is bothersome, then simply attach a switch across R9 for MM/MC mode selection- closed for moving magnet (the last stage is converted to unity gain), open for moving coil. If there is absolutely no need for MC operation then the last stage may be omitted altogether. It will save one dual op-amp and for a stereo pair a single quad package (TL074) can be used - just use a 100 ohms resistor in series with the output of U1b.