Moving magnet phono pre-amplifier - 1

Rohit Balkishan Dubla

Description: The pre-amp consists of a balanced input and gain stage U1a followed by two active RIAA equalisation stages. U1b provides the 3180µS (50Hz) equalisation and U1c provides the 75µS (2122Hz) equalisation. U1d provides a buffered output. 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. The 68pF capacitors (C2 & C3) in the input stage roll off any noise above 21 kHz and must not be altered. These need to be changed only if R4 & R5 are altered. R1 & C1 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. C6 & C9 must be non-polar types if available. Since these could be physically large to accommodate on the PCB, even 10µF NP caps can be used in place of these with some useful reduction in the low frequency gain. If non-polar caps are not available, then normal electrolytics may be used (although not recommended), since these will work more or less normally without any bias. 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 normalised response is shown in the figure below.

As can be seen from the plot, the low frequency response extends up to well below 10 Hz 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 20 Hz can be added at the pre-amp output to eliminate any rumble. The RIAA accuracy can be fine-tuned by changing C4 & C7. C4 = 1nF and C7 = 3.3nF gives an accuracy of better than 1 db w.r.t. the RIAA specification. 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 op-amps used are TL074 quad devices. Other op-amp ICs like TL072 and NE5532 can also be used - the different pin-outs will have to be kept in mind though.