Many of today’s receivers and preamplifier processors do not have phonograph inputs, so anyone with a vinyl collection will need an external phono preamplifier. My friend found himself in this predicament and asked me if I could build him one. As an aside, my Outlaw Audio Model 950 preamplifier processor doesn’t have a phono input either.
Texas Instruments (TI) has a series of high-performance op-amps specifically designed for high-fidelity applications. The datasheets include the schematic for a phono preamplifier, the LME49740, which looked like a good candidate. The LME49740 is a 14-pin integrated circuit (IC) containing four op-amps, which is just the right number for the preamplifier. I’ve had excellent experience with this series and decided to use it.
The first thing I did was design a power supply worthy of the job. I started with a toroidal transformer to minimize the stray magnetic field. Then I used a bridge rectifier with large capacitors to form the raw supply. I regulated the raw supply down to ±14 V. I used adjustable regulators (the LM317 and the LM337) because they are said to have less noise than fixed regulators. The raw supply provides power to a relay driver circuit with about a 0.5s turn on delay and almost instantaneous off delay.
The circuit for the preamplifier itself is essentially the TI-supplied circuit. I’ve made a few minor changes to accommodate the moving coil cartridges, cartridge loading, and output muting. The topology is simple. There is a linear gain stage at the input, followed by a passive RIAA (Recording Industry Association of America) filter, followed by a linear output gain stage. The passive filter consists of resistors in series and capacitors in parallel to closely achieve the proper component values. I used the values specified in the datasheet with excellent results. (The TI LME49740 datasheet is available here) It includes the phono preamplifier schematic with specified values. Other combinations are valid.
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This article was originally published in audioXpress, July 2013.
