Phono Preamps

Phono preamps haven’t disappeared; well, at least not yet. In fact, interest in phono preamps seems to grow steadily, as many audiophiles (and quite a few music lovers) transfer their old record collections to CDRs or MP3 files. Like the groom who buys his first (and last) expensive haircut before his wedding, many are seeking a better phono preamp than the one that came in their old receiver, knowing that this playing of their old records will be the record’s last playing. Thus, some adventurous home constructors are building discrete transistor-based preamps, hoping to avoid the “cheap IC sound,” while others are building op-amp-based phono preamps that use premium ICs and run on batteries, while others still are building tube-based preamps to bridge the sound on their records to the ADC in their computer. (From vinyl to diamond to vacuum to silicon to paper: this quite an odd path to your ears, if you think about it.)

Regardless of the underling technology used, a phono preamp must undo the RIAA (Recording Institute Association of America) equalization curve or the CCIR (Comité Consultatif International des Radiocommunications, the European version of the RIAA) curve used in making the record. Why was this curve imposed on the records? Using it improved the signal-to-noise ratio of the record by boosting the highs going to the cutting head, while greatly extending the play-back time by cutting the lows. The end result was a fairly even grove cut, regardless of the frequency. The inverse of the RIAA curve returns the signal to flat by cutting the highs and boosting the bass. (Bare in mind that most records made before 1950 may not have followed the RIAA curve, but some other proprietary-to-the-record-label curve.)

This reverse-equalization (de-emphasis equalization) can be performed in either an active or passive fashion. The active method works by selectively varying the amount of output signal  given to the preamp’s negative feedback input (in relation  to frequency). The passive method places a complex filter network in series with the signal, which then shapes the desired equalization curve by shunting to ground (based on frequency) portions of the signal. Active equalization overcomes the problems of loose tolerances and part aging (assuming that the equalization parts are high quality, i.e. time stable and tight tolerance). On the other hand, passive equalization cannot not be voltage overloaded nor can it bring the preamp to oscillation. Both methods have their adherents and their distracters. Nonetheless, both methods are perfect candidates for analysis in SPICE, as the formulas are usually incomplete, assuming as they do, perfect conditions, conditions that seldom arise in actual practice.