Category Archives: Preamps

RIAA Preamps

Phono cartridges need both gain and equalization to make their output suitable for an auxiliary input or direct into a power amp.  A typical moving magnet cartridge has an output of 2 to 5 millivolts. Typical Moving Magnet (MM) gain at 1 KHz is 40 dB. The RIAA recording curve adds about 20 dB of boost at low frequencies and 20 dB cut at high frequencies to compensate for the RIAA recording curve and the fact that a magnetic cartridge is a velocity transducer. When displacement transducer is used (strain gauge cartridge) the required EQ is only 12 dB starting at 500 Hz and ending at 2000 Hz for a decrease of 12 db in the high end. This was done to reduce surface noise on records by boosting the high frequencies in the record cutting process so that they could be reduced by the same amount on the playback end.

When Moving Coil (MC) cartridges hit the market in the last 1970s their superiority was quickly recognized. However they required 20 dB more gain in the phono stage to bring them up to standard level as their output is 0.2 to 0.5 millivolts (20 db lower than MM). Early adopters of MC cartridges had two options. Buy an external transformer or a pre-preamp (head amp). Of course there was much discussion about the merits of these two very different ways of solving the low output problem. Both had typical gain of 20 dB and the better ones gave you choice over both gain and cartridge loading which affects the sound of the cartridge. When not loaded many MC cartridges sound bright and edgy because they are undamped. Most cartridge manufacturers give an ideal load resistance (typically 3-10 times the coil resistance is a good place to start if none is given). This load electrically damps the cartridge mechanical system for best response. If a cartridge has too much loading (low resistance) is will start to sound dull and lifeless. There were many high quality pre-preamps made up to the point of the popularity of the Compact DIsc. With the resurgence of vinyl, pre-preamps and complete MM/MC phono stages are coming back. A complete phono preamp will have sufficient gain (60+ dB) and RIAA EQ for MC cartridges along with loading. However it is an excellent, and sometimes better solution, to split the phono stage into a separate head amp and RIAA stage to isolate those functions as the very small, delicate output from a MC cartridge is best amplified in two stages.

The first product from Music Reference (www.ramlabs-musicreference.com) was the RM-4 pre-preamp which was designed to sit in front of a standard phono preamp and raise the gain of an MC cartridge to suitable level to go into a standard phono stage such as the RM-5, Modulus, CJ, ARC or other standard preamp at the time. It is still a fine solution to implement MC phono systems. What makes the RM-4 special is that is has low noise comparable with solid state units (which is not easy), cartridge loading and gain selection of infinite range through plug-in modules. The gain can be from 0 db to 30 dB. Along with a 40 dB standard phono preamp this combination will give a total of 70 dB gain, more than most MC phono stages and in some cases can go directly to a power amp via a passive preamp (simple volume control).

Although I prefer the active (pre-preamp) solution transformers are also available. Some transformers allow independent choice of gain (step up ratio) and loading, however many do not. In that case the load presented to the cartridge is determined by the chosen step up ratio and may not be the ideal load for that cartridge. With a good pre-preamp one has independent control over gain and loading. While some pre-preamps add noise, a well designed one will have noise below the surface noise of a good record. Transformers can pick up hum and often have problems at the frequency extremes where a good pre-preamp can have bandwidth from a few Hz to 50 KHz. Needless to say, Be aware that some complete phono stages use a transformer to amplify the MC input rather than to do that function electronically. I prefer the all electronic method as long as the design is low noise.