Each component in a typical audio system plays a crucial role in delivering music to your ears. The source components (CD player, turntable/cartridge, computer/DAC, etc.) supply the essential musical signal. The line level preamp selects among input signals, adjusts the volume, and provides a modest amount of gain, typically doubling or quadrupling the voltage. The power amp takes the signal from the preamp, amplifies it by a factor of about 30, and supplies enough current to drive sometimes finicky speakers. The speakers convert the electrical signal from the power amp into the vibrations of the air we recognize as sound.

But a pretty good case can be made that when you are using a turntable the phono preamp has the most difficult task, at least among the electronic components. It must properly amplify and equalize the tiny signal produced by modern phono cartridges. The average level of those signals can be, and in high-end moving coil cartridges often is, as low as 0.2 mV (i.e., two ten-thousandths of one volt), a level barely above the noise level in many amplifiers. The first challenge for the phono amp is therefore simply to amplify such miniscule signals to a level, usually half a volt or so, that the rest of the system can use. This means the amplifier in the phono stage must multiply the level of the cartridge’s tiny output by a factor of about 2500 (almost 70 dB) or more. And that’s the easy part.

The greater challenge is to provide this high level of amplification without damaging the tiny musical signal with noise or distortion. All sources of noise have to be minimized. These include noise and hum that ride in on the input cables from the turntable, those that leak through the power supply from the AC lines, and those that are generated by the very components (e.g., resistors, capacitors, transistors) used to perform the amplification. But minimizing noise from all of these sources is only part of the job; the vulnerable signal from the cartridge can also be seriously damaged by distortion, the bending or deforming of the shape of the musical signal. Because high gain amplifiers are particularly prone to distort the musical signal, designing a low distortion phono amp demands skill and attention to detail.

The other essential task for the phono preamp is to restore to the music its original frequency balance. In order to fit 20-30 minutes of music onto a single side of an LP record, the engineers who make LPs reduce the level of the low frequencies by a factor of about 10 (i.e., 20 dB) and boost the high frequencies by the same amount. Since the mid to late 1950s, they have used primarily what is known as the RIAA curve for this purpose. Of course, played back with this altered response, the music would sound horrible, with no bass and screechy highs. So, the phono preamp must undo this alteration and restore the music’s proper balance. A highly accurate RIAA equalization curve is needed to meet this challenge.

Interestingly, no other element of the typical high end system is required to correct the musical signal in this fashion. All the others are designed to output an amplified but otherwise exact copy of the input signal.

Meeting these challenges can be a formidable task, but it can be done.