Stereo Pre-Amplifier Basics
By Chuck Hawks
The pre-amplifier is an important part of a component music system. I consider the pre-amp second only to the loudspeakers in determining system performance, although it is the power amplifier that gets the lion's share of attention in the audiophile press. The pre-amp (or "control amplifier") handles lower level signals than the power amp, so relatively small amounts of hiss, noise, distortion and other unwanted sonic characteristics can cause an audible problem.
The pre-amp is located between the program sources (CD player, Tuner, Turntable, etc.) and the power amplifier in the component chain. The signal from a program source is fed to the pre-amp, where it is amplified enough to present a useable signal to the power amplifier that, in turn, drives the loudspeakers.
The pre-amplifier is also the stereo control center. Even the most minimal pre-amp/control centers incorporate a front panel volume control and an input selector to choose the program source (CD, Tuner, Phono, Aux, etc.). Some pre-amps no longer include amplifier circuitry for magnetic phono cartridges. For convenience and increased flexibility, most pre-amps add controls for balance (left and right), tone (bass, treble and sometimes midrange) and a headphone output jack. Some pre-amps offer front panel switching for monitoring recordings, two sets of speakers, mono/stereo, muting, loudness contour compensation, low cut (subsonic) filter and other features.
As a minimum, the back panel of a stereo pre-amplifier must have an AC power cord, left/right input jacks for program sources (CD, Tuner, Aux, etc.) and left/right output jacks for connection to a power amplifier. Most add recorder/tape input and output jacks and, if they incorporate amplification for magnetic phono cartridges, a phono grounding screw. Other back panel features can include AC outlets that allow you to plug other components into the pre-amp, balanced inputs and outputs, provision for a center channel speaker and/or active bi-amping.
Most modern pre-amps include a remote control that allows the operation of most features from your listening position. This is not necessary, but it is a convenience. Another convenience is a switched AC outlet on the back panel. This allows you to plug the AC power cord of your power amp into the pre-amp, so both can be turned on and off with the pre-amp's power switch. Some good power amps do not have on/off power switches. Decide what features and controls you need and make sure that your pre-amp of choice includes them.
Minimalist pre-amp/control centers, some argue, are less likely to degrade the purity of the signal. This sounds plausible and there may be an element of truth in it, but not always. Sometimes it is just a ploy to sell a stripped-down product for higher profit. I recently purchased a new pre-amp and in the course of my selection process, I auditioned a highly regarded minimalist pre-amp. This unit had no remote control, panel lights, tone controls, or even a balance control. However, the full-featured Marantz SC-11S1 that I eventually purchased was quieter than the minimalist unit, as well as much more convenient to use!
Most pre-amps and power amps are designed to be compatible, so mixing and matching brands is not usually a problem. Neither is using components of different ages. For example, for many years I used a McIntosh C-26 pre-amp with a Marantz 250M power amp with excellent results. Now I am using a new (2011) Marantz SC-11S1 pre-amp with the same 1975 vintage power amp. However, a few pre-amp/power amp combinations may not be a good match, so it is wise to try before you buy.
Component manufacturers usually offer pre and power amps that are designed for each other. Using matched pre and power amps eliminates the possibility of mistakes. It would be hard to go wrong by matching a Marantz pre-amp with a Marantz power amp or a McIntosh pre-amp with a McIntosh power amp.
Stereo pre-amplifiers are sophisticated instruments and it is my opinion that the best are manufactured in developed countries. A great many pre-amps sold these days are made in Red China and other third world countries, but I recommend components made in the UK, Western Europe, Japan, Canada, Australia, New Zealand or the USA.
Vacuum tube electronics seem to be experiencing a modest come back. They are claimed to be more "musical" (whatever that means), as if an inanimate object could be, on its own, musical. (I wonder if they are "musical" when reproducing spoken dialog?) Good vacuum tube pre-amps can be satisfactory, but they are clearly inferior to good solid-state units in every measurable way. Modern solid-state pre-amps have the advantage in signal to noise ratio, harmonic distortion and frequency response. These are objective, measurable and repeatable--not subjective--advantages. Solid-state devices are also far more reliable than vacuum tubes and have a much longer operating life. As far as vacuum tube electronics are concerned, I have been there and done that (back when I had no choice) and moved on. The advantages of modern solid-state pre-amplifiers are obvious, while the alleged "advantages" of vacuum tube pre-amplifiers are completely subjective.
While on the subject of performance, it is worth noting that reasonably complete specifications are usually supplied by the manufacturers of stereo pre-amplifiers. (If detailed specifications are not available for a particular pre-amp, the manufacturer probably has something to hide.) Here, for example, are the specifications supplied for the McIntosh C-46 pre-amplifier:
20Hz to 20kHz, +0 / -0.25dB
TOTAL HARMONIC DISTORTION
0.002% max. from 20Hz to 20kHz
A-WEIGHTED SIGNAL-TO-NOISE RATIO
Phono: 86dB, High Level: 97dB
RATED OUTPUT VOLTAGE
5 Vrms at balanced outputs
2.5 Vrms at unbalanced outputs
Phono, MM: 47K ohms, 65pf
High Level: 47K ohms balanced
22K ohms unbalanced
240 ohms unbalanced
480 ohms balanced
Phono to Tape Out: 40dB
High Level to Tape Out: 0dB
High Level to Main Output: 15dB
MAXIMUM VOLTAGE OUTPUT
10.0 Vrms unbalanced
20 Vrms balanced
SENSITIVITY (for rated output)
High Level: 450mV unbalanced, 900mV balanced
120V 50/60Hz, 50 watts
DIMENSIONS (H X W X D)
5 3/8” (13.65 cm) x 17 1/2” (44.45 cm) x 19”
(48.26 cm) Panel clearance required in front of
mounting panel is 1 3/4 inches (4.5 cm) for
26 lbs. (11.79kg) net
40.41 lbs. (18.33kg) in shipping carton
Particular attention should be paid to the signal to noise ratio. Remember that the decibel scale is logarithmic, so a three-decibel change doubles or halves the background noise. The frequency response should be essentially flat (less than +/- 0.5 db variation) from 20-20,000 Hz and the total harmonic distortion in a modern pre-amp should be less than 0.05%.
Electrical performance is important, but so is physical quality and robust construction. The circuitry in a good pre-amp should last a long time, so it is important that things like the volume and other front panel controls are equally durable. The outer case should be damage resistant and the control layout convenient. Some excellent performing pre-amps fall down in these mundane areas.
There are many good pre-amps available today, so finding one is not difficult. When I recently went shopping for a new pre-amp, being a long time Marantz and McIntosh fan when it comes to electronics, I chose a Marantz Reference Series SC-11S1 to replace my reliable old McIntosh C-26. The Marantz Reference Series components are very well designed and built and the SC-11S1 fit my budget. It incorporates the features I wanted. The SC-11S1 is physically a very large pre-amp and it weighs 35 pounds, but it sounds great. By "sounds great," I mean that it does not "sound" at all; it just relays a pristine signal to the power amplifier. The S/N ratio is specified to be 99db (500mv input), which to my ears means silent. What more could I ask of a pre-amp?
Copyright 2011 by Chuck Hawks. All rights reserved.