Preamplifiers

The preamplifier is the Grand Central Station of your hi-fi system. It receives signals from source components—turntables, CD players, SACD and DVD Audio machines, FM tuners, satellite radio receivers, music servers—and allows you to select which of these to send to the power amplifier for listening. In addition to allowing you to switch between sources, the preamplifier performs many other useful functions, such as amplifying the signal from your phono cartridge (in some preamplifiers), adjusting the balance between channels, and allowing you to set the volume level. Fig. 1, shown earlier in Sect. 2, illustrates the stereo preamplifier’s role in a music-playback system.

In addition to providing volume control and letting you select which source component you listen to, the preamplifier is a buffer between your source components and power amplifier. That is, the preamplifier acts as an intermediary, taking in signals from source components and conditioning those signals before sending them on to the power amplifier. Source components can easily drive a preamplifier, with the burden of driving a power amplifier through long cables falling on the preamp’s shoulders. By buffering the signal, the preamplifier makes life easier for your source components and ensures good technical performance.

The preamplifier is the component you will most often use, touch, and adjust. It also has a large influence on the system’s overall sound quality. (Note: preamplifiers are built into, or “integrated” with, integrated amplifiers and receivers, instead of being housed in a separate chassis.)

Types of Preamplifiers

There are many types of preamplifiers, each with different capabilities and functions. Choosing the one best suited to your system requires you to define your needs. Listeners without a turntable, for example, won’t need a preamplifier with the ability to amplify the tiny signals from a phono cartridge. Others will need many inputs to accommodate digital recorders, music servers, FM tuners, satellite radio, and some will need multichannel capability. Let’s survey the various preamplifiers and define some common preamplifier terms.

Line-Stage Preamplifier: Accepts only line-level (low-level) signals, which include every source component except a turntable. Line stages have become much more popular as listeners increasingly rely on digital sources rather than LPs as their main signal source. If you don’t have a turntable, you need only a line-stage preamplifier.

Phono Preamplifier (also called a phono stage): Takes the very tiny signal from your phono cartridge and amplifies it to line level. It also performs RIAA equalization the signal from the cartridge. RIAA equalization, named after the Recording Industry Association of America, is a bass boost and treble cut that counteract the bass cut and treble boost applied in disc mastering, thus restoring flat response. A phono stage can be an out board stand-alone unit in its own chassis, or a circuit section within a full-function pre amplifier. If you play records, you must have a phono stage, either as a separate component or as part of a full-function preamplifier.

Full-Function Preamplifier: Combines a phono-stage with a line-stage preamplifier in one chassis.

Tubed Preamplifier: A tubed preamplifier uses vacuum tubes to amplify the audio signal. Solid-State Preamplifier: A solid-state preamplifier uses transistors to amplify the audio signal.

Hybrid Pre-amp: A hybrid preamplifier uses a combination of tubes and transistors.

Audio / Video Controller: A device analogous to a preamplifier that includes video switching, multiple audio channels (typically six), and surround-sound decoding such as Dolby Digital and DTS. (A/V controllers are described in Section 10, “Audio for Home Theater and Multichannel Music.”)

Multichannel Preamplifier A preamplifier with multiple audio channels (typically six) for playback of multichannel music. Differs from an A/V controller in that the multichannel preamplifier has no surround decoding, video switching, or other functions for film-soundtrack reproduction.

How to Choose a Preamplifier

Once you’ve decided on a line-stage, a full-function preamp, A/V controller, or separate line and phono stages, it’s time to define your system requirements. The first is the number of inputs you’ll need. If you have only a CD player, the four or five line inputs on most preamps are more than enough. But let’s say you have a turntable, CD player, FM tuner, music server, satellite radio receiver and you also want to plug into your pre amp an iPod or portable music player. You’ll need a phono input, five line inputs, and one tape loop—at the minimum. A tape loop is a pair of input and output jacks for recording onto a recording device and receiving a signal from that recording device. When you press the TAPE MONITOR button on your preamplifier, you are routing the signal from the TAPE INPUT jacks to your power amplifier fir listening.

Most high-end preamps have few features—and for good reason. First, the less circuitry in the signal path, the purer the signal and the better the sound. Second, the preamp designer can usually put a fixed manufacturing budget into making a pre amp that either sounds superb or has lots of features--but not both. Mass-market mid-fi equipment emphasizes vast arrays of features and buttons at the expense of sound quality Don’t be surprised to find very expensive preamps with almost no features; they were designed, first and foremost, fur the best musical performance. Most high-end preamps don’t even have tone (bass and treble) controls. Not only do tone controls electrically degrade the signal—and therefore the musical performance—but the very idea of changing the signal is antithetical to the values of high-end audio. The signal should be reproduced with the least alteration possible. Tone controls are usually unnecessary in a high-quality system.

Another school of thought, however, holds that a playback system’s goal isn’t to perfectly reproduce what’s on the recording, but to achieve the most enjoyable experience possible. If changing the tonal balance with tone controls enhances the pleasure of listening to music, use them. If you’re comfortable with the latter philosophy, be aware that tone controls invariably degrade the preamplifier’s sonic quality. (Some preamps with tone controls allow you to switch them out of the circuit when they’re not being used, a feature called “tone defeat.”)

A similar debate rages over whether or not to include a balance control in a high-end preamp. A balance control lets you adjust the relative levels of the left and right channels. If the recording has slightly more signal in one channel than the other, the center image will appear to shift toward the louder channel, and the sense of soundstage layering may be reduced. A similar problem can occur if the listening room has more absorptive material on one side than the other, puffing the image off-center. A balance control can also center the soundstage if you are sitting to the left or right of the sweet spot. A small adjustment of the balance control can correct these problems. Like tone controls, balance controls can slightly degrade a preamplifier’s sonic performance. It isn’t unusual to find a $10,000 state-of-the-art preamplifier with no tone or balance controls, and a $199 mass-market receiver with both of these features.

When making a purchasing decision, you should also consider the preamplifier’s look and feel. Because the preamplifier is the component you’ll interact with the most, ask yourself: Are the controls well laid out? Is the volume control easy to find in the dark? Is using the remote intuitive? Does the preamp have a mute switch? A mute switch is handy when your listening is interrupted, for protecting the rest of your system when disconnecting cables, or for maintaining the same volume level when com paring two other components, such as digital sources or cables.

Tubes vs. Transistors

Of the many components that make up a hi-fi system, the preamplifier is the most likely to use vacuum tubes instead of solid-state devices (transistors). This is because preamplifiers handle only low-level signals, making tubes more practical and affordable there than in power amplifiers. Power-amplifier tubes are large and expensive, run hot, and require expensive replacement. Further, one theory of audio design holds that if the system is to include tubes, they are best employed closest to the signal source— such as in the preamplifier.

Moreover, the qualities that have endeared many music lovers to the magic of tubes are much more affordable when used in a preamplifier. Tubed audio components are more expensive to build and maintain than solid-state units, but that cost differential is much lower in preamplifiers than in power amplifiers. If you want the special qualities of tubes but not their heat, greater expense, and higher maintenance, the tubed preamplifier is the way to go.

Tubes are often claimed to sound sweeter and warmer, and to have a more natural treble. Many solid-state preamps tend to make the treble dry, brittle, metallic, and etched. The result is steely-sounding strings (particularly violins), unnaturally emphasized vocal sibilants (s and sh sounds), and cymbals that sound like bursts of high-frequency noise rather than a delicate brass-like shimmer. Because these unpleasant artifacts can be introduced by many components (digital sources, power amplifiers, cables, tweeters), a natural-sounding tubed preamplifier can tend to counteract the system’s tendency toward these amusical characteristics.

Tubes have several advantages in their favor as audio-amplifying devices. First, the circuitry associated with a vacuum tube—the ancillary parts that make the tube work—is generally much simpler than a circuit using transistors. Second, the distortion tubes produce is much more benign than the distortion created by solid-state electronics. Tube distortion is predominantly second-harmonic, which can actually be pleasant to the ear. Transistors produce upper-order harmonics that harden the sound. Tube proponents point to these facts as evidence of the superiority of tubed preamplifiers.

Be aware, however, that some tubed preamps are intentionally designed to sound very colored. Rather than present the music with the least added effect, tubed preamps often add significant amounts of euphonic coloration. This form of distortion can at first be pleasing to the ear, but represents a departure from the original signal. This type of “tubey” coloration is characterized by a soft treble, an overly laid-back and easygoing presentation, lack of detail, and a “syrupy” sound. Many audiophiles, in their attempts to avoid the worst characteristics of solid-state, turn to euphonically colored tubed preamps to make their systems listenable.

It is a far better approach, however, to make sure that each component in the chain is as transparent as possible. If this is achieved, there will be no need for “tubey” preamps. Ideally, the listener shouldn’t be aware that she is listening to tubes; instead, she should be aware only of the music. Just as poor solid-state preamps color the sound by adding grain, treble hardness, and etch, the poor tube preamp will often err in the opposite direction, obscuring detail, adding false “bloom,” and reducing resolution. Be equally aware of both forms of coloration.

It’s a mistake to “fall in love” with either solid-state or tubes for the wrong reasons. The solid-state lover may think he is getting “more detail,” and the tube aficionado may fall for the “lush sweetness.” Both extremes are to be avoided in the pursuit of a truly musical playback system. The overly “sweet” preamplifier may become uninvolving over time because of its low resolution; the “detailed” and “revealing” solid-state unit may eventually become unmusical for the fatigue it produces in the listener. Not all tube and solid-state preamps can, however, be categorized so neatly. Many tubed preamps are extremely transparent and neutral, having very little effect on the sound.

Nor should you buy a preamp purely because it uses tubes. Certain circuits are better implemented with tubes, others with solid-state devices. There are no magic components or circuit designs that will ensure a product’s musicality. Worthy—and unworthy—products have been made from both tubes and transistors. That’s why Nome companies design tubed, solid-state, and hybrid tubed/solid-state preamplifiers. The designer picks the best device for the particular application.

Ideally, both tubed and transistor preamplifiers strive for musical perfection, but approach that goal from different directions. The gross colorations I’ve described arc largely a thing of the past, or relegated to fringe products that appeal to a minority of unsophisticated listeners. Today’s reality is that tubed and solid-state designs sound more and more alike every year. The best preamplifier will be a transparent window on the music—no matter what its design.

The best advice is to choose the preamp that has the least effect on the music; you’ll get much more musical satisfaction from it for a longer period of time. And remember: The perfect tubed preamp and the perfect solid-state preamp would sound identical.

Balanced and Unbalanced Connections

Some preamplifiers have balanced inputs, balanced outputs, or both. A balanced signal is carried on a three-pin XLR connector rather than the conventional RCA plug. (Balanced and unbalanced connections, described in Section 10, can be seen in Fig. 1.) If you have a balanced source component—usually a CD player—you should consider choosing a preamplifier with a balanced input. Nearly all source components with balanced outputs also have unbalanced outputs. You can use either output, but you may not get the best sound quality unless you use the balanced output option. In addition, not all “balanced” preamplifiers are created equal, as we’ll see.

A preamp with unbalanced inputs and a balanced output can accept unbalanced signals but still drive a power amplifier through a balanced interconnect. If your power amplifier has balanced inputs, getting a balanced-output preamplifier is a good idea. You can listen to the system through both balanced and unbalanced lines and decide which sounds better. Some products sound better through balanced connections; others perform best with unbalanced lines.

Although all balanced preamps have XLR jacks, not all balanced preamplifiers are created equal. Two preamps that have balanced inputs and outputs can be very different in how they treat the signal. Most preamps accepting a balanced signal immediately convert it to an unbalanced signal, perform the usual preamplifier functions (pro vide gain and volume adjustment) on the unbalanced signal, then convert the signal back to balanced just before the main output. A preamp with balanced inputs and out puts, but unbalanced internal topology, often adds two active stages to the signal path:

a differential amplifier at the input and the phase splitter at the output. A differential amplifier converts a balanced signal to unbalanced; a phase splitter converts an unbalanced signal to balanced. These additional circuits inevitably degrade sound quality.

The preferred, but much more expensive, method is to keep the signal balanced throughout the preamplifier. This technique requires double the audio circuitry; each portion of the balanced signal is amplified separately. Moreover, very close tolerances between halves of the balanced signal are required. Although the fully balanced circuit has more active devices in the signal path, the signal isn’t subjected to a differential amplifier or a phase splitter.

Audio/Video Controllers and Multichannel Preamplifiers

The increasing popularity of home theater, with its multichannel sound, has enticed many high-end preamp manufacturers to produce models that work in 2-channel stereo systems as well as those incorporating multichannel surround sound for home theater. These products are called A/V (for Audio/Video) preamplifiers or A/V controllers. The latter term is more technically correct, and better describes the product’s function.

Although A/V controllers are covered in depth in Section 10, a brief introduction here is illustrative. A/V controllers are distinguished by three factors: multichannel sound capability; video switching, and surround decoding. First, an A/V controller will typically have six audio channels instead of two to accommodate the 5.1 channels of audio in the Dolby Digital and DTS surround-sound formats. Second, the A/V model can switch video sources as well as audio ones. Finally, A/V controllers all have some form of surround decoding, such as Dolby Pro Logic, Dolby Digital, DTS, or all three.

A key feature of a high-end A/V controller is the ability to pass analog input signals to the output without converting the signal to digital and then back to analog. This analog bypass feature avoids unnecessary sonic degradation imposed by A/D and D/A conversions.

For those who want multichannel music reproduction without video switching and surround decoding, the multichannel preamplifier is a better choice than an A/V controller. A multichannel preamplifier is simply a conventional preamplifier with six channels rather than two (Fig. 1).

Fig. 1 A multichannel preamplifier with no home-theater features. (from Audio Research)

Although ft otters ho surround decoding, bass management, or other home—theater related functions, it will likely offer better sound quality than a digital controller, for several reasons. First, the chassis contains no digital circuitry, which creates noise that can get into the analog signal. Second, the designer can focus the entire parts budget on making the product sound good without worrying about including all that other complex circuitry. Very few multichannel preamplifiers are available, since they are only for those who will use their systems exclusively for music listening.

Phono Preamplifiers

Line-stage preamplifiers are fairly straightforward; they take in line-level signals from the source components, select which source you want to listen to, allow you to adjust the volume, and send that signal to the power amplifier.

A phono preamplifier, however, has a trickier job. The phono preamplifier amplifies the very tiny signal (between a few tens of micro-volts and a few millivolts) from the phono cartridge to a line-level (about one volt) signal. This line-level signal can then drive a line-stage preamplifier, just as any other source component would. A phono stage can be an integral part of a full-function preamplifier, an optional board that plugs into some line-stage preamplifiers ( Fig. 2), or an outboard unit in its own chassis (Fig. 3). Outboard phono stages have no volume controls; they usually feed a line input on a line-stage preamplifier.

Fig. 2 A phono preamplifier can be an optional board (or pair of boards) that plugs into a line stage preamplifier. (from Mark Levinson)

In the days before CD, virtually all preamplifiers included integral phono stages. In many of today’s preamplifiers, however, a phono stage is an option (usually about $200- $800) for those listeners who play LPs. This arrangement reduces the pre amplifier’ price for those needing only a line-stage preamplifier. It’s also less expensive to buy a preamplifier with an integral phono stage than buying separate line and phono stages. With one chassis, one power supply, one owner’s manual, and one shipping car ton, a full-featured preamp is less expensive. These integral phono stages can also offer exceptional performance.

Fig. 3 An outboard phono preamplifier can be a simple box. (Pro-Ject and Sumiko)

RIAA Equalization

In addition to amplifying the cartridge’s tiny output voltage, the phono stage performs RL4A equalization on the signal. RIAA stands for Recording Industry Association of America, the body that standardized the record and playback equalization characteristics. Specifically, phono-stage RIAA equalization boosts the bass and attenuates the treble during playback. This equalization counteracts the bass cut and treble boost applied to the signal when the record was cut. By combining exactly opposite curves in disc mastering and playback, a flat response is achieved. Attenuating bass and boosting treble when the disc is cut allows more signal to be cut into the record groove and increases playing time. Because bass takes up more room in the groove than does treble, reducing the amount of bass in the groove allows the grooves to be spaced closer together. In addition, the treble boost in disc cutting and subsequent treble cut on play back act as a noise-reduction system: attenuating treble on playback also attenuates record-surface noise. RIAA equalization is why you can’t plug a line-level source into a preamplifier’s phono input. Similarly, a phono cartridge can’t be plugged into a preamplifier’s line input.

Phono Stage Gain

The amount of amplification provided by a phono stage (or any other amplifier) is called its gain. Gain is specified either in decibels (dB), or as a number expressing the ratio between input and output voltages. Phono stages have much more gain than line stages. Where a line stage may have 10 to 20dB of gain, a phono stage typically amplifies the signal by 35 to 60dB.

The amount of phono-stage gain required depends on the type of phono car :ridge driving the phono preamplifier. Phono stages are of two varieties, each named Tor the type of cartridge with which it is designed to work. The first is the moving-magnet phono stage. Moving-magnet phono stages have their gain optimized to work with the relatively high output voltages from moving-magnet cartridges. Moving-magnet cartridges have high output voltages as cartridges go, on the order of two to eight mini volts (2-8mV). Consequently, they need less gain; a moving-magnet phono stage’s gain is toward the lower end of the range, typically about 35dB.

Moving-coil cartridges have much lower output voltages due to their different method of generating a signal. Moving-coil output levels range from 0.1 5mV to 2.5mV.

Consequently, they need more amplification (gain) in the phono preamplifier to reach line level than do moving-magnet cartridge signals. Moving-coil phono preamplifiers have about 40 of gain. Note that moving-coil output voltages vary greatly with the cartridge design, with some so-called “high-output” models reaching moving-magnet levels.

Because of this wide variation in cartridge output level, a gain mismatch can occur between the cartridge and phono stage: The phono stage can have either too much or too little gain for a specific cartridge output voltage. If the phono preamp doesn’t have enough gain, the volume control must be turned up very high for sufficient playback levels. This raises the noise floor (heard as a loud background hiss), often to the point of becoming objectionable. Conversely, a high-output cartridge can overload a moving-coil phono stage’s input circuitry, causing distortion during loud musical passages. This condition is called input overload. A high-output cartridge driving a high-gain phono preamplifier can also make the preamplifier’s volume control too insensitive. A moderate listening level may be achieved with the volume control barely cracked open; this makes small volume adjustments difficult.

Correctly matching the cartridge output voltage to the phono-stage gain avoids excessive noise and the possibility of input overload. A moving-coil cartridge specified at 0.1 8mV output needs about 55dB of gain. A typical moving-magnet out put of 3mV should drive a phono stage that has about 35dB of gain. Some phono stages and full-function preamps have internal switches that adjust the gain between moving-magnet and moving-coil levels.

High phono-stage gain carries the penalty of increased noise. Although phono stages in general have poorer signal-to-noise specifications than other components, very-high-gain phono preamps can be objectionably noisy. All other factors being equal, the greater the gain, the higher the noise. Select a phono preamp with just enough gain for your cartridge.

Cartridge Loading

Cartridge loading is the impedance and capacitance the phono cartridge “sees” when driving the phono input. Cartridge loading, specified in both impedance and capacitance, has a large effect on how the cartridge sounds, particularly moving-magnet types. Improper loading can cause frequency-response changes and other undesirable conditions. Many preamps allow you to adjust the input impedance and input capacitance to match the phono cartridge by adding resistors and capacitors to the phono stage’s input circuit. These adjustments usually require a soldering iron, however, and should be done by your dealer. Some preamps have tiny internal switches to adjust cartridge loading, while others provide front-panel adjustments. One model even allows cartridge loading by remote control while in the listening seat. More discussion of cartridge loading is included in Section 6 (“The LP Playback System”).

What to Listen For

The preamplifier has a profound effect on the music system’s overall performance. Because each of the source signals must go though the preamp, any colorations or unmusical characteristics it imposes will be constantly overlaid on the music. You can have superb source components, a top-notch power amplifier, and excellent loudspeakers, yet still have mediocre sound if the preamp isn’t up to the standards set by the rest of your components. The preamplifier can establish the lowest performance level of your system; careful auditioning and wise product selection are crucial to building the best-sounding playback system for your budget.

A preamplifier’s price doesn’t always indicate its sonic quality; I know of one $1500 model that is musically superior to another preamp selling for nearly $8000. If you do your homework and choose carefully, you’ll avoid paying too much for a poor- sounding product.

In addition to the usual listening procedures described in Section 4, preamplifiers offer several methods of sonic evaluation not possible with other components. We can therefore more precisely evaluate preamps and choose the best one for the money.

We’ll start with the standard listening-evaluation techniques. First, the same musical selection can be played on the same system, alternating between two competing preamps. Be sure to match levels between the two preamps under audition. Listen for the presentation differences described in Section 4—particularly clarity; transparency, lack of grain, low-level detail, soundstaging, and a sense of ease.

The most common sonic problems in preamplifiers are a bright and etched treble and a thickening of the soundstage. Many preamps, particularly inexpensive solid-state units, overlay the midrange and treble with a steely hardness. These preamps can give the impression of more musical detail, but quickly become fatiguing. The treble becomes drier, more forward, and etched. Cymbals lose their sheen, instead sounding like bursts of white noise. Vocal sibilants (s and sh sounds) become objectionably prominent; violins become screechy and thin. The poor-sounding solid-state preamplifier emphasizes the brightness of the strings and diminishes the resonance of the violin’s wooden body. Such preamps also reduce the saturation of tonal color, sounding thin and bleached.

The preamplifier that thickens the sound makes the soundstage more opaque. The transparent quality is gone, replaced by a murkiness that obscures low-level detail and reduces resolution. Instruments and voices no longer hang in a transparent, three- dimensional space. Instead, the presentation is thick, confused and congealed, and lacks clarity Even some expensive models impose these characteristics on the music.

Beware of the preamplifier that tends to make all recordings sound similar in timbre, tonal balance, or spatial presentation. The best preamplifiers let you hear changes in the size of the concert hall, how closely the microphones were positioned, and can clearly resolve the differences between a Steinway piano and a Bösendorfer, among other such subtleties. In addition, listen for how well the preamp resolves the fine inner detail of instrumental timbres.

Compare the preamps under audition to the very best preamp in the store. Listen for the qualities distinguishing the best preamp, and see if those characteristics are in the preamps you’re considering. This will not only give you a reference point in selecting a preamplifier for yourself, but will sharpen your listening skills. The more experience you have listening to a variety of products, the better your ability to judge component quality.

A less analytical method is to borrow the preamp from your dealer for a weekend and just listen to it. How much more exciting and involving is the music com pared to using your existing preamp? Does the new preamp reveal musical information you hadn’t heard before in familiar recordings? How much does the new preamp compel you to continue playing music? These are the best indicators of the product’s ability to provide long-term musical satisfaction. Trust what your favorite music tells you about the preamplifier.