Choosing Stereo Separates

By Michael Riggs

A music library full of two-channel CDs and LPs is teeming with possible adventures. Those seeking the best from these sources naturally gravitate to separate preamplifiers and power amplifiers. It is in such separate components that most cutting-edge audio technologies find their beginnings. And their designs are less constrained than those of receivers, which must pack so many functions into a single box of reasonable proportions, heft, and (usually) price. This is particularly evident with amplifiers, which can be considerably beefier when separated out from the the rest of the electronics. But there's more to power amps than just the number of watts per channel and more to preamps than just the number of inputs. The buyer of stereo separates should also consider what signal sources he or she will use, where the equipment will go, whether tone or balance controls are needed, and how the two components will talk to each other.

Key Questions
  • Need AM/FM or phono?
  • Need remote control or tone controls?
  • How much power?
  • What about tube amps?
  • XLR or RCA power amp/preamp link?

A stereo preamp provides source switching and volume control for the whole system. If it includes an FM or AM/FM tuner, it's called a tuner/preamp. A power amp accepts the output from a preamp and builds up (amplifies) the signal to the level needed to drive a pair of speakers. Does all that sound like a lot of trouble? Perhaps you'd prefer a one-box solution combining stereo preamp and power amp. If so, you want either an integrated amp or a stereo receiver, which includes the tuner function as well.

Features: Preamps
Stereo preamps have undergone relatively minor change over the last several years. Many now offer remote control (finally!). Many others still don’t, however. That won’t be a problem if you plan to place the preamp within arm's reach; otherwise, you probably will appreciate having a remote. Phono stages are disappearing, so don't assume any preamp you buy will automatically accommodate a turntable for LP playback. Many are line-level only. The stripped-down, straight-wire aesthetic of high-end preamp design may be for you if what you want is the simplest possible signal path and best possible performance. If that's the case, you might be tempted to consider a passive preamp–that is, one lacking active circuitry, with just a potentiometer in between the selected source and the power amp to adjust volume. Although there is a certain superficial logic to such an approach, it invites a host of problems, ranging from inadequate volume to impedance mismatches that can play havoc with sound quality. Unless you have a good understanding of these interfacing considerations (and probably even then), you should stick with conventional active preamps. Tone and balance controls are among the conveniences more common in one-box amps than in separates. Some argue that such controls (especially tone controls) color the signal even when they are adjusted to their neutral positions, which can be true if they are poorly designed. A sensible compromise is embodied in models that provide a bypass (defeat) switch for the tone controls or for all controls except volume. That preserves your ability to make alterations when needed while allowing you to select a more direct signal path when they aren’t. It also facilitates instant comparisons between the modified and unmodified sound. Tone controls usually come in pairs, one for the bass and another for the treble, which do pretty much what their names imply. A bass control manipulates low sounds (such as a bass drum or a bass guitar), while the treble control affects relatively high-pitched sounds. They are primarily useful when you encounter a recording that is good musically but not all that it might be sonically. It might have weak bass, for example, or harsh, exaggerated treble. A good set of tone controls can often make poor recordings much more listenable. And if you like listening in an off-center position, the balance adjustment can help keep the stereo image properly centered. Preamplifiers normally include at least one tape-monitor loop, which is designed for connecting an audio recorder. It consists of a pair of inputs for playing back from the recorder’s output and a pair of outputs that feed the recorder an unaltered version of whatever signal is coming in from the source chosen at the main selector switch. Usually the input from the recorder is switched separately from the main selector, which facilitates comparison of source and recording while you are making the recording (at least with three-head tape decks). A tape-monitor loop is also a convenient place to connect equalizers and other signal processing components, which can then be easily switched in and out of the signal path. Last, but not least, make sure any preamp you buy has enough inputs to handle all the source components you expect to have in your system. You’re going to be very unhappy if it doesn’t!

Features: Power Amps
Although engineers keep coming up with new approaches to amplifier design, ranging from more efficient power supplies to fully digital amplification, most still use some variation on the tried-and-true Class AB analog amplification circuit supported by a linear power supply incorporating a substantial transformer. In general, it’s best not to get swept away by how the job is done; what’s important is the quantity and quality of the result. On the most basic level, the key issue is quantity: How much power (how many watts) can each amplifier channel deliver. Power is what moves the loudspeaker diaphragms to make the sound, and all else being equal, the more power you have available, the louder you can make that sound without having it become unpleasantly distorted.
Now, in practice, with real loudspeakers at the end of the chain, it’s a little more complicated than that. But before we get to the complications, here's how to read amplifier power specs. To clean up a lot of gross exaggeration that was going on 30 or so years ago, the Federal Trade Commission (FTC) wrote a set of rules requiring that power output for all home audio and receivers be specified in a standard fashion manner designed to enforce consistency and maximize comparability between models and brands. Here is an example of an FTC-compliant power specification: "100 watts per channel continuous into 8 ohms from 20 Hz to 20 kHz with .09% THD with both channels driven." The wording will vary slightly from manufacturer to manufacturer, but it should be close to that.

Here’s what it all means:
  • Wattage is specified per channel, not in aggregate.
  • It is the continous average power the amplifier can deliver, rather than, say, instantaneously on brief peaks. Often this will be designated as RMS (root-mean-square) power. An engineer will tell you, correctly, that RMS ratings are properly applied to voltage, not power, but there’s no need to worry about the copywriter’s technical illiteracy. He means continuous average.
  • Ohms are units of electrical impedance, or resistance. This is involved in the most important complication, which is discussed below.
  • The amplifier probably can deliver slightly more power in the middle of the audio range–say, around 1 kHz–than at the very bottom or the very top. This means a manufacturer can usually stuff a few more watts into the spec by limiting the specified range to something less than the full audio band (e.g., 40 Hz to 20 kHz or simply 1 kHz).
  • THD stands for total harmonic distortion, a measure of signal purity. In general, the less the better (although you can be very confident that you won’t hear any further improvement once the level gets below about 0.1%, even on pure test tones, and on music you can safely raise that threshold to at least 0.5%.) Here is another place where watt-stuffing can occur, since an amplifier will start to produce more distortion as it approaches its power limit.
  • Almost all stereo power amplifiers can produce more power per channel when only one is driven at a time than they can when both channels are stressed simultaneously. Sometimes quite a bit more. That’s the significance of the "both channels driven" part of the spec.

Let’s go back to impedance for a moment. The FTC requires that the primary power specification be for an 8-ohm resistive load. This is great for consistency but at the same time a drastic simplification of what an amplifier has to deal with in real life, that being loudspeakers. Although speaker specifications normally include a single-number rating for nominal impedance, almost all speakers have impedances that vary significantly with frequency. And though 8 ohms is probably the most common value for the specified nominal impedance, it may be 6 or even 4 ohms.
This is important because transistor amplifiers normally will supply more current, and thus more power, for a given output voltage as the impedance of the load connected to it is reduced. (Power is voltage times current.) So a perfect amplifier that could produce 100 watts into 8 ohms could yield 200 into 4 ohms, 400 into 2 ohms, and so on. Practical amplifiers, on the other hand, have to give up at some point, either because their power supplies can’t deliver all the necessary current or because their output transistors would burn up trying to pass it. Where and how they give up is thus an important design decision. You could take three different amplifiers, each honestly rated at 100 watts per channel into 8 ohms, and find that one could produce 150 watts into 4 ohms, another 120 watts, and the third just 100 watts. If you were to drop the load to 2 ohms, the first amp might give you another 20 or 30 watts while the third might shut down entirely to protect itself.
Moral of this story: Given the nature of real loudspeaker impedances, it is a good idea to look for an amplifier that has enough current reserve to remain comfortable driving impedances at least as low as 4 ohms. And if your speakers have a specified impedance of 4 ohms, you want a bit more reserve still. It is thus a good sign if the amplifier specifications include ratings for impedances lower than 8 ohms in addition to the FTC-mandated spec. A final word about power: Because of the way we hear, doubling the power to a speaker does not double the perceived volume. The difference amounts to 3 dB (decibels), which is noticeable but not dramatic. To double the volume, you need a ten-fold (10 dB) power increase. Consequently, if you’re choosing between two amplifiers, a power-rating difference of less than 2:1 is pretty much not worth worrying about. And the difference between, say, 100 watts and 120 watts is completely trivial.

Tube vs. Solid-State
Thought this was settled in 1968, didn’t ya. In the esoteric realms of audiophilia, some hobbyists think that tubes still sound better than transistors. A more accurate characterization would be that they sometimes sound different, usually because of some distortion engendered by the tube design or its interaction with other components (especially speakers). Good modern transistor amplifiers and preamplifiers are actually superior in essentially every performance category and at least as good in literally all of them. There is no real reason today to consider anything else.

Connectivity: Preamps
  • preamp outputs (to power amp, RCA or XLR)
  • line-level inputs (RCA or XLR)
  • phono input (MM or MC)
  • tape loop inputs/outputs
  • FM (75-ohm or 300-ohm), AM antenna inputs
  • headphone jack

Linking preamp to power amp presents a choice. Balanced (XLR) connectors are better for unusually long runs because they are much less susceptible to induced hum and radio-frequency interference (RFI). In a normal home system, however, ordinary unbalanced (RCA) connections are fine. The same applies to connections between source components and the preamp.

One special input increasingly omitted from preamps is the phono input, which is designed for the low-voltage output from a turntable's pickup cartridge. There are two kinds of phono inputs, corresponding to different types of cartridges: moving-magnet (MM) and moving-coil (MC). The latter, which are less common, require more amplification. Your preamp's phono input may accept only one type or may be switchable for both types. When using an analog audio recorder, such as a cassette deck, you'll need both line-level inputs and line-level outputs, often grouped together and labeled "Tape Loop" or "Tape 1," "Tape 2," etc. If you don't need to record, the output of any cassette deck or other recorder or player can feed a standard line-level input.

Digital recording devices normally will accept direct digital inputs as well as analog signals. Some stereo preamps provide digital inputs and outputs for such devices, but in most cases you will have to make a direct link between the digital recorder and the digital source device you want to record from if you want to make a direct digital dub. Tuner/preamps include jacks for radio antennae. The FM input may be labeled either 75 ohms (with an RF-type threaded screw terminal) or 300 ohms (a wire clip or other type of terminal accepting bare wire or Y-shaped spade lugs). AM antenna inputs are normally wire clips or screw lugs. The headphone jack on a preamp is usually the larger quarter-inch phone type that's suitable for larger home headphones, as opposed to the c-inch mini-jack found on portable equipment. Adapters are widely available to mate a c-inch plug with a quarter-inch jack or vice versa. If your preamp does not have a headphone jack, you may use the headphone jack on the source component (if possible) or buy a separate headphone amplifier.

Connectivity: Power Amps
  • fixed line-level inputs (from preamp, RCA or XLR)
  • variable line-level inputs (from preamp, RCA or XLR)
  • speaker terminals (binding posts)

Linking preamp to power amp presents a choice (the same one detailed above under preamps–skip the rest of this paragraph if you've already pondered this question). Balanced (XLR) connectors can be better for unusually long runs because they are less prone to induced hum and radio-frequency interference. In a normal home system, however, ordinary unbalanced (RCA) connections are fine. A few power amps provide variable as well as fixed audio inputs. The variable ins have their own level controls, which can be helpful for correcting left/right imbalances or, in a pinch, doing without a preamp. For speaker terminals, most separate power amps provide five-way binding posts that accommodate banana plugs, dual bananas, pins, spade lugs, or bare wire. These are certainly the most flexible, although other types can work just as well.

Look and Feel
Power amps tend to be plain. Still, if you're going to look at a big, heavy thing sitting in your living room for years to come, it had better look at least acceptable. Preamps are a more hands-on component. Are you happy with the way the controls feel? Does the volume knob have a nice gliding quality; does it feel good to the touch? What about the remote (if there is one)? Does it feel right in your hand? Does it do everything you need it to do? Can you distinguish the buttons easily by feel? Over weeks and months and years, these seemingly mundane characteristics can make a big difference to your enjoyment of the equipment.