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More than knobs and convenience

By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.

The control center for your system is also called the preamplifier. It can control two channels of sound or a surround system. When it is built into the same chassis as the power amplifier, it is called an "integrated amplifier." If a tuner is included, then you have a "receiver." No matter the functions or name, you must still listen to find the best clarity, musicality, and dynamics.

The finest two-channel preamps can use transistors or vacuum tubes. Tubes are rarely used in home theater control centers. When comparing home theater control centers, though, it will be easy to hear differences if you use two-channel music. On soundtracks you know well, turn off the video screen to remove the visual distraction.

Volume controls

Some two-channel preamps are fitted with volume controls that "come on" too quickly. When the sound is too loud at the 9 o'clock position, you are left with no fine sound control. Ideally, the sound should become loud when you reach the 2 o'clock position. There may be nothing easily done about this if you otherwise enjoy the sound of that particular preamp.

A home theater control center should have a "pass through" switch for listening to two-channel music. This allows the most clarity as it bypasses all circuitry except for the volume control. If it makes little difference, make certain you are indeed using a good pair of interconnects from the CD player. If so, then this control center is not very clear.

Important
specifications to know

Compare signal-to-noise ratios, where the higher ratio is generally better. For this, the reference level must be the same, such as "relative to 1 Volt of output." For the best dynamics, the output impedance (resistance) of a preamplifier must be lower than the input impedance of the power amplifier. You should expect a preamp and power amp from the same manufacturer to work together, but preamps (and home-theater control centers) should have less than 2,000 Ohms output impedance, and power amplifiers more than 10,000 Ohms input impedance.

Whether two-channel or multi-channel, some preamps are hazy-sounding while others have no impact in the bass. Some are 'boomy' in the bass, while others sound 'closed-in.' Some preamps can simply lack dynamics. When we have heard such a condition, it seems to be present no matter what amplifier is used. An ultra-low "Harmonic Distortion" specification does not seem to correlate with what is heard, because that test uses only simple test tones. However, intermodulation distortion (IMD) specifications sometimes do correlate with the sound quality, especially when that test was performed using many tones at once instead of just two. The number of test tones used may be stated on the component's specifications sheet. Regardless, the standard of perfection for IMD is zero because it makes the sound 'fuzzy.'

Tips for use

A preamplifier and home theater control center (and CD/DVD player) sound best when left on -- not in a 'Standby' mode -- because they require a long time to reach operating temperature. A vacuum tube preamp should be turned off, though, as it quickly comes up to temperature.

The circuitry inside a preamp is sensitive to vibrations from the shelf. Everything about the sound improves when the preamp touches less shelf area. Consider placing your preamp or home theater control center on three non-metallic cones, such as those made by Marigo Labs or Black Diamond Racing. Place the cones in certain positions -- a) to support the heavy power supply; b) under the input circuits near the rear, and c) near the front edge of the unit (but off-center).

To determine which set-up produces better sound in your system, perform the following test. Set up your home theater control center to decode your two-channel music. Then set up the CD/DVD player to decode that same music through excellent-quality interconnect cables to the control center.

Home theater control centers will never be simple inside, but some do allow a DVD player's own audio signal to pass through the control center with no further processing. The result may be better or worse, depending on which one does a better job at decoding and the quality of the DVD player's audio interconnect cables. Trust your ears!

Recommendations

The very best two-channel preamplifiers we have heard are quite simple inside and have a robust power supply. They either have circuit boards with very short paths for the signals to follow, or discrete wires instead of circuit boards. Parts are important inside a preamplifier and the best designers have listened to their choices. Manufacturers who impress us the most include Canary Audio, Croft Acoustics, Herron Audio, Jeff Rowland, Leben, and Reflection Audio.