Audio Research SP-6

Published: 05/12/2020

Manufacturing date: 1980

Author: Karsten Hein

Category: Gear & Review

Tag(s): Pre-Amplifiers

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High Definition Stereo Preamplifier

As the vacuum tube was phased out in home audio appliances during the late 1960s, for the sake of cheaper production and more versatile equipment, some music lovers believed that the industry was heading in the wrong direction. While transistors outperformed tubes in terms of size and heat dissipation, their musical performance was not yet great.

Among the transistor sceptics was a man called William Zane Johnson. Bill had been running his own specialty audio store in south Minneapolis, out of which had he designed sophisticated amplifiers for those who cared deeply about accuracy and musicality in sound reproduction since 1951.

In 1970, following a failed attempt of developing his existing store model, he decided to start a company that would do justice to his designs, patents, as well as to himself. He named it ‘Audio Research’, and from the beginning it was clear that this company would put its focus on vacuum tubes for music reproduction.

While Bill held fast in the belief that vacuum tubes could convey a more realistic, and therefore convincing, sonic picture, there was still some work to be done for tubes to match transistors in terms of agility and control. The challenge was to marry the accuracy in sound reproduction that is typical of the tube with the ability to present detail in music that is common with the transistor.

The successful marriage of these properties Bill Johnson named ‘High Definition’ audio, now a well-known trademark and mission statement that has been stamped on Audio Research products ever since. While his tube designs were at times perceived to be a disruption to the progress of the industry, Audio Research proved to be well ahead of the competition during the tube revival that followed in the 1980s.

The success of the Audio Research Corporation was also helped by the fact that it was declared the official benchmark of sound by some of the most influential audio magazines of the time, such as ‘The Absolute Sound’ and ‘Stereophile’. Audio Research is today the oldest existing manufacturer of high-end audio products and is considered to have given rise to the very notion of high-end audio. Their SP-3 High Definition preamplifier was said to be the best preamplifier on the market over a period of many years.

The unit discussed here, the Audio Research SP-6 High Definition preamplifier, was built from 1978 to 1982 in only slightly deviating versions (models A-E), with the exception of the SP-6F version which has been stated to be the SP-8 MK II with a different face plate. The unit shown here is the SP-6B of 1980 fitted with Russian 12AX7 Tung-Sol tubes. For a preamplifier, the cabinet is quite tall at 13.4cm, and at 10kg also quite heavy. Although general power consumption is at a relatively low 50 watts, the internal tubes do get quite hot, so that there should be 5 cm of ventilation space kept free above the unit.

The turning knobs are of excellent touch and feel with a stepped volume attenuator. The row of switches could perhaps be a bit more refined. The muting switch can especially give an occasional plop when turned on or off. All great preamplifiers play loud, and so does the SP-6. This can make it a little difficult to set subtle volume differences for night-time listening, especially when playing from a loud CD player, DAC, or streamer as source, and when powering high decibel speakers. Due to this, we have often ended up playing our music a little louder than we would otherwise have done.

Bringing the SP-6 to operating temperature after turning it on takes about four minutes. The unit is fully warmed up when the power LED stops flashing. However, its full sonic capacity is reached only after about thirty minutes. This is when the preamplifier begins to sound the most musical. And, no surprise, what we get when the SP-6 is fully warmed up is very pleasing, indeed. Compared to our DB Systems DB1, the Audio Research preamplifier sounds wonderfully voluptuous, but it does not do so at the expense of focus. Voices simply take up more space in the virtual center. Singers are accurately allowed their own realistic dimension. Where the DB 1 can make voices sound frail at times, the SP-6 makes them sound lush and solid. Accurate tonal color and realistic musical detail are present at all times.

Similar to our DB1, the SP-6 builds a realistic and three-dimensional sound stage. Both preamplifiers play accurately and provide a warm and pleasing signature to the system; however, the SP-6 provides more bass slam and dynamic drive than the DB1. On our main system consisting of a B&K ST140 amp and Martin Logan SL3 speakers, the SP-6 offers the most balanced, musical, and entertaining performance of any preamplifier we have had the pleasure of exploring here to date.

Note: In the end I was relieved to find that returning to our DB Systems preamplifier with satisfaction was still possible after this tour of exploration. While the two preamps accentuate different aspects of the music, they both provide an endearing and convincing performance at a very high level.


  • Frequency response: 10 Hz to 30 kHz
  • Total harmonic distortion: 0.01%
  • Gain: 50dB 60dB (MM), 26dB (line)
  • Signal to noise ratio: 66dB (MM), 90dB (line)
  • Output: 5V (Pre out), 60V (Pre out Max)
  • Valve complement: 6 x ECC83 (6 x 12AX7 Tung-Sol)
  • Dimensions: 480 x 134 x 260mm
  • Weight: 10kg
  • Country of Manufacture: U.S.A.
  • Year: 1980
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