Audio Research SP15 Review

by James Leahy

                                                                     Introduction 

The Audio Research SP15 was introduced to the world in 1988 and is a full functioning pre-amplifier with inbuilt phono stage. As can be seen from the pictures (to the left) the SP15 is constructed on two chassis, utilizing aircraft-quality connectors and an interconnect cable forty-eight (48") inches long. The unit is divided into an active amplification chassis (SP15) and a power supply chassis (SP15PS). A custom interconnect cable of longer length is available from Audio Research at additional cost, although some degradation of performance will occur as the cable lengthens.

The power supply is pictured on the bottom with the control section on top. If you are a control freak this is the pre-amplifier for you. It has a plethora of functions to play with and is the ultimate in flexibility. T

he power supply has two controls, one is the main power and the other is the auxiliary power outputs for various other components that can be switched on or off via the SP15. Nifty hey. There is a five level attenuation control that is designed to equalize source levels and give a huge range of volume control to the pre-amplifier. It is graded in 6dB increments and is adjustable down to -24dB. The volume control is the usual analogue pot with graduated positions being used for this multi-ganged heavy gold platted unit designed to minimize sonic degradation of microvolt level analogue signals. There is a balance control that is useful in some poor acoustical environments.

There is one pair of Balanced outputs as well as two pairs of Single Ended outputs. As always I recommend using the Balanced output to connect up to your power amplifier if it has this available too. The SP15 has Mono, Stereo, Left, Right and Reverse listening modes. The phono circuit has impedance settings of 3, 10,30,100 and 47K ohm. There is also a 'Direct' mode that in the "Bypass" position, directly connects the Attenuator to the output of either the RIAA compensated phono amplification circuitry or a selected line-level source (such as CD. T

he "Balance" and "Mode" Controls, as well as the "Monitor" Switch are removed from the active circuit path when this switch is in operation. The Bypass Switch is provided for use with audiophile-quality program material or whenever maximum fidelity to the input signal is desired.

There are three 6922 Electro Harmonix tubes used in intermediary  gain stages in the phono section of the SP15. Each of the levels of circuitry amplification is provided with its own electronic regulation in the form of a "capacitance multiplier," providing essentially "zero" impedance power supplies with essentially infinite isolation between circuit sections. Thus, the active circuitry is allowed to operate near theoretical performance levels.

The separate "active" chassis contains both the high-gain, RIAA equalized, magnetic phono preamplifier and the "line" level circuitry, plus all of the inputs, outputs and various function controls.

Extensive use has been made of state-of-the-art audio materials, including such items as polypropylene and polystyrene dielectric capacitors, metal film resistors, wire-wound resistors, proprietary point-to-point internal wiring, gold-plated input and output connectors, heavily gold-plated switches, and other high-quality components.

The SP15 is the ultimate pre-amplifier even to this day that has an inbuilt phono section. The improvements over the much loved but not forgotten SP11 are as follows:

Like the SP11, the new SP15 is a "hybrid" preamplifier using both vacuum tubes and semiconductors as gain devices. However, the SP15 uses only three tubes compared to the SP11's six; and whereas the SP11's tubes were employed in both phono and line sections, the SP15's tubes are confined to the phono section alone. As in the SP11, these tubes are not required to be selected, low-noise types.

The changes in the SP15 line section (as compared to the SP11) are not limited to choice of gain devices. Resistance to input overloads, reduction of inherent noise levels, resistance to inter-input crosstalk and dynamic headroom have all been achieved as a result of circuit redesign in this area.

Convenience features and controls remain similar to the SP11, but with some significant additions. The familiar "Gain" control has been replaced by a 5-position "Attenuator" switch having 6dB increments, resulting in better sonics while maintaining a useful selection of settings to match the output characteristics of both phono cartridges and CD players. Tape and dubbing functions remain identical to the SP11.

On the back of the SP15 are new provisions for either conventional (i.e. single-ended) or balanced output connection. Balanced connections can be made via the RCA jacks labelled "Direct" and "Invert," or by using the new industrial "XLR" connector provided.

Like the SP11, the SP15 is a two-chassis design isolating power supply functions from sensitive preamplifier circuits, reducing the opportunity for magnetic field effects to degrade sonic performance. A toroid power transformer minimizes mechanical buzz and "hum." See Installation Instructions for details on proper placement of the two chassis relative to one another.

The SP15 reflects a further evolution and refinement of the idea that state-of-the-art preamplifier performance can be combined with real-word user convenience and system flexibility. Care has been taken to optimize the sonic performance of a variety of popular input options -- video, compact disc, tuner and analogue tape, as well as unsurpassed phono reproduction.

Sonic Evaluations

The SP15 was fired up with Kim Carnes timeless classic Bette Davis Eyes from the  'Mistaken Identity' album (EMI Records, 1981). The soundstage was both wide and deep. I was more than a little impressed with the sonic maturity of this product even by today's standards. The SP15 had an impressive level of drive and slam compared to the LS15 that I reviewed at the same time. The SP15 was in a different class sonically and had a marvellous top end that was not harsh or brittle. The midrange was smooth without any obvious flaws or trouble spots. Bass extension was well defined and quick. However the vocal's were a large step behind the Audio Research LS26 and there was a gritty texture to the lower mid bass that I did not care for. However comparing the SP15 to the LS26 is hardly a fair fight. But on the whole it is a very worthwhile unit with great features that would surpass all in the SP line up that I have heard. Even the latest SP16 that I have tried and did not take to.

For all those who wish to know about the quality of the phono stage in the SP15, read on. The internal phono stage of the SP15 is the best that I have heard in an integrated component. However this is not saying much. I have done extensive comparisons with the SP15 and the separate Audio Research PH5 Phono Stage and the difference in quality is huge. But that is as I would have expected, I was just not prepared for the level of difference that could be heard when I paired up the SP15 with the PH5. It was JAW DROPPING. If you are into your vinyl then the PH5 is a must have as you can by-pass the phono section in the SP15 while still maintaining use of all the other great benefits of this wonderful classic unit.

Associated review Equipment

• Amplifiers:         Audio Research VT200 Mk.I
                          Audio Research VTM120's

• Phono Stage:      Audio Research PH5

• Speakers:           ScanSpeak & Vifa Custom

• Cables:              Neotech

• Cartridge:           Ortofon MC Jubilee

• Subwoofer:         Velodyne HGS-15

• CP Player:          Sony SCD-XA777ES