Revox B251 Integrated Amplifier Review

Swiss-made Revox audio components have long been noted for their outstanding performance and overall quality. The Revox B 251 integrated amplifier certainly seems to follow in that tradition.

The unit’s ratings, however, are not presented in accordance with either EIA or FTC standards (for example, contrary to FTC rulings, the specifications are headed by a “Music Power” rating), making it difficult to compare it with other, more conventionally rated amplifiers. A Revox press release gives the B 251’s power rating as 140 watts per channel into 4-ohm loads, but the instruction manual rates its continuous “sine-wave” power as 100 watts per channel into 4 ohms, which is the rating we used as the basis for our tests. The distortion is rated as 0.01 per cent at 10,000 Hz instead of the usual 1,000 Hz.

Many aspects of the operation of the B 251 are controlled by a microprocessor. This gives it the ability to “remember” a turn-on volume setting and the different input sensitivities required to produce the same listening levels from several sources as well as the ability to compensate for different sensitivities in the two pairs of speakers to which its output can be switched. Even the operation of its infrasonic filter can be linked to the pro-gram-source selection so that, for example, the filter comes into use automatically when the phono input is selected but not when a Compact Disc player is the source.

Most of the operating controls of the B 251 are pushbuttons in the upper portion of its front panel. There are input selectors for two tape decks, three high-level sources (marked tuner, aux, and disc), and a record player. The phono input is designed for a moving-magnet (MM) cartridge, but an optional head amplifier for a moving-coil (MC) cartridge can also be installed in the amplifier and selected with a front-panel rotary switch. Other buttons insert the tone controls into the signal path and reduce the volume by 20 dB. An LED in the center of each source or monitor selector button shows when it has been activated.

Pressing one of two large rectangular plates (marked + and —) steps the amplifier volume up or down. A brief touch changes the level by a single increment (barely perceptible to the ear): holding the plate in causes the level to change rapidly. The rate of change depends on which end of a plate is held in: the inner ends (marked with single arrows) cover the full control range of 66 dB in about 13 seconds; the outer ends (with double arrows) cover the same range in only 3 seconds. A similar-sized plate to the right of the in-put-selector buttons is the power switch. Its “off’ setting is actually a stand-by condition. The microprocessor and related circuits are always powered when the amplifier is plugged into the power line, as is indicated by a red LED below the power switch.

To the left of the control panel is a window containing a dual-channel segmented liquid-crystal display. This can be switched (using a set of three pushbuttons below it) to show the volume-control with a front-panel rotary switch. Other buttons insert the tone controls into the signal path and reduce the volume by 20 dB. An LED in the center of each control button shows when it has been activated.

The lower portion of the front panel of the B 251 is normally covered by a tinted plastic window that is removable by pulling on its metal end strips. The right side of the panel contains a number of record output buttons that determine the signal source appearing at the tape-output jacks on the rear panel. Any source can be selected independently of the one being heard, but the monitor button can be used to provide the same source to both the tape and speaker outputs. A subsonic filter button introduces an 18-dB-per-oc-tave rolloff below 18 Hz.

On the left side of the panel are a number of pushbutton controls for the memory and display systems. In the center of the lower panel are two balance buttons that shift the channel gains over a range of +3, —10 dB in 0.5-dB steps. Above them is the infrared receiving window for the optional remote-control unit.

A number of knob controls line the lower part of the panel. The two headphone jacks have an associated switch that shifts their levels in 4-dB steps from + 4 to — 8 dB relative to the volume setting. A mode knob selects stereo or mono operation. The bass and treble tone controls each have five positions of cut and boost as well as a center (flat) setting. Finally, the phono switch selects input-shunt capacitances of 150, 300, or 450 picofarads or the optional MC head amp.

The rear of the Revox B 251 contains the various signal input and output jacks (the phono-input jacks are gold plated), insulated spring-loaded speaker outputs, a socket for remote power switching by a Revox B710 cassette deck, and a socket for controlling a B710 or B77 open-reel deck by the amplifier’s optional B201 remote control. (The remote control is a battery-operated, hand-held unit whose buttons not only duplicate all the functions of the B 251 but can operate a complete Revox system including a turntable, tuner, and tape decks.) The B 251 also has separate preamplifier outputs and power-amplifier inputs (labeled monitor) as well as a screwdriver-operated switch for electrically separating the two portions of the amplifier. There are no a.c. convenience outlets.

The Revox B 251 is finished in dark gray with a satin-silver control panel and accents. It is about 17-3/4 inches wide, 13 inches deep, and 6 inches high, and it weighs 18-3/4 pounds. Price: $1,500.

Laboratory Measurements.

The top of the Revox B 251 became very hot during our preconditioning and subsequent testing. It heated up rapidly during any period of extended continuous-power operation, and we found it impossible to touch the top over the heat-sink fins for more than a second or two during this part of our tests. In normal use, however, the top became only moderately warm.

With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 136 watts per channel. With 4- and 2-ohm loads, the respective power outputs at clipping were 127 and 90 watts. The dynamic power output of the B 251 into 8-, 4-, and 2-ohm loads was 156, 245, and 90 watts, respectively. The 8-ohm clipping- and dynamic-headroom measurements (based on a 100-watt rating) were 1.34 and 1.93 dB, and the 4-ohm values were 1.04 and 3.89 dB.

When we drove 8-ohm loads at the 100-watt level, the distortion was between 0.01 and 0.02 per cent over most of the range from 20 to 20,000 Hz, reaching a maximum of 0.035 per cent at 1,000 Hz. At lower power levels the distortion characteristic was similar but with lower distortion readings (typically between 0.002 and 0.01 per cent) at normal listening levels.

The harmonic distortion with both channels driving 8-ohm loads at 1,000 Hz was about 0.002 per cent at 1 watt, rising smoothly to about 0.02 per cent in the 100- to 130-watt range. The 4-ohm distortion rose from 0.005 per cent at 1 watt to about 0.05 per cent in the 10- to 130-watt range. At 2 ohms, the distortion was 0.007 per cent at 1 watt, 0.08 per cent at 50 watts, and 0.18 per cent at 70 watts. Distortion reached 0.47 per cent at the maximum output of 90 watts. The amplifier’s protection circuits shut it off frequently while we were driving it to clipping into low load impedances, and a thermal protection system also turned it off several times during the tests. It always returned to normal operation automatically and suffered no apparent damage. The output transistors are cooled by a sealed fluid-convection system, and we could hear the coolant boiling during much of our high-power testing into dummy loads.

The amplifier was stable with a reactive load simulating a “difficult” loudspeaker impedance and had no difficulty driving a standard EIA reactive load. Its slew factor was 1.5, with the full-power (100-watt) sine-wave output becoming triangular above 30,000 Hz.

The input sensitivity of the Revox B 251 for a standard 1 -watt output was 50 millivolts (mV) for the aux input and 0.5 mV for the phono (MM) input. The respective noise levels (A-weighted) were — 85.5 and —83 dB referred to 1 watt. (The optional MC head amplifier was not installed in our test sample.) The aux input overloaded at 10 volts (a more than safe value), and the phono preamplifier overloaded at very high input levels, from 270 to 425 mV depending on frequency. The phono equalization was within ±0.5 dB of the RIAA characteristic from 25 to 20,000 Hz, and it was totally unaffected by cartridge inductance. The phono input impedance was 41,000 ohms in parallel with a capacitance of about 160, 250, or 350 picofarads, depending on the setting of the front-panel switch.

The tone controls of this amplifier had characteristics quite unlike any others we can recall seeing. They affected only the frequency extremes, with the bass control’s “3-dB” point always under 100 Hz and the treble turnover frequency above 6,000 Hz. The maximum bass-control range was ± 12.5 dB at about 35 Hz, and the treble control had a maximum range of ± 6 dB at 15,000 Hz (see graph above). The subsonic filter cut response by only 1 dB at 30 Hz and 3.5 dB at 20 Hz.

Comment

To begin with, let us dispose of our only real criticisms of the Revox B 251, both of them minor items. First, the removable plastic cover over the lower front-panel control section was difficult to pry off and replace. But there is actually no need to use it except for appearance’s sake or to prevent unwanted changes of the control settings. Second, the lack of a.c. convenience outlets may be a definite inconvenience for anyone who uses this amplifier with non-Revox signal-source components.

Otherwise, the B 251 was a delight to use. Everything operated with the silence and precision that we have come to expect from Revox products. The LCD display proved to be remarkably accurate as a power indicator. Its fast peak response read 100 per cent of steady-state values on 20-millisecond tone bursts and was only 1 dB off with 10-millisecond bursts and 3 dB off with 1-millisecond bursts (only 1 cycle of a 1,000-Hz tone!). Some practice is needed to learn how to store the various operating levels in the computer memory, but this feature worked well and was completely effective in preventing unpleasant surprises when the amplifier is turned on.

The tone controls were almost too subtle in their action, which was almost undetectable with most program material. According to the instruction manual, setting both controls to maximum causes them to function as a loudness compensator; if so, its effects were very subtle compared with most such controls.

One of the most interesting aspects of testing the B 251 was examining the jewel-like interior of this remarkable amplifier. Everything in it exuded an aura of quality, with no cost-cutting techniques in evidence. We noted the very compact switching power supply, which has no conventional heavy power transformer. The extremely low noise levels of the amplifier attest to the effectiveness of its power-supply shielding and filtering. Such attention to detail is one of the things you expect to find in a premium-priced product. Whether or not it yields sonic dividends, it is bound to contribute to the amplifier’s long-term reliability and to the satisfaction of its user.

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