Rockford Fosgate, which is known primarily for automotive audio components, has now entered the home hi-fi market with a high-quality preamplifier and power amplifier. The two units, similar in styling and finish, offer a high degree of operating versatility.

The compact RF200 preamplifier has slightly sculptured sides that enhance the appearance of the otherwise conventional black box. All of the controls are clearly marked in large, white letters. The four inputs (three high-level and one phono) are selected by pushbuttons. Other pushbuttons operate the tape-moni-tor switch, tone bypass, and mono/stereo mode selection. Small center-detented knobs control bass, treble, and balance, and a larger knob operates the volume control. A rocker-type power switch, headphone jack, and green led pilot light complete the front-panel features.

The gold-plated phono jacks on the rear include the inputs for all signal sources and input and output connectors (normally joined by jumpers) for an external-processing loop to insert an equalizer or another accessory into the signal path. There is a single switched AC outlet.

The preamplifier has three pairs of output jacks, identified as main out, high out, and low out. The latter two are used with the preamplifier's built-in active crossover network for driving biamplified speakers through additional power amplifiers. As delivered from the factory, the RF200's internal crossover is set at 100 Hz (with 12-dB-per-octave slopes), but the frequency can be changed by the user without opening up the cabinet. Removing a small plate on the rear apron exposes a dual in-line package (dip) containing the resistors that determine the crossover frequency. The instruction manual contains the formula for calculating the resistance values for any frequency, allowing the user to make a custom crossover module. Additional dip assemblies are available from Rockford Fosgate for crossover frequencies from 50 to 9,000 Hz at halfoctave intervals.

The RF2000 power amplifier is a very powerful and rugged component. The power supply uses a large toroidal transformer, which is able to supply 1,000 watts continuously, and a total of more than 80,000 microfarads of filter capacitance.

Each output channel uses sixteen mosfet power transistors to deliver a peak output current in excess of 50 amperes to a speaker load. The amplifier is fully rated for use with 4-or 8-ohm loads, and it can operate stably into 2 ohms, although heavy, sustained 2-ohm operation may trigger its thermal-shutdown protection system.

The thermal protectors, mounted on the output-transistor heat sinks, respond to a temperature rise by turning on and gradually increasing the speed of the built-in cooling fan. Normally the fan is either off or turning so slowly as to be inaudible, and under home listening conditions it is unlikely to be heard. If the heat-sink temperature approaches a dangerous value, the amplifier shuts off altogether.

Another protective system monitors the operating current and voltage of the output devices, calculates their internal temperature, and cuts the power to safe values when the maximum limits are reached. At this time, the green power-status led on the front panel changes to red. Since the fan continues to cool the output stages, the amplifier soon returns to normal operation. There are also two channel-status led's on the panel, which are extinguished when the amplifier is off or if its power output is under about 200 milliwatts (such as during a quiet portion of a program). In normal operation, they are both green. If there is distortion in a channel's output signal, for any reason, its light changes to red.

The RF2000 is rated for 200 watts per channel into 8 ohms from 20 to 20,000 Hz with less than 0.05 percent total harmonic distortion. Into 4-ohm loads the rated output is 300 watts per channel with less than 0.1 percent distortion. A switch on the rear of the amplifier bridges the two channels, giving it a mono rating of 600 watts into 8 ohms. Another operating mode uses the bridged connection to drive a mono subwoofer or center-channel speaker while driving two satellite speakers in the normal stereo mode. Normally the input signal ground is floating (isolated from the output grounds), but a third switch connects the chassis to the output ground should this be desired.

The only front-panel features of the RF2000 are the three led signal lights and a rocker-type power switch. Its rear apron, in addition to the bridging and grounding switches, contains five-way binding-post speaker terminals (which can accept standard twin banana-plug connectors). Each speaker output is protected by a 5-ampere fuse. The input phono jacks are gold plated, and there are individual level-control knobs for the two channels. The cooling-fan exhaust is in the rear of the amplifier.

The RF200 preamplifier measures 17-1/2 inches wide, 8-1/8 inches deep, and 2-3/4 inches high and weighs about 6 pounds. The RF2000 amplifier is the same width but 12-1/4 inches deep and 4-1/2 inches high; it weighs 35 pounds. Prices: RF200, $550; RF2000, $1,198.

Lab Tests

The RF200 met or surpassed all its specifications in our tests. Its clipping-level output (from the main outputs) was 8.8 volts, and the sensitivity for a reference output of 0.5 volt was 53 millivolts (mv) through a high-level input and 0.52 mv through the phono input. The phono-preamplifier stage overloaded at inputs between 71 and 74 mv from 20 to 20,000 Hz. The phono-input impedance was 47,000 ohms in parallel with 130 pF. The A-weighted signal-to-noise ratio was 91 dB through a high-level input and 83.6 dB through the phono input.

The frequency response, essentially unaffected by having the tone controls in the circuit, was flat within 0.01 dB from 20 to 15,000 Hz, down 0.05 dB at 20,000 Hz, and down only 0.4 dB at 50,000 Hz. The RIAA equalization error was ± 0.15 dB from 20 to 10,000 Hz and -0.5 dB at 15,000 Hz. The active crossover frequency was 100 Hz, as rated, with the specified 12-dB-per-octave slopes. The tone controls did not affect the response between 500 and 1.000 Hz. The bass control had its maximum effect at 40 Hz, where the output could be varied ± 14 dB. The treble control had its maximum effect of ±11 dB at 20,000 Hz. Channel separation (through a high-level input) was about 80 dB from 20 to 2.000 Hz, falling to 66 dB at 20,000 Hz with the unused input shorted or to -40 dB with it open-circuited. The crosstalk from the CD input to the tuner input was a constant - 80 dB from 20 to 20,000 Hz. Harmonic distortion at 1,000 Hz reached its minimum of 0.003 percent just before the output clipped.

The RF2000 power amplifier proved to be very conservatively rated. After the FTC-mandated 1-hour preconditioning, during which the exterior of the amplifier became only moderately warm, its 1,000-Hz output clipped at 345 watts into 8 ohms and at 565 watts into 4 ohms. The respective clipping-headroom figures were 4.7 and 5.4 dB. With a 2-ohm load, the 5-ampere speaker-output fuses blew before the amplifier waveform clipped.

In dynamic power tests, the 8-and 4-ohm power levels at clipping were 450 and 720 watts, respectively, corresponding to dynamic-head-room figures of 7 and 7.6 dB. Since the average power is relatively low during these tone-burst tests, we were also able to measure the 2-ohm dynamic power-a most impressive 1,150 watts!

The harmonic distortion (THD plus noise) was less than 0.0023 percent at the rated 200 watts into 8 ohms for frequencies between 20 and 1,000 Hz. It rose slowly at higher frequencies to a maximum of just under 0.008 percent at 10,000 Hz. At lower power levels the distortion readings were roughly similar.

The low-level frequency response of the RF2000 was down 0.2 dB at 20 and 10,000 Hz and 0.37 dB at 20,000 Hz. The - 3-dB frequency was 100,000 Hz. With the input level controls at their maximum settings, an input of 45 mv was needed for a reference output of 1 watt, and the A-weighted noise was -95 dB referred to 1 watt. The amplifier's slew factor was 12.5 (the waveform at 200 watts output became triangular at approximately 250 kHz), and it was exceptionally stable with capacitive loads, showing no overshoot or ringing in square-wave tests.

Comments

The test results speak eloquently for the quality of both the RF200 and the RF2000, which certainly constitute an auspicious entry into the home market by Rockford Fosgate. We were very impressed by the degree to which they surpassed their key specifications in our tests. The "bulletproof" nature of the RF2000 amplifier was readily apparent. Moreover, even though it has electronic circuit protection for the output transistors, it also has speaker fuses that are likely to blow well before the amplifier reaches its own limits.

There were other encouraging features of these components, which we operated together in our listening tests. Neither had any tendency to produce an output transient when it was switched on or off, and either one can be turned on or off before the other without risking damage to the speakers or the listeners' sensibilities. The manufacturer does suggest, however, that the power amplifier be switched on last and off first since other system components may not be as carefully guarded from power-surge transients as the RF200 and RF2000. And the same care was taken with the other switches on the RF200, so that both components are true "silent partners" in a music system.

We did not operate the RF2000 in its bridged modes, in which it should be an even more formidable amplifier than it was in a normal stereo configuration. We noted with interest that although a warm breeze could sometimes be felt emerging from the fan vent on its rear apron, absolutely no sound could be heard from the fan. Neither the amplifier nor the preamplifier ever became more than faintly warm to the touch.

Finally, the user's needs have been taken into consideration in the design of these products. The instruction manuals, though unpretentious in size and appearance, tell how to install and use the components in clear and unambiguous language. The preamplifier's control markings are legible - and intelligible as well, thanks to a welcome absence of cryptic abbreviations- and the channel lights on the power amplifier are an exceptionally simple and effective source of information. They're off when the signal level is near zero, bright green during all normal operation, and red only when the amplifier's output is becoming nonlinear. This system is far more practical than either level meters or an array of led's, since it indicates unequivocally when the amplifier is operating within its linear range and when those limits are being even momentarily exceeded.

As for "how they sound"-if you believe that all amplifiers sound different, you will have to listen for yourself. I hold that all good amplifiers sound pretty much alike, and the Rockford Fosgate units are very good amplifiers. I found them a pleasure to test and to use in a music system.

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