Audio Control Richter Scale Equalizer Review

The Audio Control “Richter Scale” (henceforth referred to as the RS) is a special-purpose five-band graphic equalizer and wave analyzer that operates only at frequencies below about 200 Hz. Its filters, spaced at half-octave intervals, have center frequencies of 31.5, 45, 63, 90, and 125 Hz. Each slider control adjusts the gain in its own frequency band over a ± 12-dB range.

Adjusting any graphic equalizer by ear can be difficult, and to simplify the process the RS contains a tunable warble-tone generator covering the range from 31.5 to 250 Hz. Its frequency is adjusted by a small front-panel knob whose calibration points correspond to the filter center frequencies (the 250-Hz point serves as a reference level to which the response at the other frequencies can be matched by use of the slider controls).

A small electret microphone is supplied with the RS and is powered from it. The microphone is only about 2 inches long, and it is the same type used with the Audio Control Model C-101 real-time spectrum analyzer and equalizer. The level indication, instead of using a bar-graph LED display like the C-101, is a single meter calibrated to read relative acoustic levels from —20 to + 3 dB. It is very heavily damped to give steady readings during the rapid level fluctuations that can occur in low-frequency warble-tone measurements.

The Audio Control RS is normally connected to the tape-monitoring circuit of an amplifier, and the amplifier’s tape-recording inputs and outputs are duplicated on its rear panel. A front-panel pushbutton switch connects either the source or the tape signal to the equalizer inputs. Another button (equalize program) connects or bypasses the equalizer circuits themselves. The five center-detented equalizer sliders are calibrated over their ± 12-dB range. To use the warble-tone signal source, a small analyze button at the right side of the panel is pressed. This supplies the tone to the left-and right-channel warble-tone jacks in the rear of the unit and illuminates the front-panel meter. There is a single microphone jack in the rear, and the microphone has a 22-foot cable that allows it to be placed almost anywhere in the room (normally close to the listening position).

Although no instructions were supplied with the early production sample we tested, the operation of the RS was straightforward. With all equalizer controls at 0, the warble-tone frequency at its maximum of 250 Hz, and the microphone placed near the normal listening position, the system level was set to produce a 0-dB reading on the analyzer meter. The frequency was then moved successively to each of the other frequencies, the corresponding sliders being adjusted for a 0-dB meter reading as well. Since there is considerable interaction between adjacent (and even alternate) controls, it is necessary to repeat this procedure several times until no further improvement is possible. The final result may not be a truly “flat” response, but it is certain to be much flatter than the unequalized response (we were usually able to reduce the level variations to no more than ± 2 or 3 dB).

There are several independent, but somewhat related, pushbutton controls that also affect low-frequency performance. The subsonic filter introduces a steep (18 dB per octave) rolloff below 20 Hz. To impart the last bit of low-bass “feel" to the sound, the ultra lo boost raises the output by 15 dB at 34 Hz. The rumble reducer sums low-frequency signals in the two channels (below about 200 Hz) to reduce out-of-phase rumble without significant loss of audible bass response.

The Audio Control RS has yet another function that, to some people, could be worth a good part of its price. It contains an electronic crossover network, with the crossover frequency switchable to either 100 or 800 Hz (it can be set to other frequencies by an internal modification). A front-panel knob adjusts the low-frequency output level from the crossover circuit over a ± 20-dB range, sufficient to match almost any conceivable combination of drivers. The crossover input and output jacks for left and right channels and both frequency ranges are in the rear of the unit.

The Audio Control “Richter Scale” is a compact unit, measuring only 14-1/2 inches wide, 5-7/8 inches deep, and 2-1/2 inches high. It weighs 5 pounds. The exterior is finished in black, with solid oak end plates. Price: $249.

Laboratory Measurements

The equalizer response curves of the Audio Control RS were approximately at the stated frequencies and showed the rated control range of ± 12 dB. With all controls centered, the response was flat within ±0.5 dB from 20 to 20,000 Hz. The subsonic filter began to affect the response at about 30 Hz, and it was down about 2.5 dB at 20 Hz. The ultra lo boost raised the output by the rated 15 dB at about 35 to 40 Hz. Its effect was reduced to about 7 dB at 20 and 70 Hz.

The RS had a gain of exactly 1.0 when driving an IHF standard load. Depending on the equalizer settings, the output clipped at 6.5 to 7 volts. The unweighted output noise was unmeasurable, being less than 100 microvolts. The level of the warble output was 70 millivolts, and the warble rate was about 6 Hz. The frequency deviation appeared to be about ± 30 per cent of the center frequency.

Although we did not use the RS as an electronic crossover (this function is completely independent of its equalizer circuits), we did measure the frequency response of its low- and high-frequency channels. The manufacturer claims to have an asymmetrical crossover design, with phase shift controlled to produce a constant-voltage total output when the two channels are summed. To this end, the upper end of the woofer range is rolled off at a 6-dB-per-oc-tave rate, while the lower end of the tweeter range is rolled off at an 18-dB-per-octave rate. Our measurements confirmed this characteristic and in addition showed a broadly raised output (about +3 dB) near the upper end of the woofer’s range. When we combined the two outputs resistively, however, the sum was absolutely uniform with frequency. Not a trace of the crossover could be found in the summed signals, which had a response of ±0.25 dB from 20 to 20,000 Hz.

The distortion rating of the RS is 0.015 per cent, with no level stated. Our measurements showed the distortion to be less than 0.003 per cent up to about 0.3 volt output and between 0.04 and 0.1 per cent from 1 to 6 volts output. These values are completely negligible.

Comment

We used the Audio Control "Richter Scale’’ in a couple of different music systems in different rooms. The equalization adjustment took only a minute or two, and the results were most gratifying.

Unlike the usual room-equalization procedure, which attempts to obtain a “flat” response over the audio range in the search for greater accuracy of reproduction (an ambiguous target at best), the bass equalization of the RS makes little claim to greater “accuracy," whatever that might be. It is, quite frankly, a means of restoring the “feel” of low bass in systems in which that part of the spectrum is missing or severely attenuated; at the same time it removes much of the upper-bass heaviness that normally mars the sound of many speakers. That, at least, is our interpretation of its purpose and the basis for our evaluation here.

The RS did both jobs very well. Together with a more natural overall bass balance, it managed to give some speakers a skin-tingling low-bass output that we had never before experienced from them in the same room. It won’t get more from a speaker than it is capable of giving, but it will surely exploit any speaker’s capabilities to the utmost. The ultra LO boost was responsible in good measure for this performance, even without using the half-octave equalizer. It could give a remarkably solid deep bass with no danger of overdriving either the speaker or the amplifier in the infrasonic range.

The Audio Control “Richter Scale” is not inexpensive, but its specialized design makes it more effective as a bass equalizer than any of the full-range octave-band equalizers we have used. It is worth its price for that function alone if, for one reason or another, you are not presently realizing your full share of bass from your speakers. The RS is an equally good value as a very fine electronic-crossover accessory for biamplified systems. If both capabilities can be used, the "Richter Scale” will prove itself a worthwhile addition to almost any music system.

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