Audio Control C-101 Series III Equalizer/Analyzer Review

Audio Control, true to its name, is a specialist manufacturer of products for controlling or modifying the frequency response of audio systems. For home and car audio, it produces a number of graphic equalizers, signal processors, and real-time spectrum analyzers. The C-101 Series III is the latest incarnation of a home product initially introduced in 1979.

The C-101 Series III combines a ten-band octave equalizer with a realtime spectrum analyzer whose frequency bands correspond to those of the equalizer. The gain in each band is adjusted by a vertical slider control, of which there are twenty for the two channels. The display window shows the instantaneous program level in each of the ten bands, which are centered at frequencies of 32, 60, 120, 250, and 500 Hz and 1, 2, 4, 8, and 16 kHz. The levels are indicated by vertical rows of red LED’s; the level intervals can be set at 2 or 4 dB by a front-panel control.

The C-101 Series III is normally installed in the signal path to the power amplifier, typically through a tape-monitor loop of a preamplifier or receiver (the C-101 III provides a replacement tape loop). Several alternative configurations are also shown in the instruction manual. If you use the equalizer in a home theater system, it should be placed in the signal path after the surround-sound decoder.

Except for the small Display Level knob and the vertical sliders, all the front-panel controls of the C-101 Series III are pushbuttons. Operating with a light touch and a short “throw,” the buttons trigger electronic switches located close to the controlled circuits, minimizing crosstalk and noise pickup.

There are buttons for selecting a slow or fast LED time constant (display decay rate) and for activating an internal pink-noise generator for use in speaker/room equalization. (Pink noise, a random signal having equal power in each frequency octave, ideally shows as flat on the analyzer display.) Other buttons switch on the equalizing function, equalize the tape-recorder outputs, activate the tape-monitor loop, and insert a “subsonic filter” (more accurately, an infrasonic filter) into the signal path. There is even a button to disable the display if it is too distracting.

In addition to its program-equalization functions, the C-101 Series III can be used for room and loudspeaker equalization and for optimizing speaker placement in the room. For this purpose, it is furnished with a small microphone (apparently an electret type) that plugs into a jack on the equalizer/analyzer’s front panel. The microphone’s frequency response, though not specified, is said to be “very flat,” and it is specifically designed to give the best results with the C-101 Series III. The manual warns against trying to use it for recording or with other audio components.

Although the C-101 Series III is an exceptionally full-featured graphic equalizer, it might be difficult to justify its purchase merely as a highly sophisticated tone control (which it certainly is). Probably the most useful application for the instrument is room/loudspeaker equalization, which simply cannot be done effectively with conventional multiband “equalizers.”

The instruction manual details the procedure for optimizing speaker placement and then equalizing for the room characteristics so as to obtain the flattest overall frequency response at a preferred listening location. The process takes a little time, but we were able to produce an octave-to-octave response flat within +2 dB from 32 Hz to 16 kHz without any difficulty. It is important to add, however, that we used speakers capable of such performance at moderate equalizer settings. In order for a system to be equalizable, the amplifiers and loudspeakers must be able to deliver the desired output levels; even numerically small boosts can greatly increase an amplifier’s output at the affected frequency. Good as it is, even the C-101 Series III cannot perform the sonic equivalent of converting a sow’s ear to a silk purse.

One of the most striking features of Audio Control’s instruction manuals is an unstuffy, plain-speaking style. The manual for the C-101 Series III is no exception, and it makes clear that the purpose of equalization is to make something sound the way you like it to sound, and not merely to produce the horizontal line of LED’s that indicates a flat response. Many years ago, I learned that an indicated flat response at the listening position, which the C-101 Series III can produce quite readily, is not necessarily the best-sounding response. I have always found, for example, that such a response is both brighter at the top end and heavier in the bass than I prefer.

The reasons for the discrepancy between measured flat response and perceived “flat” response are psycho-acoustically complex, but can be simply stated: A microphone-analyzer combination picking up the sound of a speaker in a listening room does not “perceive” the frequency response the same way the human ear does. The real value of the C-101 is its ability to achieve a flat response as a starting point from which you can easily modify the settings to suit your own taste.

The C-101 Series III can also be used to equalize tape recordings, and it is the most sophisticated tone control you could desire. Its LED display, which normally shows the constantly changing spectral content of the program, is fascinating (and very educational) to watch, yet it can be extinguished at the touch of a button if you find it distracting.

Our measurements of the C-101 Series III confirmed its basic ratings, including its awesomely low A-weighted noise level of -118 dB relative to its rated maximum output of 8 volts. Even if you refer the noise voltage (a mere 11 microvolts at the outputs) to the more standard 0.5-volt output level, the resulting -93-dB figure puts the C-101 Series III into the CD player range—and with the equalization bypassed, the noise drops another 7 dB.

The indicated center frequencies of its ten bands were right on the nose, and each had a control range of at least ±12.5 dB (rated at 15 dB). With the sliders in their flat positions, the C-101 Series III had a frequency response of ±0.06 dB from 20 Hz to 80 kHz. At a 1-volt output (1 kHz) the total harmonic distortion was an inaudible 0.002 percent. With the switchable infrasonic filter on, response was ruler-flat (within 0.1 dB) down to 30 Hz, falling off to -3 dB at 20 Hz and -20 dB at 10 Hz.

Our tests and use of the Audio Control C-101 Series III left no doubt that it would be a first-rate addition to any good audio system. Used wisely, it can only improve the final sound, whether you prefer a flat measured response or something else that sounds right to you. The better your system is, the more likely you are to improve its sound by judicious room equalization. Don’t expect miracles, but if you can invest another $459 in your audio system, you may be pleasantly surprised at the results.

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