AudioControl R-130 Analyzer and C-131 Equalizer.

Manufacturer: AudioControl, 22410 70th Avenue West, Mountlake Terrace, WA 98043; 425/775-8461; www.audiocontrol.com

Price: R-130, $649; C-131, $529 each (see text)

Source: Manufacturer loan

Reviewer: James T. Frane

What's an equalizer and why would one use it? An equalizer is a means to alter the frequency response from speakers. Equalizers can be as simple as a bass or treble boost/ cut control or as sophisticated as is the C-131 equalizer, which permits boost or cut at 30 different frequencies, spaced at 1/3 octave. The C-131 is a monaural device, which means you need two of them for stereo use.

All sound systems interact to some degree with their surroundings. The sound from loudspeakers travels directly to the listener, but it also spreads out in many other directions. The result is that some sound is reflected and reaches the listener at different and delayed times compared to the direct sound. The distances between listener and speakers, listener and room surfaces, speakers and room surfaces, and the natures of those surfaces are all variables having different effects on the sounds reaching the listener's ears. For this reason, altering speaker and/or listener positions can have a significant effect on the sound the listener hears. Altering the surfaces along the paths of the sound can also have substantial effects. When speaker location, listening position, and room treatments have been optimized, there may still be some unwanted peaks or dips in the frequency response.

These peaks and dips can be corrected to some degree by all equalizer. The ability to alter 30 frequencies provides a greater degree of control than being able to control just bass and treble at two different frequencies. It also, of course, provides greater potential for the user to make the sound worse.

Enter the analyzer. An analyzer uses a microphone to pick up sounds and displays the sound pressure levels over a range of frequencies. The more frequency sampling points there are, the more meaningful the representation. The range of human hearing is nominally given as 20 Hz to 20,000 Hz, or a range of 10 octaves. A one-third octave analyzer such as the AudioControl R-130 samples 30 frequencies over those 10 octaves. To test the flatness of system frequency response, one needs a constant sound. Music would not be suitable because it is constantly changing. Pink noise, which is sound from 20 Hz to 20 kHz with equal energy in each octave, is generally used for analysis of frequency response. The pink noise signal shows as a straight line across the face of an equalizer. The pink noise picked up in the room by the analyzer microphone will enable the user to see any peaks and valleys in the sound at the microphone location.

One can buy the AudioControl R-130 One-Third Octave Precision Real Time Spectrum Analyzer with or without the pair of monaural C-131 One-Third Octave Precision Constant Q Equalizers. The analyzer and two equalizers are designed to work together, but it's not mandatory that they be used that way.

General Description: The R-130 and the C-131 each come with a 29-page manual that gives information about how to connect and use the equipment. The manuals are well written, but my one complaint is in the area of connection between the AudioControl units and the rest of the system. The manuals for the respective units show drawings for connecting those units in different ways, such as to a tape loop of a receiver or preamp, or between a preamp and power amp. None of the drawings showed the complete connection loop for both C-131s and the R-130 with each other and the rest of the system. Specifically, when one is using both the analyzer and one [or two] equalizers, the sequence of connection to the preamp is not clear. As it turns out, after correspondence with AudioControl, the proper connection schematic is as follows:

- The left and right R-130 output jacks connect to the input jack on each of the C-131 equalizers

- The C-131 equalizer main output jacks connect to the tape loop input jacks on the preamp

- The preamp tape loop output jacks connect to the main input jacks of the R-130.

- Alternatively, the R-130 can be installed between the preamp and the power amp. With the above connections in place, the preamp tape loop is engaged to put the R-130 and the C-131s in use.

R-130 Analyzer Description: The black R-130 cabinet is 17"W x 3.5"H x 11"D. The power switch is in the upper left corner of the front panel; below it is the pink noise level control, and a 0.25" headphone jack. To their right is a selector to toggle the display range between 2 dB and 4 dB and a button to select "bar" or "dot" display. "Bar" lights all the LEDs under the highest one, and the latter mode lights only the highest LED in each of the 30 columns, which results in a line display of the frequency response. Below the dot/bar switch is the selector for fast or slow display speed and the pink noise on-off switch. The display window contains 300 LEDs, 10 in each of 30 columns spaced at 1/ 3 octave frequencies. With the 4 dB per step position selected, a 40-dB range will be covered.

Another column of buttons is to the right of the display. The top two buttons permit selecting either or both channels being displayed at once with either line or microphone input. Then there is the tape loop selector, which can be used as a replacement for the preamp tape loop if that were used to connect the R-130. Below the tape loop button is the "low/high gain" switch. The low gain is used when the R-130 is connected to the preamp tape loop, and the high gain when the R-130 is connected between preamp and amp.

A display level potentiometer to the right allows centering the LED display. Below this control is a 0.25" jack for the supplied, calibrated electret microphone. The user is cautioned to not use any other microphone in this jack, as the microphone receives power from the R-130, which could damage another microphone.

The R-130 back panel has both balanced XLR input connectors and unbalanced input main and loop RCA phono jacks. The loop in and out jacks are designed to replace the tape deck connections on the preamp when the latter are used to connect the R-130 to the system. An unswitched 200 watt, 110 volt outlet resides to the far right of the panel above a 1/4 amp slow-blow fuse.

C-131 Equalizer Description: The C-131 cabinet is also black, and the same size as the R-130. This makes for a visually appealing combination. The C-131 front panel has a power switch in the upper left corner. To its right is a switch to put the C-131 in or out of the circuit, and an LED that lights when the unit power switch is on. Further to the right are 30 vertical sliders with an indicated +/- 15 dB range. Each C-131 can control one channel.

The C-131 back panel has XLR style balanced input and output connections as well as unbalanced RCA-type phono jacks. There are main and auxiliary output jacks. There is a selector switch for the 18 dB/octave "subsonic" filter. The manual acknowledges that infrasonic is the more appropriate term, as "infrasonic" refers to frequencies below the level of human hearing and "subsonic" means below the speed of sound. The subsonic selector switch attenuates frequencies below a frequency that is selectable at 15 Hz, 25 Hz, or 35 Hz, or the filter can be bypassed entirely. AudioControl recommends that this filter be engaged, stating that turntables playing records and many CDs generate these frequencies that can rob amp power and cause woofer flutter, and that the filter has no negative effect.

Associated Equipment: My normal system components included a Carver TFM-55x power amplifier controlled by a Carver CT-17 preamp-tuner with a Denon TU-767 tuner. Speakers were Mach One model M-Two 2-way speakers used with NHT SW3 subwoofers run in dual mono and separately driven by an NHT SA-3 power amplifier. I alternated a Parasound HCA-1000A power amp with the Carver. Interconnect cables were by Gotham and Monster Cable, and speaker cables were Kimber 4PR to the Mach Ones and Original Monster Cable to the subwoofers. A Dual CS5000 turntable with Grado Prestige Red cartridge, Sony XA20ES CD player, and the Denon tuner were the signal sources.

Testing and Listening: I set up the AudioControl calibrated microphone at the listening chair at ear height and plugged it into the R-130 analyzer. I turned on the system, including the subwoofers and experimented with moving the speakers about in minute amounts. The 30-column, 1/3-octave display made determining the effects of location changes quite evident. Essentially, I found that the position in which I had been using the speakers was best, give or take a couple of inches. A small subwoofer level adjustment made the system response relatively flat at the microphone. The range of 25 Hz to 4 kHz was 80 dB +0, -1 dB, and 5 kHz to 20 kHz sloped from 78 dB downward to 74 dB. The R-130 also showed the effects of altering the subwoofer phasing with respect to that of the satellite speakers in 90-degree increments. When I altered the phasing in such a way as to create a small frequency imbalance, I was able to improve it using the C-131 equalizers without affecting the rest of the response spectrum.

I did not attempt to flatten the treble range, as that would have resulted in excessive treble emphasis and might have stressed the tweeters, but I did experiment with the equalizer controls a bit. I found that a boost or cut at any given frequency affected only the two adjacent channels, and at a maximum value about 2 dB less than the boost or cut of the frequency being adjusted. Small adjustments of 2-3 dB had no measurable effect on adjacent frequencies. This is a narrow band of adjustment and is attributable to the constant-Q feature of the design. It permits more precise control than if several frequencies were affected above and below the one being adjusted.

At no time did the units add any discernable electronic noise to the system. When the equalizers were set to flat frequency response (no boost or cut at any frequency), there was no audible difference with them in or out of the system. AudioControl states they consider an equalizer the final resort. They advocate making all the possible physical changes before resorting to equalization -- sound advice (no pun intended). It is important for the listener to experiment with speaker and listening positions, as well as with the room itself in order to obtain the best sound. A good analyzer such as the R-130 can be a great help in this process. Once all practicable changes have been made, and there are remaining sonic problems, equalization may resolve these problems.

AudioControl has designed and manufactured some very good products in the R-130 analyzer and the C-131 equalizers. These units performed their intended functions well. They have the potential to make improvements to a wide variety of situations. If all other avenues have been followed in an unsuccessful attempt to correct frequency response anomalies in your system, the C-131 equalizers may be the solution. - JTF