The Graphic Equaliser has become an essential component of most advanced audio systems: a multiband equaliser can be used to alter the frequency response of various items of studio equipment. The instrument can be linked between two other units, such as the mixing desk (some of which have individual equalisation on each channel) and perhaps a tape unit, thus achieving an optimum frequency response for recording purposes. A common use for such a device is in studio monitoring systems or for PA work.

An equaliser is capable of 'tuning' equipment to the acoustics of a room, studio or auditorium, producing a uniform response. Another extremely important use for this device is in high power work where room resonant frequencies can be eliminated and so acoustic feedback can be minimised. It accomplishes this by filtering out the main resonant frequencies causing the problems. I have used an equaliser on many occasions for just this purpose and its effectiveness is unparalleled. As a result of this particular feature PA systems are capable of being driven at much higher levels, without the threat of feedback.

A Graphic Equaliser such as the SR 271 can also be used as a simple spectrum analyser, where the LEDs will indicate predominant frequencies — an important facility when trying to locate a feedback frequency.

Every musician who uses a PA system can benefit from an Equaliser, but up to now this kind of unit has been very expensive. However, the SR 271 has broken the price barrier and now must be well within the budget of most bands.

The SR 271 Graphic Equaliser is totally screened by a matt black metal case which is designed to slot into a 19in rack mounting (PA:CE have made flight-cased racks available for all models). The dimensions of the unit are 5¼in high and 9in deep.

The 27 bands of the equaliser are controlled by an equivalent number of 60mm slider potentiometers. These are calibrated for: 0dB, ±3dB, ±6dB, ±12dB over a frequency range of 40Hz to 16KHz at intervals of ⅓ of an octave.

Above each of the slider controls there is an LED which is precisely tuned to the band centre frequency. All of the 27 LEDs are controlled by a common threshold potentiometer which is situated on the top right hand side of the front panel. Next to the threshold pot there is a rotary gain control. Beneath these there is a push button power on/off switch and adjacent to this there is a push button Bypass switch; both of these controls have LED status indicators. The artwork on the front panel is all white and of a clear, unpretentious nature.

The connectors on the rear panel are arranged for flexibility, with two XLR Cannon input and output sockets plus two ¼in jacks. Situated to the right of these is a small slide switch, an input sensitivity selector calibrated at 0dB and —20dB. To the far left there is a standard (IEC) Euroconnector mains socket with a 20mm miniature fuse holder just to the right. That's all you have to play with on the outside.

Removing the top cover, I was pleasantly surprised by the overall neatness of the unit and the easy accessibility of all of the components, which will make for fast repairs by a service engineer. All the components which go together to form 27 filter/equaliser circuits are mounted on one large well constructed PCB. Items such as the 20VA (2x12v) toroidal mains transformer and the power supply capacitors are mounted on the side panel. The transformer is complete with an interwinding screen between the primaries and secondaries which ensures safety and electrostatic screening.

The LED indicators are soldered directly to the PCB and are connected to special driver circuits. All interconnecting leads are of the screened variety; with a virtual absence of magnetic fields PA:CE have managed to reduce hum to a minimum. To add to this total mains hum screening the user is totally protected from mains voltages within the unit by the use of rubber sleeving on all otherwise exposed electrical contacts.

The whole filter design is based on resistive/capacitive circuits, thus making replacement simpler. Of course, replacement should not be necessary, for the design and workmanship of this instrument are of the very highest order. This is clearly demonstrated by the accuracy of the band centre frequencies, individual gain alignments being very close and overall cut and boost on each band being a 12dB minimum.

The unit can be regarded as a very reliable piece of equipment and should do any PA justice. It also proved very versatile for studio work, and will most certainly withstand rough road treatment.

rrp £190/$600