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ACES 27 Band Graphic Equaliser



It has taken man millions of years to progress from making music by means of hitting his neighbour's skull with rocks to the point where he builds and plays programmable polysynths, but it has taken only twenty years or so for the humble knob labelled 'tone' to evolve into the bewildering variety of EQ systems currently available.

At the end of the Sixties, tone controls were limited to 'bass' and 'treble', but very soon, a newcomer called 'middle' started to appear on all fashionable amplification.

From that time onwards, the words 'tone control' were banished and anyone uttering them was given the same type of contemptuous look that you might receive from a hi-fi salesman if you called a compact disc player a gramophone!

The types of EQ system available rapidly diversified and, after taking some pretty bizarre dead ends, we now have a choice of bandpass, shelving, parametric or graphic designs with derivatives and hybrids being offered as further alternatives.

Application



One area in which the graphic equaliser is used almost exclusively is in the correction of control room acoustics, made necessary by the frequency dependant absorbing or reflecting properties of its walls and surfaces and by the choice of monitoring system.

Unfortunately, EQ is not the universal panacea that it might at first appear to be; although it can be used to reduce the amount of acoustic energy at the frequency of a troublesome resonance or standing wave, it won't cause these effects to disappear.

Given a sensibly designed room however, the graphic equaliser is probably the most convenient equalisation device to use as the position of the sliders gives a graphic indication of the frequency response curve, and quite detailed corrections can be implemented due to the number of separate filter bands available. It follows that a unit with more frequency bands is more flexible in this application and this is particularly true at the low frequency end where most room problems are encountered.

It is, of course, true that a badly set up EQ unit will produce a poor result and there are some purists who believe that the amount of damage that can be done using an equaliser is proportional to the number of knobs on it!

It is, however, likely that a professional control room would be set up using a pink noise source and a spectrum analyser to ensure that the equaliser settings are optimised and an equaliser, having upwards of fifteen bands would be usual for this application.

The same problems are encountered in live PA work and here the requirements are similar except that the room acoustics are generally too bad to correct completely and a compromise must be accepted, control of acoustic feedback being the first priority.

Circuit Design



The revised model is a 27 band, single channel device offering more than 12dB of cut or boost in each frequency band. Being a single channel (mono) device, two would be required for stereo applications and the ACES 15 band stereo unit may appeal to home studio users as it costs only £153 plus VAT.

The audio spectrum is covered in bands of one third of an octave and covers the range of 40Hz to 16kHz.

This is achieved by means of a bank of inductorless filters, based on the gyrator design, connected into the feedback circuit of an op-amp. Each filter band is active and is based around the TL071 op-amp which is a good compromise between quality and cost.

This type of circuitry is used in almost all budget graphic equalisers and in general, gives very acceptable results if its limitations are borne in mind.

The filters use standard resistors and capacitors and any deviation from the optimum values will show up as an error in the filter's centre frequency.

A more severe limitation imposed by this type of circuitry is that adjacent bands will interact and if one filter band is set at a drastically different level to the ones at either side, then the ideal response curve goes out of the window to be replaced by something altogether more lumpy!

In all fairness, it is not normal to set up a graphic in this way and a circuit free from these vices would be considerably more expensive.

Construction



The equaliser is built into a conventional but sturdy steel 3U high (5¼") rack unit and has heavy gauge aluminium front and rear panels. The front panel is finished in the characteristic ACES brown paintwork with white, silk screened legends, and contains all the sliders, the PPI LED, the bypass switch, and the illuminated power switch.

The sliders (which have a convenient centre detent) are mounted directly to the PCB to reduce manufacturing costs by keeping wiring to an absolute minimum, which is also likely to improve reliability. The rest of the box is mainly empty except for the small power supply.

In Use



Tested in a studio environment, the unit performed well without fuss or undue background noise, as is to be expected from this type of circuitry.

The close spacing of the filter bands enables troublesome areas to be effectively dealt with, without unduly compromising the overall sound, whilst the bypass switch permits instant pre-post EQ comparisons.

The PPI (peak programme indicator) is a sensible warning feature as over-zealous use of any equaliser can give rise to overload problems.

Conclusion



In spite of being totally unremarkable in terms of appearance or design (brown paintwork apart), the ACES 27 band graphic offers a very acceptable performance at a reasonably low cost and so is worthy of consideration if you're in the market for a new graphic equaliser.

In addition, the tough construction means that it should have a long life expectancy in the studio or on the road.

The ACES 27 band Equaliser retails for £180 plus VAT.

Contact: ACES (UK) Ltd, (Contact Details).



Previous Article in this issue

Phono Guide


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - May 1984

Donated & scanned by: Mike Gorman, Gab

Gear in this article:

Studio FX > Aces > 27-Band Graphic EQ


Gear Tags:

EQ
Graphic EQ

Review by Paul White

Previous article in this issue:

> Phono Guide

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> Amcron DC-300A Series II Pow...


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