Dual 31 Band
Graphic Equaliser &
Dynamics Processor
User Manual
User Manual
Copyright 2004. All rights reserved.
The information in this document is subject to change without notice.
Audient plc makes no warranty of any kind with respect to the material in this
document and shall not be liable for errors contained herein or for incidental or
consequential damages related to the use of the material.
No part of this document may be reproduced without the prior written consent of
Audient plc.
© LA Audio 2004
User Manual
Thank you
Thank you for buying this
As can be found throughout the LA Audio range, your EQ231G-SP has been designed
and manufactured to the highest possible standards of performance and reliability. To
make the most of your unit, we suggest that you take some time to acquaint yourself
with the contents of this manual before you get started.
The safety and installation information will help you set up your EQ231G-SP with the
minimum of effort, while the operational guide describes the unit’s controls and
functions. The applications section gives useful hints on how to get the most out of
your unit in numerous situations, along with a couple of special effects with which you
can experiment.
© LA Audio 2004
User Manual
Safety Information
1. Introduction
2. Installation
3. Operating Instructions
4. Applications & Techniques
5. Block Diagram & Specifications
6. Repairs and Warranty
© LA Audio 2004
User Manual
© LA Audio 2004
User Manual
Safety Information
Safety Information
This unit is connected via its power cord to the mains safety earth.
DO NOT remove the covers. Refer servicing to qualified personnel
The EQ231G-SP is set to operate only at the voltage shown on the
outer packaging. CHECK that the correct voltage is available before
connecting the AC mains supply.
CHECK that the fuse fitted is of the correct type for the local mains
voltage. ALWAYS replace fuse with the correct type.
DO NOT expose the unit to rain or moisture. If your EQ231G-SP
should become so exposed REMOVE the mains power immediately.
ALWAYS site the unit away from sources of heat including direct
sunlight and ensure adequate ventilation around the unit.
© LA Audio 2004
User Manual
1. Introduction
1. Introduction
The EQ231G-SP is a high quality dual 31-band 1/3 octave graphic equaliser including
variable Hi and Lo-pass filters, Shelving EQ, Gate and Limiter.
The Graphic Equaliser features centre detented, 45mm faders which give up to 12dB
of cut and boost at 31 ISO centre frequencies, covering the entire audio spectrum (20
to 20KHz).
The Hi-pass filter uses 4th order bi-quadratic topology giving a 24 dB/octave slope with
corner frequency variable from 16 Hz to 160 Hz.
The Lo-pass filter is a 2nd order design (12dB/octave) with the corner frequency
variable from 3KHz to 32KHz.
The Shelving Equaliser faders enable control of the amount of cut or boost, a centre
detent is provided at 0dB. Two rotary potentiometers enable control of the LF and HF
shelf frequency.
A bypass switch allows the whole section to be taken out of circuit.
The Dynamics section uses a fast RMS detector with soft-knee action.
The Limiter has a 3 segment LED gain reduction display.
The Gate has an LED indicator illuminating when the signal is below threshold.
A delayed by-pass relay allows silent on/off switching and also provides direct
throughput when the unit is not powered.
Inputs and outputs are electronically balanced. Each channel has a Peak Indicator
illuminating for an equivalent output signal of +18dBu.
The unit can be augmented by parallel outputs offering a unique transformer distortion
cancellation circuit. This reduces distortion by an order of magnitude compared to
conventional designs and also provides galvanic isolation from connected devices for
the reduction of interference and hum.
© LA Audio 2004
User Manual
2. Installation
2. Installation
It is important to read this section before applying power to the unit.
2.1. Inspection and unpacking
The EQ231G-SP is carefully packed at the factory in a carton which is designed to
withstand handling in transit.
However if transit damage is evident, DO NOT discard any of the packaging material
and notify the carrier and your dealer immediately.
The shipment should contain the following:
The Unit, EQ231G-SP Graphic Equaliser
Power cord
Technical Manual (this book)
If any of the above items are missing, notify your supplier immediately.
2.2. Operating Environment
The unit is designed to operate between 0û and +50ûC (32û - 122û F) in an atmosphere
of relative humidity up to 80%.
Should the unit be installed in an equipment rack, it is important to ensure that the
operating temperature inside the rack does not exceed the upper limit. This may be the
case where the rack contains power amplifiers and a cooling fan may be necessary in
such installations.
Although designed to withstand hum pick-up, it is VERY important that the unit is not
installed close to any strong electro-magnetic fields, such as power amplifiers.
2.3. Power Requirements
The voltage is pre-selected at the factory for the country of destination.
Fuse rating
(Note: Fuse must be Slow – Blow T-Type)
115-125 Volts AC 50/60Hz
200-250 Volts AC 50/60Hz
(A 100 Volt special transformer is available for Japan).
© LA Audio 2004
User Manual
2. Installation (contd.)
2.4. External connections
The input and output connectors are wired on female and male XLRs respectively,
wired as follows:
Pin 1: Ground
Pin 2: Hot balanced signal
Pin 3: Cold balanced signal
2.5. Impedance & Termination
Most modern audio systems have cable runs of less than 100 meters; the devices
used usually have very low output impedance and relatively high input impedance.
This allows equipment to be connected in series without regard to "impedance
matching" (as long as the succeeding input is greater than the stated minimum for the
output it is connected to).
LA Audio products conform to this standard; therefore inputs and outputs are usually
indifferent to varying load or source impedance. The only exception to this rule is
where the unit is connected to a transmission line or some older vacuum-tube
equipment, which require proper impedance matching. This may be achieved by
shunting the correct matching resistor across the inputs or outputs. The EQ231G-SP
has an input impedance of approximately 20kΩ and an output source impedance of
100Ω, (20Ω with the transformer option fitted). Its minimum following stage input
impedance should equal or exceed 600Ω.
© LA Audio 2004
User Manual
2. Installation (contd.)
2.6. Signal levels
The unit is designed to accept an input signal level of up to +20dBu (dB referred to
0.775Vrms). Above this, clipping (distortion) of the signal will occur.
It should be noted that the filter amplifiers may be driven into distortion by excessive
boost in the unit, and where this is the case the input signal should be reduced
sufficiently to allow enough headroom.
© LA Audio 2004
User Manual
3. Operating Instructions
3. Operating Instructions
3.1. Controls
1. EQ faders allow high-resolution control over EQ gain at 31 ISO frequencies.
The frequency is indicated on the front panel, as are the amounts of cut and
boost, between –12 and +12 dB. Each fader has a centre detent, which allows
you to quickly return it to its inactive (0dB) position.
2. Graphics ON switch allows the graphic filters to be taken out of circuit for
comparison checks. The adjacent LED illuminates when the filters are IN circuit
(i.e. the graphic is functioning).
3. Gain control adjusts gain at the input stage; ranging from -∞ to +6dB. The
centre detent allows the unit to quickly be set to unity gain (0dB).
4. Peak LED. This illuminates when the audio signal is within 2dB of clipping
(+18dBu) at the output. When this light is on, there is a good chance that boost
applied in the equaliser’s filters will cause clipping. To combat this, reduce the
input gain or the source’s output level.
© LA Audio 2004
User Manual
3. Operating Instructions (contd.)
5. Shelving Filter Bypass switch allows the filter to be taken out of circuit. This
allows you to hear its effects when compared to the unfiltered sound, and also
to allow low frequencies to pass unaffected.
6. Shelving Filter Frequency rotary potentiometers enable control of the LF and
HF shelf frequency.
7. Shelving EQ section faders enable control of the amount of cut or boost, a
centre detent is provided at 0dB.
8. The High-pass filter is a 4th order design (24dB/octave) with the corner
frequency variable from 16 Hz to 160 Hz. The Lo-pass filter is a 2nd order
design (12dB/octave) with the corner frequency variable from 3KHz to 32KHz.
9. The Dynamics Bypass switch allows the whole section to be taken out of
10. The Dynamics section features a Limiter and a Gate. The Gate Threshold
potentiometer controls the level of signal necessary to turn on the gate. Below
this level, the signal is attenuated in such a way that unwanted noises such as
hiss and hum are reduced. The Gate LED illuminates when the signal is below
threshold, indicating the gate is "closed".
11. The Limiter Threshold potentiometer controls the level above which gain is
reduced in order to keep the level constant, thus avoiding damage to
succeeding equipment. A 3 LED progressive luminosity bargraph indicates the
gain reduction or "amount of limiting". The Threshold ranges of the Gate and
Limiter are such that in the extreme positions (Gate at -70dB, Limiter at +20)
they can be considered out of circuit.
12. The Phase Reverse switch reverses the connections of the output XLR
connectors. This passive arrangement guarantees that, even when the unit is
off, the phase condition is retained. A red LED illuminates to indicate the
Reverse condition.
© LA Audio 2004
User Manual
3. Operating Instructions (contd.)
© LA Audio 2004
User Manual
4. Applications & Techniques
4. Applications & Techniques
A graphic equaliser may be used in numerous situations, in many different ways. It is
impossible to document every implementation, but we hope that the following
examples will serve as a useful starting point.
4.1. Some Examples
a) Room equalisation
Every room, whether it be in a studio, theatre or home has a different acoustic
response, depending upon room dimensions, fixtures and fittings and any other
absorptive or reflective materials such as people, tables or mixing consoles. A graphic
equaliser may be used to "correct" the room, i.e. tailor the sound being reproduced so
that the room appears to have an even or "flat" response throughout the audio
frequency range, thus providing a correct reproduction of the original sound, improved
intelligibility, or helping to achieve loud volumes without an abrasive tonal character.
There are several ways of measuring a room's frequency response, and these are
usually all similar in technique, although differing in hardware. Using a Real-time
analyser, inconsistencies in the system’s frequency response can be detected and
corrected with the graphic EQ. Alternatively, adjustments can be made by ear in
response to trial system output, such as CD playback. It is important to remember that
the ear responds differently to the frequency spectrum at different volumes, so
adjustments made by ear should be done at or around the intended operational
While the graphic equaliser is a powerful tool, it is worth noting that speaker
positioning and balancing along with room adjustments such as using drapes will often
prove to be more effective solutions to balance issues than heavy-handed equalisation.
The graphic EQ is usually best for fine-tuning system response.
b) Sound-shaping
The graphic equaliser may be used to change an original sound to make it more
pleasing to the ear or to blend it in with other audio signals without overlap. This
© LA Audio 2004
User Manual
4. Applications & Techniques (contd.)
technique is quite common in recording or production studios to tailor a sound for its
eventual reproduction medium.
c) Elimination of "sibilance"
Sibilance is the noise which occurs when someone is speaking who either has a
"hissy" voice or who is using words with the letter ‘s’ in them, e.g. ‘sibilance’. This can
become annoying to a listener and the graphic equaliser can be used to overcome this
problem by either removing or reducing the offending frequency.
Graphic equalisers can also be used in conjunction with compressors to create a ‘deesser’, which can respond to ‘s’ sounds in a signal and reduce the level accordingly. To
create this effect, insert the graphic EQ into the compressor’s sidechain and boost the
frequencies in which the sibilance occurs. While this seems counterintuitive, boosting
increases the compressor’s sensitivity to these frequencies, meaning that when they
are most present, the compressor will reduce the signal level, thereby stopping the
sibilance from reaching its full volume. Because the EQ is inserted into the
compressor’s sidechain and not the signal path, the EQ boost will be inaudible at the
compressor’s output.
d) Elimination of "proximity effect"
Cardioid microphones usually have a low frequency emphasis, which in certain
situations can present problems. For example, speech over a public address system
can sound muffled and lose intelligibility if the microphone’s response is particularly
bassy. The graphic equaliser can eliminate this by reducing or removing the unwanted
e) Sound effects
By using the graphic equaliser creatively a sound may be completely changed and
effects obtained. The most common sound effect created in this way is the "telephone
voice" which can make even an expensive studio microphone sound like a telephone
handset. To create this effect, first set the hi-pass filter to its highest frequency
(160Hz). Then cut completely frequencies between 20 and 315Hz and those between
4 and 20kHz. The remaining band is roughly equivalent to the bandwidth of a phone
© LA Audio 2004
User Manual
4. Applications & Techniques (contd.)
Another useful effect that can be created with a graphic EQ is a ‘Comb Over’. To give
a mono source some interest in a stereo mix, the graphic equaliser can be used to
create variances in each side of the stereo signal, thereby widening the sound. Route
the mono signal to both channels of the EQ231G-SP and on each channel cut
alternate frequencies by around 6dB. With certain source material, you may need to
cut less at low frequencies to avoid an imbalance in the stereo image.
f) Reduction of feedback
In many auditoriums, a graphic equaliser may be used to increase the gain before
feedback (GBF). Essentially, this involves cutting the frequencies associated with
feedback to allow the system to be safely turned up.
The usual method is to set the graphic equaliser to a "flat" position, (i.e. all sliders at
0dB) and to increase the overall level in the auditorium, until feedback commences.
Then reducing the level to just under the feedback point, each frequency band in turn
is boosted until the feedback frequencies have been identified. Some practice will
enable you to identify frequencies by ear to speed this adjustment. By notching the
offending bands down slightly, the overall level may be increased.
This may be
repeated until the overall gain reaches the required level.
It is worthwhile to note that if boosted sufficiently, all frequencies will eventually feed
back, so a balance should be struck between gain and tonal integrity, as extreme EQ
adjustments can have a detrimental effect on audio quality. Moreover, the graphic
equaliser is not the only tool at an engineer’s disposal for improving GBF: good
microphone and speaker placement will often reduce the severity of EQ required to
achieve sufficient gain.
g) Graphic equalisers and dynamics
In most instances, it is good practice to place graphic equalisers before any dynamics
processors such as compressors or limiters. This is for two main reasons:
As the graphic equaliser is essentially a set of volume controls (albeit frequency
specific ones) and dynamics units respond to volume, it is best to cut or boost
frequencies as desired before applying compression. This ensures that the
compressor or limiter only acts on frequencies that will be present at the output. This is
particularly important with low frequencies, which are more likely to trigger the
© LA Audio 2004
User Manual
4. Applications & Techniques (contd.)
compressor or limiter. Placing the graphic equaliser after dynamics could cause the
compressor or limiter to react to frequencies not audible in the output, creating unusual
undulations in volume not associated with the program material.
The second reason for placing the graphic equaliser before dynamics occurs where
the dynamics processor is being used to safeguard PA systems or digital recorders. As
the equaliser can create changes in volume (both with EQ and input gain), placing it
after a limiter or compressor, may cause increases in gain beyond safe limits.
4.2. A word of caution
Sadly, graphic EQs can’t cure every problem you’re likely to encounter as an engineer.
Not only that, but it’s highly unlikely that you’ll ever get a truly flat room response
without calling in some builders. Consequently, a compromise must be struck between
technical perfection, sonic quality and, very often, the time taken to reach such a
There are three things worth remembering when using your graphic EQ: Avoid
extreme settings unless for an effect; they will most likely sound unnatural and may
exacerbate balance problems. Boost tends to sound more obvious to the ear, so cut
troublesome frequencies rather than boosting nice ones. And finally, remember that
while the legend and calibration markings on your EQ231G-SP are a useful guide, they
are just that, a guide so be sure to trust your ears.
© LA Audio 2004
User Manual
5. Block Diagram & Specifications
5. Block Diagram & Specifications
5.1 Block Diagram
© LA Audio 2004
User Manual
5. Block Diagram & Specifications (contd.)
5.2 Specifications
Frequency Response
20 Hz - 20 kHz, -1dB
Signal to Noise Ratio @ +4dBu output
Input impedance
20 kΩ balanced
Output impedance
100Ω balanced
Maximum Output Level
(into 600Ω Load, 1 kHz, 600 Ohms Load)
Total Harmonic Distortion
0.003 %
(1kHz, +4dBu Output)
Peak indicator Threshold
2dB before clipping
Centre frequencies :
20, 25, 31.5, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1k,
1k25, 1k6, 2k, 2k5, 3k15, 4k, 5k, 6k3, 8k, 10k, 12k5, 16k, 20k.
Limiter Threshold
-6 to +20dBu
Gate Threshold
-70 to -20dBu
Limiter Ratio
20:1 soft-knee
Gate Ratio
10:1 soft-knee
Hi-Pass Filter corner frequency
16Hz to 160Hz
Hi-Pass Filter slope
24dB/Octave Butterworth
Lo-Pass Filter corner frequency
3KHz to 32KHz
Lo-Pass Filter slope
12dB/Octave Butterworth
LF Shelving EQ Frequency
16Hz to 160Hz
HF Shelving EQ Frequency
2.5KHz to 25KHz
Power Consumption
15 Volt-Amps
(115/220 VAC, 50-60 Hz)
7.2 kg
Dimensions (mm)
483 W x 230 D x 135 H
Note: 0dBu is referenced to 0.775 V RMS.
Product information and specifications are subject to change without notice. E&OE.
© LA Audio 2004
User Manual
6. Repairs and warranty
6. Repairs and warranty
The EQ231G-SP is a ruggedly constructed solid-state unit that requires no
preventative or routine maintenance.
The unit is constructed from the highest quality components, which are selected to
provide consistent, long-lasting performance. Each unit is ‘soak-tested’ before
despatch – sifting out any rogue components before they leave the factory. If correctly
installed and operated the unit should give years of problem free operation.
Your EQ231G-SP comes with a manufacturers warranty for one year from the date of
despatch to the end user.
The warranty covers faults due to defective materials used in manufacture and faulty
workmanship only.
During this warranty period LA Audio will repair or at its discretion replace any faulty
unit provided it is returned carriage paid to an authorised LA Audio service centre.
We will not provide warranty repair if in our opinion the fault has resulted from
unauthorised modification, misuse, negligence, act of God or accident.
We accept liability to repair or replace your EQ231G-SP as described above. We do
not accept any additional liability.
This warranty does not affect any legal rights you may have against the person who
supplied this product – it is additional to those rights.
© LA Audio 2004