Rebis Audio RA402 User manual

REBIS AUDIO RA402
PARAMETRIC EQUALISER
OPERATORS MANUAL
CONTENTS Page
Introduction 2
Installation 2
Earthing 3
Position 3
Operation 4
Feedback Prevention 5
Maintenance 6
Guarantee 7
Shipping 7
Specifications 8
Circuit Description 9

INTRODUCTION
The RA402 provides two four section equalisers in a standard 19"x 3.5"
(482.6mm x 88.9mm) rack mounting unit. Each section has controls for
centre frequency, bandwidth, equalisation and an In/Out switch.
The frequency controls cover the entire audio spectrum from 20Hz to
18kHz in 4.5 octave ranges, giving 2.7 octave overlap between sections.
A Q range of 0.89 to 13 (5.5dB/octave to 36dB/octave) is available on
the bandwidth control and up to 21dBs of boost or cut can be applied
with the equalisation control.
Each equaliser has an input gain control with 20dB of make up gain
and an overall in/out switch.
The peak LED will light and stay on for 100mS if any of the sections
should should detect an instantaneous overload.
INSTALLATION
Immediately the RA402 is unpacked it should be inspected for transit
damage. The unit left the factory in perfect order and any damage
should be reported to the carriers immediately.
CHECK THAT THE MAINS SELECTOR ON THE REAR PAN-
EL IS SET TO THE CORRECT MAINS VOLTAGE FOR YOUR
AREA.
Signal connections are made via the XLR type connectors on the rear
panel. Both inputs and outputs are electronically balanced and should
be wired as shown in diagramatic form on the rear panel alongside the
sockets. Signal ground and chassis ground are separate, they can be
linked inside one of the cable XLRs if required.

EARTHING
Some experimentation may be required to find the best system of earth-
ing. In many studios equipment racks are earthed via a master earth star
point and earthing the RA402 directly to mains earth via its supply con-
nector may result in an earth loop.
If this is the case it may be necessary to disconnect the mains earth in
the supply connector. However it must be emphasised that this should
only be done after ensuring that the RA402 chassis is making good elec-
trical contact with the rack and that the rack is correctly earthed.
WHATEVER EARTHING SYSTEM IS USED THE CHASSIS OF
THE RA402 MUST CONNECT TO MAINS EARTH.
If there are still any problems try disconnecting the output cable screen
wires at the RA402 end and the input cable screen wires at the system
end. In this case a separate cable should be run from 0 volts in one can-
non to the system master earth point. This should prevent any earth
loops.
It should be noted that signal 0 volt connections are floating.
POSITION
Wherever possible the RA402 should not be mounted in close proximi-
ty to excessive heat producing equipment such as power supplies and
power amplifiers.

OPERATION
How do you use a parametric equaliser? The answer could not be sim-
pler, merely listen! This might sound obvious but only a parametric
equaliser with its continuously variable control of all parameters allows
you to zero in on exactly the sound you want.
Try the unit out with programme material. Switch all but one section
out on one channel and try out the following on the section you have
left switched in. Set the bandwidth control central and apply about
12dBs of boost.
Sweeping the frequency control you can now easily select the frequen-
cy you wish to equalise. once you have found the centre frequency the
amount of boost or cut can be varied and the range of the equalisation
can be altered with the bandwidth control.
Wide bandwidth settings give conventional sounding equ alisation. Nar-
row bandwidth boost produces a ringy effect, whilst narrow bandwidth
cut removes a particular frequency without being audible in the pro-
gramme.
Phasing effects are produced when sweeping the frequency controls,
the type of effect produced depending on both the equalisation and
bandwidth control settings.
The input gain control is used to compensate for overall changes in
level due to equalisation. The overall In/Out switches bypass the chan-
nel gain controls and therefore allow either direct comparison of
straight and equalised programme or switching in of pre-set equalisa-
tion during mixdowns etc.

FEEDBACK PREVENTION
The RA402 can be utilised to prevent acoustic feedback in sound rein-
forcement systems. Set all sections for full cut with narrow bandwidth
and switch them out. Increase system gain until feedback just starts, se-
lect the section which covers the band in which the frequency of oscilla-
tion lies, switch the section in and sweep the frequency control until the
oscillation stops. Now reduce the amount of cut until the feedback starts
again and then back off a little.
This procedure is then repeated at higher system gains, the next three
feedback frequencies being notched out with three remaining sections.
With the great overlap on frequency controls even several close frequen-
cies of oscillation can be counteracted without trouble. It should be not-
ed that an audience will almost certainly alter the resonances in an
auditorium and therefore the anti-feedback technique described here
should be carried out in as near to live situation as possible.

MAINTENANCE
A service manual containing circuit diagrams, board layouts and set up
instructions is obtainable from Rebis Audio Ltd.
Only properly qualified personnel should attempt repair of the RA402.
WARNING: THIS UNIT CONTAINS PRINTED CIRCUIT
MOUNTING HARDWARE CARRYING MAINS VOLTAGES.
USE EXTREME CARE WHEN MAKING REPAIRS.
Should it prove necessary to effect repair of the electronics proceed as
follows.
To gain access to the circuit boards remove both lids. These are held
on by six self tapping screws, two in each side and two in the back, and
one M3 countersunk socket screw in the centre of the front panel.
If it is necessary to remove the lower circuit board first remove the
front panel knobs. After removing the four self tapping screws from the
sides of the circuit board it can be eased back and lowered on its flying
leads.
Note that the circuit boards are coated with a protective lacquer. If any
components are changed clean the boards and respray with a suitable
laquer.
When refitting the lower circuit board ensure that the peak LED is cor-
rectly located.

GUARANTEE
The RA402 parametric equaliser is guaranteed against defects in work-
manship or components for a period of one year from date of purchase,
this guarantee applying only to the original purchaser. Should a fault
develop within this period the unit will be repaired with no charge for
parts or labour, shipping costs being met by the owner.
Any claim under the guarantee must be accompanied by a copy of the
original customer invoice showing date of purchase and serial number.
Any misuse of or modifications to the unit will render the guarantee
void.
SHIPPING:
NEVER RETURN A UNIT WITHOUT FIRST PHONING OR
WRITING.
Should it prove necessary to return a unit it should ideally be packed
in its original carton. If this is not available place the unit in a plastic
bag and pack in a stout carton surrounded by at least 1.5" of shock ab-
sorbing material, Pelaspan polystyrene chips, bubble- pack blanket or
foam chips are suitable. Make sure the packing is firm enough to pre-
vent the unit from shifting in transit. Seal the carton with wide rein-
forced tape.
The carton should be marked:
FRAGILE. DELICATE INSTRUMENTS.

SPECIFICATION
Input Gain controls: Continuously variable up to 20dBs of boost.
Equalisation controls: Continuously variable boost or cut up to 21db.
Bandwidth controls: Continuously variable from
5.5dB/octave to 36dB/octave.
Peak Indicator: The LED will light and stay on for 100mS if the peak
positive or negative level anywhere in the system exceeds +21dBm.
Frequency controls: Continuously variable over 4.5 octave ranges;
Section 1: 20Hz to 450Hz
Section 2: 70Hz to 1.6kHz
Section 3: 250Hz to 5.6kHz
Section 4: 800Hz to 18kHz
Section overlap 2.7 octaves
Bypass: Each section has an In/Out switch.
Each channel has an In/Out switch.
Frequency Response: +1dB from 20Hz to 20kHz.
Input Impedance: 100 kilohms per leg.
Maximum Input level: +21dBm.
Output Impedance: Less than 50 ohms.
Maximum Output: Balanced; +28dBm into 600 ohms.
Unbalanced; +22dBm into 600 ohms.
Noise: EQ controls flat, frequency controls midrange, bandwidth
controls on broad. Better than -80dBm (20Hz to 20kHz).
Distortion: Below 0.05% THD @1kHz @+12dBm.
Power Requirements: 110 to 240 volts 50 to 60Hz.
Dimensions: Width; 19" rack mounting.
Height; 3.5".
Depth; 9.75" (behind front panel).

CIRCUIT DECRIPTION
The signal enters the unit via balanced input buffer IC1. The output
from IC1 connects to the 'out' side of the EQ In/Out switch and the top
of the input gain control.
The input gain control passes signal to a non- inverting 20dB gain
stage IC2 which feeds the first of four virtually identical equalisation
sections. The signal passes from the the highest frequency section
through to the lowest frequency section. The output of the last section is
connected to the 'in' side of the channel In/Out switch S4.
The parametric section functions as follows. IC6b is an active band-
pass filter with adjustable centre frequency, IC5b is an inverter which
compensates for signals passed by the filter which are well outside its
frequency range.
IC6a is an inverter which provides negative feedback, through one half
of the Q control, around IC6b.
The input to the section feeds the boost side of the EQ control and the
input of summing amplifier IC5a. the output of IC5a feeds the cut side
of the EQ control.
The outputs of IC6b and IC5b are fed via the section In/Out switch S4
to the input of IC5b where they are mixed with the input signal.
Boost is produced by mixing the input signal with the output of active
filter IC6b, which is constantly boosting by 20dB. Cut is achieved by
utilising this boost in a negative feedback loop around summing amplifi-
er IC5a.
One half of the Q control is used to increase the negative feedback
around IC6b thus broadening its response. This however also lowers
peak boost so the second half of the control increases the input level to
IC6b and IC5b, thus maintaining the amount of boost across the range
of the Q control.

Circuit Description (cont.)
R40 provides a small amount of negative feedback around IC6b to
make the peak boost the same on both high and low Q settings.
The EQ In/Out switch S3 passes signal to the input of the balanced
output driver IC3.
The peak detector IC4 is a comparator which monitors the output volt-
ages of IC1, IC2 and IC5a in each parametric section. It does this via
pairs of diodes from the output of each IC.
If the output voltage on any IC comes within 1.5 volts of either supply
rail the peak LED L1 will light and, due to hysteresis provided by C18,
stay on for about 100mS.