LA Audio Electronic SPX20 User manual

STEREO SOURCE SELECTOR/PREAMPLIFIER
OPERATION MANUAL
Version - 1 April 2000

Page 2
CONTENTS
Page
1. Introduction 3
2. Installation 4
2.1 Inspection and un-packing 4
2.2 Operating environment 4
2.3 CE standards 4
2.4 Power requirements 4
2.5 Signal levels 5
2.6 External connections 6-7
3. Warranty 8
4. Description of controls 9-10
4.1 Inputs section 9
4.2 Outputs section 10
4.3 Headphones section 10
5. Applications 11
5.1 Using the SPX20
6. Circuit description 13
7. Specifications 14

Page 3
1.0 INTRODUCTION
The SPX20 is a six input stereo source selector designed to extend the
number of inputs and range of monitoring facilities available on a typical
mixing console. However the SPX20 is suitable for a wide range of
monitoring applications including tape dubbing, audio-visual and digital
workstations and for installation in background music applications.
Each of the inputs on the SPX20 has a level trim with sufficient range to
cope with +4dBu and -10dBV equipment and for balancing the levels
between the different sources. Inputs are selected by momentary action
buttons which also allow more than one input to be selected at a time.
Multiple inputs are mixed (summed).
Two separate outputs with level controls and on-off switching provide a
means of selecting and balancing levels between two types of monitor
speakers or alternatively to add a 'monitor mix to studio' facility to your
mixer. The outputs have relay switching which is delayed at power on to
prevent annoying and potentially dangerous thumps and clicks reaching
your monitor speakers. The output section also has an overall Balance
control, Mono and Dim switching.
The SPX20 also has a built-in headphone amplifier with separate level
control and front panel accessible connector.
Main features of the SPX20 include -
• Six stereo inputs with level adjustment
• Momentary action input select with led indicators
• Front panel accessible insert and level adjust for Input 1
• Mixing of multiple input selects
• Balanced and unbalanced inputs on XLR, 1/4" TRS jack and phono/RCA
connectors
• Output Balance, Mono and Dim
• Two separate outputs with level control and on-off switching
• Power on-off output relays
• Tape record output
• Headphone output with level control

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2.0 INSTALLATION
IMPORTANT: PLEASE READ THIS SECTION BEFORE USING THE
SPX20
2.1 INSPECTION AND UNPACKING
The SPX20 has been carefully packed at our factory in a carton designed
to withstand handling in transit. Should the unit appear to have been
damaged in transit notify your dealer immediately and do not discard any
of the packing. The carton should contain -
• The SPX20 unit
• Power cord
• Operator Manual (this book)
2.2 OPERATING ENVIRONMENT
The SPX20 is designed to operate between 0 and 50°C (32-122°F) with
relative humidity no more than 80%. Should the unit be installed in an
equipment rack, ensure that the ambient temperature conforms to these
levels.
2.3 CE STANDARDS AND THE LOW VOLTAGE DIRECTIVE (LVD)
The SPX20 has been designed to meet the latest Electromagnetic
Compatibility (EMC) regulations. However we recommend you do not
operate the unit close to strong emitters of electromagnetic radiation such
as power transformers, motors, mobile telephones or radio transmitters.
The unit should only be connected to a power supply of the type described
in 2.4 POWER REQUIREMENTS or as marked on the unit. The unit must
be earthed for safe operation. Disconnect the mains supply before
removing any cover.
2.4 POWER REQUIREMENTS
There is a mains fuse located inside the unit which is accessed by
removing the cover. This fuse is for protection against unit faults and as
such will not normally need replacing. If you think you have a power
related problem please refer to your dealer.
The voltage and fuse rating have been factory set for -
230V 50mA UK
220V 50mA Republic of Korea
115V 100mANorth America
• The correct voltage and fuse rating is indicated on the rear panel.

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• Please check that your unit is correctly rated for the voltage in
the country of operation.
If the fuse requires changing at any time please ensure the correct fuse is
fitted. An incorrect fuse could cause damage to the unit and may constitute
a fire hazard.
If you carry your SPX20 from one country to another it is very important to
ensure the voltage is compatible.
For your information the unit can be internally rewired for use in 230V, or
115V and 100V countries - contact your dealer for information. A special
220V, 60Hz version is available for the Republic of Korea.
The mains lead connections to the appliance are coloured in accordance
with the following code:
Green-and-Yellow Earth
Blue Neutral
Brown Live
• WARNING: THIS APPLIANCE MUST BE EARTHED
2.5 SIGNAL LEVELS
For input level ranges see 'TRIM' control in the Description of Controls
section. The TRIM range allows the SPX20 to work with +4dBu and -
10dBu equipment.
IMPORTANT NOTE: To ensure the best performance from the SPX20 it is
important to adjust the input TRIM controls to give the correct operating
levels within the unit.
Signals in excess of +20dB will cause the unit to distort.
2.6 EXTERNAL CONNECTIONS
R
654 3 2 1
R
L
R
LLRLR
INPUTS L
XLRs
1 = GND
2 = Hot
3 = Cold
Jacks
T = Hot
R = Cold
S = GND
INPUTS section:
INPUT 1
This input is balanced on XLR connectors and is wired as follows -

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Pin 1 = ScreenPin 2 = Hot (+)
Pin 3 = Cold (-)
PUSH
Fig 2.6.3: Pin arrangement for XLR inputs on the SPX20
Pin 1 Screen Signal ground
Pin 2 Hot Signal +
Pin 3 Cold Signal -
For unbalanced operation join pins 1 and 3 and connect to Screen (signal
ground) and use pin 2 as Hot (signal +).
Input 1 also has a pair of front panel inserts on TRS jacks which are wired
as follows -
Sleeve
Screen
Ring
Cold (-)
Tip
Hot (+)
Fig 2.6.3: Pin arrangement TRS jack inserts on Input 1
Tip Hot Signal +
Ring Cold Signal -
Sleeve Screen Signal ground
For unbalanced operation join Ring and sleeve and connect to screen
(signal ground) and use Tip as Hot (signal +).
Plugging a mono jack into these inserts effectively unbalances the
connection.
Sleeve
Screen
Tip
Hot (+)
Fig 2.6.4: Mono jack pin arrangement
Tip Hot (signal +)
Sleeve Screen (signal ground)

Page 7
Plugging into the inserts jacks on the front panel breaks the signal from the
rear panel XLRs
INPUT 2 & 3
These are balanced inputs on TRS jacks - see Fig 2.6.3
For ubalanced operation wire as described above for Input 1 inserts or use
a mono jack plug.
INPUT 4,5 & 6
These are all unbalanced inputs which have RCA/phono connectors.
These are wired with the centre pin as hot (signal +) and the sleeve as
screen (signal ground).
TAPE RECORD
This is line level output of the selected input or inputs. Output level
depends on the settings of the input TRIMs and on how many inputs are
selected at the same time. If the input TRIMs are set up as described in
the 'Description of Controls' sections then the level will be approximately
0dBu.
OUTPUTS section:
SERIAL No:
230V 50Hz
FUSE T50mA
Manufactured in the UK by
LAAudio
No user serviceable parts inside
Disconnect from mains before removing cover
This equipment must be earthed R
OUTPUTS RECORD OUT
12
R
L
LRL
~
OUTPUT 1 & 2
Outputs on the SPX20 are via 1/4" TRS jacks - see Fig 2.6.3
Tip Hot Signal +
Ring Cold Signal -
Sleeve Screen Signal ground
Output amplifiers on the SPX20 are 'ground compensated balanced
impedance' type.

Page 8
3.0 WARRANTY
Your LA Audio SPX20 has been manufactured to a high standard using
quality components. If correctly installed and operated the unit should give
years of problem free operation.
However in the event of a defect in material or workmanship causing
failure of the unit within one year of the date of original purchase wewill
agree to repair, or at our discretion replace, any defective item without
charge for labour or parts. To receive service under this warranty it is
necessary to return the unit to an LA Audio authorised service centre or to
the factory with a dated receipt as proof of purchase. After repair the unit
will be returned to you free of charge.
Limitations:
This warranty does not cover damage resulting from accident or misuse.
The warranty is void unless repairs are carried out by an authorised
service centre. The warranty is void if the unit has been modified other
than at the manufacturers instruction. The warranty does not cover
components which have a limited life, and which are expected to be
periodically replaced for optimal performance. We do not warrant that the
unit shall operate in any way other than as described in this manual.

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4.0 DESCRIPTION OF CONTROLS
4.1 INPUTS SECTION
INPUT 1
Left & Right sockets
These are insert points for Input 1. Plugging into these connectors breaks the
signal from Input 1's rear panel XLRs. They are intended to allow a quick and easy
way of temporarily connecting an external piece of equipment to the SPX20.
TRIM control
The TRIM control for Input 1 has a range of ±10dB which should provide adequate
adjustment for levels provided by pro, semi-pro and commercial equipment. The
centre position (12 o'clock) provides unity gain (0dB) and has a mechanical detent
for easy positioning.
1button
Pressing this switch selects Input 1 as the signal source. It is fed to the TAPE
RECORD output, on the rear panel, OUTPUT 1 & 2 and the Headphone amplifier.
The associated led lights to show Input 1 selected.
2, 3, 4, 5, 6 button
These select Inputs 2 through 6 as the Input source. Each select button has an
associated led.
To select more than one input at the same time press and hold the first input and
whilst holding this button press another button. It will be necessary to hold down all
the inputs you want selected as each time a button is pressed it tries to cancel all
other selections. Release all buttons.
TRIM control
Each input TRIM provides a gain range of -6dB (attenuation) at counter-clockwise
(CCW) to +10dB clockwise (CW).
Setting up the SPX20
To ensure maximum headroom, noise and crosstalk performance it is important to
have the correct signal levels within the SPX20. Please follow the following simple
procedure.
When using equipment with a +4dBu output turn TRIM to almost CW and to fully
CCW for -10dBV equipment as a starting point. Connect one of the outputs to your
monitoring system and set LEVEL 1 or 2 control to give a suitable listening level.
Then select each input in turn and adjust TRIM to give the same listening levels for
each input. It should not be necessary to turn TRIM far from these settings.

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4.2 OUTPUT SECTION
BALANCE control
This control allows adjustment of the stereo centre position. The centre detent
position gives equal left and right levels.
Turning BALANCE fully CCW cancels all right output signals and vice versa.
BALANCE affects OUTPUT 1 & 2 and Headphones.
DIM switch
Pressing this switch in reduces output levels by 20dB.
DIM affects OUTPUT 1 & 2 and Headphones.
MONO switch
Pressing the MONO switch sums left and right signals.
MONO affects OUTPUT 1 & 2 and Headphones.
LEVEL 1 control
This control adjusts the level of OUTPUT 1. Fully CCW is off and fully CW is
+10dB. Unity gain (0dB) is around '7'.
ON switch + led
Switches OUTPUT 1 on and off.
LEVEL 2 control
Similar to LEVEL 1
ON switch + led
Similar to OUTPUT 1.
4.3 HEADPHONES SECTION
VOLUME control
Controls the headphones signal level. Fully CCW is off.
The headphone amplifier on the SPX20 is designed to drive headphones with an
impedance from 8Ωto 400Ω.
POWER indicator
The green POWER indicator is lit whenever power is applied to the unit.

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5.0 APPLICATIONS
The majority of mixing consoles, even quite expensive models offer only 2
or possibly 3 monitoring inputs. So if you want to simultaneously connect a
CD player, DAT machine, stereo cassette deck, 2 track analogue tape
machine etc. you will have to use up input channels. These are often
already used for tape returns or for FX so some repatching and gain
adjustment will necessary in order to replay these sources.
The SPX20 has been designed to extend the number of monitor inputs
and to offer some extra facilities not found on budget priced consoles. The
SPX20 is intended to be used in one of two ways -
5.1 AS THE MAIN MONITOR CONTROLLER
In this application the SPX20 is used as the control centre for selecting
sources and controlling levels in the monitors. Although this requires the
SPX20 to mounted in a readily accessible place it does offer a number of
advantages -
The two separate outputs with on-off switching and level controls can
be used to drive different types of monitor speakers.
Alternatively one output can be used for the control room monitors and
the other provide a studio room feed.
The monitoring system benefits from the added protection of the
delayed power-on and rapid power-off switching included in the SPX20.
A number of playback sources with different output levels can be
balanced to provide a consistent listening level across all the inputs.
The TAPE RECORD output allows a stereo recorder to be connected to
record a rough mix whilst laying down tracks.
Mixing console
MONITOR
OUTPUT CD
SPX20 Stereo source selector
DAT
Cassette
2 track
Power amp
Power amp
INPUTS RECORD OUTPUTS

Page 12
Fig 5.1.1 Connecting the SPX20 as the main monitoring controller
5.2 AS A MONITOR EXTENDER
If you prefer to keep control of the control room monitors within the mixing
console the SPX20 can be connected as shown in Fig 5.1.2. The SPX20 is
now selected by pressing the 2T (2 track) return on the monitor section of
the mixing console.
Mixing console
MONITOR
OUTPUT
CD
SPX20 Stereo source selector
DAT
Cassette
2 track
Power amp
INPUTS OUTPUT
2T INPUT
Fig 5.2.1 Connecting the SPX20 as a monitor extender

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6.0 CIRCUIT DESCRIPTION
TRIM
Left
Right
Select #1
LR
MIX
•
•
•
Mix amp
Tape Send L
•
Mix amp
Tape Send R
••
BALANCE
•
•
MONO DIM
LR
•
•
•
Left
On-Off #1
•
LEVEL 1
Right
Left
On-Off #2LEVEL 2
Right
HEADPHONES
VOLUME
•
•
TRIM
Right
Select #2
•
•
Left
TRIM
Right
Select #4
•
•
Left
•
•
••
INPUT 1
INPUTS 2 & 3
INPUTS 4, 5 & 6
•
Select
#1
•
Select
#2
•
Select
#3
•
Select
#4
•
Select
#5
•
Select
#6
1
2
3
4
5
6
Input
Select
Power
on-off
Delay
On-Off #1 On-Off #2
•
OUTPUT 1 ON-OFF OUTPUT 2 ON-OFF
~
OUTPUT 2
OUTPUT 1
Fig 6.1.1 SPX20 block diagram showing main signal paths
INPUT 1
The input is routed through a pair of half normalled TRS jacks and into a
differential amplifier. The TRIM varies the gain of the differential amplifier so
ensuring maximum headroom at all gain settings. The output of this amplifier is fed
via CMOS switches onto the mix buss.
INPUT 2 & 3
Inputs 2 and 3 are similar to Input 1 without the insert jacks.
INPUT 4, 5 & 6
These inputs are similar to Inputs 1-3 but are unbalanced.
The CMOS switches are controlled by the input select logic block. This consists of
six set-reset latches. Pressing a select button sets its latch and sends a reset
signal to all the other latches.
Signals on the mix buss are summed by two virtual earth amplifiers. These feed
the TAPE RECORD output. The mixed signal also feeds the BALANCE pot,
MONO and DIM switches.
OUTPUT 1 & 2 are relay switched. These relays are controlled by the OUTPUT on-
off switches and the power on-off circuitry. The output amplifiers are configured as
ground compensated, balanced impedance.
All inputs and outputs have RFI filtering.

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7.0 SPECIFICATIONS
INPUTS
1 electronically balanced, 20kΩ, XLR-F, pin 2 = hot
2 & 3 electronically balanced, 20kΩ, 1/4" TRS, Tip = hot
4, 5 & 6 unbalanced, 20kΩ, RCA/Phono, Centre pin = hot
OUTPUTS
TAPE RECORD 100R, RCA/Phono, Centre pin = hot
OUTPUT 1 & 2
Type Ground sensing, balanced impedance
1/4" TRS jack 0dBu (nominal), wired Tip = hot
Impedance <100Ω
Max. output +20dBu
PERFORMANCE
THD+N < 0.015%
Noise <-90dBu, 20Hz to 20kHz
Frequency response +0, -0.5dB, 20Hz to 20kHz
Crosstalk <-70dBu @ 10kHz
CONTROLS
TRIM 1 ±10dB
TRIM 2 - 6 +10dB, -6dB
DIM -20dB
LEVEL 1 & 2 -∞to +10dB
POWER SUPPLY
Mains voltages 230Vac, 50Hz, 6VA Europe
115Vac, 60Hz, 6VA North America
220Vac, 60Hz, 6VA ROK
Mains connector Standard IEC receptacle
MECHANICAL
Dimensions 1.75" (44mm)H x 19" (482mm)W x 6.5" (165mm)D
Weight Unit = 2.2kg, Shipping = 2.85kg
LA Audio reserves the right to alter any feature or specification without prior notice.
E&OE