LA Audio Electronic Stereo Amplifier SPX20 User Manual

STEREO SOURCE SELECTOR/PREAMPLIFIER  
OPERATION MANUAL  
Version - 1  
April 2000  
 
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  
Page 3  
 
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  
220V  
115V  
50mA  
50mA  
100mANorth America  
UK  
Republic of Korea  
• The correct voltage and fuse rating is indicated on the rear panel.  
Page 4  
 
• 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  
Blue  
Earth  
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  
Jacks  
XLRs  
INPUTS  
6
5
4
3
2
1
T = Hot  
R = Cold  
S = GND  
1 = GND  
2 = Hot  
3 = Cold  
R
L
L
L
R
L
R
L
R
R
INPUTS section:  
INPUT 1  
This input is balanced on XLR connectors and is wired as follows -  
Page 5  
 
PUSH  
Pin 2 = Hot (+)  
Pin 1 = Screen  
Pin 3 = Cold (-)  
Fig 2.6.3: Pin arrangement for XLR inputs on the SPX20  
Pin 1  
Pin 2  
Pin 3  
Screen  
Hot  
Cold  
Signal ground  
Signal +  
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 (-) Hot (+)  
Tip  
Fig 2.6.3: Pin arrangement TRS jack inserts on Input 1  
Tip  
Ring  
Hot  
Cold  
Signal +  
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 6  
 
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:  
No user serviceable parts inside  
Disconnect from mains before removing cover  
This equipment must be earthed  
RECORD OUT  
OUTPUTS  
2
1
L
R
L
R
L
Manufactured in the UK by  
LAAudio  
R
230V  
FUSE T50mA  
50Hz  
~
SERIAL No:  
OUTPUT 1 & 2  
Outputs on the SPX20 are via 1/4" TRS jacks - see Fig 2.6.3  
Tip  
Ring  
Hot  
Cold  
Signal +  
Signal -  
Sleeve Screen  
Signal ground  
Output amplifiers on the SPX20 are 'ground compensated balanced  
impedance' type.  
Page 7  
 
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 we will  
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.  
Page 8  
 
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.  
1 button  
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.  
Page 9  
 
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 8to 400.  
POWER indicator  
The green POWER indicator is lit whenever power is applied to the unit.  
Page 10  
 
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.  
MONITOR  
CD  
OUTPUT  
DAT  
Mixing console  
Cassette  
2 track  
Power amp  
Power amp  
INPUTS  
RECORD OUTPUTS  
SPX20 Stereo source selector  
Page 11  
 
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.  
CD  
MONITOR  
OUTPUT  
2T INPUT  
DAT  
Mixing console  
Cassette  
2 track  
Power amp  
OUTPUT  
INPUTS  
SPX20 Stereo source selector  
Fig 5.2.1 Connecting the SPX20 as a monitor extender  
Page 12  
 
6.0 CIRCUIT DESCRIPTION  
Left  
MIX  
L
R
Tape Send L  
Mix amp  
L
R
Left  
MONO  
DIM  
OUTPUT 1  
INPUT 1  
Right  
On-Off #1  
BALANCE  
LEVEL 1  
Tape Send R  
Mix amp  
Right  
Select #1  
TRIM  
Left  
1
2
Left  
Select  
#1  
OUTPUT 2  
Right  
LEVEL 2  
On-Off #2  
Select  
#2  
TRIM  
Select #2  
INPUTS 2 & 3  
Right  
3
4
Select  
#3  
Input  
Select  
Select  
#4  
Left  
5
VOLUME  
Select  
#5  
6
Select  
#6  
TRIM  
Select #4  
INPUTS 4, 5 & 6  
Right  
HEADPHONES  
On-Off #1  
On-Off #2  
Power  
on-off  
Delay  
~
OUTPUT 1 ON-OFF  
OUTPUT 2 ON-OFF  
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.  
Page 13  
 
7.0 SPECIFICATIONS  
INPUTS  
1
electronically balanced, 20k, XLR-F, pin 2 = hot  
2 & 3  
4, 5 & 6  
electronically balanced, 20k, 1/4" TRS, Tip = hot  
unbalanced, 20k, 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  
Impedance  
Max. output  
0dBu (nominal), wired Tip = hot  
<100Ω  
+20dBu  
PERFORMANCE  
THD+N  
< 0.015%  
Noise  
Frequency response  
<-90dBu, 20Hz to 20kHz  
+0, -0.5dB, 20Hz to 20kHz  
Crosstalk  
<-70dBu @ 10kHz  
CONTROLS  
TRIM 1  
±10dB  
TRIM 2 - 6  
DIM  
+10dB, -6dB  
-20dB  
LEVEL 1 & 2  
-to +10dB  
POWER SUPPLY  
Mains voltages  
230Vac, 50Hz, 6VA  
115Vac, 60Hz, 6VA  
220Vac, 60Hz, 6VA  
Europe  
North America  
ROK  
Mains connector Standard IEC receptacle  
MECHANICAL  
Dimensions  
Weight  
1.75" (44mm)H x 19" (482mm)W x 6.5" (165mm)D  
Unit = 2.2kg, Shipping = 2.85kg  
LA Audio reserves the right to alter any feature or specification without prior notice.  
E&OE  
Page 14  
 

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