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ModelLA-2A
LevelingAmplifier
UniversalAudio,Inc.www.uaudio.comPOBox3818
SantaCruz,CA95063-3818
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ii
TeletronixLA-2AbyUniversalAudio
Thank you for purchasing this reproduction of the Teletronix LA-2A. The LA-2A was
originally
produced
inthe
early1960s
by
Teletronix,
which
was
later
acquired
byBabcockElectronicsCorporation. My fatherpurchased theproductrightsand thenameTeletronix from Babcock Engineering in 1967, folding it into his Studio ElectronicsCorporation, shortly before he changed the name to UREI. There were three differentvariations of the LA-2A during this period before productionwas discontinued around1969.
Straightforward initsdesign,and initially intendedforbroadcastapplications, theLA-2Aquicklybecamestandardequipmentinstudiosworldwide.Manyhavebeenpainstakinglymaintainedfor the30yearsthat theyhaveremained inusesinceproductionstopped.Atube-based compressor, the LA-2A features hand-wired components and two simplecontrols.Atthetime,itselectro-luminescentopticalgainreductionwasquiterevolutionary:applying
the
audio
signal
to
an
electro-luminescent
light
panel
which
shines
on
aphoto-
electriccellwhichinturncontrolsthegain. Incontrasttotheelectro-opticaldeviceswhichpreceded it, the electro-luminescent light source provided the fast attack necessary forbroadcast applications. Additionally, the cadmium-sulfide photo-cells provided a verynatural two-stage release which resulted in a compression characteristic more
transparent than the other compressors of its day. To this day the LA-2A delivers atrademarksoundtreasuredbyengineersworldwide.
Here at Universal Audio, we have gone to great lengths to recreate the LA-2A withcompleteauthenticity. Itsbeennosmall feat to locateobscureparts,putqualitycontrolprograms in place that didnt exist 30 years ago and find personnel capable of the
handworkthat
goes
into
eachunit
.
Every
stepwas
necessary
to
ensurethat
each
LA-2Adeliversthesoundyouexpect.
As for the future,we areworking on additional reproductions beyond the LA-2A and1176LN. To thatend,weare interested inyouropinionsaboutyour favoritepiecesofvintagegear. Whatcantyoulivewithout? Whatwouldyoulovetoown,butcantfindany longer? Inaddition,UniversalAudio isworkingonnewproductsdesigned tomeetthedemandsofthemoderndigitalrecordingstudio,yetretainingthecharacterofclassicvintageequipment.
Developing theseproducts hasbeenquiteanadventure,enjoyedbyall involved. Wethankyouforyoursupportandwethankmyfather,BillPutnam.
Thankyou,
BillPutnam
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iv
LA-2A!!!!UsersGuide
UniversalAudio,Inc.
PO
Box3818
SantaCruz,CA95063-3818(831)454-0630voice
(831)454-0689faxwww.uaudio.com
UniversalAudioPartNumberLA2A-M01.Revision1.3
Notice
Thismanualprovidesgeneralinformation,preparationforuse,installationandoperatinginstructionsfortheUniversalAudioLA-2ALevelingAmplifier.
Theinformationcontainedinthismanualissubjecttochangewithoutnotice.UniversalAudio,Inc.makesnowarrantiesofany kindwith regard to thismanual, including,butnot limited to, the impliedwarrantiesofmerchantability and fitness for a particular purpose. Universal Audio, Inc. shall not be liable for errorscontained hereinordirect, indirect, special, incidental,or consequentialdamages in connectionwith thefurnishing,performance,oruseofthismaterial.
Copyright
"2000UniversalAudio,Inc.Allrightsreserved.
Thismanual
and
any
associated
software,
artwork,
product
designs,
and
design
concepts
are
subject
to
copyright protection.No part of this document may be reproduced, in any form, without prior writtenpermissionofUniversalAudio,Inc.
Trademarks
LA-2A,1176and theUniversalAudio, Inc. logoare trademarksofUniversalAudio, Inc.Othercompanyandproductnamesmentionedhereinaretrademarksoftheirrespectivecompanies.
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v
TableofContents
TeletronixLA-2AbyUniversalAudio................................................................................... ii
Specifications ....................................................................................................................... 1
OperationoftheLA-2A .......................................................................................................2
InputandOutputConnections ............................................................................... 2
PeakReductionControl ........................................................................................ 2
GainControl ....................................................................................................... 2
VUMeterOperation ............................................................................................ 3
Limit
/
Compress
Switch........................................................................................ 3Calibration ........................................................................................................................... 4
MeterZeroAdjust............................................................................................... 4
Side-ChainPre-Emphasis(R37).............................................................................. 4
StereoBalanceAdjust(R3) ................................................................................... 4
TheoryofOperation ............................................................................................................ 5
CompressorBasics ............................................................................................... 5
GainReductionCircuit.......................................................................................... 8
Side-ChainCircuit .............................................................................................. 11
OutputCircuit.................................................................................................... 11
MeteringCircuit................................................................................................. 11
Appendix .......................................................................................................................... 12
CreativeClassics:The1176SolidStateLimitingAmplifierandtheLA-2A
LevelingAmplifier .............................................................................................. 12
TheLA-2A ..................................................................................................... 12
Developing
the
1176..................................................................................... 13
MoreThanaVintageFad:ClassicSound.......................................................... 13
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1
Specifications
! Gain
Reduction:up
to
40
dB
! Distortion:lessthan0.35%totalharmonicsat+10dBm,andlessthan0.75%totalharmonicsat+16dBmoutput
! Response:+/-0.1dB,30cyclesto15kilocycles
! Noise:75dBbelow+10dBmoutputlevel
! Gain:40+/-1dB
! OutputLevel:+10dBmnominal,+16dBmpeaks
! InputLevel:+16dBmmaximum
! Very
fast
attacktime
! ReleaseTime:approximately0.06secondsfor50%release,0.5to5secondsforcompletereleasedependingupontheamountofpreviousreduction
! InputImpedance:600ohmsbalanced
! OutputImpedance:600ohmsbalanced
! PanelSize:19x5
! DepthBehindPanel:7
! PanelControls:Gain(Outputlevel),PeakReduction,andMeterSelectorSwitch
! Meter:
dB
Gain
Reduction
and
dB
Output
! TubeComplement:(2)12AX7A,(1)12BH7A,(1)6AQ5
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2
OperationoftheLA-2A
InputandOutputConnections
Input
and
Output
connections
are
made
using
standardXL
R
style
connectors. T
erminalstripsarealsoprovided.
Outputconnections Inputconnections
6) StereoLink 1) +600ohms7) Ground 2) +250ohms8) +600ohms 3) Centertap9) Centertap 4) -250ohms
10) -600ohms 5) -600ohms
BarrierStripConnections
IfyouwishtousethebarrierstripinsteadoftheXLRconnectors,connectasfollows:INPUT: Connect +600(1)toHOTand600(5)toNEUTRAL.OUTPUT:Connect+600(8)toHOTand600(10)toNEUTRAL.
ConnectGround(7)tothecableshield.
PeakReductionControl
OperationoftheLA-2Aisverystraightforward. Thereareonlytwocontrolsthatonemustdealwith. The first is thePeakReductioncontrol. Thiscontrolshouldbesetso that the
compressor
exhibitsthe
desired
amount
ofcompression
.
Thiscontrol
should
be
setindependentlyoftheGainControl.
GainControl
This controldoes not affect the compression. The gain control should be set after thedesiredamountofcompressionisdeterminedusingthePeakReductioncontrol. OncethePeakReductioncontrolisset,adjusttheGainControltoachievethedesiredoutputlevel.
PowerTransformer
12AX7
6AQ5
12AX712BH7
R3
R37HA-100X
T4A-24
IEC
10 9 8 6 5 4 3 2 17 Output Input
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3
VUMeterOperation
TheVUMeter is used forboth output levelmonitoring and gain reductionmonitoring.
When used tomonitor the output level, the usermay choose either +4 dB or +10 dBsettings. Themiddle setting is for gain reduction, and the meter reads the amount ofcompressionindB.
Limit/CompressSwitch
TheLimit/CompressSwitchchangesthecharacteristicsofthecompressorIOcurve. WhenintheCompressposition,thecurveismoregentle,andpresentsalowcompressionratio.AhighercompressionratioresultswhentheswitchissettotheLimitposition.
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4
Calibration
MeterZeroAdjust
The
zero-
adjust
of
the
meter
is
set
by
the
screw-
adjust
potentiometer
located
next
to
theLimit/CompressSwitch. Toadjust this,put themeter intoGainReductionmode. Makesurethatthereisnosignalpresent. Loosenthelockingnutanduseascrewdrivertoadjustthispotentiometeruntilthemeterreadszero.
Side-ChainPre-Emphasis(R37)
The LA-2A was designed for use in broadcast applications. The audio signal in FMbroadcastingundergoespre-emphasisandresultsina17dBboostat15KHz. Duetothisincreaseinsignallevel,transmittersaresubjecttoover-modulation. TheLA-2Aprovidesacontrol(R37)whichcontrolstheamountofhigh-frequencycompression.
This
potentiometer
is
factory
set
for
a
flat
side-chainresponse
(clockwise)
.
Increasingthe resistance of this potentiometer by turning it counter clockwise will result incompressionwhichisincreasinglymoresensitivetothehigherfrequencies.
StereoBalanceAdjust(R3)
For stereooperation, two LA-2A unitsare interconnected inorder to achieve the sameamountofgainreductionfromeachofthetwounitstomaintainstereoimaging. Inordertoaccomplishthis,terminals6fromeachunitshouldbeconnectedtogether. Additionally,theirgrounds(terminal7)shouldbeconnectedtogether.
Theinterconnectingwireshouldbelessthan2feetinlengthandshouldbeshielded. The
shield
should
beused
to
connect
terminals
7
from
each
ofthe
units
together.
Tocalibratetheunitsforstereooperation:
! Connecttheunitstogetherasdescribedpreviously.
! TurnthePeakReductionknobcounterclockwise(nocompression).
! SetR3oneachunittoaclockwiseposition.
! SeteachmetertoreadGainReduction.
! Adjust thePeakReductioncontrolon the leftchanneluntilapproximately5dBofgainreductionisachieved.
! Adjust
R3
onthe
unit
that
showsthe
greatest
amount
of
gainreduction
until
the
gainreductionindicationsareequal.
! When operating, set the Peak Reduction controls to the same setting on bothchannels.
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5
TheoryofOperation
CompressorBasics
Before
we
dig
in
to
a
description
of
theL
A-
2A
circuit,
it
is
useful
to
examine
the
generalcharacteristics of compressors and review some terminology. Figure 1 depicts theinput/outputcharacteristicsofacompressor,anexpanderandaperfectamplifier. Whenoperated within its specified range, an amplifier provides a constant amount of gainregardlessof the levelof the inputsignal. InFigure1, themiddle linedepictsaperfectamplifierwithagainof10dB. Toseethis,noticethatasignalwithaninputlevelof30dBwillresult inanoutput levelof20dB,which isan increaseof10dB. Similarly,aninputlevelof0dBwillresultinanoutputlevelof10dB,hencethegainstaysfixedat10dBregardlessoftheinputlevel.
-30
-20
-10
0
+10
-30 -20 -10 0 +10
InputLevel(dB)
Output
Level(dB)
Compression
Expansion
Perfect
amplifier
Figure1 - Input/output characteristics of a compressor, an expander and a perfect
amplifier.
Incontrasttoanamplifier,whosejobistopresentaconstantgain,acompressorvariesitsgaininresponsetotheleveloftheinputsignal. Largeinputsignalsresultinlessgain,thusreducingorcompressing thedynamic rangeof thesignal. Referringagain to the line
marked
compression
in
Figure
1,
we
see
that
an
input
level
of
30
dB
results
in
anoutputlevelof20dB,indicatingagainof10dB. Repeatingthisforinputlevelsof20dBand10dB,weseethatthecompressorexhibitsgainsof5dBand0dBrespectively.Fromthis,itisclearthatthegaindecreasesastheinputsignalincreases.
Referringtothediagram,weseethatthecompressorwillincreaseitsoutputlevelby5dBforevery10dBthatweincreasetheinputlevel. Thecompressionratioisdefinedastheratioofthesetwonumbers. Inthiscasethecompressionratiowouldbe10:5,whichcanbereducedto2:1.
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6
Asanaside,anexpanderisadevicewhichincreasesthedynamicrangeofasignal. Forexample,a10dBchangeintheinputsignalmightresultina20dBchangein theoutputsignal,thusexpandingthedynamicrange.
There are several other terms related to compression that can be demonstrated byreferringtoFigure2. TheamountofcompressionorgainreductionistypicallygivenindB
and
is
defined
asthe
amount
bywhich
the
signal
level
isreduced
bythe
compressor.
Graphically, thiscanbeunderstoodby lookingat thedifference in levelsbetweenwhatwouldhavebeen theuncompressed (theoutput fromanamplifier)output leveland thecompressedoutput level. Thisvalue iswhat isdisplayedby the LA-2Ameterwhen it isswitchedtogain-reductionmode.
Asmentionedpreviously, thecompressionratio isdefinedas theratioof the increaseofthe level of the input signal to the increase in the level of the output signal. In thisexample,theinputlevelisincreasedby10dBwhiletheoutputlevelonlyincreases5dB.Thiswouldbeacompressionratioof2:1. Lowerratiossuchas2:1resultinmoregentlecompression. (Notethatacompressionratioof1:1isnocompressionatall).
Typically,compressors
letyou
choose
a
threshold.
This
isthe
point
atwhich
gainreduction starts to take place. When an audio signal is below this threshold thecompressoractslikeanamplifierandthereisnogainreduction. Abovethethresholdtheslopebecomeslessthan45degrees,indicatinggainreductionandhencecompression.
Thepointatwhichacompressortransitionsintocompressioniscommonlycalledtheknee.
In practical compressors, this transition is more gentle than what is depicted in thediagram.
Manymoderncompressorsprovideacontrolwhichadjusts the thresholddirectly. In thecaseoftheLA-2A,thePeakReductionknobcontrolsboththethresholdandtheamountofcompression.
-30
-20
-10
0
+10
-30 -20 -10 0 +10
InputLevel(dB)
Output
Level(dB)
Compressionregion
knee
2:1Compression
10dBof
compression
Figure2 - Input/outputcurveofacompressorwitharatioof2:1anda thresholdof -20dB.
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7
TRIM
IN
PU
T
IN
PU
T
TRA
N
SFO
RM
ER
O
PTIC
A
L
AT
TEN
U
A
TO
R
M
O
D
U
LE
T4
VO
LTA
G
E
A
M
PLIFIER
(12
A
X7
A
)
C
A
TH
O
D
E
FO
LLO
W
ER
(1
2
BH
7
)
PRE
-EM
PH
A
SIS
V
O
LTA
G
E
A
M
PLIFIER
(1
2
A
X7
A
)
ELEC
TRO
LUM
IN
ESC
EN
T
D
RIV
ER
(6
A
Q
5
)
O
U
TPU
T
TRA
N
SFO
RM
ER
O
U
TPU
T
N
EG
A
TIV
E
FEED
B
A
C
K
G
A
IN
C
O
N
TRO
L
PEA
K
RED
U
C
TIO
N
C
O
N
TRO
L
STEREO
IN
TERC
O
N
N
EC
TIO
N
Figure3-BlockdiagramoftheLA-2Acompressor.
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LA-2ABlockDiagram
AfunctionalblockdiagramoftheLA-2AisprovidedinFigure3. Abriefoverviewoftheoperationwillbeprovidedhere. Theinputtransformerprovidesisolationandimpedancematching. Afterthisthesignalisfedintoboththeside-chaincircuitandthe gainreduction
circuit.
The
side-chain
iscomprised
of
a
voltage
amplifier,
a
pre-emphasis
filter,
and
adriverstagewhichprovides thevoltagenecessary todrive theelectro-luminescentpanel.Thissignalcontrolsthegainofthecompressor. Afterthegainreductioncircuit,thesignalissentthroughanOutputGaincontrolandatwo-stageoutputamplifier,followedbytheoutputtransformer.
Electro-luminescentPanel
Photo-ElectricCell
Figure4-DiagramoftheT4electro-opticalcell.
GainReductionCircuit
Asmentionedpreviously,compressorsaredevicesthatvarytheirgaininamannerwhichisdependentupon thelevelof the inputsignal. Inorder todo this, thecompressormustfirsthavesomemethodofdeterminingthelevelofthesignal,andmustthenbeabletousethis tocontrol thegain. Therearemanydifferentschemes toaccomplish these tasks. InthecaseoftheLA-2A,bothofthesefunctionsareperformedbytheT4,whichisanelectro-opticalelement.
AT4 iscomprisedofanelectro-luminescent (EL)panelandaphoto-electriccell. TheEL
panel
is
essentially
anight
-
light.
As
you
would
expect,
the
larger
the
signal
that
isappliedtoit,thebrighterthelightthatisgenerated. This lightshinesuponthephoto-electriccell.
Aphoto-electriccell isalightsensitivedevicewhoseresistancechangesdependinguponthe intensityof light towhich it issubjected; thebrighter the light, the less resistance thephoto-cellwillhave.
AsdepictedinFigure5,thephoto-cellisusedtocontrolthegainofthecircuit. Essentially,thephoto-cellactsasthebottomleginavoltagedividercircuit. Thelowertheresistanceof thephoto-cell, the lower thesignalvoltagewillbeat theoutputof thegain reduction
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stage. To seewhy this is true,we can look at the extreme cases. If the resistance isextremelyhigh(thisisthecasewhenthereisasmallinputsignalandthelightisoff)thenthephoto-celldoesnotaffectthecircuitand thereisnogainreduction. Thesecondcasewecan lookat iswhen there isa largesignalpresent. In thiscondition, the lightshinesbrightlyandthephoto-cellexhibitsverylowresistance. Iftheresistanceofthephoto-cell
becomes
zero(a
dead
short),
then
the
signalwould
be
grounded
andthere
would
beno
output. Inreality,thephoto-cellresistancecannotgocompletelytozeroandhencetherewillalwaysbesomesignalpresent.
7
R5 68 K
R6
68 K
R7
2.7K
8
9
10
TO V O LTA G E
A M PLIFIER
TO G A IN
RED U C TIO N
C O N TRO L
TO A TTEN U A T O R
D RIVE A M PLIFIER
G A IN R 1
100K
PH O TO C O N D U C TIV E C E LL
E LEC TR O LU M IN ESC EN T
E LEM E N T
H A-100X
.0
0
4
7
u
F
Figure5-Schematic oftheLA-2Ainput andgainreductioncircuit.
The T4 electro-optical device is the heart of the compressor and its gain reductioncharacteristics. ItsuniquecharacteristicsaffecttheoverallsoundandcharacteroftheLA-2A.
Inaddition to thecompressioncurve, thecombinationof theELpaneland thephoto-celldetermine the attack and release characteristics of the LA-2A. This is one of themostimportantcontributors to thesoundof theLA-2A. Unlikeothercompressorswhichallowtheuser toadjust theseparameters, theattackand releaseof theLA-2Aarecompletelydetermined bytheT4.
There
are
several
importantcharacteristics
ofthe
T4which
playcrucial
roles
inthe
soundoftheLA-2A. Thefirstistheattack. TheLA-2Awasthefirstelectro-opticalcompressortouseanelectro-luminescentpanelfor the lightsource. Previousattemptsatelectro-opticalcompressionemployedeitherneonorincandescentlights. Bothofthesetooktimetolightup,and thisdelay resulted in slowattacks. Theelectro-luminescentpanel resulted inafasterattackthanexhibitedbyothercontemporarydevices.
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The next important aspect is thatof the releaseof the compressor. This is determinedalmostentirelyby the characteristicsof thephoto-cell. The LA-2A uses cadmium-sulfidephoto-cells. Thefirstimportantaspectofthecellisitstwo-stagedecay. Afterthelightisremovedfromthecell,itreleasesquickly(40-80milliseconds)toapproximatelyhalfofitsoff resistance. The remainder of its release can take place over as much as several
seconds.
Thenextaspect is thememoryof thecell. This results in two importantaspectsof thecharacteroftheLA-2A. Theamountoftimeittakesforthecelltorecoverafterthelightisremoveddependsonhow long lighthadbeenshiningon itandhowbright the light. InthecaseoftheLA-2Athisresultsinbehaviorwherethereleasetimeissloweriftheunithaseitherbeenincompressionforawhile,ortheamountofcompressionislarge. Thissignaldependentreleasecharacteristiciscriticaltothesoundoftheunit.
The amountof compression, aswell as the compression threshold, is controlled by thePeakReductionpotentiometer. Thispotentiometer controls thegainof the side-chaincircuit. Thegreater thegainof thiscircuit, the lower the threshold and thegreater the
amount
of
compression
will
be.
Many
modern
limiters
and
compressors
allow
for
thedirectadjustmentofthethreshold. Otherunitssuchasthe1176LNuseafixedthresholdandprovideaninputlevelcontrol,whichadjuststhesignallevelbeforeitisappliedtothecompression circuit. In contrast, the LA-2A, while also having a fixed threshold,does not control the input level, but rather controls the amountofsidechaingainappliedtotheinputsignal.
C 8
.03
R3 1 1 K
R2
100K
C 9
.02
R3 3
220K
1 MegLIMRESP
R37
R3
1M
STEREO
A D J
O U T PU T
R3 6
1K
C 10
50 uF
C 7D
30 uF
R3 5
220K
R3 4
22K 2W
V4
6 AQ 5 A
IN PUT
R3 0
47 K
.01
ATTEN UATO R
D RIVE -A M PLIFIER
C12
.001
STEREO
PARA LLELPO W ER
SU PPLY
R3 2
1K
C 6
12AX7A
V3 B V3 A
C 11
.1
Figure6-Schematic diagramoftheLA-2Aside-chaincircuit.
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11
Side-ChainCircuit
Thepreviouslydescribedgainreductioncircuitiscontrolledbythecontrolvoltagewhichissuppliedbytheside-chaincircuit. TheLA-2Aisafeed-backstylecompressor. Thisisduetothefactthatthesignalthatisusedtodrivetheside-chaincircuitisaffectedbythegainreducedsignal. ThissignalisfirstfedintothePeakReductionpotentiometer(R2),which
controlsthe
amount
of
side-chain
drive
and
inturn
controls
the
compression
threshold
andamountofgain reduction. A12AX7 is thenusedasavoltageamplifier to increase thesignallevel. Apre-emphasiscircuit isprovidedon theoutputof the12AX7. Originallydesignedforbroadcast,theLA-2Aallowedforside-chainequalization,whichallowedtheoperator tomake thecompressionmoreor less sensitive to thevoice frequencybands.Formusicalapplications,thisequalizationisusuallysettoaflatfrequencyresponse.
Subsequent to the filter, a 6AQ5 provides the signal necessary to drive the electro-luminescent panel. EL panels were often used for night-lights and hence are usuallydesignedtobedrivenwith120volts,60HzAC. Theywerenotdesignedforaudio,andapplyingthewide-bandwidthsignalsthatariseinaudioapplicationsresultsinashortened
lifetime
of
the
part.
OutputCircuit
Figure7-Schematic diagramoftheLA-2Aoutputcircuit.
The
output
circuit
is
comprised
of
a
12AX7
which
operates
as
a
voltage
amplifier
followedby a 12BH7A which operates as a cathode-follower. This is followed by the outputtransformer,whichprovidesimpedancematchingandabalancedoutput.
MeteringCircuit
Themeteringcircuit in theLA-2Ahas3modesselectedbya front-panelswitch,allowingforoutput levelmonitoringat+4and+10dBaswellasgain reduction. Asmentioned
R9
220K
R10
1.5K
R12
470K
R14
2.7K
R11
68K
C1
.02V1A
C7C
30uF
R13
220K
C2
.1
R16
68K
R17 10K
R15
470K
R18
1K
R20
1K
R19
470K
C3
.1
R21
100K
C2
.1
R16
68K
C5
10 uF1
34
8
A-24
2
5
V1B
12AX7
12BH7
V2B
V2A
CATHODE
FOLLOWER
VOLTAGE
AMPLIFIER
FROM
R1
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previously, thegainreduction iscontrolledby thephoto-cell in theT4el-op. Inorder totrack theoperationof thiscellanddetermine thegain reduction,a secondphoto-cell isalsoilluminatedby thesameELpanel. Thisphoto-cellishand-selected tomatchthegainreductionphotocellandhencegivesanaccurateindicationoftheamountofcompression.
Appendix
CreativeClassics:The1176SolidStateLimitingAmplifierand
theLA-2ALevelingAmplifier
The LA-2A and 1176 compressor/limiters long ago achieved classic status. They're a
given
in
almost
any
studio
in
the
world
relied
upon
daily
by
engineers
whose
stylesrange from rock to rap, classical to country and everything inbetween.With somanynewerproductsonthemarkettochoosefrom,it'sworthlookingatthereasonswhytheseclassics remain a necessary part of any professional studio's outboard equipmentcollection.
Thebasicconceptofacompressor/limiter, isofcourse, relativelysimple. It'sadevice inwhichthegainofacircuitisautomaticallyadjustedusingapredeterminedratio thatactsinresponse to the inputsignal level. Acompressor/limiter"ridesgain" likearecordingengineerdoesbyhandwith thefaderofaconsole: itkeeps thevolumeupduringsoftersectionsandbrings itdownwhen the signalgets louder. Thedynamicprocessing thatoccursatratiosbelow10or12tooneisgenerallyreferredtoascompression;abovethat
it'sknown
as
limiting.
Modernday compressorsofferagreatdegreeofprogrammability and flexibilitywhileolderdevicessuchas the1176and theLA-2Aaremorestraightforward in theirdesign.Perhaps it is this fact thathascontributed to theirappealing soundand the longevityoftheirpopularity.
TheLA-2A
TheLA-2A levelingamplifier,a tubeunitwithhandwiredcomponentsand three simplecontrols, was introduced in the mid 1960s. It utilized a system of electro-luminescentoptical gain control that was quite revolutionary; gain reduction was controlled byapplying
the
audio
voltage
to
aluminescent
driver
amplifier,
with
asecond
matched
photoconductive cell used to control themetering section.With its0 to40 dB of gainlimiting,abalanced stereo interconnection, flat frequency responseof0.1dB from30-15,000hzandalownoise level(betterthan70dBbelowplus10dBmoutput,)theLA-2A quickly became a studio standard. Originally patented by Jim Lawrence, it wasproduced by Teletronix in Pasadena, California, which became a division of BabcockElectronicsCorp. in1965. In1967 Babcock's broadcast divisionwas acquired by the
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inconsistentpiece topiece than the1176s, becauseof the tubes and thedifference infatigueofthetubes.
"MybigmentorswereAndyJohnsandLeeDeCarloandRonNevisionbecausetheywereallRecordPlantguys.Ilearnedhowtomakearockandrollrecordfromthem.Althoughovertheyearsit'sbecomemyownthing,mystylestilltendstobethatRecordPlantstyle,
U87s,1176s,
LA-2As,47
F.E.T.s...it'swhat
I
like."
MikeClink
Producer/EngineerMikeClink (GunsN'Roses,SammyHagar,Pushmonkey)alsocomes
fromtheRecordPlantschoolofrecording."IfindthatIactuallyuse1176smorenowthanIeverdid,"hecomments."Ilikethembecausetheybringoutthebrightnessandpresenceofasound--theygiveitanenergy.ItseemslikewhenI'mmixingIendupusingan1176onthevocalseverytime.And ifIwant tocompressaroomsound I'll takeamono roommic,putan1176acrossitandpushinallthebuttons."
BruceSwedein
Bruce Swedien is amaster engineerwho needs no introduction.He also is a die-hard1176fan."Ihave twosilverface1176LNs inmyrack thatBillPutnampersonallypickedoutforme,"hesays."IremembersittingatBill'splaceintheChannelIslands,andtalkingabout the1176andhow Iwantedapair .Thenext timewewentoverhe'dpicked thispairoutandtheyweresitting inhisgaragewaiting forme.I love themonvocals.AlloftheMichael Jackson and James Ingram vocals thateveryone has heard somuchweredonewithatleastoneofthose1176s.Icouldn'tpartwiththemforanything.Theysoundfabulous."
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