music, recording music and building recording studios

8/10/2019 Music, Recording Music and Building Recording Studios

1/21

ModelLA-2A

LevelingAmplifier

UniversalAudio,Inc.www.uaudio.comPOBox3818

SantaCruz,CA95063-3818


2/21

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


3/21


4/21

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.


5/21

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


6/21

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


7/21

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


8/21

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.


10/21

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.


11/21

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.


12/21

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.


13/21

8

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


14/21

9

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.


15/21

10

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.


16/21

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


17/21

12

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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16

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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