The Professor Amp - 1984 - Gitec | Electric guitar & amp ......The preamp includes a first stage...

GITECMemberProject:"TheProfessor"Amp Page1of7

©TilmannZwicker&GITEC,2018 www.gitec-forum.de&https://www.gitec-forum-eng.de

„TheProfessor“–anoldschoolampprojectfrombackintheday....

TilmannZwicker,GITECMember,Munich,Germany

1.Somepersonalhistory(justskipitifyouwanttogettothecircuitsrightaway)I was just going through some old papers and I stumbled across two hand-drawncircuits from some DIY-projects I tackled in the early 1980’s. In view of all thediscussions I experienced about guitar circuits over the years, I felt I should takeadvantage of the “Member Projects” page on the GITEC website and post these oldhardware approaches. Since the circuitswere used to do some recordings I can evenpresentsomeaudioexamples.This amp project originated sometime in the late 1970’s when even in provincialSouthernGermanyword got around amongst the guitar playing folk that therewas anewwonderweapon fromCalifornia: theBOOGIEamp. I somehowevengotholdof asalesbrochureandleftthatlyingaroundinmyparents’housesomeday.Mydad(whoas a professor held the chair of the Institute of Electroacoustics at the TechnicalUniversityinMunich)sawitandhetooknote.Notoftheampsassuch(forhimallthetube technologywas a bit boring because he could honestly say: “been there – donethat”) but of the beautiful amp-cabinets shown in the color pictures. In his free time,(amongother things likegardeningorhiking inBavarianmountains)mydad loved toworkwithwoodandbuiltlotsafurnitureforthehouse,andheloverthelookofthoseBoogieamps.



ItdidnottakelongforhimtosuggestthathecouldbuildmeamahoganycabinetforanampifIwantedtodoaDIYproject.OfcourseIjumpedatthechance-andsoitbegan.Byearly1981,IhadseenadrawingofthecircuitofaBoogieMk.1andwasshockedtofindnexttonodifferencetothesouped-upfenderTwinReverbIhadbeenplayingforawhile.Ialsostartedtobefrustratedwithcertainaspectsoftubeamps,andhadreadthebooks by Helmuth Lemme on guitars and amps –which pointed to the possibility ofusing solid-state technologydespite thebad rap ithad inguitarist circles. In addition,some very usable devices such as the Ibanez Tube-Screamer and the Boss OD-1 hadcomeoutandindicatedthatsolid-statewasnotnecessarilybadatall.With this as a background, and given that I was just getting my own place to live, IdecidedthattheampIwasgoingtobuildwithmydadshouldbeliving-room-compatible(i.e.good lookingandsmall),and includeonlysolidstatetechnology(that tied inwiththesizeaspect).IhavehadaloveaffairwithJBLspeakersallmylive(well,sinceIcouldaffordthem–myTwindidn’thaveany! ) andhadgoodexperienceswithaK110.Sucha speakerwasthusgoingtobethebasisfortheamp,andthecabinetwasgoingtobesosmallthatitcould just accommodate the 10”-speaker. The overall design was going to be that ofearlyFenderorVoxampswiththecontrolsaccessiblefromthetopoftheamp.Mydadfound somenicemahoganyandwent towork, even carving thedove-tail joints all byhand.Hefinishedthecabinetquitequickly(andnicely!)soIhadtofollowupwiththeelectronics.



2.AmpElectronics(withalittlehistorythrownin..,)–seetheFiguresTheelectronicsoftheampcometogetherinanumberofstages.-Powerampandpowersupply(notshown):itwasclearfromthebeginningthattheprojectwasgoingtobeasolid-stateonesincetheampwasgoingtohaveaverycompactformat but also decent power output. Only a transistor power amp could offer thiscombination.Also,withmyknowledgeandexperienceatthetime,itwasclearthatasaconsequence any distortion had to be generated in the preamp and not in the poweramp.I later learned that there were other approaches possible but at that time, theexperimentsIhaddonewithvariousequipmentIhadowned,builtandusedoverthe10yearsorsosinceIhadstartedplayingtheelectricguitarpointedthatway.ThepowerampIchosewasakitavailablefromaMunichcompanycalledRIM,capableofdelivering160Wintoa4Ωload.TheK110speakerusedintheampwasratedat16Ωimpedanceand75Wpowerhandlingcapacitysoitwasreasonablysafegiventhatthepower amp could deliver (nominally) approx. 50W into that load. However, I couldconnectfurtherspeakerstotakeadvantageofthemassivepowercapacity.Thebigheatsinkofthepowerampmadeasubstantialpartoftherealsurfaceoftheamp.Thepoweramprequiredmerelyaverysimplepowersupplywith1rectifierand4capacitors–thiscouldbefittedtothebottomoftheampcabinetwhiletheprettybig(andHEAVY)mainstransformerwasbolted to thespeakerbafflenext to theK110. (Thisparticularpowerampdied 20 years later due to negligence onmypartwhen servicing the amp. Sinceexperiencehadshownthatmassivepowerwasneverrequiredfromtheamp,thepoweramwasreplacedbya40WPowerModulewhichmadetheampconsiderablelighter☺)-Linearpreamp(Fig.1):inafirstconstructionstage,IdidnotfitanydistortioncircuittotheampsinceIdidnotreallyhaveagriponwhattodothereatthetime.The preamp includes a first stage (IC1), a verymuch Fender- andMusicMan-inspiredtonestackwith trebleandbasscontrolsplusbrightanddeepswitches,a send/returnstageanda(master)volumecontrol.ThebrightswitchcontrolsasimpleRC-filterinthefeedback loop of IC1which produced a boost of 6 dB above about 2,5 kHz. The deepswitchisinfacta“normalorlessbass”circuitcuttingfrequenciesbelow230Hzwithashallow6db/Octslope.Thecutofffrequencyofthishigh-passhighpassisdeterminedbythe6,8nFcapacitorandtheinputimpedanceofthesubsequentstage(100kΩ).Thepreamphasatrimmertoadjustthesensitivity–infactInevertouchedthatagainaftersettingitoncesoasnotto(audibly)distortwithGibsonDirtyFingerspickups.Thepreamp also includes a pre-wired optional additional treble boost (dashed in thefigure)thatwasneverconnectedbecausethecircuitgeneratesalotoftrebleasis.Twovoltageregulators fedfromthepowerampsupplydeliverthe+/-12VfortheOP-amps.



Fig.1:Preamplifier(linearpart)with“Treble”&“Bass”controls,and“Bright”-and“Deep”-switchesAtthispoint,theampbecameoperational,andIusedaBoss“HeavyMetal”stompboxconnectedtotheSend/Return-stageforsoundshaping.- Distortion circuitry (introduced a year later): investigating circuits used in thedistortiongeneratingstompboxesmentionedabove(whichIgenerallylikedbutfoundabitunnaturalsounding),andalsobasedonwhatIhadreadintheLemme-books,itwasclearthatOP-ampsplus(asnon-lineardevices)diodesinthefeedbackpathwouldgiveadecentsound.ExperimentingwithaBossOD-2,IfoundthatIcouldnevergetenoughdistortionfromitbecauseofthewaythenon-inverting-OP-Ampdesignworks(acharacteristicofmanyofthesimplerdistortionboxesincludingtheTubescreamerTS-808).Thisledmetothinkthat I would require two such circuits (IC2, IC3) in series to obtain the degree ofdistortion Iwanted.An additional diode clipper between the two stageswas going toroundoutthecircuit.Thisclipperwasswitchableasakindofboost.Originally, I used SI-diodes in the OP-amp feedback circuits and GE-diodes for theclipperinbetweenthetwostages.Later,IreplacedtheSI-diodesinthefirstdistortionstagewith(green)LED’stoobtainahigheroutputlevelofthatstage.

Fig.2:Overdrive/distortioncircuitrywith“Drive”-controland“Boost”-switch



-Send/Returnandrecoverystage:priortotheS/Rthereisasimple16kHzlow-passthathasnoeffectonthesound,andaftertheS/RthereisasimplerecoveryOP-amptomatchtheoutputofthepreamp(andoftheeffectsdevices)tothepoweramp

Fig.3.:Send/return-circuitryandgainrecoverystage.-Descriptionofoverallcircuit(seelastpage):IC1ismerelyapreamplifierwithaveryhighinputimpedancetoaccommodatetheguitaroutputcharacteristics,increasesignallevels,anddrivethetonestack.Myinvestigationswithotherdeviceshadshownthat,foranaturalampsoundIpreferred, thepositionof thetonestackaheadof thedistortionstagewasbest(againinlinewiththescenariointheclassicamps).Thedistortionstageconsisting of two OP-amps (IC2, IC3) with diode clippers (D1, D3) in the feedbackcircuits, and a further diode clipper (D2) between themwas fitted between the tonestack and the S/R-stage so that any effects could be routed between distortion andpower amp. No post-distortion filtering was introduced since the loudspeaker wouldtakecareofthis–justasitdoesinanyclassicamp.The“Deep”-switchcutsthelow-frequencycontentofthesignalaheadofthedistortioncircuit.Thispositionhelpstoavoidthe“muddy”soundsometimesassociatedwithpre-distortion-tone-stackampssuchastheBoogieMk.I.Nofrequencyresponseshapingisdoneanywhereinthedistortionstages–thisisverymuch like the situation inmost classic tube amps.My feeling is that this results in arathernaturalsound-butnotnecessarilyinaverymodernandaggressiveone.Both active distortion stages are controlled simultaneously by a stereo “Drive”potentiometer. They also both include a trimmer setting the minimal gain when the“Drive” pot is atminimum i.e.when adjusted for a clean sound. Formy taste, a great“clean” tone may well include a bit of distortion that is not noticeable as such butprovides more of a “coloring”. This can be adjusted via these trimmers – I set them(again using a high-output guitar) so that both stages do provide gain but no actualimmediatelyaudibledistortion.Thisseemstonicely“warmup”thesound.The“Boost”switchcircuit includesalsoatrimmerthatadjustshowmuchthesignal isattenuated before hitting the GE-diodes. Since all three distortion stages operatesomewhat differently (1st with LED’s, 2nd with GE-diodes, 3rd with SI-diodes), thistrimmercanadjusttosomedegreethecontributionofthevariousstagestotheoveralldistortionalthoughIthinkthatissuedoesnotmakemuchofadifference.



3.That’s(almost)all,folks!So,that’sit:averysimple,straightforwardpreampwhichhasaccompaniedmeformorethan30years,anditstillsoundsgoodtomeinconjunctionwithalinearpowerampandthe JBLK110. The date on the figure (1984) is the datewhen the final changesweremadeintheformofintroducingtheLED’stothefirstdistortionstage.Inretrospect,Icouldhavegoneadifferentwayinthedistortionstagebyintroducingainverted-amplifierapproachwhichwouldhavegivenmeallthedistortionIcouldhavewantedinonesinglestage(manydistortiondeviceslaterthantheTS-808andtheBossOD’suse that successfully, including theRockman). I cannot sayhowmuch themulti-stagedistortionsoundsdifferently.Isuspectnotmuch.TherearesomeaudiosamplespostedontheGITECwebsitewhichpresentthispreamprunthroughasimpleDYI-directboxwithanevensimplerspeakersimulator(discussedin my “other” member-projects posting). Check them out – the guitars are easilydiscerned: a Strat, a Tele, a ES-335, and an el-cheapo Guyatone lap-steel – all playedthroughTheProfessor and recorded straight into theboardviamydirectbox.NoEQfromtheboard.Ah…yes,thename:Inamedtheamp“TheProfessor”becausemydad(theprofessor)hadsuchalargepartinit!1Thanksforyourinterest&takecare,Tilmann,inOct.2017

1Ishouldmentionthatyearsbeforethisproject,whenIwasjuststartingoutplayingtheguitar,IinquiredwithmydadhowthatguitarsoundonSantana’sAbraxasalbummightbegenerated.He–i.e.somebodywhohadnoconnectiontomusical-instrumentelectronicsandcertainlynonetothedesired„distortion“,butwhohadalifetimeinaudioelectronicsandacousticsunderhisbelt–justlikethattookapencilandmadealittledrawingshowingtwoanti-paralleldiodesandasimplelow-pass....HEWASSORIGHTONTHEMARK!

The Professor Amp - 1984 - Gitec | Electric guitar & amp ......The preamp includes a first stage...

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