Analog Audio Amplifier Design 1997

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19 J anua

Anal og Audi o Power Ampl i f i er Desi gnThe Met hod of DePal ma

"Art whi ch does not have t he appearance of art i s t r ue art . "

- Ol d Roman sayi ng

nt roducti on

author ' s exper i ence wi t h anal og audi o ci r cui t desi gn ext ends over a per i od of 50 years - - t he era of t he vacuumg pl ayi ng r ecords, FM r adi o, t ape r ecordi ng, and t r ansi st ors. I t al so i ncl udes t he di gi t al er a wher e comput ers becol ved wi t h audi o. Si nce t he subj ect of t hi s paper i s anal og audi o, di gi t al audi o si gnal pr ocessi ng i s not di scusse

Hi st or i cal Backgr ound

hi s youth, t he aut hor was f ort unate i n growi ng up and bei ng educat ed i n a geogr aphi cal r egi on, east coast U. S. A. ,adel phi a, New Yor k, Bost on, M. I . T. - Harvar d; dur i ng a t i me ext endi ng f r om t he 1940' s t o the l ate 70' s. I n 1954, d Haf l er and I r vi ng Fr i ed, who l i ved near my f ami l y home i n Phi l adel phi a. Thr ough t hei r associ at i on and f r i endshi

umber of t he east coast audi o f i gur es of t he t i me. At t hat t i me Davi d Haf l er and Herb Ker os were manuf act uri ng ACRnsf ormers f or t he Wi l l i amson ampl i f i er. Later on, Haf l er went on t o f orm Dyna Company, manufacturi ng audi o output

nsf ormer s, ampl i f i er and pr e- ampl i f i er ki t s, Dynaki t s.

Chi ef Engi neer i n the earl y vacuum t ube days wi t h Dynaco, I met and spoke wi t h other audi o desi gners, i . e. Stuarteman, Ben Dr i sko, Fr ank McI ntosh, and Henr y Kl oss. Out of t hese meet i ngs a phi l osophy of desi gn was emergi ng whi chompassed the whol e audi o r eproduct i on process. As t i me went on I became acquai nted wi t h Emory Cook, Rudy Bozak, Paher s, Edgar Vi l chur , Ar t hur J anszen, and Donal d Chave ( LOWTHER, U. K. ) .

hough my pr i mary i nter est devel oped i n the basi c physi cal sci ences, my i nter est i n musi c and sound repr oduct i on hai st ed t o t hi s day. Over a per i od of decades, experi ence and i nt r ospect i on have r esul t ed i n t he evol ut i on of cer t aepts whi ch compri se t he Met hod of DePal ma i n anal og audi o ci r cui t desi gn. The phi l osophy and worki ng out of t heseexempl i f i ed i n t he t hree power ampl i f i er desi gns pr esented her e.

Phi l osophy

og ci r cui t desi gn I woul d char acteri se as an Ar t . Di gi t al ci r cui t desi gn I woul d char act er i se as a geomet r y. Ar t be i n t he eye of t he behol der, consequent l y i t i s not al t oget her a l ogi c pr ocess whi ch governs t he choi ce of a ci rol ogy. The obj ect of our desi r e i s a pl easi ng and sat i sf yi ng musi cal exper i ence comparabl e on some basi s wi t h thegi nal perf ormance. Over t he hi st ory of evol ut i on of t he art i f i ci al - - and now el ect r o-mechani cal r epr oduct i on of sai n not i ons more commonl y associ ated wi t h the musi c bei ng r epr oduced i nj ected themsel ves i nt o t he speci f i cat i on o

ctr i cal par amet er s.

i deas of st abi l i t y, l i near i t y, harmony, bal ance, and power express t hemsel ves i n el ect r oni c ci r cui t r y. I mageryel oped; i t coul d be concei ved the anal og audi o power ampl i f i er expr essed i t sel f out of t he pr opert i es of t he repr oc. Out of t he ampl i f i er conf i gur at i ons and possi bi l i t i es assembl eabl e t hr ough t he known ( f i xed) l aws of el ect r i cipr opert i es of t he known act i ve ampl i f i er devi ces, of t hese whi ch are t he musi cal l y accept abl e t opol ogi es?

s post ul at ed her e that t he musi cal l y accept abl e topol ogi es must necessar i l y sat i sf y al l t he el ectr i cal l aws as wemusi cal ones. Conf ormi t y wi t h musi cal l aws conver t s an exper i ment i n el ectr omechani cal l y i nduced t r ansduct i on i ntocal exper i ence.

Tubes and Tr ansi st ors

i f i er s desi gned usi ng act i ve devi ces, i . e. t ubes and t r ansi st ors, are si mi l ar i n concept i on t o modern mul t i - el emeh- speed photogr aphi c l enses. I n l ens desi gn, mat eri al s wi t h def i ni t el y non- l i near pr oper t i es, chr omati c and spherirat i ons, ar e bal anced of f agai nst each ot her t o resul t i n a magni f i cat i on (gai n) wi t hout chr omat i c aber r at i on (f l

quency response). Hi gh si gnal t o noi se and reduced i nt er modul ati on, ( modern l ow r ef l ect i on l ens coat i ngs) . Theept i onal qual i t y of modern opt i cs speaks t o t he achi evement of t hi s bal anci ng. I n f act t he resol ut i on of a moder nf ract i on l i mi t ed) l ens i s r egul at ed by the propert i es of t he l i ght passi ng t hrough i t . Thi s pri nci pl e carr i es ove

ct r oni cs where gai n and l i near i t y can be achi eved by bal anci ng t he i nherent cur vat ur es of act i ve devi ces. A push- pss A t r ansf ormer coupl ed ci r cui t demonst r at es thi s. Ther e i s pr obabl y no i nherent musi cal superi ori t y of t ubes vsnsi st ors. Al l known gai n - t r ansconduct ance devi ces have separ ate and i ndi vi dual char act eri st i cs. The desi gn pr obl

og Audio Amplifier Design http://depalma.pair.com/Analog/an

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nt erf ace t he acti ve el ement s wi t h t he ci r cui t t opol ogy i n the most harmoni ous manner .

ety years of devel opment of vacuum t ube ci r cui t r y dur i ng t he anal og er a have pr obabl y r esul t ed i n bett er Hi - Fi t hars of t r ansi st or ci r cui t r y devel oped dur i ng a t i me when anal og t echni ques ar e on the decl i ne and di gi t al t echnol ogascendency.

The Fi rst Ci rcui t

f i r st ci r cui t i s t he ol dest , maki ng use of a. c. coupl i ng, vacuum t ube act i ve el ements, t r ansf or mer coupl ed push- pal i near out put . Havi ng st ewed i n t he f erment of 50' s gol den age audi o, i t i s a combi nat i on of t r i ed and tr ue wi t hi l l at i on of t he i deas of D. T. N. Wi l l i amson and Norman Cr owhur st .

dr i ver ci r cui t r y i s t he i nput pent ode di r ect coupl ed t o a t r i ode spl i t l oad phase i nver t er . The pent ode sect i on, rates at a l ow pl ate vol t age maki ng f or sat i sf act ory di r ect coupl i ng to t he phase i nver t er gri d. Low f r equency gaise shi f t i s cont r ol l ed by the si ze of t he scr een bypass capaci t or.

s wel l known that even though si gnal cur r ent i s common t o both t he cat hode and pl ate l oad resi st or of t he spl i t lse i nver t er, because of t he di f f eri ng i ncrement al i mpedances at t he cat hode and pl ate t he ci r cui t can become unbalhi gh audi o f r equenci es. A ci r cui t has been devi sed, f i gur e (1) , t o pr ovi de l ow i mpedance dr i ve t o the out put t ubes

as cor r ect t he hi gh- f r equency unbal ance of t he phase i nvert er . I n thi s ci r cui t t he vol t age out put i s cont r ol l ed upper ( cat hode f ol l ower ) gr i ds and the cur r ent i n the l oad by t he dr i ve t o the l ower t ubes' gr i ds.


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h the combi nat i on as shown usi ng a 6AN8 i nver t er dri ver and t wo 6SN7s, 6CG7s, or 6FQ7s, t he f r equency response i snt i cal f or bot h si des, mi nus 3db @35kc.

ver out put i mpedance i s 600 ohms. The f r act i on of t he pl ate or cat hode l oad resi st or of t he i nver t er st age tapped ve t he l ower t ubes' gr i ds i s det ermi ned by exper i ment . The dr i ve t o one of t he l ower t ube' s gri ds i s adj ust ed f or ance or l owest di st or t i on.

pl i ng t o t he out put t ubes' gr i ds i s vi a a "st ep network" whi ch gi ves zero phase shi f t at d. c. Thi s i s i mpor t ant f obi l i t y. Out put i s push- pul l Ul t r al i near wi t h a t er t i ar y wi ndi ng f or di f f eri ng pl at e and screen vol t ages. Negat i ve d bi as vol t age i s suppl i ed t o t he out put t ubes by a separat e d. c. power s uppl y.

output cat hodes ar e t i ed t o ground t hrough a 10 ohm wi r e- wound common cat hode r esi st or . The si ze of t hi s r esi st or

ul at es out put st age di st or t i on at hi gh l evel s. Di st or t i on at hi gh l evel s i s r educed at t he expense of a smal l i ncrl evel I . M. The 10 ohm val ue r epr esent s a compromi se.

s essent i al f or Hi - Fi use that t he vol t ages appl i ed t o t he ci r cui t bui l d up gradual l y. Vol t age sur ges can causeshovers i n the out put t ubes whi ch coul d damage del i cate l oudspeakers.

- The Power Suppl y -

al l vacuum t ube f i r st ci r cui t r equi r es a r at her el aborat e power suppl y. At f ul l output , 250- 300 wat t s, i t must bemai nt ai n the out put t ube pl ates at 800 vol t s and the scr eens at 400 v. d. c. The l ow l evel st ages and the dr i ver ci ri sol at ed vi a L-C f i l t eri ng f rom t he out put t ube pl at e ci rcui t .

h si ngl e ended st ages i n the i nput dr i vi ng a cl ass AB out put st age unwant ed f eedback t hr ough t he power suppl y si ck

r t he audi o i mage.

goes wi t hout sayi ng t hat d. c. del i vered t o al l ampl i f i er st ages must be pur e d. c. wi t hout r i ppl e. Under cer t ai ndi t i ons t he ef f ect s of power s uppl y r i ppl e under l oad may be amel i orated by compl ement ar y ( t r ansi st or) ci r cui t r y. er hand t here i s no subst i t ut e f or pur e d. c.

ci r cui t one t he power t r ansf ormer provi des 800 v. a. c. cent er t apped t o a f ul l wave br i dge rect i f i er composed of f4 damper di odes. Under l i ght l oad, qui escent condi t i ons f or t he ampl i f i er, t hi s ci r cui t gi ves one-t housand vol t s fes of t he out put EL34s. The t r ansf ormer cent er t ap gi ves 1/ 2 vol t age, i . e. 500 vol t s d. c. f or t he scr eens.

vi dual f i l t eri ng of t he pl ate and scr een busses i s by t wo st ages each of pi sect i on 15 henr y chokes wi t h 10 mi cr oer f i l t er i ng capaci t or s, except f or t he i nput capaci t or whi ch i s 5 mi crof ar ads. The 500 vol t buss i s f ur t her f i l t e

more pi sect i ons usi ng 20 henry chokes and 10 mi cr of arad paper capaci t ors. Thi s l i ne feeds the dr i ver sect i on of cui t . The B+ f or t he i nput 6AN8 tr i ode-pent ode i s f ur t her i sol ated wi t h another pi sect i on usi ng a 20 henr y choke mi crof ar ad f i l t er capaci t or .

put t ube negat i ve gri d bi as i s suppl i ed f rom a separat e hal f wave si l i con di ode rect i f i er wor ki ng i nt o a si ngl e pi

i on f i l t er usi ng a 20 henr y choke and 100 mi crof arad l ow- vol t age el ect r ol yt i cs.

6AU4 damper di odes have good enough heater cat hode i nsul at i on so t hat t hey may be operat ed f r om a common 6. 3 v. a.di ng. The sl ow warm- up of t he heavy dut y f i l ament s appl i es t he hi gh vol t age t o t he ci r cui t wi t h a smoothl y r i si ng d- up. A separate 6. 3 v. a. c. wi ndi ng suppl i es t he f i l ament s of t he power amp.

of hi gh- vol t age paper f i l t er capaci t ors gi ves essent i al l y unl i mi t ed l i f espan as compared t o el ect r ol yt i cs whi ch aabout 20 years when operat ed wi t hi n r at i ngs. Use of paper capaci t ors wi l l ensure a const ant , l ow power suppl y i nte

edance over t he audi o r ange.

power suppl y shoul d provi de t wo f unct i ons. Fi r st l y t o suppl y st eady d. c. t o al l st ages, and secondl y t o i sol at e tges i n such a way that unwant ed f eedback l oops ar e not set up between them. Thi s i s especi al l y i mport ant wi t h si nged l ow l evel st ages. Because of compl ement ar i t y, t r ansi st or st ages can be bal anced to el i mi nat e common mode ( powerpl y) f l uct uati ons f r omt he out put . Tubes come i n one sex onl y so that t he bal anci ng oper ati on i s much more di f f i cudi ng t o the use of di f f er ent i al ci r cui t r y whi ch has i t s own compl i cat i ons.

s har d t o make gener al st atement s about power suppl i es f or audi o ampl i f i er s. Even wi t h perf ect bal ance, i nt ermoduresul t i f a gai n par amet er i s sensi t i ve t o curr ent . The pent ode- l i ke characteri st i cs of sol i d- st at e devi ces make

s sensi t i ve t o power suppl y vol t age f l uct uati ons t han t r i ode vacuum t ubes.

i ce i t t o say that power suppl y conf i gur at i on and topol ogy i s j ust as i mpor t ant as ampl i f i er desi gn. I f t her e i spl ement ar i t y i n vacuum t ube ci r cui t desi gn i t i s expr essed i n the r el at i onshi p of t he act i ve ci r cui t t o i t s power s i n t hi s t hat t he musi cal as wel l as t he el ectr i cal l aws are sat i sf i ed.

- Negat i ve Feedback -

he ci r cui t descr i bed 20db of negati ve f eedback i s t aken f r omt he out put t r ansf ormer secondary 8 ohm t ap t o t he cahe i nput pent ode. The ci r cui t i s set up by adj ust ment of t he f eedback resi st or bypass capaci t or t o pr oduce a cri t

ped 20kc square wave i nt o an 8 ohmnon- i nducti ve l oad. The f eedback network i s a st ep ci r cui t t o pr event excessi veh- f r equency f eedback i n a r ange where i nt er nal ampl i f i er and tr ansf ormer phase shi f t s coul d pr oduce osci l l ati on,


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eci al l y wi t h capaci t or ( el ectr ost at i c l oudspeaker ) l oadi ng.

h al l other l ow- f r equency ti me const ant s as i ndi cat ed a l ow- f r equency 2 c. p. s. d. c. f unct i on generator i s used f or ustment of t he scr een bypass of t he i nput pent ode. Per f ect r epr oduct i on of t he l ow- f r equency square wave i s what ir ed. I n t hi s ci r cui t wi t h 20db of over al l negat i ve f eedback thi s capaci t or shoul d be adj ust ed t o r epr oduce t he 2

are wave at t he out put wi t hout sl ope or t i l t . Too l ar ge a scr een bypass capaci t or wi l l pr oduce over shoot , i ndi cat iesi r abl e peak i n l ow- f r equency response. These t est s must be done at a l ow si gnal l evel i nto an 8 ohm l oad t o avoirat i on of t he core of t he out put t r ansf ormer.

ome r espect s ci r cui t one i s deri ved f r omt he earl i er ( 1948) Wi l l i amson ampl i f i er desi gn. The Wi l l i amson ampl i f i er bi t a const ant l ow- f r equency osci l l at i on r esul t i ng i n a "breat hi ng" acti on of woof er cones movi ng i n and out , t r i

wow or r umbl e i n t he r epr oduct i on of l ong-pl ay vi nyl di scs.

Wi l l i amson ampl i f i er was st abi l i sed by t he use of "st ep" networ k coupl i ng of t he dr i ver pl at es t o t he output t ube modi f i cat i on cor r ect ed t he l ow- f r equency phase r esponse and was or i gi nated by Nor man Cr owhurst .

cui t one i ncor porat es st ep ci r cui t coupl i ng to t he out put t ube gr i ds as wel l as the l ow- f r equency phase and gai nustment af f orded by var i at i on of t he scr een bypass capaci t or of t he i nput pentode.

a vacuum t ube ampl i f i er t here i s essent i al l y zer o ti me del ay between t he si gnal gr i d dr i ve and a cur r ent r esponse e. Thi s not t he case wi t h tr ansi st ors wher e t he f l ow of char ge car r i ers t hr ough sol i d semi - conduct ors i s much sl o

n el ect r ons movi ng through a vacuum.

l i mi t at i on i n t he use of negat i ve f eedback t o r educe di st or t i on occur s because of phase shi f t s i n t he ci r cui t el eng up to 180 degr ees at subsoni c and ul t r asoni c f r equenci es and t hus t urni ng negat i ve f eedback i nt o posi t i ve feed i s especi al l y t r ue when an out put t r ansf or mer i s one of t he ci r cui t el ement s. Experi ence has shown t hat 20db of

dback i s opt i mum i n a ci r cui t where i t t akes 30 - 35db t o make t he ci r cui t osci l l ate. Negati ve f eedback i s ver y usor t i on reduct i on i f not overdone.

al ma l i kes t o desi gn hi s ci r cui t s so t hat t here i s onl y one vol t age gai n stage, i . e. t he i nput pent ode. The subseqges are uni t y gai n cat hode f ol l ower s.

- Di scussi on -

eni ng test s by thi s aut hor have determi ned that di f f erent power out put t ube t ypes have di f f erent sounds. Di f f er ennds of t he same t ube al so sound di f f erent . The t wo generi c t ube t ypes are t he beam t et r ode and t he pentode exempl iKT- 66 and t he EL- 34/ 6CA7. The KT- 66 has a sweet , smooth sound, and the EL- 34 a sound whi ch has been charact er i sedI do not pr efer t he sound of t he 6L6, 6550 or KT-88. I t hi nk t he EL- 34 pent ode or i gi nal l y manuf act ur ed by Phi l i p

er by U. K. Mul l ard, and German Tel ef unken, i s t he best audi o tube ever made. For what i t ' s wort h, I l i st en t o a stem wi t h one channel EL- 34s and t he other KT- 66s.

uum t ubes have l ong rel i abl e l i f espans when oper ated wi t hi n rat i ngs, and especi al l y at hi gh vol t ages and l ow cur r e

f ast est way t o shor t en power t ube l i f e i s t o over - di ss i pat e t he t ubes.

re was a t i me i n t he hi st or y of audi o power ampl i f i er desi gn when output t ubes wer e r un at excessi vel y hi gh cur r eneve l ow I . M. di st ort i on. Thi s l ed t o shor t t ube l i f e necessi t ati ng r epl acement every si x mont hs or so of cont i nuoeni ng t o mai nt ai n specs.

end devel oped of operat i ng pl ates and screens at t he same vol t age. At t he same ti me, ampl i f i er manufacturer s t r i emi se output power, necessary because of t he ext r emel y l ow acoust i cal ef f i ci ency of acoust i c- suspensi on, AR, KLH,

ct r ostat i c t ype l oudspeakers. Operati on of pl ates and scr eens at t he same vol t ages, obt ai ned f r omUl t r al i near t appaudi o output t r ansf ormer pr i mary, l eads to a danger ousl y unst abl e si t uat i on as pl ates and scr eens ar e taken up t ovol t l evel . Excessi ve el ect r on cur r ent can cause the scr een wi r es to become i ncandescent , i ncreasi ng t ube curr ent r uct i on. The screens of EL- 34s shoul d be i n the "shade" of t he cont r ol gr i d wi r es. Lat er versi ons of EL- 34s manufhe 80' s and 90' s may not be as car eful l y assembl ed as the or i gi nal Phi l i ps const r uct i on.

best way t o get hi gh power and l ow di st or t i on i s t o operat e EL- 34s at hi gh vol t age. The EL- 34 can gi ve 100 wat t s/h 800 vol t s on t he pl ates and 400 on t he scr eens. Thi s necessi t ates a t ert i ary scr een wi ndi ng on t he out put t r ansfbest t r ansf or mer f or t hi s purpose i s t he Dynaco A-440, or one of i t s cl ones. At t he 800- 400 pl at e - scr een vol t ag

t r ansf ormer can gi ve 300 wat t s output bet ween 30 cycl es and 15kc, 200 wat t s 20- 20kc.

he ampl i f i er desi gn descr i bed no l oad vol t ages on the out put t ube pl ates and scr eens are 1000, 500 respect i vel y. 40 year s of exper i ence I have had wi t h t hi s desi gn no speci al probl ems have devel oped wi t h the except i on t hat t heages shoul d be appl i ed wi t h a cont r ol l ed bui l d- up whi ch i s obt ai nabl e f r om t he vacuum t ube damper di ode rect i f i erempl oyed. Tubes al so f l ash occasi onal l y f r oml oose i nt ernal part i cl es. A f ew sharp raps general l y l oosens t hem.

al ma and Haf l er di scover ed t he di st ort i on r educi ng pr opert i es of a smal l common cathode r esi st or i n cl ass AB ci r cun they were l ooki ng f or an easy way for t he cust omer t o set t he out put st age bi as. A 12 ohm r esi st or equated t o a c. dr op whi ch was equal t o the vol t age of a 1. 5 vol t dr y, zi nc- car bon cel l . I t al so t ur ned out t hat t hi s r esi st or

uced hi gh l evel I . M. by 2/ 3, down t o . 25 percent , whi l e l ow l evel , one-watt I . M. was onl y per cept i bl y i ncreased.

of a 10-12 ohmcommon cathode r esi st or i n cl ass AB push- pul l audi o power out put st ages al l ows a r educt i on i n qui ei c cur r ent t o achi eve t he same di st or t i on at maxi mum out put . Any reduct i on i n t ube cur r ent wi l l i ncrease t ube l i fl common cat hode resi st or t ogether wi t h hi gh- vol t age, 800- 400, operati on of t he EL- 34 can r esul t i n a t ube l i f e i

ess of 20 year s.


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i mport ant paramet er i n vacuum t ube l i f e i s bul b t emperat ure. Out put t ube t emperat ures hi gh enough t o burn f i nger s chi ng t he bul b evapor ate gett ers on t he i nsi de of t he envel ope. These tubes event ual l y become gassy and l ose emi ssone seri ous about vacuum t ube ampl i f i er desi gn must ei t her cont empl ate manuf actur i ng hi s own tubes or devel opi ng cch maxi mi se t he per f ormance and l i f e of t he exi st i ng r emai ni ng st ocks.

he earl y days of Hi - Fi t he eart h had not r eached i t s l i mi t of pl anetary r esour ces. Economi c consi derati ons had noched a l i mi t where pr i ce and "perf ormance" were i ntr a- conver t i bl e or even r el evant . The i mpor t ant f act was t o achical per f ormance f or one' s Hi - Fi syst em. The contemporar y aphor i sm i s not whet her i t i s musi cal but good val ue f or

ey. As we dr i f t i nto our uncert ai n f ut ure we have l ost t ouch wi t h musi c.

The Second Ci r cui t

second ci r cui t was devel oped as a hybr i d t ube- t r ansi st or audi o power ampl i f i er . I n t he or i gi nal desi gn a DynaKi t age ampl i f i er phase spl i t t er drove a 6360 si ngl e envel ope twi n tet r ode, Ul t r al i near connect ed. The out put t r ansf oa Dynaco A- 410 modi f i ed wi t h normal l y par al l el ed 16 ohm secondar i es separ at ed. These wi ndi ngs dr ove one or more p

e sex t r ansi st ors i n the manner s hown, f i gure ( 2).

ngl e power t r ansf ormer wi t h t wo capaci t i vel y i sol ated 40 vol t wi ndi ngs and a 250 vol t wi ndi ng f or t he two tube pher t er dri ver was used. The 6360 power st age need onl y put out a f ew watt s t o dr i ve the t r ansi st ors t o sat ur ati on. cui t const r ucted a bypassed common cathode resi st or pr ovi ded bi as.

second ci r cui t can al so be dr i ven by a t r ansi st or i sed vol t age-amp- phase- i nver t er - dr i ver . The sal i ent f eat ures of put ci r cui t are t hat i dent i cal same sex t r ansi st ors can be used and because of t he f l oat i ng nature of t he tr ansf or

pl ed dri ve, ei t her end of t he out put l oad resi st or may be gr ounded. Negat i ve f eedback to t he i nput st age cat hode (aken f r omt he ungr ounded end. Ei t her same sex bi pol ars or FETs can be used i n thi s ci r cui t .


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- Phi l osophy -

uum t ubes come i n onl y one gender . Transi st ors can be N t ype or P t ype. An N t ype t r ansi st or can be synt hesi sed f re and vi ce- ver sa, by what i s known as a quasi - compl ement ary ci r cui t . Quasi - compl ement ary ci r cui t s usi ng combi nat i oP type t r ansi st ors work wel l but di r ect f abr i cati on of f ai r l y wel l matched N and P type compl ement ary power t r anss has put t hese ci r cui t s on t he back bur ner.

cour se a puri st desi gni ng cl ass B t r ansi st ori sed audi o power ci r cui t r y woul d never consi der usi ng compl ement ary pang that t he matchi ng i s not perf ect . On the ot her hand thi s necessi t at es t he use of an audi o dr i ver t r ansf ormer, ter f ecti ons of whi ch l i mi t t he amount of overal l usabl e negat i ve f eedback.

r eason I consi der t he t r ansf or mer coupl ed dr i ver ci r cui t accept abl e i s t hat exact mat chi ng of t he output powernsi st or s i s possi bl e. Thi s i n i t sel f mi ni mi ses even or der har moni c di st or t i on. Smal l l ow- power , 10 wat t s or l ess, nsf ormers have excel l ent f r equency r esponse ext endi ng to 100kc f or t he A- 410. Capaci t i ve ef f ects bet ween wi ndi ngs r at t endant r esonances i n a smal l t r ansf ormer are wel l out si de t he audi o r ange.

ger and bi gger audi o out put t r ansf ormers normal l y used i n t he r ange 100 - 500 wat t s have def i ni t e hi gh- f r equencyonance and l eakage pr obl ems whi ch l i mi t t he over al l amount of negat i ve f eedback whi ch can be used. For exampl e, t h

out put of t he f i r st ci r cui t can be obt ai ned at about . 5% I . M. usi ng 20db of negati ve f eedback. Osci l l ati on woul d32db f eedback. I n t he second ci r cui t 100 watt s coul d be obt ai ned f r oma pai r of di f f used- base bi pol ars @ . 25% I . M. desi gn 25db of f eedback coul d be used wi t h osci l l ati on occur r i ng at 35db.

ause of st ored base char ge and pr opagati on del ay ef f ect s, negati ve f eedback i s not as ef f ect i ve i n di st ort i on r edut r ansi st or i sed power ampl i f i ers as f or t ube-t r ansf or mer desi gns. I n the same vei n, excessi ve negat i ve f eedback ma

at e non- har moni ous di st ort i on i n t r ansi st or ci r cui t s. Consequent l y whatever t he desi gn, t he pr i nci pl e shoul d be t ol owest di st ort i on bef or e f eedback i s appl i ed. Negati ve f eedback as a cur e f or di st or t i on when pr operl y appl i ed cahar moni ous and dest r ucti ve (osci l l at or y) si de ef f ects i f over used.

ke the second ci r cui t because i t can empl oy same sex matched and bal anced t r ansi st or pai r s t o achi eve any desi r eder. Even though a smal l l ow- power dr i ver t r ansf ormer i s r equi r ed, t he probl ems i t sol ves more t han compensat e f or a wel l desi gned and const r uct ed audi o t r ansf ormer produces.

s wel l known, t he vani shi ngl y smal l di st or t i ons whi ch can be obt ai ned i n f ul l y t r ansi st ori sed audi o power amps areessari l y ref l ected i n t hei r audi o l i steni ng qual i t y.

The Thi r d Ci r cui t

t hi r d ci r cui t was devel oped t o f ul l y ut i l i se compl ementar y symmet r y mat ched P and N t ype f i el d- ef f ect t r ansi st or s

e devi ces, a f ul l y d. c. coupl ed audi o power amp was desi gned. I n r ecent year s audi o qual i t y has been adver sel y afhe use of pl asti c di el ect r i c coupl i ng capaci t or s. There are wel l known memory and el ect r i c hyst eresi s ef f ect s i n ect ri cs. I f coupl i ng capaci t or s are used i n audi o ci r cui t s t hey shoul d be paper or met al i sed paper di el ectr i c. Ofse, d. c. coupl i ng removes al l capaci t or s as shown i n t he thi r d ci r cui t , f i gur e ( 3) .


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hi s ci r cui t t he musi cal as wel l as el ect r i cal l aws are f ol l owed. Compl ement ary symmetr y, FETs and bi pol ar t r ansi sal l ut i l i sed i n a compl etel y bal anced, symmetr i c, d. c. coupl ed conf i gur ati on. The exper i enced desi gner wi l l see t ci r cui t compl ement ary symmet r y bi pol ars can be subst i t ut ed f or al l t he FETs.

order t o achi eve d. c. coupl i ng t he f i r st st age of t hi s ci r cui t has a vol t age gai n of l ess t han uni t y. Bi pol ar s or be used here. The second st age i s t he vol t age gai n st age. Compl ement ary bi pol ars shoul d be used here t o i nsure maage, r ai l t o rai l , dr i ve. The second st age i s i nt erest i ng because i t i s a combi nat i on of t wo dr i ven cur r ent sour c

ng each other as l oad resi st or .

di odes i n t he emi t t er ci r cui t ar e used i n a dual mode. Usi ng compl ement ary bi pol ar s as r ecommended, t hese di odes per atur e compensat i on i n conj unct i on wi t h t he 1K base to gr ound r esi st ors . The ot her f uncti on i s t o ensur e the l owe i nput r esi st ance i n t he vol t age ampl i f i er st ages. The cur r ent suppl i ed t o t he gai n st ages by the i nput t r ansi st o

des between t he base t o gr ound resi st or and the i nput r esi st ance of t he gai n stage, i . e. t he l ower t he base i nputst ance t he more dr i ver curr ent f l ows i nt o t he gai n st age t r ansi st ors.

he base i nput r esi st ance were zer o, al l t he dr i ver cur r ent woul d f l ow i nt o the bases and the gai n of t he two-st agbi nat i on woul d be hi ghest . Subst i t uti on of compl ement ary FETs i n the gai n st age changes t he i nput s i t uat i on si nce vol t age act uat ed.

he gai n st age each t r ansi st or acts as t he l oad r esi st or f or t he ot her . Ver y hi gh gai n and l i near i t y i s possi bl e. ol ar s, t he dr i ve out put of t hi s st age i s al most peak to peak, r ai l t o r ai l , l ess t he sat ur at i on vol t ages and emi t tde vol t age drops. The gai n of t hi s st age usi ng bi pol ars i s 1000- 1500. Si nce the second st age operat es as a cur r ent ari abl e ser i es resi st or i n the common col l ect or ci r cui t can be used t o regul ate t he out put st age qui escent cur r ent

be appr eci at ed fr om t he ci r cui t schemat i c, any suppl y vol t age vari at i ons ei t her f r om mai ns f l uct uat i ons or l ong tmal t emper ature coef f i ci ent ef f ects i n the act i ve devi ces appear onl y as a common mode ef f ect and do not s hi f t t

ance poi nt of t he ci r cui t .

cui t t hree r equi r es a power suppl y wi t h exact l y equal and opposi t e out put vol t ages bal anced agai nst gr ound. Thi s eot her t hi ngs bei ng equal , t hat no d. c. of f set wi l l appear across t he out put l oad r esi st or f or zero vol t age i nput

i f i er . A pr oper ci rcui t uses a f our di ode f ul l - wave bri dge rect i f i er f ed fr om a cent er- t apped power t r ansf or merondar y wi ndi ng. The cent er- t ap becomes a 1/ 2 vol t age poi nt whi ch i s gr ounded. Two 1000 mi cr of arad el ectr ol yt i cs coer i es are used f or f i l t er i ng. The cent er poi nt of t he ser i esed capaci t or s i s al so gr ounded, pr ovi di ng adequat e f ihe pl us and mi nus 40 v. d. c. out put s.

d. c. bal ance cont r ol i s an i nt er nal set - up adj ust ment whi ch becomes i nef f ect i ve when t he overal l d. c. negat i ve f ep i s cl osed. Compl ete el i mi nat i on of any remai ni ng of f set vol t age i s ef f ect ed by a mi nut e t r i mmi ng of t he resi st an

of t he 1 megohm i nput r esi st ors .

e of a hi gh- power di r ect - coupl ed ampl i f i er f or hi gh- f i del i t y audi o r equi r es a l ow f r equency cut - off t o pr event damdspeaker s. )

ecti on of t he emi t t er resi st or s of t he i nput st age det er mi nes t he qui escent curr ent of t he gai n stage. I nt er est i ngn st age may be operat ed cl ass A or B.

emi t t er - f ol l ower f ol l owi ng t he gai n st age can be ei t her bi pol ar s or FETs. An i mpor t ant paramet er here i s t he i npuaci t y of t hese devi ces whi ch can cause a hi gh- f r equency rol l - of f . Thi s r ol l - of f can be used as hi gh- f r equency phas

pensat i on. When usi ng FETs, t hei r ext ended f r equency r esponse can l ead t o R. F. osci l l ati ons. Thi s can be cont r ol l ees "s t opper" r esi st ors of up t o a f ew hundr ed ohms i n t he gate ci r cui t s.

h FETs i n the i nput st age and bi pol ars i n the gai n st age, negati ve and posi t i ve t emperatur e coef f i ci ent s work agaih other . I f qui escent out put st age cur r ent i s t emperature sensi t i ve i n the sense t hat d. c. compensati on of t he i npges does not of f er compl ete cont r ol , t hen a ser i es di ode st r i ng bet ween t he out put dr i ver t r ansi st or bases ( or gatace t he r esi st or cont r ol shown bet ween t he dr i ver t r ansi st or emi t t ers.

t hi r d ci r cui t can dr i ve sever al pai r s of paral l el ed output t r ansi st or s. A compl ementary symmet r y FET emi t t er - f ol lves t he out put t r ansi st or s. The emi t t er - f ol l ower dr i ver f ed f r om t he dr i ven curr ent sour ce gai n stage pr ovi des l owor t i on symmetr i cal r ai l t o rai l dr i ve t o t he out put st age. Here agai n compl ement ary bi pol ars can be subst i t ut ed f

s.

output st age can be ei t her bi pol ar or FET. An al t ernat i ve met hod of r egul at i ng qui escent cur r ent i s a ser i es di odng i n t he emi t t er ci r cui t s of t he dr i ver s t age. Thi s mi ght be more i mport ant wi t h a bi pol ar out put st age si nce

per ature compensati on wi t h negati ve coef f i ci ent di odes i s possi bl e. The bui l t - i n negati ve t emperature coef f i ci enent i n FETs renders t hi s unnecessar y. As a str i ct l y pur i st comment , I do not l i ke t o use di odes i n a si gnal pat h.

be f orward bi ased and i n conduct i on, but t hey st i l l have a non- l i near curr ent - vol t age r el ati onshi p as compar ed t ostor .

- Negat i ve Feedback -

at i ve f eedback i n the t hi r d ci r cui t i s f r om t he hot end of t he l oad resi st or ( l oudspeaker ) t o the i nput emi t t ers. ai n best square wave response and tr ansi ent st abi l i t y t hi s r esi st or may be bypassed by a smal l capaci t or. I f t he fst or i s connect ed between t he t wo poi nt s marked f b and bypassed wi t h a smal l capaci t or, t he i dent i cal emi t t er r eshe i nput s t age can be adj usted t o be some f r acti on of t he mai n f eedback r esi st or. Thus a "s t ep" network i s cr eat et s f eedback at hi gh f r equenci es and i mpr oves st abi l i t y. Experi ment s wi t h t hi s ci r cui t showed t hat osci l l at i on ens

h 55db of f eedback.

desi gner shoul d produce a ci r cui t whi ch has i nher ent l y l ow di st or t i on bef or e f eedback i s appl i ed. Odd harmoni cor t i on can be bal anced out by symmetr y. Even harmoni c di st or t i on i s r educed by choi ce of i nherent l y l i near ci r cui


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emi t t er - f ol l ower out put and out put dr i ver ci rcui t s, and a l i near dri ven cur r ent sour ce symmet r i cal hi gh- gai n sta

h the above combi nat i on di st ort i on does not cont i nue to decrease above 30db of f eedback. J ust under cl i ppi ng and aput i nt o 8 ohms, I . M. di st ort i on of t he t hi r d ci r cui t i s unmeasurabl e or i n t he noi se l evel of t he i nst r ument . Powponse i s f l at f r om d. c. t o 100kc where t he out put i s r ol l ed of f del i ber at el y t hr ough i nt er nal compensat i on. One- ha

i nput dr i ves t he ampl i f i er t o f ul l out put . At 30db of negat i ve feedback no compensati ng capaci t or was r equi r ed af eedback r esi st or t o pr oduce a perf ect 20kc square wave.

- Phi l osophy -

t hi r d ci r cui t was desi gned t o t ake advant age of t he most advanced semi - conduct or t echnol ogy avai l abl e t oday ( 1990e f el t t hat t r ansi st or audi o power ampl i f i ers coul d equal or out - perf orm t he best vacuum t ube desi gns. Anal og audived i n the days of vacuum t ubes, but desi gn has t ur ned away f r omanal og t o di gi t al and the seri ous evol ut i on of a

nsi st or power ampl i f i cat i on has never t aken pl ace.

nsi st or s have made i t possi bl e t o el i mi nate t he audi o power out put t r ansf ormer f r om audi o ampl i f i er s. Compl ementarnsi st ors make new desi gn opt i ons possi bl e. The pent ode-l i ke hi gh i ncrement al i mpedance of t r ansi st or col l ect or ci ruces f i l t er i ng r equi r ement s of power suppl i es. Tot al d. c. coupl i ng i s possi bl e i n compl ement ary ci r cui t s t herebymi nat i ng al l coupl i ng capaci t or s.

- Per f ormance -

ke t he sound of al l t hr ee ci r cui t s, but t he t hi r d ci r cui t repr esent s al l I know about el ectr i cal and musi cal l awsnd i s sweet , smoot h, eff ort l ess and tr ansparent . 100 wat t s of audi o can be gott en f r om8 t r ansi st or s and 6 di odes. or y of t he devel opment of el ect r oni c audi o rests on t he cont r ol of t he f l ow of cur rent , f i rst l y vi a the vacuum t u

d, t hen by car ri er i nj ect i on i nt o a back bi ased semi - conductor j unct i on, f i nal l y cont r ol of t r ansconduct ance vi a actr i c f i el d pr oj ected i nto a semi - conduct or. Ot her means of curr ent cont r ol may be possi bl e and some mi ght becti cabl e.

rni ng to t he gol den r ul es of audi o, t he si mpl est desi gn wi t h the f ewest st ages i s pref erabl e and i f bal ance,pl ement ari t y, l i near i t y and power are t he musi cal f act or s I woul d pr ef er ci r cui t 3.

A f i nal t hought

r eproduct i on of musi c el ect r o- mechani cal l y does not l end i t sel f t o t he mani pul at i ons of t he di gi t al computer and genci es of appar ent l y "per f ect " anal og t o di gi t al and di gi t al t o anal og conver si on.

hoot s of cont empor ar y i nat t ent i on t o per f ect i on ar e t he oper at i onal and di f f erent i al ampl i f i er ci r cui t chi ps. Anagn has been rel egat ed to t he mi ni st rat i ons of speci al i st s who desi gn l ar ge scal e i nt egr at ed ci r cui t chi ps. Ampl i fnot desi gned but speci f i ed. Di f f er ent i al ampl i f i er s are i nst al l ed as i nput ci rcui t s wher e negat i ve f eedback i s noi ed t o the same act i ve devi ce as the si gnal . Thi s t act i c, of t en hal ed as a gr eat f r eedomi n t he desi gn of amps wianced" negat i ve f eedback, bui l ds i n i r r educi bl e di st ort i on f r om a st age of ampl i f i cat i on not i ncl udabl e i n t he ne

dback l oop.

degr ee of per f ect i on and subt l et y of desi gn r equi r ed f or Hi - Fi anal og audi o mandat es di scr et e desi gn of audi o ci rt he avoi dance of prepackaged i ntegrat ed ci r cui t s wi t h t hei r at t endant "commerci al " mass- produced sound.

ati ve f eedback must be appl i ed i n a si ngl e l oop f r omt he out put l oad ter mi nal t o an i nput devi ce. The si gnal and f i nter act i n the same act i ve devi ce. Those who care t o st udy t he subj ect of f eedback f ur t her shoul d r ead: Val l ey man, VacuumTube Ampl i f i ers , M. I . T. Rad- Lab Ser i es. Fur t her wi sdom on t he subj ect of negat i ve f eedback can be got

m t he works of Norman Cr owhur st .

paper has been wr i t t en f or prof ess i onal s i n t he f i el d of act i ve anal og audi o ci r cui t desi gn. I t i s not i nt ended w- t o" constr ucti on art i cl e. Wri t i ng a desi gn paper i s di f f erent t han a sci ent i f i c art i cl e. Desi gn i s t he sel ecti onopr i atel y chosen set of concept s t o moti vat e and act ual i se a part i cul ar f unct i on, i . e. el ect r o-mechani cal anal og

nd r epr oduct i on.

has been sket ched out i n thi s paper ar e the set s of concept s and choi ces DePal ma woul d, and has, made i n hi s puraudi o i mage expr essed el ect r oni cal l y. Ot her set s of choi ces ar e avai l abl e and can be woven i nto a r ati onal l y accepo i mage. The choi ce i s i n the eye of t he behol der and i n the ear of t he l i st ener.

ce DePal ma


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