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8/20/2019 Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)
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RL ORD OBR 1979 5
Electrosatic hadphnesConstructinal design wth imprved aust utu
The eectrostatic headphone desgnsrvously pubshed n Wree Wodwhe gng good resuls, cannotroduce h sound ressure ees Thecan ao be dffut o construc due othe ma daphram-topate cearancesrequred. The headhone and machngdrver apfier presened here atemp toaevae hee probems
THE SUND PRESSRE output of apush-pull elctrostati transduc isdirtly proportional to th diffrialplat driv votag, th diapragmpolarizn potnia and th rcipocalof t suar of t plat spacin. n rnaly poarisin potial isdndnt on t pat spacin an ahi coustic ouput ca only bobtaind by using igh pat driv votag
far as I awar all commrcial
ctrostatic adpons us widan stpup transfomrs to product ncssary pat vola whn drivny a ow voa powr ampr
Opetionl amplfr
+ 15 - 6 Vols
by N. Pollock ME, M Aust
fortunat t dsign and construcionof suita tansfomr s vy ificuldu to t i inductanc owcaacitanc and ood inltorquirmn Bcaus of ts duts most construcional digns edrivn ctl by ig voltgaplifir Vav 1• and ansistor 3ampifs av bn used ut in al
cass t t sppy as n lss tan4 V Fo convninc th apifir htuppy is norally usd to provd thdiprag polarzin potntial With a400V potntial th minium spacrticks (plat spacn/2) is about 05mm whic placs srinnt rqrmts on pt flatnss and diaphragmtnsion
Hadpon units wi t ab ov scificatons a iitd to a aximum
fr fid rs sound prs sur (rl 002 a of abut 93 For ochsta
Fig Amplifie f r one hnel. Ntthat th bs sources A ad B suplyoth cannls.
I L9 A o woamplrs
c,
5p
ck
5p6
sic at raistic vou lvls a sonprs capaility of arond OB isncssy and fo rock usi vn
Apier dsignMst transistors urntly avaaav a mamum VCE of ss than 4 00 Hovr tr ar a ubr o spcal
dces dsignd fo tv horiontal dcion circuit which hav pa CE valus of about 5kV and powr ratinsof O or mor T psnt dsin isbasd on Matsushia 2S 200 totr s suc as t 206 BU209A105 PC 16RT or SK 115RT can
drcly sbsutdT cicit in Fi is a dvlopmnt
o an amplifir dscibd in rfrnc To ih volta cass A mplfirs arusd on of wic is drivn by a unityan invin bur pation of hamplifr s quit straihforard but i
must b rmmbrd tht th LM900 sa curt nput dvic wit both inptsclapd ar art A o th mplfirsar as om t h nd ow vol
SOOY
"
OYPoaizngsupy
Hdho u
W
1nM !kV
B o fuampe�
k l) lO 6V
T & Tr2 • 50 o r Quvl
8/20/2019 Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)
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5
tage supplies so no circuit changes oradjustmens are require if he suppl voltages re aere e suese hof 8V ows te trasistors to operatewel withn the sfe opertig e.The rtotype fuctied stisfctoth 900V suppl, bu above this thetrsists my suer from secondarbreakdowA headphoe amplifier sensitiit of
28V r.m.s. for maximum output wasseected so that it could be drien fomte eadpone output proided o mostamplifies.A kQ resistor fom t inputo gound prevens amplifier oscillatiowen te input is not connected Teprootpe as built on two pieces ofVeroboard to keep the ig and owvol tg ccuts sprt. Thi arrnge-ment limis the possibility of damagdue to costruction errors ad elpsamplier sbilit If instability occrsthe vale of C can e sligh icreasedTe tansistors are mounted on smal
separate et sinks o aoi the isua-tio problems of a common heat sin
Te power suppy design depeds onte trasformers avaiabe. The proto-ype used a vae power transfomerith a 300V widing to drive a fullwave
volage douer for the amplifier .tsuppy s shown in Fig 2. Te poarizigsupp was produced by a afwae voltage dober conected to a potentiometer cross te amplier t. supplyBcause the eadpone diapragmshave a og chargig time constatleg of te polrizing supp was
o considered ecessay. A separate
Fi g. 2 Powe suppe The t suppyhoud be ecouped ith 0.1 µF iccrac capaito near to eah
1N4007
Polrizngs
u ply800V-600
Amlfer sy'50 pen t 70V a 26An voe
JM peteer
H.T supy
25 V 3A7µ35 V
Pate
Plte15V rasfomer was used to providete amplifie Lt supply u if te ig
votage tran sformer as o as a 6Viamnt winding, tis can be used wiotage doulin
It i mpott tht th mplf dowe supp are housd i a mta casewic is entiated ad connected tmai eath To avoid goud loops theheadphoe amplifie signal easou derived frm t ampifie
WIRELES WORLD. NOVEMBER 179
Fig 3 Css-etion f a edpeasseb
Fig 4 Fixed plate onstuctin w symmetrcal pates are required fora tansdu The tix f 3hes shold ve th etngundutive a Aqudg is avai able f BDH Chemias, Bom RadPo Dset
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8/20/2019 Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)
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5e diaphagm mateal that I used
was a 127mm soft plastic foodstffappig film om its beaiou itapas to be idetica_I to Vit'fm '·2•Ae extensive expemetat10 wtig eisace coatigs tat theucoated m w s ey igh s acresistance, gave excellent eslts p vd tathe diaphagm cotacts weeaanged as descie Ths methodemoved oe of the most diicult stepsin headphoe costuctio
To assemble the diaphagm cut a holesomewhat lage tha e plaes i aigid sheet of cadoad stetch a piceof diaphagm mateal css the holead attach it to the cadboad withadhesive tape When the film as bemade winkle fe place a plate andspacer asemly on a se o e m,hold them mly togethe ad bolt thecomplete uit via the peilled holesad diaphagm inally cut the potudig diaphagm fim aoud the oside of the asemly with a azo blade
To test the headphoe uits, connectthe ve amplifie wit the polaizingpotential set to its mnimum value of800V ad check that the diaphagemains cental f the diaphagmaaches iself o oe plate o oscillaesat a low euency it must be tensionedThis ca be doe by heatig theeadphone asmbly with a adiato oght bulb until the diaphagm winklesat which poit it is left to cool
Afte testing both units conect theleads to the eadphones and insulate aepos contacts with silicone ubbe
e dve its should then be enclosedin acoustic dampes constucted omenvelopes of 6mm foam plastic Theseeveopes may be sewn aound teedges of glued with ubbe cotactadhesive The dive it mountingaangeet will depend on te peence of he constucto he ptoye used an acylic bidge between woof the assembly scews which potudedhough the foam plastic dampes Thisbdge was then attached to the head-bad om an old pai of headphoe asshow in g 6 The conecig wies
etwee he headphones and amplifieshould be less tha .5m lng a ndlosely bundled athe than twisted tominmize thei capacitace
With oth ansduces conected oe amplifie incease the polazgpotetial to jut below the value whichcases diaphagm collapse o aigapbeadow Fo the pototype a maxmum ential of 3kV was set by theose of low fequecy clicing sounds
Safethough he ig impedace of hepolaizig supply esues that it s noletal, a uncomfoabl shock can beeeve om he laes a high oupuTe foam plasc evelopes ad hesula o he coecig leads mus speced a egula erals Pod a ese pcauios ae ake
WILSS WOLD. OVMB 979
Fg 6 Authr's prototype headphoe and power supply.
FREQUNCY Hz
10 00
R H0k
- icrophone i if ear(b)
Fg 7. (a) Heaphne frquency ress withut fam pltic dmpers
and ) with dampers
and common sense is execised, theheadphoes ae completely safe ow y a no ecommne o useby childe
Headphone perrmace
Peomance measuemets wee madewith a 15mm diamete B & K condese cophoe i conact with hecene of the dampe o the cete of theplae whe the dampe was nt fiedo absolute sound pesue measuemets the system was calbaed with aB & K pisophoe
Whot the pastic dampes thehaphones had a esonance at abouz see Fg a ad a pooceoveshoot on the ecovey fom ta
sients Fig 8a Additio of he dampespoduced a euency esponse within±B between 40Hz and 3kHz wih ooveshoo o tasiens as sown iFigs 7b and 8b Placng he mcophoei a cude icial ea ompising a60mm caviy wih a 20m ameeoce conacting the dampe exededhe low feuecy 5dB poi down o0Hz Bef tests wih he cophoe
8/20/2019 Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)
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WIRELESS ORLD, NOVMBER 1979 55
ig 8 Headphon e respose to 20µs pulses at kz (a) without dampes n ) with dampe s.
ov o th ee o eheadphoe ut dicated tha heaeage esponse oe he wholeaphagm aea wa uch ate thaho Fg 7b. h m soudpessue podce by a 1400V peakto
peak ffeetia pate oage at OOHzo 5kH was 102dB (e 00002 Pa)Duig extese sts o dffclies
ee expeeced it the ucoateddiaphagms When the polaizigpotetia is appied the chae speadsove he diaphagm ad he headphonouput se o s fu aue ithi a fewsecods Ude extemely dy codi-to t s coceiabe that th faceestace may become suffcietly higho pevet chage speag If hsoccs gete beahig o he daphag hough the dape shou po
de a ce t shoud be noted, hoeethat ee a fe pece of fuff bdgghe daphamopate gap ill beedaway the aphag chage a ecethe heaphoe oupu
Lsteng tet hae been ade wth ade age of muc he udb peac is make by geat caty andI hae deected o ults The acousticoutput ees ae moe ha adeuae most lee but oe oc uscethusats mght pefe aohe dBThe quaty of the ecodg adeucto equipet is ery mot
a because a efects ae hea uchoe cealy ha wh oudspeaes ofeo headhoe T pobe wihpogae ouce quaty as asooe ef Howee th he bestaaable ecodngs ad good quatyepe e pe esuts aobtaed
Rfcs1 Wilson JP High quaty elcrostatic
adphos Wireless World, Dec 19682. Hy PD, Elcsc pn dsign Wirss Wrd Nov 1713 Polock Elecrostatc haphone
ampifrWires Wrld
Circui Ieas Jy1976 4 Fderisn T.M, Howar WM andSeth RS Te LM 900 a e cure
dff ercig quad f ± input am plif ie rsationa Seconduco applicatios not2, Set 19725. Baxda P.J Ao power ampi esgn Wirss Wrld Jan 786 Ht FV lecroacouscs HarardUiersty Prss 4
ppendFactors affctig acoustic outt
or a ush-pu eectostatic rasducr with a apram cntray mountd betw n wo pats
eQ F=-zd(I)
hre s th frc actng on h daphragm s the difrena pate otage Q s th tota charge on th daphragm and d s th sacer thcnss Aso
Q= EC 2 whr E s th polarzg poentia C s h total daphragmtoplate capaci tac nd
AC- 3
d
whreA s th daphragm area y combg
the aboe quatons w can show thatEA
f_ 4
In practce E and ar telatd b y argap breawn and daphragm stabty unt6 has shown that for daphragm stabty thamum vau of E is lmtd n th fo lowg mannr ( Td3 ) E maxc A
5
where T s th diaphragm tnsonFrom the ast two quations th maxmm diaphrag orce a henc th maximum
acoustc ou s gv n by
F T )" 6
ma d
If he transducer competey cors the ear t forc per ut area is a more reean prmt and he aboe exprssion ca be reiten o g e
7
or
(! ce )'h
Amax\
8
Th mamum oarizng potntia that can b usd bfr the daphagmopat arga arcs s gvn by
herfo
l!) :A d
1
or
· �A
ma
11
For th daphragm teson and desons of a ypcal adphone transducer ahagm ns tab y usa ly oc cu rs b fo re a rg ap badown so uatos 7 ad 8 sho b sd For high ecy transdcers sing high tnson apragms equaions 1 woud probaby be apopate
From eqatons 7 ad 8 h d s g n rqemets for a hgh ouput transdc r ar; a hg a hra tension and sma l dapam area o gi th higes accep ab daphragm reso nant freq nc y Th e smals pae spacng cset wh ow fr e u nc y di ap hr ag m m o m e n t ad maufcrng fcty
I s imo to no tha a reducion in oup cause by reasing d canno be ofst smpy by nresn E.
e urNel Polock studd mechanica n-gnerng at Melbon Unvrsty nd,
afer gauatg spent te yas workng wt acrat ght tst nstumentaton. N tn gand a masters dege
or wor on th dsgn o aoo sctonso tansonc spds He a so has nnterst n ectronc and optcal nstrumetton or wnd tunnes N says
hs man obby s desgnng unconventona mechanca and ectonc devcswhch avod th noal txtbook method
H aso adts that athogh teresting
hs apach s not aways as good as thestandad mthod
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