Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)

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



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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er

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c

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q

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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



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

Documents

Electrostatic Headphones - N. Pollock (Wireless World, Nov 1979)