what hard to get on with by my rejoinder to the above can be no morę enthusiastic than “Yes, but . . . ” First off, the impedance and sensitivity of hifi headphones can vary widely with make and model. An 8(1 rating may indicate either the nominał internal impedance, or signify that they are suitable for use with an 8(1 Circuit, even though their actual impedance is much higher.

Furthermore, because headphones are frequently bought as an afterthought and plugged into an existing system, am-plifier designers have to be cautious about how they feed the headphone jack. If they wire it directly to the output Circuit of a power amplifier, and the user unthinkingly plugs in a pair of actual 8(1 phones, he may zap his phones or his ears or both!

Even if the user avoids that situation by discreet use of the volume control, connecting actual 8(1 phones directly across the output Circuit may expose residual hum and noise not au-dible in the loudspeakers — a point mentioned by A.J.

The simplest and most obvious answer is to insert resistors in senes with the respective left and right phone feeds so that, if actual 8(1 phones are plugged in, the level will be limited to something subjectively comparable to that from the loudspeakers. With higher impedance phones,the voltage divider effect will be proportionately less and the level hope-fully still adequate. (In the case of the Playmaster 60/60, the senes components comprised a 330(1 resistor and a 22/iF ca-pacitor).

But what about the damping factor? Frankly, Tm not too surę!

With rare exceptions, loudspeakers are expressly designed for use with a Iow impedance driving source and their performance is measured and rated on that basis. If used with a high impedance drive amplifier, approximating constant cur-rent rather than constant voltage feed, the bass may be un-derdamped, particularly in non-sealed enclosures, and the mid/upper frequency response may begin to look like the impedance curve.

But headphone diaphragms operate in a much morę con-fined environment, at a much lower amplitudę and power level (a few milliwatts) and directly into the ear canal. Some are cushioned on both sides by entrapped air, some by layers of foam, while others are said to be "open”, whatever the implications may be.

I simply cannot recall ever having seen morę than passing reference in articles and textbooks to the effect of drive impedance on headphone damping and frequency response.

In his “The audio Handbook” (Newnes-Butterworth, 1975) Gordon J. King shows both a series and a series-paral-lel feed, similar to that suggested by A.J. and observes:

“Better attenuation is provided by the network shown . . . with the resistor values arranged so that (each headphone) is loaded across a Iow value resistancc for the best damping, though the relative importance of damping for headphones is somewhat debatable”.

On the other hand, he admits in the very next sentence that, while many manufacturers are content with a series feed, some go to the trouble of including the headphone feed in the feedback Circuit — which is really doing it the hard way.

The problem is that both arrangements — shunting and feedback — tend to render the headphone feed less versatile, meaning that it really needs to be optimised for headphones in a particular impedance rangę.

On reflection, AJ., you probably cast a linę with a new', nicely baited hook!    *    £)

REMEMBER THE FIRST TIME YOU HEARD DOLBY SURROUND® SOUND IN A THEATRE?

WAIT ’TILL YOU HEAR IT AT HOME!

AUDIO

About the Shure HTS 5000

"Once you hove . soeo ond hecyd O p»ope» Dotoy Stewo movie - i

P»t“iŁ'fYfoVXT m

ycxy owyi home youH new be sotni* <o *rth ord-nory. goKJen vnriety tetewsmn "

(Wth me Shure HTS SOOO) “... me

ottoct «iow-wbetmłnę. OoKjg v*» crópondcleon <tnd the strroaphor*- musie and speckji ettects vwO fGproduced Dy Itw system with jfun ning Ctarty and import."

"You ran or.tuoiiy ocnłeyr* a much hlgh«* quolify ot sourw) ttxxi in most iXjtDy Stewo fheati* iTstoiiolons "

Bert Whvte

If you have any cłoubt about how good Dolby Surround* Sound is with home TV. read these brief comments by independent authofities.

STEREOPHILE

¹ About m« Shme HT5 5000 ' tt n rare In

oudto to find o , c^eoicut besr ot anything. but m iunoufTd

decode<s me Shure me honch-down wmner. Simply put. me rooion o its supefb sound it nos focus. defoit. detimtkxi and oi<veness that l'd not p»eviousiy heofd »tom <sny suround decodłKS o synmesi/e^s...”

' The owal etter t i$ spec tocuiai and oumoritarłvo - p«ecise»y w*yjt one wants ttom theotrtcal sound"

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SHURE HTS 5000 HOME THEATRE SYSTEM

HTS 5000

AUDIO ENGINEERS PTY. LTD 342 Kent Steet. Sydney, NSW 2000 Ph: (02) 29-6731

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Ph: (03) 850-4329

ELECTRONICS Australia. September 1987

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