ACTIVE POTENTIOMETER
The potentiometer introduced by Panasonica little while ago is of a qualityexceeded only by the likes of the Penny &Giles potentiometer (which cost in excess of£ 100). The Panasonic devices have multilayertracks made from conductive plasticsand carbon, which are linked to the terminalsby silver electrodes. The five-foldwiper is also made of silver and guaranteeshigh accuracy (tracking within 0.8 dB) andsmooth operation. In other words, this is anattractive, reasonably priced, high-qualityvolume control.
The potentiometer is a standard devicewhich is preceded by an input amplifier andfollowed by an output buffer. It can be insertedinto a line connection, so that appliancesthat have no volume control can beexpanded to complete control amplifiers.With the component values specified inthe diagram, each op amp amplifies 2.24to give a total amplification per channel of5.This is sufficient to raise the line level of200 mV to the standard output amplifierinput level of 1 V.
It is possible to alter theamplification to some extent, but it is advisableto carry any changes only to the bufferstages (IC2 and IC4). For example, the amplificationof IC2 is 1+R6/R5. In most applications,this will do fine. With an input signalof 2 V (for instance, from a CD player),there is still a headroom of 6 dB.If there is a need to add a selector switchat the input, R1 and R8 may be omitted.Bear in mind, however, that it must be possiblefor a bias current to flow.