LAYOUT AND THE CONTROL OF INTERFERENCE 



should be double- (i.e. cathodally) screened, polythene insulated, in the 

 interests of good high-frequency response. 



Amplifier protection — magnetic fields — Whilst the greater part of this 

 section is concerned with minimizing the area of the loop formed by the 

 input circuit, this is an appropriate point to draw attention to the fact that 

 some kinds of amplifier, e.g. tape recorders and microphone amplifiers, 

 possess an input transformer to match the playback head or microphone to 

 the first valve grid. The secondary winding of such a transformer consists 

 of an extremely large number of turns, each of which represents a small loop, 

 and is in consequence very prone to hum pick-up from magnetic fields. The 



Figure 42.11 



solution is to enclose it in a mumetal box. The highly permeable mumetal 

 provides a by-pass for the magnetic flux, as indicated in Figure 42.11. The 

 magnetic properties of this useful material are destroyed by any sort of cold 

 mechanical working, and a special heat treatment is necessary to recover 

 them. For this reason the fabrication of such boxes is best left to profes- 

 sionals; most manufacturers of signal transformers can supply suitable 

 boxes for their products. 



With biological amplifiers the mechanical layout of the first amplifying 

 stage is all-important. If the amplifier is double-sided, the valves should be 

 placed close together, and the leads joining them to the electrodes, whether 

 via cathode follower probes or not, must run close together {Figure 42.12). 

 In addition the preparation earth should be made by a short wire to the 

 screening of one of the input cables. In the case of single-sided amplifiers, 

 the braiding of the screened cable must be used to provide the return path 

 for the signal current after passage through the input circuit of the amplifier, 

 and this braiding must be earthed at the amplifier at one point only, namely 

 the earthy end of the first stage cathode biasing resistor {Figure 42.13). The 

 screening must not make electrical contact with the amplifier case where it 

 enters it. If a single pole connector is used of the popular coaxial type in 

 which the fixing lugs are electrically continuous v/ith the outer conductor, 

 then these lugs must be insulated from the case by suitable fibre washers. If 

 an input potentiometer is included it must be wired as shown in Figure 42.14. 



Preparation protection — electric fields — In any form of electrophysiological 



661 



