Preamplifi ers and Input Signals 221In high-quality equipment, this problem is sometimes avoided by replacing R 1 by a
precision resistor chain ( Ra – Rz ), as shown in Figure 7.51 , in which the junctions between
these resistors are connected to tapping points on a high-quality multiposition switch.
By this means, if a large enough number of switch tap positions is available, and this
implies at least a 20-way switch to give a gentle gradation of sound level, a very close
approximation to the required logarithmic law can be obtained, and two such channel
controls could be ganged without unwanted errors in the differential output level.
7.18.1.2 Circuit Capacitances
A further practical problem, illustrated in Figure 7.50 , is associated with circuit
capacitances. First, it is essential to ensure that there is no standing DC potential across
R 1 in normal operation, as this will cause an unwanted noise in the operation of the
control. This imposes the need for a protective input capacitor, C 1 , which will cause a loss
of low-frequency signal components, with a 3-dB LF turnover point at the frequency
at which the impedance of Cm is equal to the sum of the source and gain control
impedances.C 1 should therefore be of an adequate value.
Additionally, there are the effects of the stray capacitances, C 2 and C 3 , associated with the
potentiometer construction, and the amplifi er input and wiring capacitances, C 4.
RzRyRxRwRcRbRa
0V 0VEoutEin C^1Figure 7.51 : Improved gain control using a multi-pole switch.