Practical microphone equaliser circuit. - SSB TX audio equaliser
CIRCUIT DESCRIPTION. The circuit diagram of the equaliser is shown in Figure 1. The circuit is based on a fairly conventional single transistor amplifier. R1 provides a DC bias voltage for the electret mic. R3 biases transistor TR1 and R4 provides the collector load. C1, C2, C7 & C8 provide high-pass audio filtering, whilst C3 & C7 provide lowpass audio filtering. RV1 sets the low frequency response of the equalisation curve.
R6 can be changed to give a more suitable output level if required (minimum 470Ω, maximum 10K). RF filtering is provided by L1, L2, L3 & C6, but these can be omitted if they are not required. The inductors can be almost any general-purpose chokes. R7 is wired in line with the DC supply lead and is intended to prevent excessive current being drawn from the supply in the event of a fault or mistake occurring during construction.
If you wish to use a dynamic microphone in place of the electret insert, a small adapter circuit is required. This is the one, shown to the left of the circuit. This is a simple common-emitter amplifier and serves mostly to increase the output from the mic. Join the extra circuit at point A (arrowed) and omit the electret.
More..... Improving the intelligibility of SSB transmissions