Speech Eroder
Nowadays, the speech quality on our telephone systems is generally very good, irrespective of distance. However, there are occasions, for instance, in an amateur stage production, or just for fun, when it is desired to reproduce the speech quality of yesteryear. The eroder circuit accepts an acoustic (via an electret micro-phone) or electrical signal. The signals are applied to the circuit inputs via C1 and C2, which block any direct voltage. The input cables should be screened. The signals are brought to (about) the same level by variable potential dividers P1-R1-R4 and P2-R2-R3, and then applied to the base of transistor T1. The level of the combined signals is raised by this preamplifier. The preamplifier is followed by an active low-pass filter consisting of T2–T4, C3, C4, R6–R8, and P4. Circuit diagram: Speech Eroder Circuit Diagram Although, strictly speaking, P3 serves merely to adjust the volume of the signal, its setting does affect the filter characteristic. Note, by the way,...