P1______________10K Log. Potentiometer R1,R10__________10K 1/4W Resistors R2_______________1K 1/4W Resistor R3______________50K 1/2W Trimmer Cermet or Carbon R4,R6,R7,R14___100K 1/4W Resistors R5______________47K 1/4W Resistor R8______________68K 1/4W Resistor R9_______________2K2 1/2W Trimmer Cermet or Carbon R11_____________33K 1/4W Resistor R12_____________18K 1/4W Resistor R13_____________15K 1/4W Resistor C1,C2,C3,C8,C9_100nF 63V Polyester Capacitors C4______________10µF 25V Electrolytic Capacitor C5_____________220nF 63V Polyester Capacitor (Optional, see Notes) C6_______________4n7 63V Polyester Capacitor C7______________10nF 63V Polyester Capacitor C10____________220µF 25V Electrolytic Capacitor IC1___________LM358 Low Power Dual Op-amp IC2_________TDA7052 Audio power amplifier IC MIC1__________Miniature electret microphone SPKR______________8 Ohm Small Loudspeaker SW1____________DPDT Toggle or Slide Switch SW2,SW3________SPST Toggle or Slide Switches J1____________6.3mm or 3mm Mono Jack socket B1_______________9V PP3 Battery (See Notes) Clip for PP3 Battery
Although this kind of voice effect can be obtained by means of some audio computer programs, a few correspondents required a stand-alone device, featuring microphone input and line or loudspeaker outputs.
This design fulfills these requirements by means of a variable gain microphone preamplifier built around IC1A, a variable steep Wien-bridge pass-band filter centered at about 1KHz provided by IC1B and an audio amplifier chip (IC2) driving the loudspeaker.
- The pass-band filter can be bypassed by means of SW1A and B: in this case, a non-manipulated microphone signal will be directly available at the line or loudspeaker outputs after some amplification through IC1A.
- R3 sets the gain of the microphone preamp. Besides setting the microphone gain, this control can be of some utility in adding some amount of distortion to the signal, thus allowing a more realistic imitation of a telephone call voice.
- R9 is the steep control of the pass-band filter. It should be used with care, in order to avoid excessive ringing when filter steepness is approaching maximum value.
- P1 is the volume control and SW2 will switch off amplifier and loudspeaker if desired.
- C5 is optional: it will produce a further band reduction. Some people think the resulting effect is more realistic if this capacitor is added.
- If the use of an external, moving-coil microphone is required, R1 must be omitted, thus fitting a suitable input jack.
- This circuit was intended to be powered by a 9V PP3 battery, but any dc power supply in the 6 - 12V range can be used successfully.