Telephone call Voice Changer

Voice manipulation device specially intended for props
9V Battery operation

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.


Parts:

• 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

Notes:

• 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.