Help with Voice Changer Circuit

Thread Starter

jj_alukkas

Joined Jan 8, 2009
753
Hi everyone,
I started with a new project yesterday for voice changing using dual op-amps.. Its a band pass filter but when I tried building it, it returns a houling sound all the time.. The pre-amp(1st section) and power amp( 3rd stage) works fine when coupled directly without the band pass stage in between.. but when i connect the 2nd stage without even giving an input, it starts to houl.. something is making it oscillate so badly.. I replaced the op-amp chip and changed power supplies and batteries but no gain.. Somebody please help me with this.. i suggested this circuit for my friend's mini project at college and now it doesnt give output, so I tried it myself and no gain..
This is the link for the site
http://www.redcircuits.com/Page112.htm

This is the circuit

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
 

Thread Starter

jj_alukkas

Joined Jan 8, 2009
753
I have breadboarded the whole circuit and mic stands on a 2 inch wire and not directed to the speaker.. but that is not important as the circuit gives a siren like sound even without the mic or preamp connected.. The middle section is the problem.. The houling sound is so large that i cannot distinguish any other sound in it even if the preamp is feeding in a good signal.. the preamp with the audio amp alone works fine..
 

bertus

Joined Apr 5, 2008
20,181
Hello,

It sound like the middle section is oscillating.(acting as a wienbridge oscillator).
Does it happen with all positions of R9 ?

Greetings,
Bertus
 

flat5

Joined Nov 13, 2008
403
R13 & C7 are providing positive feedback, right?
With 15k & 10nf...I don't know what freq. to expect.

Also, he's using a breadboard. Are the impedances high enough to cause problems with stray capacitance?
 

Audioguru

Joined Dec 20, 2007
11,251
The middle opamp is supposed to be a Wien Bridge resonator tuned to 1050Hz.
It oscillates when its gain is 3 or more.
R9 adjusts its gain from 2.48 to 3.02.
The values for R10 and R12 must be correct or it will oscillate all the time.
 

bertus

Joined Apr 5, 2008
20,181
Hello,

As audioguru said, the gain must be lower than 3.
Change R10 to 12K and R9 to 4k7.
This will give a gain cange between 2.08 and 2.89.
Thsi way the circuit will not oscillate.

Greetings,
Bertus

PS I do not reply to PM's
@ venkatapathy: The way you ask the question is hijacking the thread, the thread belongs to jj_alukkas
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,251
it works but the voice change is insignificant.
can we make tat significant
someone pls reply
It is a stupid circuit. It does not change the sound of a voice, it just makes frequencies near 1050Hz resonant.

If you turn up the pot then a voice might sound like a single-frequency robot.
 

Thread Starter

jj_alukkas

Joined Jan 8, 2009
753
Sorry guys for the long delay but thanks to all for the replies... I changed only the R10 to 12k from 10k and oscillating problem went away... Now signal is clean but works just as a preamp.. Can someone help to modify it to produce a small voice change?? This project was accepted as my friend's project and now cant change the title.. Its ok if its a robot voice...
 

bertus

Joined Apr 5, 2008
20,181
Hello,

Change R9 to 4.7 K , R10 to 12 K and R12 to 22 K.
Now the gain range is from 1.76 to 3.23 (greater than 3 will oscillate).

Greetings,
Bertus

PS did you see the other schematic, it will give a much greater voice change.
 

Thread Starter

jj_alukkas

Joined Jan 8, 2009
753
Bertus thanks a lot for the other circuits, i have saved them for my future use. I couldn't use those for the current project as circuit and title once submitted to college can't be changed once they are accepted.. Also I will try your resistance suggessions and give feedback.. Hope capacitance change is not required.. Thanks a lot Bertus..
 
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