Voice Modulator

[nucleopolymere]

Hello,

For University, we have to do a project which I decided would be a Voice Modulator. I can use bipolar , FET , op amp, capacitor and incuctors. For now, I am just trying to build the amplifier. I tried a circuit which will be in attachments but it did not work. With my gear, I measured the output voltage of my voice through a microphone and it gave me 10 mV of magnitude. For the output of my amplifier i think i would be fine with a 10 V of magnitude. The speakers i use are 5 W, 8 Ohm. Can you please tell me what is wrong with my circuit because i cannot figure it out, also why is my output signal (V2) is like a square wave ?

Thanks

[tpowell1830]

Too much gain.  Put a 1k resistor between Q1 and Q?. Adjust as needed.

[Audioguru]

The circuit shows a class-A heater, not an audio amplifier. If the collector of the 2N3904 is at 6V so that it can swing up 6V and down 6V then the collector current without a signal is 6V/R2= 750mA and the transistor heating will be 6V x 750mA= 4.5W. But its datasheet shows that it works poorly above 50mA (its max allowed current is 200mA) and it is destroyed if it heats with more than 0.625W. The transistor current and heating will nearly double with a high signal level when it drives 750mA into the speaker plus the 750mA on R2.

Why not look up "audio power amplifier circuit" in Google to see many class-AB complementary output stages that use power transistors. Class-AB has an idle current of only about 30mA. The NPN emitter-follower pushes up and the PNP emitter-follower pulls down. A push-pull output stage.

Your SIM software does not know that the 2N3904 transistor will not work and will be destroyed.
Why is the input 1V peak and not 0.01V peak like from your microphone?? Then the 2N5400 transistor is cutoff then is saturated, producing a squarewave.

Transistors in an audio amplifier are never cutoff or saturated and the circuit has negative feedback (yours has none) to cancel distortion.
Look at this amplifier circuit:

[nucleopolymere]

So i should use power transistor in the final stage of the amplification and have some feedback so it dosen't saturate. But like if i have a 5W speaker should I use 5W transistors ?, I can't figure it out how to estimate the power requirements for the transistors.
I did not know about class A, AB amps, I'll check that out thanks !

[orolo]

I think you are mixing the preamplifier, which takes the signal from the microphone, with the output amplifier, whose work is to give a lot of power. Between preamp and output amp, you would typically place the modulator.

Your MIC probably has a very high impedance, so the common emitter amplifier that you are using will probably load it too much. A good option is to use a common drain JFET as the first step in the preamplifier, to avoid loading the mic. Then you can amplify the resulting signal as much as your modulator needs. Since you can use op amps, the best is just use one buffer and then add as much gain as you need. It can't be easier.

[nucleopolymere]

Hello sirs,

I order to make my modulator work i have to make a pre-amplifier for the voice input. But I don't understand why it gives me a negative voltage at the input of the buffer (at mic_to_buffer),  can you please help me ? 

The multi-vibrator circuit seems to work fine. mic_to_buffer is the signal before the buffer and micvo is after. both the signal coming from the multi-vibrator and the pre-amp are multiplied together in the multiplier module.

Also why there is no voltage flowing through my speaker ?

elec and elec1 are the signal that were simulated. elec2 is the multivibrator and the pre-amplifier(voice) elec3 is the multiplier and the transistor driving the speaker
VCC is 9V and VEE is -9V

Thanks!

[Audioguru]

Your Mic-vo signal is negative because its opamp's (+) input is missing a biasing resistor to ground.
You do not show a value for Cout. It must be at least 470uF to pass voice frequencies. I think yours is less than 1uF so it passes only the very high frequency edges of the squarewaves.
Your Mosfet is a class-A heater with no negative feedback and it is not biased properly. It is probably almost fully turned on instead of being a linear amplifier.
Your transistor T3 at the microphone is also not biased properly. It might be turned on or turned off.

What do you want the "modulation" of your voice to sound like? Extremely distorted with a loud buzzer sound in the background?
There is no "modulator", it is simply an audio mixer. An AM modulator changes the amplitudes of your voice. An FM modulator changes the frequencies of your voice. You simply have your extremely distorted voice mixed with a very distorted buzzer sound.
Why do you have so many rectifiers?

[nucleopolymere]

hi, sorry for the delay,

Why do you have so many rectifiers?

I have 3 rectifiers to multiply the two input signals. I've took this circuit from this site https://en.wikibooks.org/wiki/Electronics/Analog_multipliers

An FM modulator changes the frequencies of your voice. You simply have your extremely distorted voice mixed with a very distorted buzzer sound.

I don't understand what do you mean by that, will not my multivibrator change the voice frequency by changing the resistances at their bases ?

It must be at least 470uF to pass voice frequencies.

How do i know what capacitance I need to output the voice ?

Your Mic-vo signal is negative because its opamp's (+) input is missing a biasing resistor to ground.

I dont understand how to correct this. Do I have to take a small resistance and connect it to the input of the op amp and the ground ?


Show me wisdom master

[Audioguru]

I do not believe that you are in a university making this project and you do not know the basics of electronics. Buy a voice modulator instead.

[nucleopolymere]

men if you don't wanna help just fuck off

"buy one instead "


i'm gonna build this project with or without you,

[danmcb]

that's a pretty bad circuit. (I am talking about the first one you posted.) There is no proper biassing, and no collector load. Also why have you chosen to make your front end "upside down" (ie.e PNP based)? What kind of  mic anyway? dynamic? electret?

for 10mV you need a gain of about 100. An input impedance of around 2k is fine for most dynamic mics. You *could* in a pinch do this with a standard common emitter, that will get you a good part of your gain of your gain. A better way is to have a balanced front end with a differential pair in combination with your op amp. Gain is then set by a resistor between the transistors. There are standard circuits for this, google and you will find. For a  small speaker you could then just have an opamp with a push pull stage at the output. Again, google.

you need to do more research and just learn how basic transistor stages work I think.  Judging by this, you don't really understand that yet. get a cheap breadboard, a  scope and a multimeter and  play around a bit.

[danmcb]

basically you need to think in blocks - first get your mic signal  up  to a sensible level, around 1V is OK. Then do the modulation. Then add a small power amp  to drive the speaker. Get each block working right, by itself, and then put it all together. Each block has its own challenges and requirements that you need to work out.

[Zero999]

Hello sirs,

I order to make my modulator work i have to make a pre-amplifier for the voice input. But I don't understand why it gives me a negative voltage at the input of the buffer (at mic_to_buffer),  can you please help me ? 

The multi-vibrator circuit seems to work fine. mic_to_buffer is the signal before the buffer and micvo is after. both the signal coming from the multi-vibrator and the pre-amp are multiplied together in the multiplier module.

Also why there is no voltage flowing through my speaker ?

elec and elec1 are the signal that were simulated. elec2 is the multivibrator and the pre-amplifier(voice) elec3 is the multiplier and the transistor driving the speaker
VCC is 9V and VEE is -9V

Thanks!

I'm still not sure what you're trying to do?

By the looks of it you want amplitude modulation but what's the application?

The usual way to do this is to use a common emitter amplifier and modulate the supply voltage.

[danmcb]

The usual way to do this is to use a common emitter amplifier and modulate the supply voltage.

I've never seen this done in audio. Interesting idea though. Before chip VCA's became commonplace, either a FET as voltage controlled resistor, or lamp/LDR pair were the usual approaches. Also valves with variable gm.

[nucleopolymere]

Hey thanks for your reply,

What I am trying to do is to change the voice frequency with the signal of the multi-vibrator by multiplying them and then output it on a speaker and thats all.

So today I did build the circuit and did some slight modifications, and the output of both my voice from the pre-amplificator and the multivibrator work as expected. I have positive output at around 1 V , so both of the multivibrator and pre-amplificator module works fine. I do understand now what you did mean by class A heater. It's because the transistor force a current but the resistance R5 is too big so all the energy is dissipated through the resistor and nothing goes to the speaker, so I did lower the R5 resistor from 8 to 1 ohm and did put the 470u capacitor at the output as you recommended so now it works ok I guess i did change the power mosfet also and yes i forgot to tell you I was using an Electret microphone . But one thing i've learned is that  next time I will take the simulation of circuits with a grain of salt.


The only thing that doesn't work is the multiplicator module. The formula of the circuit is V1*V2 /(R*Isaturation of diode) the formula is on this site  https://en.wikibooks.org/wiki/Electronics/Analog_multipliers . At first, I though it was because I did put diodes with too little Is so I replaced the 1N4148 diodes with 1N5822 which as a much higher saturation current. I then adjusted the resistor to have a 1 factor at the denominator so its basically V1*V2. But the circuit doesn't works. I've put the same strictly positive triangle wave at V1 and V2 so i should have a quadratic output but the circuit won't multiply.

Thanks again

EDIT :
Hi, it seems I did not wired well my multiplicator,  i forgot some resistances, I did mesured and  it seems it is multiplying but it saturate  at 8 V and my inputs is a triangle wave of 500 mVp-p so i think by increasing the resistance R of the multiplicator I would decrease the gain and then it would not saturate. the output is the bottom wave and  the input is the upper waves V1 and V2.

[orolo]

Follow danmcb's great advice and try a jfet modulator. See 5.9 at http://instrumentationlab.berkeley.edu/Lab5 .

You can also modulate using a bjt CE amp, injecting the carrier at the base and the signal at the emitter: this method is less linear, I think, that the jfet one. For extra originality, you could try a dual gate MOSFET, or a couple jfets for the same effect (explained in the modulation chapter of the ARRL book, IIRC). If you have an adequate audio transformer around, you could try a diode mixer. If you want to impress, try a discrete bjt Gilbert cell. V2aew has a really great video on the subject:



Any of these options looks much better, IMHO, than the opamp multiplier you aré trying to copypaste into your design.

[RoGeorge]

You know that if you do a voice FM modulation with 30 Hz you will get a Dalek voice, right?
 



Lookup "Dalek ring modulator", very easy to implement, very fun results and a damn clever circuit, so you will have a lot to learn about modulation and electronics. Also, very didactic, your teacher will love it. Good luck!

[Hideki]

You know that if you do a voice FM modulation with 30 Hz you will get a Dalek voice, right?
 

You know that a ring modulator doesn't do FM modulation, right?
 

[RoGeorge]

You know that if you do a voice FM modulation with 30 Hz you will get a Dalek voice, right?
 

You know that a ring modulator doesn't do FM modulation, right?
 

For some unknown reason, frequency mixer mixed into my head with frequency modulation. 

I will go hide myself under a rock now. Bye!

[Hideki]

I will go hide myself under a rock now. Bye!

No no, come back. It was such a cute mistake

You will not be EXTERMINATED!

[Audioguru]

The mic preamp still has many problems.
The waveform from the mic buffer is clipped on the top because it is still missing an extremely important resistor from its input to ground so that the input does not float up to a DC voltage too high.

[Zero999]

The mic preamp still has many problems.
The waveform from the mic buffer is clipped on the top because it is still missing an extremely important resistor from its input to ground so that the input does not float up to a DC voltage too high.

1) How do you know he's using an electrect mic? It could be a dynamic mic which doesn't need a bias?

2) The resistor in your schematic is in the wrong place. It should be to 0V, not VEE.

[danmcb]

because he says it on the schematic. But I have my doubts because he previously said he measured 10mV out - which he couldn't have done without biassing the mic if it was electret. So i wonder if he is actually using a dynamic - or even knows  what he is using?

The corrected schema by audio guru shows the resistor from the opamp input to Vee, it should go to GND.

The OP doesn't say what power supplies are available - does this design have a true spilt supply? something like +/-15V? I have my doubts. I have a feeling it is single rail. Anyway, he doesn't say.

If it is single rail, the better way to bias this circuit would be to use an emitter resistor (as audio guru says) to set the collector of the transistor to more or less mid point, and then direct couple the transistor to the output, with a blocking cap there.

But I am wondering why the transistor is even there. Why not just use the opamp in non-inverting configuration as a gain stage - with an artificial  mid rail (two resistors and a cap) if it is single supply? This is fine for this. And use a 5532 instead of a bloody 741 ...


 

[nucleopolymere]

Thanks again for your replies,

I have changed my circuit because the other one with the multiplicator circuit is too hard to implement ( and i don't really know what i'm doing ) the power amplifier is now a class B amplifier but the circuit is almost the same except for the modulator which is now done by a mosfet.

The output from the Electret microphone is  now amplified by U5 as danmcb recommended so when i speak I've a wave of around 5V of amplitude at the ouput of. I am aware that i 'm using an electret microphone i just forgot to put the resistor on the schematic.

But now the modulation is done by the MOSFET VN2222l, the multivibrator wave has frequency of around 50 Hz and it is inputted at the gate of the MOSFET Q3 and my voice at the drain. When not plugged in the amplifier, it is not working properly, I only get the positive part of the modulated voice and I've tried to put Q3 to ±9V instead of +9V and GND it does not work it just "adds" the wave instead of modulate it.  The image on the oscilloscope shows what do I get when I connect Q3 to +9V and GND.

Then when I connect the output of the modulator to the power amplifier, I'm talking to the microphone but the speaker seems to just buzz, i do not recognize my voice in the speaker. I do not know where the problem is.

So I my questions are:

How to get the positive and negative of the modulated voice ?

Is my amplifier circuit good or it is dog shit ?

thanks

[BrianHG]

If you want a simple 4 quadrant multiplier, just use this IC:
http://www.analog.com/en/products/linear-products/analog-multipliers-dividers/ad633.html#product-overview
The data sheet is full of examples on how to use.  I believe it's 7$ a piece.

Also, your output amplifier need class AB biasing & you should use it's output as the feedback for it's driving opamp.
It also needs a cap to GND on the negative 9v supply.
I would recommend at this point get rid of it and only output a line level signal into a cheap stereo amp until you debug your primary modulator before adding an output amp to your design.