One last thing to try before you give up on getting acoustic feedback. Reverse the wires on either the "mic" or the output speaker, and then put the "mic" and speaker close to together.

Also, increase the collector current to 100 mA, because the gain is proportional to the gm of the BJT, and gm is proportional to Ic.

Another thing. I wonder if there's something wrong with your "mic" or speaker. If I take my 2" speaker and set my Fluke 189 to measure AC uA, connecting the speaker terminals directly to the meter, I get 200 uA AC if I put my lips .5" away from the speaker and speak loudly, but not yelling. If I yell, I can get 4 mA AC. What do you get if you do this?

 

What is the DC resistance and power rating of the speakers you're using?

Mine are 2" speakers salvaged from computer speakers I bought at Goodwill.

They are rated 0.2 watts and have 7Ω DC resistance. This means I could pass .169 A through the voice coil without frying it.

If you make the same calculation on your speakers, then set your Ic at the maximum the speakers can stand without frying, you will get maximum gain.

 

Yea its really the speakers that are to blame here. Yours sound much nicer.

I am using this for both the mic and speaker. I have 2 of them.

http://www.radioshack.com/product/index.jsp?productId=2062406

 

Why not use a real microphone, a real amplifier circuit and a real speaker?

 

Why not use a real microphone, a real amplifier circuit and a real speaker?

Because this is an experiment for beginners. The point of this is to make things easy to understand for people just beginning to learn about transistors. A real amplifier is pretty complex, and there are entire courses in college dedicated to them.

Do you see how I am gradually stepping things up, beginning at the most simple theoretical amplifier and learning its limitations and learning the NEED for 6 transistor, capacitors, diodes and all the other things in a good amplifier.

Yes its a tedious process and might seem slow for you pros, but you were new once. Remember back in the day when they used vacuum tubes and you were just in college getting your BE?

 

NEED for 6 transistor, capacitors, diodes and all the other things in a good amplifier.

Not exactly.You need 3 BJT

And this


I design this circuit long time ago and its work quite well as a headphones amplifier.

 

Yea its really the speakers that are to blame here. Yours sound much nicer.

I am using this for both the mic and speaker. I have 2 of them.

http://www.radioshack.com/product/index.jsp?productId=2062406

I went to Radio Shack and got two of those speakers. I connected them to the circuit I described in post #134.

With Ic=50 mA, I was able to get acoustic feedback with the speaker polarities connected one way, but not the other.

This means you should try the suggestion I made in post #181; reverse the wires on one of your speakers and try again. In fact, try both possibilities again.

I also found that increasing Ic to 100 mA helped quite a lot.

To get acoustic feedback, the speakers had to be almost touching, so you are right, these speakers are not as sensitive as the 2" speakers I used at first.

I connected the speakers to my Fluke DVM and measured the AC current when talking into them. I was able to get about 300μA when yelling into the Radio Shack speakers, but I could get about 4000μA with the 2" speakers I got from Goodwill.

Nevertheless, it is possible to get acoustic feedback (just barely) with the Radio Shack speakers and a single BJT.

So, increase your Ic to 100 mA and try both possible polarities of your "mic" wires. The speakers must be face-to-face, nearly touching to get a squeal.

 

Thank you so much Electrician for your dedication to this experiment. I hope those radio shack speakers come in handy for you.

And I don't get it, why does polarity matter its just a dynamic coil speaker? I noticed there was a black and red wire, but didn't think it mattered. Any idea why this is?

So I will try this and lets hope the darlington will give me actual sound, not just acoustic feedback.

 

And I don't get it, why does polarity matter its just a dynamic coil speaker? I noticed there was a black and red wire, but didn't think it mattered. Any idea why this is?

The squeal we're attempting to get is occurring because the circuit with the speakers close together becomes an oscillator. Oscillators work because of feedback, and the feedback has to be in the proper phase to sustain the oscillations. When you switch the wires to one speaker, you are changing the phase of the feedback by 180°, which will give you the phase you need, we can hope.

In reality, the phase of the speaker response changes rapidly and drastically near resonance , so with either connection, you will probably get squeal if the overall gain is high enough. The squeal occurs near the speaker resonance because with the rapid phase changes that occur there, it is likely that there will be the needed phase shift somewhere in that neighborhood. That's the case when I use my 2" speakers.

But, one of the connections will probably be more favorable to squeal than the other.

So I will try this and lets hope the darlington will give me actual sound, not just acoustic feedback.

I don't think the Darlington is going to help much because the gain of a common emitter stage is roughly Rc/re (Rc is the 8Ω speaker impedance), where re is the intrinsic emitter resistance of the second BJT of the Darlington. Notice that this formula tells you that if you make re smaller, the gain goes up. This is why you should increase your Ic, because that makes re smaller.

You shouldn't make Ic much larger than 100 mA, or you will fry the voice coil.

 

...give me actual sound, not just acoustic feedback.

You will never get enough gain (with a single transistor) to get a sound out of the output speaker that you can hear over your own voice speaking into the "mic".

However, you can tell that you are getting gain if you gently rub your finger on the cone of the "mic", while you hold the output speaker up to your ear.

 

I don't think the Darlington is going to help much because the gain of a common emitter stage is roughly Rc/re (Rc is the 8Ω speaker impedance), where re is the intrinsic emitter resistance of the second BJT of the Darlington. Notice that this formula tells you that if you make re smaller, the gain goes up. This is why you should increase your Ic, because that makes re smaller.

You shouldn't make Ic much larger than 100 mA, or you will fry the voice coil.

Bill_Marsden did get good results with a Darlington. I am reffering to this post.

http://forum.allaboutcircuits.com/showpost.php?p=159101&postcount=111

and this http://forum.allaboutcircuits.com/showpost.php?p=159198&postcount=113

I want to try his circuit and see what happens. He didn't say if he heard his voice or not from the speaker, but he said "halfway result" whatever that means. I hope it means actual sound. I will have to try.

 

You'll notice in the first post he said that he was unable to get squeal with the Darlington.

I'm not suggesting that you won't be able to get a Darlington to work, but it's not necessary, and it probably won't work any better.

 

I just bought some new speakers, wanna bet they make a diff? We're talking efficiencies here.

 

Okay for some reason I'm still not getting acoustic feedback. I tried switching the wires on either the mic or speaker. Doesn't help. Maybe I didn't try all the possibilities I don't know.

I need a favor from either The_Electrician or Bill_Marsden or anyone else who has been following this thread. First of all I need someone who is familiar with this type of breadboard http://www.ecall-elec.com.tw/pics/104-1-B.jpg

My question is:

Is this schematic below

Equivalent to what I did on my breadboard in real life here?

(Big pic, used link)

http://i67.photobucket.com/albums/h304/john_ukranian/gjftrf.jpg?t=1249972093

The big green and black circles where it says 4.5V is the connection to the battery holder. I used 3 AAA batts. Duracell. Brand new.

I used a little program called Lochmaster 3.0. Its the only way I can get you guys to see exactly what I did.

I have checked my wiring many times with my friends in my engineering department who have more expereince than me, and they said everything is perfect. I think I am pretty darn good at transferring schematics to real life. I just did this to double check in case I made any errors.

I hope you know which rows and columns are connected and which ones aren't. That is why I ask someone who has used this type of breadboard before. Its an extremely common one.

So please help me out. Thanks.

 

Funny you should mention that, you mean this kind of breadboard?

I'll take a look at it tomorrow, when I have more time.

 

OK, first thing I see is the capacitor, which is a short to audio, is shorting the mic out. The schematic should look like this instead...

The mic is equivalent to a small AC generator, the cap allows the loop to be closed for it's current flow, which also happens to be through the BE.

Meanwhile, the cap blocks the DC, which is biasing the transistor into a linear amplification state.

See?

BTW, I didn't break it down, once I realized where the cap was I stopped looking, but your breadboard matches the original schematic from what I saw.

The previous graphic image I uploaded was my way of drawing protoboard layouts, for my various articles.

 

Why in the following configuration, the cap is shorting the mic out?

In small signal analaysis, all the cap does is shorting out R1 & R2, the mic stays connected to the BE junction, and therefore keeps driving it.

 

You may be right. I'm not comfortable with it though. If you're going to add a cap get the mic out of the DC bias path, first thing I did.

 

You may be right. I'm not comfortable with it though. If you're going to add a cap get the mic out of the DC bias path, first thing I did.

Hey,
I agree with you.
Actually, what you said is exactly how they teach us in U to design amplifiers.

Similarly, the load is usually capacitively coupled from the DC collector current path (for CE configuration).
But in such case, we will need to use another RC resistor (which will take over the speaker's place), and have RC >> R_Speaker, for impedance matching .

 

Hey,
I agree with you.
Actually, what you said is exactly how they teach us in U to design amplifiers.

Similarly, the load is usually capacitively coupled from the DC collector current path (for CE configuration).
But in such case, we will need to use another RC resistor (which will take over the speaker's place), and have RC >> R_Speaker, for impedance matching .

Don't really think so, the transistor has some resistance, though low. It will amplify the current you get across it's path. All a resistor would do is restrict the little current you have.

I tried getting 8Ω:600Ω audio transformers. Now those would do some good things, I would think.

At this level a lot of it speculation, the experiment is the thing.