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Megaphone circuit

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George L.

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Hello,

I am trying to make a Megaphone or a device in which you can speak into a electret mike, the circuit will amplify the signal and use a large speaker to make what you are saying much louder. I breadboarded the circuit from this site:

https://cnx.org/content/m12514/latest/figure14.jpg

The supply I am using is a 12V 1.2A transformer. I just connected a 3 inch 8 ohm speaker to the output. As soon as I turn in on, I get a loud buzzing sound, If I speak into the electret, I can barely hear my voice in the background of the loud buzzing comming from the speaker.

Does anyone know what the problem is or how to fix it?

thanks,

George L.
 
George L. said:
Hello,

I am trying to make a Megaphone or a device in which you can speak into a electret mike, the circuit will amplify the signal and use a large speaker to make what you are saying much louder. I breadboarded the circuit from this site:

https://cnx.org/content/m12514/latest/figure14.jpg

The supply I am using is a 12V 1.2A transformer. I just connected a 3 inch 8 ohm speaker to the output. As soon as I turn in on, I get a loud buzzing sound, If I speak into the electret, I can barely hear my voice in the background of the loud buzzing comming from the speaker.

Does anyone know what the problem is or how to fix it?

It sounds like acoustic feedback, an electret mike isn't a good choice as they are very sensitive - also, the mechanical construction of a megaphone is critical, it's usual to use a horn speaker, which is highly directional. If you're using an electret mike, and a normal speaker, you will have to reduce the volume massively to stop feedback, probably making it too quiet to be useful?.
 
Nigel makes a good point. While on the subject of megaphones in general though, is the output actually any louder, or is it just more distorted making it sound louder? I remember a teacher at school using one, he happened to have a very loud voice anyhow, but he certainly wasn't any louder using it, and certainly a lot less clear :lol:
 
Dr.EM said:
Nigel makes a good point. While on the subject of megaphones in general though, is the output actually any louder, or is it just more distorted making it sound louder? I remember a teacher at school using one, he happened to have a very loud voice anyhow, but he certainly wasn't any louder using it, and certainly a lot less clear :lol:

Yes, it's a lot louder, it's amplified quite a bit, then output via a horn, which makes it quite directional. We've got a VERY old one at work we hire out occasionally, and play with it sometimes :lol:

When I say 'old', it uses a stud mounted germanium power transistor - something like an OC19 if I remember correctly?.
 
Record your message then play it back through the amplified speaker. Then you won't have any acoustical feedback howling.
 
Do you have a rectifier between the transformer and the circuit? Even with a full wave rectifier, you will get close to a half volt of ripple at the output, just due to quiescent current. I don't know that this would cause a "loud" buzz, but it would certainly be annoying. I would change the cap on pin 7 to 47uF. This will max out the supply rejection. It will do nothing to avoid acoustic feedback.
 
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