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# circuit help

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:) i biult this circuit i collected from the net but it dident work ,all i got was some hissing from the speaker , could someone look at this and tell me if its correct, id like this circuit to be able to pick up sounds useing a dish, like a parabolic microphone type circuit but use a pair of headphones instead of just a speaker , and id like this circuit to be able to amplify any sounds, more from the microphone, if anyone can make any corections to this circuit, i would really apreciate it

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Hi Steven, the two elements you have to suspect are R3 and R1. I would forgo the potentiometer and bias the opamp with two resistors. The resistor to ground and a series resistor that you might be able to forget. The output will be zero volts. As far as R1, it will prevent a changing voltage if the value is too low. A mic can be represented by a coil that has a voltage induced to it by the movement of a diaphram. The change in voltage across the coil produces a change in current. The lower the resistor, the higher the change in current. The mic might not be able to produce such a change in current. So what is the limit. Why not just use a high value resistor. I think the reason is that the mic simply needs enough voltage to produce the N and S on the coil.

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hey kevin thanks but could you print that circuit out and make the changes or modifications then repost it to this topic, ive seen it done in the forums before, it would be easyer for me to .

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Hi Steven,
Your circuit is exactly the same as the output section of my Electronic Stethoscope-2 project, and the same as on the LM386's datasheet. The only difference is that it doesn't have enough gain.
Connect a 10uF cap between pins 1 and 8 of the LM386 with its positive wire at pin 1.

I connected an electret microphone to a circuit like that and could hear my voice in headphones very loudly with the microphone at arm's reach and the volume turned down. I could even hear a TV that was in another room. Your circuit has a gain of only 20, and adding the cap boosts the gain to 200, at which point the LM386 has a noticeable hiss output. Without the cap, a low noise preamp would boost the gain without hiss. A dish feeding the mic would make it much more sensitive. Use a rubber isolating sleeve around the mic.

Please disregard what Kevin said (sorry, Kevin):
1) The mic must be an electret type (not a coil and magnet type) and of course must be connected with the correct polarity. An electret mic is very common , has high quality and is inexpensive. It has a much higher signal level output than a coil and magnet mic.
2) R1 is used to power the FET transistor inside the mic. Its current may damage a coil and magnet mic.
3) The LM386 power amp (it is not an opamp) operates normally with both inputs at ground voltage and has built-in biasing. With its single supply, the output voltage will be near half supply voltage. That is why it has an output cap to the speaker.
4) The output of an amplifier (not an LM386) would be zero volts if it had a dual supply.

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:)audio guru your a geniuos you make it so cool and easy

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:)audio guru where is this sthoscope 2 circuit it sounds intrestingso i only have to add a 10uf electro between pin 1 and 8 of the ic to get better gain if ive got it corect

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Hi Steven,
My Electronic Stethoscope-2 project is here:
http://www.electronics-lab.com/projects/science/019/index.html
It uses the LM386 for a headphones/speaker amp, but has reduced gain because it has a separate preamp. circuit.

That's correct, to increase the gain of the LM386 by 10 times, connect a 10uF cap between its pins 1 and 8, with the cap's positive wire at pin 1, just like on its datasheet.

See, it is cool and easy.

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:)great stuff audio guru

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Here is an Inhanced View of yours

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here is another more powerful.

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Here are some more interesting Links.

Parabolic Microphone

http://ourworld.compuserve.com/homepages/G_Kunkel/microphone.htm
http://ourworld.compuserve.com/homepages/G_Kunkel
http://ourworld.compuserve.com/homepages/G_Kunkel/cardboard/cardboard.htm

Cardboard Dish

http://ourworld.compuserve.com/homepages/G_Kunkel/cardboard/cardboard.htm

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• 2 weeks later...

:)thanks hotwaterwizard, i should of receaved an email telling me of reply to my topic but i dident get one, unless my misses acidently deleted it when she cleaned out the junk mail have you got a better clarity circuit for that bull horn circuit and the details of the ics used and the parts to as the details are a bit hard to read

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