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Old 05-06-2004, 12:01 AM   #1
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Default If you know how a mic amp is supposed to work, Please help/

http://www.hut.fi/Misc/Electronics/circuits/micamp.html

I built this thing exactly as it'* shown, with a 2N2222 and add on the volume **** as he showed it...

It doesn't work. all it does is make some static when you turn the volume ****. WTF?
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Old 05-06-2004, 12:17 AM   #2
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Whats the voltage drop of D1?
Can you confirm the transitor is good? What voltage is the mic?
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Old 05-06-2004, 02:14 AM   #3
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Umm It'* a 1.8V Red LED, I have no idea what voltage drop would be...
I got the transistor in a baggie in the mail... How would I test it?
I got the mic out of my old Nokia 3585i, It'* a mic...

M'kay, I know how to solder, how to read that diagram, and where to buy the parts. I'm lost and hoping on the rest... If you can tell me where to hook up the multimeter and in what mode, I know how to stick that sucker on there...

I paralleled a wire with the LED to see if it was holding back power, That didn't change anything... I tested for continuity accross everyone of my joints, They all tested good, But I didn't test the resistane of each joint...
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Old 05-06-2004, 12:44 PM   #4
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Quote:
Originally Posted by Damemorder
Umm It'* a 1.8V Red LED, I have no idea what voltage drop would be...
If it'* a 1.8V LED, the voltage drop across it (multimeter set for DC voltage) should be about 1.8V when it'* on. If it'* off, the voltage should be less. If it'* off but the voltage is significantly higher than 1.8V, it'* open; if it'* off and the voltage is significantly lower than 1.8V, it'* either shorted or backwards.

Quote:
Originally Posted by Damemorder
I got the transistor in a baggie in the mail... How would I test it?
There'* no really easy way to test a transistor unless a) you have a multimeter with a transistor tester built into it -- usually it'll have a 6-pin mini socket with the holes labeled E, B, C in various orders, or b) you feel like building a circuit with which to test it. If you want to test it using a test circuit, you'll have to have a multimeter capable of reading 10s of milliamps (mA), as the 2N2222s can't really handle a whole lot of collector current. If you want to build such a circuit, let me know what resistor values you've got laying around and I'll design something for you and tell you what to look for.

Before you try that, though, make sure you've got the transistor oriented correctly; I've had issues in the past (see my homebrew scantool thread) with incorrect pinouts on the packaging. The 2N2222'* pins, with the flat side of the package facing you and the pins pointing down, are (from left to right) emitter, base, collector. There'* a chance that the transistor is toast if you had this oriented differently when you energized the circuit.

Quote:
Originally Posted by Damemorder
I got the mic out of my old Nokia 3585i, It'* a mic...
I'm not really familiar with what manufacturers are putting in cell phones these days, but there are two kinds of small microphones, electret (a little box filled with carbon and a built-in FET preamp) and dynamic (moving coil, like a speaker). As you can see from the circuit, the two types aren't interchangeable; there'* one set of hookups for an electret (which requires some current flowing through it to power the FET preamp) and another for a dynamic (which doesn't). I'd assume that the mic you've got is an electret, so make sure that'* hooked up right.

Quote:
Originally Posted by Damemorder
M'kay, I know how to solder, how to read that diagram, and where to buy the parts. I'm lost and hoping on the rest... If you can tell me where to hook up the multimeter and in what mode, I know how to stick that sucker on there...
As soon as I get home I'll set the circuit up in PSpice and simulate it to give you an idea of what voltages you should see at various points in the circuit.

Quote:
Originally Posted by Damemorder
I paralleled a wire with the LED to see if it was holding back power, That didn't change anything... I tested for continuity accross everyone of my joints, They all tested good, But I didn't test the resistane of each joint...
If you've got an electret hooked up to the electret'* terminals and it'* drawing current, the LED should be on, since it would have a path to ground. Without a mic hooked up, or with a dynamic hooked up but with no sound into it, the LED should be off once the capacitors are charged (a fully charged capacitor is like an open circuit to DC). In short, with a dynamic mic, I think the LED will only blink when the mic provides a signal to turn on the transistor; with an electret mic I think it'd be on all the time. But don't quote me on that.

The resistance of solder joints in such a low current circuit is negligible, so if there'* continuity across them, they're fine.

I'll have those test voltages for you sometime later today.

-b
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Old 05-06-2004, 12:56 PM   #5
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I read articles about this all day long, but I'm only an editor...so the only advice I can give you is if nobody on here can help, try to e-mail the designer... It'* completely possible that they can help you figure out what to tweak or they might even realize there'* a mistake in the schematic or directions. At the magazine where I work we get questions very similar to this all the time, and we send them to the authors/designers and they help people figure out what went wrong...
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Old 05-06-2004, 02:57 PM   #6
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Okay, so I might have been wrong about the LED -- it looks like it'* always got a complete path to ground, so it may be on anytime the power'* on. Not sure, it'* hard to model LEDs in PSpice.

Anyway, assuming the LED *is* on, and is dropping about 1.8V, here'* the bias point detail of the circuit with the microphone *disconnected*:



All the voltages are referenced with respect to ground. Ignore the load resistor RL on the far right side, it'* there only because PSpice doesn't like incomplete circuits.

So after application of power, and the LED being on (maybe), you should have about 7.2V at the cathode (negative -- the leg not tied to the battery) of the LED, and about 7V at the positive end of C5. You should have about 3V at the positive end of C4 (tied to the collector of the transistor), and about 0.7V at the base of the transistor.

Let us know what you *really* get, and we can troubleshoot from there.

-b
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Old 05-10-2004, 05:28 PM   #7
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Once again, I post and the thread dies. I think I'm cursed. :(

-b
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Old 05-10-2004, 07:10 PM   #8
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Quote:
Originally Posted by enmityst
Once again, I post and the thread dies. I think I'm cursed. :(

-b
you live in missouri were all cursed here, as for the topic I didnt fair to well in electrons so I'm not of much help but whats the point of it in the first place?
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Old 05-10-2004, 07:50 PM   #9
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Holy crap, I figured it was a lost hope to get help here....

I tested the "Hfe" of the transistor on a multimeter after I looked up what pins are what. I got a max of 20, but normally around 10-15. No idea what that means. The little gragh I got from www.digikey.com shopwed I should have an "Hfe" of like 30...

I'm pulling out the multimeter to test those voltages, and I swapped out to a $10 "Unidirectional" mic from walmart. I used the Ociloscope to look at the output of the mic, It looks like sound waves, So I guess that part'* working.
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Old 05-10-2004, 08:27 PM   #10
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Quote:
Originally Posted by Damemorder
I tested the "Hfe" of the transistor on a multimeter after I looked up what pins are what. I got a max of 20, but normally around 10-15. No idea what that means. The little gragh I got from www.digikey.com shopwed I should have an "Hfe" of like 30...
hfe is the transistor'* current gain. (A BJT is basically a current-controlled current source; if the current into the base is, say, 1 mA, the collector current will be about hfe * 1 mA.) Don't worry if your transistor'* hfe is a bit different than the spec sheet says, they're all like that. 15-20 will be fine. The important part is, since you got a reasonable value of current gain, you've got the pins identified correctly. Congratulations

Quote:
Originally Posted by Damemorder
I'm pulling out the multimeter to test those voltages, and I swapped out to a $10 "Unidirectional" mic from walmart. I used the Ociloscope to look at the output of the mic, It looks like sound waves, So I guess that part'* working.
Is it a computer-style unidirectional mic with a tiny plastic head and a mini (headphone-sized) plug? Or one of the larger ones with a wire mesh screen at the top and a phono plug? The computer-style ones are electret mics, while (generally) the ones with wire mesh are dynamic.

It'* hard to follow mic inputs with an scope, since the sound generally varies greatly from one time division to the next so the waveform is unstable. If you can put the mic in front of your computer speakers, use your computer to generate a pure sinusoidal signal at, say, 1kHz. That way you should be able to see a stable waveform on the scope screen, and then you can check various points in the circuit and see if you're still getting a sinusoid. For example, you've got a 1kHz sine wave going into the mic. Put the scope on the mic terminals and determine the amplitude (peak-to-peak) of the mic signal. Let'* say it'* 20mV. Then put the scope on the output terminals of the amplifier and measure the signal amplitude again. It should be sinusoidal, unclipped, with the same frequency as the original (i.e., you shouldn't have to fiddle with the time per division ****) and the amplitude should be greater than the input amplitude (anywhere from 20-40 times the input amplitude, hopefully). Otherwise, something'* jacked up. (By the way, this is something to do *after* you've verified that the circuit voltages without the mic hooked up are somewhere near the values I posted above.)

-b
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