High current op amps can drive headphones, or you can use an LM386 to power a very small speaker.
If you want more than a watt, you have to use an amplifier chip or an amplification circuit capable of it.
For the input, sound.westhost.com has a lot of projects you can look at.
Audio Electronics basics
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11th September 2008 04:24 PM #1bugmenot
Audio Electronics basics
If I have a working circuit, that I've tested an audio freqiencies with input from a frequency generator and output to an oscilloscope, how can I hook up a microphone as input, and/or a speaker on the output?
The background:
I've learned basic circuits, such as RC high/low pass filters, transistor amplifiers, oscillators, and simple linear ICs. I'd like to hook some of these up to audio equipment, but I'm a bit confused how to do this.
Let's say I have a filter... I'd like to take an audio via a microphone, pass it through my filter, and play it back via a speaker. I know that I can't just go and connect the wires directly - I need to know about the impedance of the microphone, preamping, the impedance of the speaker, matching transformers, etc. I haven't been able to find any guide about these - the few electronic texts I have don't discuss these very much. Can someone help me find a good guide for these?
Again, I don't need anything fancy. I'd like to know, given any circuit, how can I hook up a microphone to it, and how can I hook up a speaker to it.
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11th September 2008 05:05 PM #2Speakerguy
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12th September 2008 01:52 AM Thread Starter #3bugmenot
I'd like more specifics: What do I need to do to interface a standard mic to a circuit, so that I can use the mic instead of my function generator?
What do I need to do to interface a standard speaker to my circuit, so I can listen to my signal (assuming it's audio frequency), instead of watching it on my scope?
(Looking at circuits I've seen, some couple directly, some do it through a transformer, some do it through a capacitor, and some do it view more complicated things...)0Last edited by bugmenot; 12th September 2008 at 01:55 AM.
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12th September 2008 02:11 AM #4BeeBop
I don't know a lot about microphones, but do know there are different types, and I'm not sure one type is 'standard.' Perhpaps what they call dynamic?
I know this site has tons on audio, and lots on impedance matching:
http://sound.westhost.com/articles.htm
There is lots about microphones:
http://sound.westhost.com/articles/microphones.htm
and if you go to his main index, there is a search feature, very nice indeed!
http://sound.westhost.com/index2.html
When I was in school and we built amplifiers (AB with tip31 32 output stage and 2" speaker) the instructor became annoyed with all the wailing. He brought around 8ohm resistors to use as output loads.
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12th September 2008 02:40 AM #5Externet
Find the amplitude your signal generator translates into output amplitude.
You do that by connecting alternatively the oscilloscope probe to the input and output if you do not have a 2 channel oscilloscope to observe both simultaneously.
Make a chart of what input level yields what output level at different settings of your amplifier volume control or your frequency generator level in sinewave mode.
Example :
-With volume control at 11 o'clock, if the input is 0.1 Volts peak to peak, the output is 3 Volts peak to peak, your amplifier has a gain of 30.
-With volume control at 11 o'clock, if the input is 0.5 Volts peak to peak, the output is 6 Volts peak to peak
Reduce the frequency generator level to about 5 millivolts peak to peak and check what is the output.
Connect a 10 to 20 ohm resistor in parallel to the oscilloscope probes and the amplifier output and write down all readings again. ( The more the output levels differ from the first readings without the resistor, the higher the output impedance is; but do not pay too much attention to that now )
The output sinewave observed must not be distorted compared to the input, peaks must not be flattened when making notes.
Come back with findings to continue...
Miguel0Last edited by Externet; 12th September 2008 at 02:47 AM.
Abolish the deciBel !
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12th September 2008 10:09 PM Thread Starter #6bugmenot
Thanks for the pointers - I'll be doing some research and experimentation, and hopefully come back with my results.
One important question: What type of voltage (RMS) and current should I use in the final stage to drive the speaker? (I'm using a small, standard speaker that came with my breadboard - no specs included.)0
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13th September 2008 12:52 AM #7audioguru
Your little speaker is probably 8 ohms and can be driven not very loud with an LM386 little power amplifier powered from only 9V.
If you have the microphone near the speaker then it will hear the speaker and you will have acoustical feedback howling. BeeBop called it "wailing".
In audio circuits, impedances are not matched anymore. Very old vacuum tube amplifiers had an output transformer that matched the speaker. The output impedance of a modern amplifier is 200 times (or more) lower than a speaker. Then the resonances of the speaker are damped properly.0Last edited by audioguru; 13th September 2008 at 12:53 AM.
Uncle $crooge
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