Cheap microphone - simple amp circuit - Page 1
Forum section: electronics / beginners

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 25, 2018, 07:40:11 pm
content: I very new to audio circuits.I created a very simple audio amplifier circuit based on the NE5532 (using a split single DC supply)which I am testing with a simple signal generator for the ac input, and a small pc speaker on the output.http://www.ti.com/lit/ds/symlink/ne5532.pdfIn my tests, I am sending in a .05 V input sine waveand getting a fairly clean 5 Vpp sine wave on the output(which sounds as I would expect on the pc speaker ....I tested ranges of 2khz to 8khz frequency)That said,I was hoping to use this same small test circuit, to see if I can amplify input signals from a super cheap pc microphone such as this one:https://www.cdw.com/product/Cyber-Acoustics-ACM-51B-Desktop-Microphone/1755669?pfm=srhI noticed that the microphone website lists its input and output impedance of 2k ohmsHowever, I am wondering if any cheap microphone (such as the one linked above) would produce enough millivolts to be picked up by my circuit.I welcome wisdom/guidance.As I do not know how many milli volts typically can generated directly from a cheap microphone.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: September 25, 2018, 08:18:26 pm
content: Your posted link takes me to a news report, not to a microphone.

author: Buriedcode
author_rank: Super Contributor
timestamp: September 25, 2018, 08:20:48 pm
content: I'm thinking you have linked the wrong link - thats a quack website for conspiracy theories (at least it looks like that to me).With an output impedance of 2k I think you may be talking about an electret mic.. but I@m just guessing here. If your circuit does indeed have a gain of 100 (50mV p-p in, 5v p-p out) thats a gain of 40dB which is quite high and should be more than enough to amplifier even the lowest output microphones. Most PC inputs have around 20dB gain.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 25, 2018, 09:07:02 pm
content: Indeed I did not mean to post about Nancy Schaefer (however that is indeed a very interesting story in and of itself).I corrected the link above for a sample inexpensive pc mic.

author: Audioguru
author_rank: Super Contributor
timestamp: September 26, 2018, 01:03:33 am
content: The NE5532 is an opamp, not a power amp. Its datasheet says that its minimum load is 600 ohms but most speakers are 8 ohms that needmuchmore current than an opamp can provide.The spec's for your microphone say that it is an electret type that is very common today. Its output is about 10mV when 10cm away from your mouth and you are talking at a conversation level. One resistor value sets the gain of the amplifier, 200 is very sensitive but 500 is more sensitive. High sensitivity causes an amplifier to make severe distortion when a normal sound level is at the microphone, and to cause acoustical feedback howling if the microphone can hear the sounds from the speaker.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 26, 2018, 01:39:59 pm
content: Thank you for your reply/feedback.I agree the datasheet indicates they use 600ohm, however I am not sure why.That seems very impractical for an audio op amp.When I drive my small pc speak using my existing circuit with 5v dc (which gets split), the volume seems plenty high/loud enough.I will likely have more follow up question, but let me ask this one first:Why would an audio opamp be spec'ed for 600ohm load?(As most speakers are no where NEAR that reactance in typical audio frequency ranges).

author: dmills
author_rank: Super Contributor
timestamp: September 26, 2018, 01:58:15 pm
content: 600 ohms is a perfectly good sort of impedance for a minimum load on an opamp intended for audio use (And in fact the NE5532 is a perfectly fine audio opamp, most of your music collection has likely been thru dozens of the things).Your basic problem is that you need a power amp not a device intended to drive line level inputs (impedance measured in kohms), try following the 5532 stage with something like a LM386 or such (You will also want much less gain out of the 5532 stage).Regards, Dan.

author: Bassman59
author_rank: Super Contributor
timestamp: September 26, 2018, 05:30:51 pm
content: Quote from: mikeinfodoc on September 26, 2018, 01:39:59 pmWhy is an audio -- or, really, any other op-amp specified to work into a 600-ohm load?First, remember that an op-amp is a voltage-amplifying device. It doesn't need to drive heavy loads at all in order for it to do its thing. Many op-amps are specified into 2k-ohm loads. Driving an 8-ohm speaker is an example of "heavy load." Not only does it need to swing to the desired output voltage, it needs to source enough current into the load in order to maintain that swing. We mistakenly call this a "power amplifier," although it really isn't, because it still amplifies voltage, but again, it is capable of sourcing/sinking enough current into the load. (And of course voltage times current is power.)600 ohms is an old telecom spec, back when the amplifier had to drive long cables that were terminated (for signal-integrity reasons) in a 600-ohm load.Now carefully read some op-amp specs. You'll see something along the lines of "maximum output swing" and then specify a load resistance. The max output into 600 ohms will be less than the max output into 2k ohms. And that's perfectly OK if you don't expect to swing to the maximum output.You may also see specs for distortion vs output load. The 600 ohm load may have higher distortion than the 2k load.When the manufacturer states "capable of driving 600-ohm loads," what you're being told is that the device has higher output current capability than other op-amps, and that it's stable into that load. Again, note possible reduced max output swing.All that said, your NE5532 can drive a small 8-ohm speaker. The maximum output voltage swing may be greatly reduced from ±15V rails, and it might have high distortion, but it will work.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2018, 06:31:07 pm
content: There seems to be some confusion about the minimum load remark. The NE5532 is specified to give a peak to peak voltage of 26V, that's 13V peak, into a load with a minimum impedance of 600Ω. This works out to be a peak current of just under 22mA, so reducing the load impedance below 600Ω, when the output voltage is 13V peak, would cause the current to rise and it might cause it to current limit, resulting in clipping and distortion. Of course, there's no reason why the NE5532 can't drive a lower impedance than 600Ω, with a lower supply voltage and output voltage. It's perfectly reasonable expect it to drive a 150Ω load with 3.3V peak and a supply voltage of +/-6V.Yes, it's an electrect mic, which requires a biasing current. A 2k2 resistor in series, with a 3V supply is the norm, for powering it.https://en.wikipedia.org/wiki/Electret_microphoneYou'll probably want to use a higher voltage, than 3V, so use a higher value resistor, or better still two resistors with a larger capacitor to 0V. This will help to improve the supply rejection, which will mean that any noise on the power supply rail won't appear on the output.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 27, 2018, 10:19:37 pm
content: If I follow with LM386 stage, what is the advantage of using the ne5532 at all?I ask this earnestly as I am wondering when the ne5532 would/should be used...Also is the term "line level" considered a norm for small signal transfer between devices? (I take it that LINE LEVEL means kohms and very small millivolts, but I could be wrong about that also)

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 28, 2018, 12:00:46 am
content: Quote from: mikeinfodoc on September 27, 2018, 10:19:37 pmThe LM386 is designed for a line-level signal input. So you need some amount of additional voltage gain if your source is a low-level source like a microphone. The NE5532 is designed for amplifying low-level signals (microphone, phonograph pickup, electric guitar, etc.) up to line-level. And with better signal-to-noise ratio than common general-purpose monolithic op-amps.QuoteYes most audio gear uses "line-level" internally. And also "line-level" externally to connect between different pieces of gear.There are roughly three levels of audio signal standards:1.Low-level- this includes mic-level, phono-level, instrument-level, etc. Typically down at a few millivolts. Modern microphones are low-impedance sources (a few hundred ohms), Phono-level are typically 47K ohm impedance, and electric instruments (guitar, bass, etc.) expect to see no lower than 100K load impedance (or higher)2.Line-level- There are two varieties of this, Consumer line-level is typically -10dBv (somewhere around 300mV), and Professional line-level (+4dBv to +10dBv) which is typically around 1.2V Modern gear has rather low output impedance (perhaps a few hundred ohms to maybe 1K ohm source impedance. And modern gear typically has rather high input impedance (perhaps 5K to 10K or 20K ohms. That convention generally eliminates cable effects (parallel capacitance and series resistance) from consideration, and makes "impedance matching" something that only our grandfathers remember.3.Speaker-level- Typically a very low source-impedance from a power amplifier designed to drive a 8 ohms (or 4 or 16 ohms) load from a passive speaker cabinet. Voltage depends on the amplifier power. Around 9V for 10 Watts into 8 ohms, or 28V for a 100 Watts into 8 ohms. Ohm's law will translate watts into volts for a given impedance (http://rcrowley.com/eirp.htm)QuoteNo. Line-level does not mean K ohms of impedance, and certainly does not mean small mV of signal level. See outline above.As @Hero999 observed, you are trying to use an electret microphone capsule. Electret microphones do not generate any signal on their own (as a dynamic microphone would). They actually have a FET transistor inside with "sound" connected to the gate. So you must put some current through it in order to get a voltage signal out of it. There are hundreds of representative circuits on the interweb showing electret condenser microphone circuits connected to the input of an NE5532.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: September 28, 2018, 11:57:48 am
content: Because there are other uses for op amps used in audio circuits than to drive speakers. The word audio does not make it a power amp/ speaker driver.Quote from: mikeinfodoc on September 26, 2018, 01:39:59 pmSent from my SM-G900V using Tapatalk

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 28, 2018, 03:18:03 pm
content: Your feedback is immensely useful.You cleared up several concepts for me which I have wondered about for a while.It seems in electronics, there are those who know a lot, and those who know a little, and not many "in betweener's" (which is where I find myself).My day job is oracle database programming and IT related stuff (30 years strong), but I have a keen interest in electronics as a hobby (and wish I had more time to devote to it). My son is 10 yrs old, so not quite old enough to help dad with this stuff yet :-)

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: September 28, 2018, 03:21:47 pm
content: Quote from: 6PTsocket on September 28, 2018, 11:57:48 amThank you, as I understand now, there are often situations in which you only need to take low-level to line-level, etc...I have ordered some LM386 power audio amp dip's. Once they arrive, I hope to add a 2nd stage to take my line level to speaker level.I welcome any guidance on doing that (or schematics) if you so choose.

author: Audioguru
author_rank: Super Contributor
timestamp: September 29, 2018, 12:43:33 am
content: If you split 5V then the NE5532 has plus and minus 2.5V power supply, but its minimum allowed is plus and minus 3V for a 6V total. Maybe you are lucky yours works.Since your pc speaker is plenty loud enough then does it have a built-in power amplifier?

author: Teledog
author_rank: Regular Contributor
timestamp: September 29, 2018, 04:02:53 am
content: Perhaps lightly off topic,This arrived today;https://www.ebay.ca/itm/DC-3-7V-6V-MIC-AMP-MAX9812L-TDA1308-Stereo-Microphone-Headphone-Amplifier-Module/112487497429?hash=item1a30c6fed5:g:azAAAOSwvDpa2wASUsed a high end noise cancelling electret mic (Polycom surplus), and yes, there is a DC bias on the mic input.~5-6VDC batteries works best, added a 100uF electrolytic & a 0.1 cap at the DC input (USB chargers all have 60hz hum, good Li batt charger packs have little/no noise)For the price, easy-peasy, plug & play.Cranked up, you will hurt your ears with earbuds,.. if someone even burps in the near vicinity.my 2 cents...