No sound from this circuit - Page 1
Forum section: electronics / beginners

author: RobH
author_rank: Regular Contributor
timestamp: October 23, 2022, 09:00:37 am
content: I have put this circuit together purely as an experiment etc. It is from a book by Charles Platt called Make: More ElectronicsIt uses an electret microphone as an input source to a LM741, then to a LM386 which send the signal / sound a speaker.The problem I have is that there is no sound at all coming from the 8 ohm 1w speaker.Using a 9v supply, I get 4.5v DC between pins 2 and 6 on the 741, and 0.6v DC between the 10uf capacitor and the 10k pot when turned fully anti clockwise. If I switch my DMM to AC , I get upto 0.130v when I tap the microphone, but nothing from a speaker. The same electret mic works on another circuit just fine but not this oneOn pin 3 of the LM386, I get 0.5v DC and upto 0.02v AC. On pin 5 of the 386 I get 2.49v DCThanks

author: bidrohini
author_rank: Regular Contributor
timestamp: October 23, 2022, 02:47:59 pm
content: Have you checked the OPAMP separately? Make sure that it is good.

author: RobH
author_rank: Regular Contributor
timestamp: October 23, 2022, 04:08:06 pm
content: Quote from: bidrohini on October 23, 2022, 02:47:59 pmI have tested the LM741 with a 741 circuit tester I found, and the led flashes to say it is ok.I don't know how to test the LM386 tho'When I tap the electret mic , I get upto 0.3v AC on pin 3 and upto 0.15v AC on pin 5 of the LM386Also just found that I can get sound from a beeper type sound sensor when I connect the +v wire direct to pin 5 or the resistor, but nothing when using a speaker

author: JuanGg
author_rank: Regular Contributor
timestamp: October 23, 2022, 08:21:26 pm
content: Hi RobHI'm glad it doesn't work! That means you get to figure it out and learn in the process.The problem with those schematics is that they may be nice for building the circuit up, but they do not help much in understanding what it does.Here I've drawn it for you following the standard conventions, signal flowing from left to right, positive supplies up, negative supplies down. And using symbols that actually represent what's going on.I would advise you to build the circuit from left to right as I've drawn it, check at each step wether if it matches the notes. You may likely not understand what's going on, please do not hesitate to ask.Good luck,Juan

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2022, 08:53:23 pm
content: Quote from: RobH on October 23, 2022, 04:08:06 pmHave you tested the speaker? Does it click when briefly connected to a battery?

author: RobH
author_rank: Regular Contributor
timestamp: October 23, 2022, 10:17:26 pm
content: Quote from: Zero999 on October 23, 2022, 08:53:23 pmYes it does

author: RobH
author_rank: Regular Contributor
timestamp: October 24, 2022, 04:22:15 pm
content: @ JuanGgFirst of all in the said book, there is a sort of explanation of how the circuit works: ie: feedback from pin 6 of the LM741 circles around through the 100k feedback resistor to the inverted input , and is modified by the 10k ground resistor, etc , etc.Anyways , thanks for drawing it out and it was easier and clearer to understand. I did have a faulty lm386 as I was getting 8.32v on pin 5 , where it should be 4.5. After I changed it , I now get some sort of buzzing noise from a speaker, as well as scratching noise when I rub across the electret mic with my finger.I cured this by changing the 0.047uf capacitor to 0.1uf, and grounding pin 7 through another 0.1 capacitor. Also I put a 10uf capacitor between pins 1 and 8. Now there is no buzzing, but the mic is not as very sensitive as I would like.It doesn't work when either shout or clap above the mic, only when I rub it.Thanks

author: boB
author_rank: Frequent Contributor
timestamp: October 24, 2022, 05:36:12 pm
content: I *think* I see the problem but, to get closer, I would take your meter and check the amplifier's DC output voltage.Much easier to see with Juan's schematic layout.boB

author: RobH
author_rank: Regular Contributor
timestamp: October 24, 2022, 05:56:37 pm
content: Quote from: boB on October 24, 2022, 05:36:12 pm.The output voltage on the LM386, pin 5, is currently 4.85v with 9v input from a power supply.This drops by about 0.1v when I clap over the mic, and when I do a loud aaargh, it drops by about 0.3v

author: StillTrying
author_rank: Super Contributor
timestamp: October 24, 2022, 07:03:22 pm
content: "The output voltage on the LM386, pin 5, is currently 4.85v with 9v input from a power supply."That's about right.The 100R in series with the 8R speaker looks strange to me, that will reduce the speaker power a lot, I can't see why it's needed.

author: RobH
author_rank: Regular Contributor
timestamp: October 24, 2022, 07:34:55 pm
content: Quote from: StillTrying on October 24, 2022, 07:03:22 pmOk but it doesn't say what resistance the speaker should be, and that is what I have. What would recommend I should use, 4R??Thanks

author: rstofer
author_rank: Super Contributor
timestamp: October 24, 2022, 08:32:47 pm
content: According to the 741 datasheet, theminimumvoltage is +- 10V or 20V between V+ and V- and anominalvoltage of +-15V. I'm not convinced that operating an op amp at less than half the recommended voltage is a great idea.https://www.digikey.com/htmldatasheets/production/94208/0/0/1/lm741-series.htmlApparently it works from time to time.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 24, 2022, 09:38:52 pm
content: Try this.You don't need to remove everything, only disconnecting the potentiometer from the 741 output, and modifying the load.Set the potentiometer cursor all the way to Gnd, connect the power and start turning it slowly while you measure the voltage at input(3) and output (5) pins.The LM386 has 26dB voltage gain, that means you should see 20x the input voltage at the output.The 100K resistor limits the max input voltage to 800mV and provides finer adjustment.So with 500mV input you should completely saturate the output @9V.Not sure about the output voltage capabilities of this amp, didn't find anything on the datasheet.The output might not go lower than 1.5V or higher than 7.5V, that's pretty normal in a lot of amplifiers.

author: RobH
author_rank: Regular Contributor
timestamp: October 24, 2022, 09:41:29 pm
content: Quote from: rstofer on October 24, 2022, 08:32:47 pmThat maybe so, but it doesn't do anything for the lack of sensitivity of the mic when I clap or shout. Thanks anyway

author: Zero999
author_rank: Super Contributor
timestamp: October 24, 2022, 10:40:39 pm
content: TI's data sheet says the 741 will work down to 10V, but going by the internal schematic I would say this is very conservative. It should work down to around 6V, if the input and output voltage swings are minimal.https://www.ti.com/lit/ds/symlink/ua741.pdfDo you have a signal generator? The circuit has a total gain of about 220: 11 from the op-amp and 20 from the LM386. If you connect a 10mV signal to the input, you should measure 2.2V at the output. The microphone and speaker should be removed before performing this test. Check the voltage at the signal generator with a multimeter. It doesn't matter if the meter is designed for mains frequency and isn't true RMS. It's the ratio of the two voltages which is importantBy the way, there's no need for that 100R resistor in series with the speaker. The LM386 can drive an 8 Ohm speaker with no trouble. All the resistor does is reduce the volume and damping factor, which results in a large resonant peak, around its resonant frequency.

author: RobH
author_rank: Regular Contributor
timestamp: October 25, 2022, 02:49:30 pm
content: Quote from: DavidAlfa on October 24, 2022, 09:38:52 pmThanks and I have added the 100k resistor as per the picture, and that has made a big difference.I am now using a 47R from pin 5 of the 386 to the 330uf capacitor for the speaker. the speaker crackles and whistles louder, but adjusting the 10k pot down to about 400mV input to pin 3 stops that. It still crackles when I make a noise over the mic.I tried a sound sensor beeper, and that seems to work better, with 8.6v out from pin 5, and that drops to just over 5v when the beeper is activated.if possible , I'd like to know if it , the beeper noise, can be turned off when the external noise stops, ie only beep when a noise is made. Presently I can only stop it by putting my finger on the mic.Thanks

author: RobH
author_rank: Regular Contributor
timestamp: October 25, 2022, 02:55:40 pm
content: @Zero999I have changed the 100R resistor to 47R on pin 5 of the 386 now and the speaker still works ok.I don't have any test equipment except a DMM, so all I can do is measure voltages and resistance.

author: Zero999
author_rank: Super Contributor
timestamp: October 25, 2022, 05:59:49 pm
content: Quote from: RobH on October 25, 2022, 02:55:40 pmYou mean it works now? I meant replace the 100R resistor with a wire. It's not needed.

author: RobH
author_rank: Regular Contributor
timestamp: October 25, 2022, 06:09:53 pm
content: Quote from: Zero999 on October 25, 2022, 05:59:49 pmYes it does work now and I have changed the resistor with a wire.Thanks

author: Zero999
author_rank: Super Contributor
timestamp: October 26, 2022, 08:55:08 pm
content: That makes sense. The power output would've been tiny with that series resistor.I wonder why the schematic included the resistor in the first place? Perhaps they thought the LM386 can't drive an 8 Ohm speaker.

author: boB
author_rank: Frequent Contributor
timestamp: October 27, 2022, 05:51:38 am
content: Maybe the high resistance was for 600 Ohm headphones ?I still would have used zero Ohms