Questions regarding LM386 Amp circuit experiments - Page 1
Forum section: electronics / beginners

author: mart1n
author_rank: Contributor
timestamp: February 01, 2017, 09:35:44 am
content: I built the following LM386 amplifier circuit from Afrotechmods recent YT videoafrotechmods.com/tutorials/2017/01/17/how-to-make-a-simple-1-watt-audio-amplifier-lm386-based/It works great, but in my experimentation I ran into some things that I need help explaining...Question #1It's not clear from the schematic, but the left and right audio channel comes from a 3.5mm audio jack.With the 3.5mm jack flapping in the breeze (no audio source plugged in), I was picking up voices...or radio stations. It worked better when I touched the end of the jack with my fingers.The jack had wires soldered to it and they were plugged into my breadboard.The question is...how was I picking up voices with this simple circuit? I'm a beginner, but I thought a radio receiver was more complicated, so was I picking up radio? If so AM or FM? Or was it folks talking in cordless phones?Question #2I replaced the speaker in the circuit with an LED. There was not enough current to light it, but I noticed on my oscilloscope earlier when playing with the 3.5mm jack that when I squeezed the audio jack with my fingers that the amplitude of the signal went up, so I squeezed the audio jack and sure enough the LED light came on.Why does that happen? Why does the voltage go up when I squeeze the audio jack between my fingers?Question #3I replaced the 3.5mm audio jack with my new Rigol DG1022Z function generator and sent in a 2hz, 5vpp sine wave to blink the LED.I had my scope hooked up to the LM386 output, along side the LED and got this goofy looking waveform, can anyone explain why I get this?FYI - after this I put the speaker back and removed the LED and got a normal looking waveform at higher frequencies. But still not a sine wave...bonus question...does the LM386 modify the signal? I was sending in sine waves with my FG and typically got square wave looking signals on the output.Thanks for all the help and all the replies on the questions I've been asking!Sent from my iPhone using Tapatalk

author: danadak
author_rank: Super Contributor
timestamp: February 01, 2017, 12:21:38 pm
content: The waveform problem is the LM386 input common mode rangedoes not extend negative below its ground. So offset your generatorto ~ Vdd / 2 and amplitude to [ (Vdd /2 ) / G ] - ~ 1V to take care of thefact it cannot common mode all the way to the positive rail.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: February 01, 2017, 12:26:04 pm
content: The radio pickup not unusual, especially in a station is fairly close to you.Reason for shielded cables, short lead runs to jack.You can get demodulation from the device parasitic diodes. People have experienceddemodulation from fillings in their teeth. Imagine living with that, especially if its astation/music you don't like.Regards, Dana.

author: LazyJack
author_rank: Frequent Contributor
timestamp: February 01, 2017, 12:38:30 pm
content: Quote from: danadak on February 01, 2017, 12:21:38 pmAlso, it would be useful to AC couple the input.

author: mikerj
author_rank: Super Contributor
timestamp: February 01, 2017, 01:19:28 pm
content: Quote from: LazyJack on February 01, 2017, 12:38:30 pmEssential for any realistic use.A single LED on the output won't work because the output coupling cap has no current path to discharge after it has charged up. Connect a second LED in parallel with the first LED but connected in the opposite polarity (i.e. anode to cathode, and cathode to anode) and you should also add a current limiting resistor in series as even the little LM386 can deliver enough current to over-drive a standard signal LED. Use the lowest forward voltage LEDs you have (e.g. high brightness red LEDs, which tend to be around 1.8v).

author: xtoffer
author_rank: Contributor
timestamp: February 01, 2017, 01:35:12 pm
content: The LM386 actually expects its input to be around the negative rail while the output will be biased to half the supply voltage. So AC coupling the input might be the easiest way to achieve that requirement.The maximum ratings specify the input signal at +/- 0.4V. So a 5Vpp (even if AC coupled) is definitely too much..and will clip.

author: Afrotechmods
author_rank: Unknown
timestamp: February 03, 2017, 07:40:19 am
content: 1) AM radio is transmitted around 1MHz or so, and the usable bandwidth of an LM386 is well over that. So what was happening is the audio cables were acting like an antenna, picking up stray AM signals and they were getting amplified.2) It depends which part of your fingers were touching which part of the audio jack, but in general you can expect the human body to act as a fantastic antenna for 50/60Hz mains electricity hum that is all around you. Try poking your oscilloscope probe onto your finger - you can easily get a 1Vp-p 60Hz waveform just by doing that. Then the LM386 would have been amplifying it.3) The LM386 circuit I showed is not meant to be used with signals as massive as 5vpp. Instead try starting out with 0.2Vpp, AC coupled i.e. the signal swings from -0.1V to +0.1V. Then crank up the amplitude and see what happens. This would be closer to what you'd get from an AC coupled audio source like a phone.