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lm386 phenomenons

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aalauzier

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Hello everyone, I was working on my 386 bass boost project, and I went by the schematic in the datasheet, and it is also attached. It does have good bass, but it also buzzes and acts strangely. Is that just because you need to sacrifice some of the sound quality for better bass? I made a few changes to the schematic, although it follows the same basic principals. The two smaller caps are .047uf because they were more common, and the output cap is 470uf. I started with a 1000uf cap but heard the buzzing, so I changed it to a smaller one but it still didn't go away. Does anyone know what the problem might be? I thought it might be the gain, but I am not sure what I should put across the pins. But when I did experiment with that, something strange happened, the amp was powered on, and I touched my soldering iron to pin one (it also happened with pin 8) and i heard an Asian radio station. It sounded Japanese, but I am not sure exactly what it was. I heard a few others, but the Asian one was the most prominent. Was that just because my soldering iron was acting like an antenna? what happened? Advice would be great. Thank you all.
 

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It almost sounds like you are picking up 60HZ noise on the input and by improving the low frequency response you are making it sound worse. The input to the amp should be shielded wire and short. As to the radio.... Maybe a Japaneese part? :=) - No idea.
 
But when I did experiment with that, something strange happened, the amp was powered on, and I touched my soldering iron to pin one (it also happened with pin 8) and i heard an Asian radio station. It sounded Japanese, but I am not sure exactly what it was. I heard a few others, but the Asian one was the most prominent. Was that just because my soldering iron was acting like an antenna?

Yes it was.
 
The supply voltage Vs needs a capacitor to ground, try 100uF to 470uF.
The output power from an LM386 is only 0.45W at clipping when it uses a 9V supply and drives an 8 ohm speaker. If the volume is turned up higher then the bass will be clipping so badly that it will sound like buzzing.
 
So instead of only running 9V directly into pin 6, i should also have a cap connecting it to ground? I also have a 100uf smoothing cap along the +and- rails
 
Your "smoothing" cap is not shown on your schematic so I recommended adding one. 100uF is so small then of course the amplifier will have mains hum at its output unless it is powered from a battery.

The filter capacitor should be very close to the supply pins of the amplifier IC to avoid it from oscillating. If it is further than a couple of inches then add another 0.1uF capacitor at the supply pins of the amplifier IC.
 
The smoothing cap (i don't know what else to call it is 100uf across the power strips on the pcb, so I should add a .1 or 100uf cap with the supply pin to ground? Sorry if I seem ignorant, I am still new to electronics design. Would changing the gain also improve the sound quality? if so, how should I change it?
 
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You forgot to say what is your 9V power supply. Is it a 9VDC wall-wart adapter or is it a little 9V battery?
Both need a 470uF to 1000uf capacitor from V+ to ground but your 100uF capacitor on th pcb should be OK for a battery if the battery is fairly new.

The bass-boost parts increase the gain at low frequencies so that you hear mains hum is the filtering is poor or if you did not use shielded audio input cables.

The LM386 has low output power. Maybe you need much more power.
 
I am not exactly sure what shielded wire is, and I am using a wall wart, so the supply is around 11VDC. The 100uf cap is good? Do I need any caps connected the the Vs pin on the ic? Thanks for your help
 
I am not exactly sure what shielded wire is, and I am using a wall wart, so the supply is around 11VDC. The 100uf cap is good? Do I need any caps connected the the Vs pin on the ic? Thanks for your help

ALL audio equipment is connected together with shielded audio wire to avoid mains hum pickup by the wires. If you are in North America then the shielded wires usually have RCA plugs on them. Look in Google Images for RCA plugs.

We don't know if your wall-wart is cheap with lots of hum or if it is good with low hum. Adding a filter capacitor might help reduce the buzzing.
 
I am looking into the tda7240 amp because i have some old car speakers that sounds good and wouldn't be too bad after some cleaning. Are they mono or stereo? there are 2 output pins, is one left and right? they are 1 of the 2 connections for the speakers, while the other one goes to ground? and where is there a volume control on it, or is it just controlled by the input. This is the schematic that I was looking at. https://www.circuitstoday.com/wp-content/uploads/2010/08/20W-amplifier-using-TDA7240.png and one last question, what does the first switch, the standby switch do? is it necessary? Thank you
 
Why don't you look at the datasheet of a TDA7240A??
It shows two amplifiers that are bridged so their voltage swing across the speaker is almost doubled into one speaker then the current in the speaker is also almost doubled then the power in the speaker is almost 4 times as much as a single amplifier. The speaker IS NOT grounded.
The input to the input coupling capacitor can have a volume control like every other amplifier.

If you want stereo then you need two of this amplifier circuits.
 
Alright, thank you, I am not sure how to read a datasheet and I only saw one schematic so I will try that. Is a bridged amplifier an amp that has both wires to the speaker coming from the amp, instead of one to the amp and one to ground?
 
Alright, thank you, I am not sure how to read a datasheet and I only saw one schematic so I will try that. Is a bridged amplifier an amp that has both wires to the speaker coming from the amp, instead of one to the amp and one to ground?
The datasheet of the TDA7240A has a schematic of an amplifier and a recommended pcb design for it.
The bridged amp has two amplifiers. One amplifier feeds one wire of a speaker and the other amplifier feeds the other wire of the speaker out-of-phase. Then the voltage swing to the speaker is almost doubled so its max output power is almost 4 times.

If you short one output of a bridged amplifier to ground then the amplifier will blow up if it does not protect itself. The speaker also might blow up because a bridged amplifier does not use an output coupling capacitor.
 
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