LM386 with Voltage Divider @ Input Question

Thread Starter

drmanmachine

Joined Mar 12, 2016
68


At the Vin stage, I have a voltage divider dropping down the voltage from around 3-3.5V to 300-350 mV. (I'm not in front my circuit, but I believe the voltage divider is a 100k (R1) and 10k (R2) connected to the high end of the 10k pot. It's not shown in this schematic, but this is essentially the same idea minus the divider.

At one point, the volume sounded very loud and clear. Now, I'm left with about half that volume. I thought it was the 10k pot, so I changed it out, no difference. When I run my meter on pin 3 to check the voltage at the input, I'm getting a lot of noise and reading no voltage. When I place my fingers on the shell of the 10k pot, I also get a lot of noise.

Can anyone provide me some insight on what may be the problem?
 

AlbertHall

Joined Jun 4, 2014
12,328
If you remove the 10:1 divider at the input and remove the capacitor between pins 1 and 8 you will have the same overall gain but you can expect lower noise.
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
Not without more information.

Show us the input and output waveforms.
I'll post them when I get home.

I took three shots. One screenshot represents the output of my oscillator that I have built. The second is the waveshape via the voltage divider and the last shot is the waveshape of the output of the LM386.
 

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Audioguru

Joined Dec 20, 2007
11,248
The squarewave from your oscillator makes the raspy sound of a buzzer that is not a clear smooth sinewave tone.
If you use your multimeter to touch the input of the amplifier that has a voltage gain of 200 times then of course the meter leads will pickup hum and other interference noises.
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
The squarewave from your oscillator makes the raspy sound of a buzzer that is not a clear smooth sinewave tone.
If you use your multimeter to touch the input of the amplifier that has a voltage gain of 200 times then of course the meter leads will pickup hum and other interference noises.
Okay. Good to know. With that said, I am feeding a squarewave into the LM386.
 

Audioguru

Joined Dec 20, 2007
11,248
The output waveform might look odd because the 'scope timebase shows only half a cycle and the input of the LM386 is severely over-driven.
Why are you making a buzzer?
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
The input isn't severely overdriven. The datasheet mentioned that the maximum is 400mV. I'm at around 300mV at the input. I'm not making a buzzer. I'm using the IC 40106 as squarewave oscillators outputting to an LM386 for amplification. I'm making it for a friend who is a noise musician.
 

Audioguru

Joined Dec 20, 2007
11,248
The input of your LM386 is 300mV. The voltage gain is 200 times. Then is the output 300mV x 200= 60V?
No, the input is simply over-driven or the gain is too high.
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
I removed the cap between the gain pins and it didn’t improve.

I’ll be honest and maybe I shouldn’t read into it, but the way you deliver feedback is a bit on the rude side. I felt like you’re trying to make me feel like an idiot. I don’t appreciate it.
 

DickCappels

Joined Aug 21, 2008
10,122
I doubt anybody is trying to make you feel any particular way, he's just trying to get his point across.

With the capacitor between pins 1 and 8 out of the circuit the gain should be about 26 db or 20x to get a 6 volt output. What is your power supply voltage?

What is the problem you are having with the circuit? You wrote "At one point, the volume sounded very loud and clear. Now, I'm left with about half that volume." <= I think this is the problem you are trying to solve, not the distortion -is that correct?

Did you make any changes to the circuit between the time it sounded very loud and clear and the time it is only at about half volume?
 

be80be

Joined Jul 5, 2008
2,072
Here from the datasheet it show a gain of 20 hows the OP getting a gain of 200
Sorry I see he has cap from 1 to 8 which set's it to 200


Screenshot from 2017-12-08 03-07-17.png

So as been said remove it and go with the above
 
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Thread Starter

drmanmachine

Joined Mar 12, 2016
68
I doubt anybody is trying to make you feel any particular way, he's just trying to get his point across.

With the capacitor between pins 1 and 8 out of the circuit the gain should be about 26 db or 20x to get a 6 volt output. What is your power supply voltage?

What is the problem you are having with the circuit? You wrote "At one point, the volume sounded very loud and clear. Now, I'm left with about half that volume." <= I think this is the problem you are trying to solve, not the distortion -is that correct?

Did you make any changes to the circuit between the time it sounded very loud and clear and the time it is only at about half volume?
I removed the 10 microfarad capacitor between pins 1 and 8 and left them open. My power supply voltage is a 9V battery. The main problem I am having is that the output was very loud and clear and now it's about half. Granted, this was with the 10 microfarad capacitor between pins 1 and 8. There were no indications of distortion or the signal clipping. However, I noticed a problem when I turned it on one day and found my 10k pot to be pretty loose. I would tussle it a bit and started noticing that the connections were loose. I re-soldered new wires to the 10k pot and soldered them onto the board. After that, my output level shrank.

I'll take a picture of my completed board and draw on where I believe the problem lies. I'll also provide a full schematic once I get home. That may help solidify where my problem lies.
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
Hello,

What is the powersupply voltage?
What is the impedance of the used speaker?

Bertus
The power supply voltage is 9V.
My output is to a 1/4" Mono Audio Jack and I am using my headphones right now to test the output. To be honest, I'm not 100% sure what the impedance of those headphones are. If I had to guess, they're probably 8 ohms.
 

bertus

Joined Apr 5, 2008
22,264
Hello,

Do you use a decoupling capacitor accross the powersupply?
I have heard that a 100 uF and a 0.1 uF accross the powersupply will give some improvement.
With a 9 volts supply, you may have a max output swing of 6 Vpp, assuming the 8 Ohms impedance.

LM386_output_swing.png

Bertus
 

DickCappels

Joined Aug 21, 2008
10,122
He has 250 uf according to the schematic.

The power supply voltage is 9V.
My output is to a 1/4" Mono Audio Jack and I am using my headphones right now to test the output. To be honest, I'm not 100% sure what the impedance of those headphones are. If I had to guess, they're probably 8 ohms.
Probably between 4 and 32 ohms, depending upon what they were designed to do.

8 ohms would be marginal in your case. BUT that's not your complaint. Even with the capacitor gone you still have enough gain to turn 300 mv P-P into 6 V P-P. How many volts P-P do you get out of the amplifier?

Did you experience the loud and clear output with the same headphones you were using when the output was only half?
 

Thread Starter

drmanmachine

Joined Mar 12, 2016
68
Hello,

Do you use a decoupling capacitor accross the powersupply?
I have heard that a 100 uF and a 0.1 uF accross the powersupply will give some improvement.
With a 9 volts supply, you may have a max output swing of 6 Vpp, assuming the 8 Ohms impedance.

View attachment 141183

Bertus
I have a 100 uF across the power supply.

He has 250 uf according to the schematic.



Probably between 4 and 32 ohms, depending upon what they were designed to do.

8 ohms would be marginal in your case. BUT that's not your complaint. Even with the capacitor gone you still have enough gain to turn 300 mv P-P into 6 V P-P. How many volts P-P do you get out of the amplifier?

Did you experience the loud and clear output with the same headphones you were using when the output was only half?
Honestly, I'd have to wait until I got home to check with my meter. Last time I did check was when I still had the 10uF across pins 1 and 8 and it was reading about 4-4.2V with a 9V battery reading 8.6V.

Since then, I put a new 9V battery, but didn't check the output yet. I was going by what I was hearing. I'll check the output voltage when I get home tonight.

I will post my own schematic when I get a chance.
 

Audioguru

Joined Dec 20, 2007
11,248
I am sorry that you think I was rude to you. You did not explain everything before.

Now you say the LM386 was not clipping. But you said the input was 300mV and the gain was 200. I showed you that the output could not possibly be 300mV x 200= 60V! So of course it was severely clipping.

You did not show your 'scope showing the output of the LM386, instead your scope's timebase was too fast and showed only 8us which is a tiny piece of the output. Please turn down the timebase to show 3 or 4 cycles of the waveform then the timebase will be about 5ms.

Is your 9V battery 9V or has it drained to 6V? The datasheet of the LM386 shows that with a 9V supply and a 4 ohm speaker (your 8 ohm headphones are 4 ohms if both earphones are driven in parallel) the output clips at 3.5V peak-to-peak.
 
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