Building simple amplifier using 386 Amplifier chip

[VacuumTube194]

I built a simple audio amplifier from a 386 amplifier chip. The sound coming out of the speaker is not too bad, but sometimes a little staticy.

Is there a way to cut down on the static? Also would piping the signal through an audio transformer help?

Should the transformer be before the chip or after? (I mean should the audio signal go into the transformer - then to the chip and speaker, or put the transformer after the chip, right before the speaker?)

Any help would be great, thanks!

[Bigglez]

Greetings (No Name Supplied),

I built a simple audio amplifier from a 386 amplifier chip. The sound coming out of the speaker is not too bad, but sometimes a little staticy.

What does "staticy" mean? Are you hearing something
when there is no input signal? Is the reproduced sound
defective in some way? Does this happen at all volume
levels? Are you listening with headphones or a speaker?

Is there a way to cut down on the static?

It depends on what it is and what's causing it.

Also would piping the signal through an audio transformer help?

No.

Should the transformer be before the chip or after? (I mean should the audio signal go into the transformer - then to the chip and speaker, or put the transformer after the chip, right before the speaker?)

How do you expect the 'transformer' to improve the
circuit? What type of transformer are you considering?

It would be very helpful if you can provide a circuit
diagram of your project. A PIX of how you constructed
it would also help.

Comments Welcome!

[VacuumTube194]

Its the 386 Audio Amplifier (typical application) in the Forest Mimms Radio Shack books, page 11 in my copy (though mine is from the 80's). Basically its just the 386, a capacitor and variable resistor.

There is no sound without an input, but when the input goes in sometimes the audio is good and other times it comes with a little static - I don't hear anything in the static not like an overlapping signal, its like a radio station slightly off. Its also weird that sometimes when I touch the circuit the sound improves - then when I remove my hand the static comes back. I'm just wondering if there are ways to improve the clarity of the output.

Any ideas?

[Bigglez]

Greetings (No Name Supplied),

Its the 386 Audio Amplifier (typical application) in the Forest Mimms Radio Shack books, page 11 in my copy (though mine is from the 80's). Basically its just the 386, a capacitor and variable resistor.

I don't have that particular reference handy. Here's
a well known application circuitand here's a
construction article for another amp using the LM386 device.
Yours sounds very similar.

There is no sound without an input, but when the input goes in sometimes the audio is good and other times it comes with a little static - I don't hear anything in the static not like an overlapping signal, its like a radio station slightly off.

An AM radio "off tune" will sound clipped or muffled,
would you say that's a good description of your amp?
Co-channel interference is usually a series of whistles
or tones, while two or more signals on top of each other
is heard as distortion.

Distortion is usually caused by clipping of the output
which will happen with any single supply amplier without
AC coupling.

If you only have one capacitor in the circuit it most likely
is at the output. I would have expected two other capacitors,
one at the input (to block DC through the signal source)
and another across the power supply to bypass the
battery ro DC power supply.

Its also weird that sometimes when I touch the circuit the sound improves - then when I remove my hand the static comes back. I'm just wondering if there are ways to improve the clarity of the output.

Now that sounds like a RF effect. Either your circuit is
oscillating at RF, and your hand 'tunes' it slightly, or
your hand is bring RF into the circuit from a radio
broadcast.

Again, not having the correct decoupling capacitors
in place can lead to self-oscillation of any amplifier.
Usually any high gain amplifier will also have a
roll-off filter (capacitor and resistor) at the
output to provide stabilty. Take another look at the
links above, add these parts if you don't have them.

Comments Welcome!

[evahle]

Hi VacuumTube194. Bigglez is right, in that it is probably due to self oscillation. This is a common problem with the LM386 chip. It seems that you may have left out the 10ohm resistor and .047uf ceramic capacitor in series, on the output of the chip to ground.

I've included an old schematic of my stereo amp using two LM386's. It works great and I still use it for experimenting. Hope this helps a little more.

evahle

[GoingFastTurningLeft]

I'm no expert on RF but if proximity to the circuit affects noise, you might need to ground your stuff better.

Interesting thing about the AM radio comment... I remember when I got into electronics building guitar distortion pedals, which are pretty much simple op-amp circuits.

On a few of them I would get BBC world news coming through my amp if my guitar volume knob was turned all the way down (IIRC). I think somehow it was turning my guitar and pedal into a crystal radio with an amplifier.

If you look at a few datasheets for 386's from different manufacturers, you'll find a bunch of example circuits. Compare those to what you have and you might find an improvement.