LM380 oscillates on positive

Thread Starter

jmoffat

Joined Jul 18, 2012
42
I breadboarded the audio amplifier in the circuit below. My O'scope indicates that it is oscillating on the positive half of the sine wave. Any ideas? I used the inductor symbol to represent the speaker. If anyone knows where I can find a LTspice model for a LM380 please share. Also what would be a reasonable model for a 2" speaker?
 

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dl324

Joined Mar 30, 2015
9,633
Do you have the power supply decoupled? Is the oscillation frequency low enough to be audible?

Post a schematic for those of us who might be using a device that doesn't have LTspice installed.
 

bertus

Joined Apr 5, 2008
20,240
Hello,

Can you post a readable schematic?

Do you have the resistor/capacitor combination on the output of the LM380?

LM380_output.png

Bertus
 

Thread Starter

jmoffat

Joined Jul 18, 2012
42
Do you have the power supply decoupled? Is the oscillation frequency low enough to be audible?

Post a schematic for those of us who might be using a device that doesn't have LTspice installed.
Yes the power supply has a 220 uf cap which I moved to be physically closer to the opamp.

What is a good way to generate a jpeg of my schematic?
 

Thread Starter

jmoffat

Joined Jul 18, 2012
42
Hello,

Can you post a readable schematic?

Do you have the resistor/capacitor combination on the output of the LM380?

View attachment 95534

Bertus
I will have to find a way to generate a jpeg or something that can be displayed easily.
and yes I do have the resistor / capacitor in place. Ironically the oscillation diminished
substantially when I disconnected it. This led to me dress the leads differently. Problem solved!
I never thought lead dress would be critical at 1KHz!
 

RichardO

Joined May 4, 2013
2,271
I never thought lead dress would be critical at 1KHz!
Your signal may be at 1 KHz but the amplifier has gain at hundreds of kHz or more. This gain at high frequencies with some feedback can cause oscillations from the parasitics dl24 mentioned.
 

Roderick Young

Joined Feb 22, 2015
408
In post #3 by Bertus, there is a 2.7 ohm plus 0.1 uF snubber on the output. Do you have that installed? In 1979, I worked on a commercial product where the original engineer did not put in the snubber, and there were severe problems. The LM380 worked as an amplifier, but was very soft. My theory was it was putting out most of its power at the high frequencies of the ringing (oscillation). Once the 0.1 uF cap was installed, the output became normal.
 

AnalogKid

Joined Aug 1, 2013
8,259
As a rule of thumb, audio power amp chips, old and new, are notorious for breaking into oscillation. They are very sensitive to ground impedance (long wires/thin traces), decoupling, etc. Almost all have reference designs in the datasheet or app notes, and that board layout is not just recommended. Often it is the only thing that will keep the device under control.

ak
 
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