LM386 circuit

[bugmenot]

The data sheet for LM386 shows on the output, in parallel with the capacitvely coupled speaker, a 10Ω resistor in series with a 0.05µF capacitor. Why are these there, and what do they do?

 

[audioguru]

The series output coupling capacitor blocks the DC output of the amplifier which averages at half the supply voltage, and it passes the AC signal.

The 10 ohm resistor in series with the 0.5uF capacitor at the output is a load to the amplifier at high frequencies so that the amplifier does not oscillate when a speaker is a high impedance at high frequencies due to its inductance.

 

[bugmenot]

The 10 ohm resistor in series with the 0.5uF capacitor at the output is a load to the amplifier at high frequencies so that the amplifier does not oscillate when a speaker is a high impedance at high frequencies due to its inductance.

Can you elaborate? Why should a high impedance load cause oscillation?

 

[Leftyretro]

Can you elaborate? Why should a high impedance load cause oscillation?

Circuits like this audio amp rely on negative feedback for defining the gain, good frequency response and stable operation. This feedback is designed with a particulr output load impedance range over a specific frequency range. When the load is outside the specified impedance range it can cause a change is phase relationship in the feedback circuit and at certain frequencies the negative feedback will change to be a positive feedback. An oscillator by definition is an amplifier with positive feedback applied.

 

[bugmenot]

When the load is outside the specified impedance range it can cause a change is phase relationship in the feedback circuit

Thanks - I understand the concept of phase shift causing positive feedback causing oscillations. But why does a high impedance cause a phase shift?

 

[audioguru]

The LM386 is not very powerful so it has a substantial output resistance. A load reduces the output level and phase shift. an inductive speaker is a high impedance at high frequencies so the RC network becomes the high frequency load.

An applications note for the LM380 which is an earlier similar amplifier says that the RC network stops a 5MHz to 10MHz oscillation. I think the little LM386 would melt if it oscillates at a frequency that high.

 

[Patrick Simons]

When I take out pieces it still works?

Hello, can anyone tell me what the 104 cap linked in parallel with the 100uf cap across positive and ground does? If I take it out it still works, likewise the 473 cap in series with the 100K resistor when taken out still allows the circuit to work. I am however getting issues with the 5K pot i.e. when at max the output is < the output when the pot is disconnected and it cuts to muffle from halfway down to zero resistance.

Appreciate any help on this

Patrick

 

[duffy]

The 104 is a .1µf cap used to remove high frequency power supply noise the big cap can't get.

 

[audioguru]

Hello, can anyone tell me ....

You forgot to attach the schematic you found, so we don't know which part does what.

All battery powered circuits need a supply bypass capacitor to keep the supply voltage from jumping around or oscillating as the internal resistance of the battery rises as it runs down.
For the LM386 audio amplifier the 100uF capacitor is an audio frequencies bypass and the 0.1uF (104) capacitor is for high frequencies because an LM386 has plenty of gain at 1MHz where the 100uF capacitor is a lousy bypass.