LM386 Amplifier

I've set up a simple LM386 amplifier on a breadboard according to the above schematic. When connected, the amplifier works like a charm driving a spare 8Ω speaker with impressively low (audible) noise.

On the input (pin 3) I'm connecting just the left channel of a 3.5mm audio jack.

When no source is connected to the jack and the power is switched on I get a significant buzz coming from the amplifier, which is worsened when a 3.5mm audio cable is plugged in. However, as soon as I connect the other end of the cable to a source (phone/MP3 player etc.) the buzz stops completely.

I assume this is a grounding issue and that I'm essentially making a big aerial the moment I connect an unterminated cable. This is inconvenient as there might well be times when the amplifier is switched on but a source isn't hooked up.

Question: Why does this happen, and how do I prevent noise/buzz when no source is connected to the input?

  • As I've reported elsewhere on the site, "A sound engineer dies somewhere every time you do this.". Best practice if you can't turn the amplifier volume down or power it off is to connect the source first and then plug the lead into the amplifier. Electric guitar players do this all the time. – Transistor Aug 21 '17 at 11:32
up vote 3 down vote accepted

Use a 10 k pull down resistor at the input rather like as shown in the standard circuit that uses a potentiometer: -

enter image description here

Without an impedance to ground (such as a 10 k resistor) you will get pick-up because the input is high impedance and any wire conencted to this will pick-up some signal. The pull-down resistor will reduce hum/noise when the input is not connected to a signal source.

Alternatively you can arrange for a jack socket to have a grounding contact the shorts the input to signal ground when nothing is plugged in: -

enter image description here

  • Perfect, thanks - I have since done the requisite reading on pull up / pull down resistors. Makes sense, and worked like a charm! – abza Aug 22 '17 at 9:00

Try experimenting with putting a resistor between the LM386 pin 2 and pin 3 (GND). This will lower the input impedance and remove the AC line frequency pickup from the open connection or cable. Start with something like 10K ohms and lower as necessary to remove the problem.

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