# How do I start debugging this circuit?

I recently got a small LM386M-1 based amplifier kit, mainly to practice my SMD soldering. I managed to solder it, but it's giving me unexpected results.

Here's the schematic:

The input signal is supposed to be connected to NF (for positive) and GND connectors. ZV is obviously a speaker. Pin 1 is not connected.

Now about the results: When I power on the device and sent a 1 kHz sine wave to it, I get this:

The peak to peak voltage here is around 190 mV.

When I turn it off, I get this:

and the peak to peak voltage is around 1.46 V.

So it works much better when it's off than when it's on.

Also after reading the LM386 datasheet, the circuit looks a bit strange to me.

Here's the pinout of LM386 from the datasheet:

So is the PCB I got plain wrong or should I start checking out each individual component?

• The pin numbers in the schematic don't agree with the schematic pinout at the bottom of your post. – Federico Russo Apr 21 '11 at 13:59
• @Frederico I'm aware of that. – AndrejaKo Apr 21 '11 at 16:38

Where to start?

1. Your input signal is being applied to pin 4 which should be 0V
2. Your speaker is connected between $V_{OUT}$ & Gain instead of between $V_{OUT}$ & 0V
3. Both inverting & non-inverting inputs (pins 2 & 3) are connected to 0V
4. Bypass (Pin 7) is connected directly to 0V instead of through a capacitor (as per the data sheet)

The schematic is clearly wrong. Where did you get the kit from?

• I got it from a local electronics store called „Kelco”. They actually make the kits, so I'll contact them about the problem. I wanted a second opinion because they've been making various kits for some 20-30 years and I didn't expect such a basic error from them. – AndrejaKo Apr 21 '11 at 16:36

Also, the speaker should be connected to Vout via a capacitor (250 uF).

Forget about that kit and wire the chip according to one of the schematics in the data sheet.

• I'm aware of that, but when I connect a capacitor, I get horrible distortions on the output. – AndrejaKo Apr 21 '11 at 16:38
• Probably caused by all the other problems with the circuit. – Leon Heller Apr 21 '11 at 17:13
• heller why 250uF specifically? How did you get that value? I am in a learning stage of electronics. I think its better understand these things now instead of blindly copying. – 0xakhil Jun 11 '11 at 16:40

A bridge bucket design will fight you on your schematic, its reactions are that of a mirror false grounding.

LM3386 and LM386 are not the same.

• I haven't come across the term "bridge bucket" in several decades of hobby study. You might want to expand your answer to make it useful. – Transistor Jun 16 at 21:54