Overheating Little Gem

[shaman]

I recently build my second DIY amp (the first one was simple TDA2030 hifi amp on 9V), using design Little Gem Mk1. I hooked it up to my Peavey's Rage 258 speaker (8ohm), a fresh 9V and my guitar. The amp was producing good sound for about 1-2 sec. Then it died. LM386 was buring hot, so I disconnected the battery quickly to avoid damaging the IC, but I fear it could have died.
I took my time and built the amp very carefully, checked the solders with a multimeter. The only differences are that I didn't connect the gain pot, and master vol pot (hard to get in my area). The gain pot isn't necessary for the amp to run according to the instruction. I thought the master vol pot isn't necessary (it should pass full power to the speaker when at certain setting right? ) so i skipped it and hooked speaker to - of 220u cap, and ground, does this cause the problem, or something else ? Too low speaker impedance ? Bad IC ? I guess in few places my soldering skill could be lousy, but I don't think it did cause the problem.

Link to the design, just in case 
http://www.runoffgroove.com/littlegem.html

[joecool85]

I run my Little Gem (also Mk1) with no volume control and a switch for the gain control (no connection or straight connection, no caps/resistors etc).  It is fine driving 4 ohms or greater.  So an 8 ohm speaker is fine.  Is the speaker in good working order?  Check the resistance of it, it may have a shorted coil.  Also, check that your output wires haven't crossed.  If they short out you would lose all volume and the chip would get hot as you described.

Pictures of your build would help us diagnose it as well.

[DJPhil]

My first LM386 build went like this. I had a bad salvaged capacitor that'd failed short in the output filter, and it worked out just like Joe said and gave the chip a viking funeral. This taught me to always invest in chip sockets.

Definitely start with the speaker and speaker connections as per Joe's instructions so you're sure there isn't a short in there.

With the battery and speaker disconnected (to be safe) you should see a very high resistance or open circuit between pins 4 [gnd] and 5 [out] (either side of the arse end of the chip from the dot at pin 1). If you don't see at least 1MΩ then it may be toast. If you're measuring with a meter that has a multi-megaohm range you might get some odd readings, especially if the chip's in circuit or you're wearing a static strap. That's normal, as high impedance measurements are difficult to do well. If the chip failed as a short (hard to tell without removing it) it might hamper your ability to find the short elsewhere in the circuit.

The big output cap (220µF) and the low pass filter cap (47nF) block the output from having a DC path to ground. If either of those caps goes short, or if there's any way for the output to just dump current to ground, it'll be likely to toast the output stage of the chip.

On these chips too much gain won't hurt anything, it just sounds horrible. You can stick any resistor you like between pins 1 and 8 to change the gain, and for super duper gain you can stick a 10μF cap in there, but that's only really useful for ultra low signals (RF mixers and magnetic tape heads, etc.). I've never seen an LM386 with open gain pins, so if it were me I'd try 10k in there just to have something to make me feel better. The volume pot's also optional, as it just provides an adjustable parallel load to go with the speaker, so it's absence won't really contribute to your problem.

Hope that helps some.

[joecool85]

On these chips too much gain won't hurt anything, it just sounds horrible. You can stick any resistor you like between pins 1 and 8 to change the gain, and for super duper gain you can stick a 10μF cap in there, but that's only really useful for ultra low signals (RF mixers and magnetic tape heads, etc.). I've never seen an LM386 with open gain pins, so if it were me I'd try 10k in there just to have something to make me feel better. The volume pot's also optional, as it just provides an adjustable parallel load to go with the speaker, so it's absence won't really contribute to your problem.

Like I said, I run mine open for clean and shorted for gain/dirty.  It sounds great.  I don't see the need for a cap or resistor between pins 1 and 8.  Mine is in a small clip on FM radio enclosure that I gutted.  I kept the 2" speaker and rock that out with the LM386 circuit.  I have mine powered by 3 AA batteries because that's what the enclosure held stock and it works perfectly so I left it.

[J M Fahey]

You *might* have the output (220uF) capacitor backwards or have the speaker "grounded" to B+ (9v) instead of ground.
In either case the reversed electrolytic will look like a short to DC.

[shaman]

I wrote this thing below, then it struck me. Im using an universal board (first time), something like a PCB with lots of holes. I realized that some holes are connected. Then I facepalmed myself. The whole circuit is one big short. I have to desolder everything and start over. At least I learned how to test a circuit. Thanks for help everybody !
Btw. I have one question before I get to putting it back together again. I want to put a LED between battery +9V and pin 6. Is this ok ? I don't know what kind of LED it is (salvaged). Won't it eat too much power ? After all Little Gem has only 1/2W, and the LED can have about 1W. Can I limit the power it gets somehow ? I include pictures of my failure

The speaker is fine, it reads 7.3Ohms and I use it in my combo everyday. I measured resistance between pins 4 and 5 of the IC it reads 2 MOhms in circuit, 11 when I take it out.

I decided to torture my circuit again today. I made sure that the connection to the speaker is ok, hooked a 9V. It takes less than a second for the IC to get burning hot, this time however nothing from the speaker. I disconnected speaker and measured voltage between speaker output + and gnd. I discovered that whenever I connect power, there is a constant voltage of 4.96V. Also it showed that at least some of the caps are good, cause the voltage drops slowly after disconnecting the power source.
I read in another topic that I can test the electrolytic caps by measuring the resistance on them. Both read over 2MOhms, that indicates they are fine.

[joecool85]

Any reason you want the LED on pin 6 as opposed to ground and +9v?

**edit**
I guess I'm confused.  Pin 6 IS +9v....

[J M Fahey]

Use a 2200 ohm, 1/4W in series with the LED.
Remember it's a diode and has polarity.
The flat side should go to ground.

[joecool85]

Use a 2200 ohm, 1/4W in series with the LED.
Remember it's a diode and has polarity.
The flat side should go to ground.

But if you go from Pin 6 to +9v it won't do anything, pin 6 IS +9v!

[J M Fahey]

I think I was clear the right connection is Pin6(9V)->2200ohms->Led Anode->internally through LED to its cathode (flat side)->Ground.
If not clear I'll add a drawing later.

[polo16mi]

Not clear yet about LED function that you want.

If perhaps you want to use it like a "power on" indicator, may i suggest use a resistor of 330 ohms connected in serie (like J M explained above) instead of 2200?

I say it because as rule of design, i think that a tipical LED has a 2V drop of tension, and is necesary flow about 22 mA of current trough it to bright it on. 

So, 9V-2V = 7V -->  by ohms law,  7V/22mA = 0,318 Kohms---  around 300 ohms.--- ¿330 ohms more aprox commercial value?

About power used by LED connected like that...

If we use an 330 ohms resistor, current will 21.2 mA,  so, Power = V*I = 9V * 0.021 mA = 0.19 W

Hope help you

[J M Fahey]

Hi Polo.
Your calculation is right, I agree with it, and I also hit Leds even harder, with 50mA, because I want them visible even outside under sunlight; but in this case we have a weak 9V battery powered amplifier, so I did want the LED to use the lowest current possible, in this case around 3mA.
With this low current they are still visible in bedrooms at night, which is the most probable use this amp will see.

[joecool85]

I think I was clear the right connection is Pin6(9V)->2200ohms->Led Anode->internally through LED to its cathode (flat side)->Ground.
If not clear I'll add a drawing later.

That's fine.  I thought I had read that he wanted to connect one end of the LED to Pin 6 and the other to +9v.