Little Gem Amp problem

[BJM]

Hi all,

I've built a Little Gem LM386 Amp from the schematic at Runoffgroove, got it working and it sounded quite nice too. I've built it with battery clip and wall wart connection. The plug from the wall wart disconnects the battery, Today I picked up a cheap 12 Voltl wall wart, took out the battery and plugged the wall wart in. No sound/led light (the gem turns on when I insert the guitar jack) but smoke..... I think one of the capacitors, it got very warm. The LM386 is supposed to handle 12 Volt. Re. parts etc. it's not really a problem, it's a cheap design but I'm wondering what caused the problem? Any ideas/comments would be welcome, I hope you can help.

BRgds,

Bert

[GibsonGM]

Could be a lot of things, BJM (welcome to the forum!) - can you post exactly what it says on your wall wart?  I am interested in the voltage AND the current rating.

12 volts DC???   (I hope)

If you didn't draw enough current (the "500mA" or whatever it might say on it), you COULD have popped a capacitor rated at like 6, or 16, volts.   Perhaps.     See, it says "12V", but that's AT the rated current...so if you draw maybe 1/100th the rated current, it could REALLY be giving you 18V or more!    Again, perhaps.   If that's the case, the chip might have gone.

If it is an AC adapter, the problems will be bigger.    It's ok, we've all had things like this happen, ha ha!

Do you have a digital multi-meter handy?  (DMM).   If so, try to measure the wall wart's output voltage when not plugged into anything.

[BJM]

Hi,  I think you've hit the nail right on the head. I already measured the wall wart (12 Volt DC 600mA) and it turned out to be almost 17 V. I'm not sure about what the capacitor can handle but it's probably less.

Thanks for your help.

BRgds,

Bert

[bool]

Lots of small (non-stabilized) warts are designed to hold the specified voltage at the specified load (0.6A in your case) - when their voltage actually droops under the load. Unloaded, they will naturally have to have quite a bit higher voltage (most of "12-volters" I have seen at 16-something volts), and just running a small amp at whatever quiescent current it has will of course present a negligible load to such a wall-wart.

Hence the consequence.

[GibsonGM]

I would 'examine' any caps you have in there, try to see if they're rated low (6v, etc).   They MAY be ok if they're rated at 16v, you never know.  If in doubt, change them out!   They're cheap enough.
Change the chip, it's most likely a dead bug now.

Lastly, I would either find a REGULATED wall wart, or get an LM7809 and see if you can run the one you have thru it. That will output a stable 9V for you.  I does take a couple of volts to operate, but I think you'll have that covered...that's why I'm not saying you can get 12V out of what you have.     You just need the regulator IC, and a couple of caps, to use it.   

To get a 12V supply, you need an LM7812 regulator and a supply rated to give a few volts over that.  16V would do.   If you dropped much below say 14V, the regulator will "drop out".    It is questionable whether the current wall wart can get you by at 12V WITH regulation...you could try it, though! 

To test it, you can always use a 9V battery for now.

Like bool pointed out...if you're not drawing that rated current, they are unreliable.  There are a few ways to "fool" the things into thinking you ARE drawing the rated current, but it's really not even worth trying that since the regulators are so inexpensive...

This is for the whole "family" of 78XX regulators, take a look. Available anywhere such as Digikey, Mouser, etc. 
https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf   

Power supply building is FUN!     lol

[pappasmurfsharem]

Also...

There are different forms of LM386, not all can handle 12Vs

[duck_arse]

erm, why did you take the battery out first go, just outta curiosity? and polarity? no-one has yet mention polarity. does the board have a revese protection diode? test the output of the wall wart the way you think it is, and see if the meter reads minus instead.

[mth5044]

and polarity? no-one has yet mention polarity. does the board have a revese protection diode? test the output of the wall wart the way you think it is, and see if the meter reads minus instead.

Came in to say this. I have made the mistake before - it seems that a decent amount of laptop power supplies have the opposite polarity.

[BJM]

Hi,

i took out the battery to make sure the power would be from the wall wart. The polarity was mentioned on the thing itself, + on the inside, - outside, looked o.k. to me.

Anyway thanks for your replies, I think I'll go for a regulated power supply but  will take a look a the LM7809/7812 also.

BRgds,

Bert

[mth5044]

I would 'examine' any caps you have in there, try to see if they're rated low (6v, etc).   They MAY be ok if they're rated at 16v, you never know.  If in doubt, change them out!   They're cheap enough.
Change the chip, it's most likely a dead bug now.
lol

Would they not have gone up when the battery or a 9V adapter was used? Do any of the normal distributers even carry 6V electros? I guess I could see accidentally getting them from mouser.

[GibsonGM]

I kind missed that he did the battery test in my first reply, ha ha.  Yes, of course they may have been damaged by 9V.

However - if the "12V" was really putting out 17 or 18V, that would've done it for SURE.   I've gotten away with 6V caps running on 9V when I was a noob, but going over 12 is probably going to do them in....sounds to me like the old case of unregulated wall-wart put into a circuit that doesn't draw enough to drop the voltage down to nominal. 

[mth5044]

I kind missed that he did the battery test in my first reply, ha ha.  Yes, of course they may have been damaged by 9V.

However - if the "12V" was really putting out 17 or 18V, that would've done it for SURE.   I've gotten away with 6V caps running on 9V when I was a noob, but going over 12 is probably going to do them in....sounds to me like the old case of unregulated wall-wart put into a circuit that doesn't draw enough to drop the voltage down to nominal. 

I believe you - you are most wise 

[GibsonGM]

Nah - it's the wisdom passed down to us all, from the great gurus of stompboxology!!!!   

[BJM]

Hi all,

It worked fine with the 9 Volt battery. Practically all designs for LM386 amps (Ruby, Cricket) have a 9 V battery. I'm not sure about the specs of the capacitor, it's an Elco, I think it was rated for 16 V,  and deceased now.......  Anyway it served it's purpose, I'm new to building stuff like this and  learning some new things now.

Thx,

Bert

[GibsonGM]

Happy it worked out, Bert!    You just need to get a 12V regulated supply or otherwise regulate what you have....the LM7812 is great, as is a properly-rated Zener diode with series resistor, too (test before attaching Gem, ha ha).   Another topic to read about.     

Solving these kinds of problems helps you to grow into the hobby, and provides you with "ways out of tough situations" in the future.    Keep at it, there is a lot to learn just in what you're doing there, and it will pay off in spades down the road!!        I think you might like the Gem more on 12V, if you can get a good solid supply.

[BJM]

Hi people,

It's been a while but I've rebuild the Little Gem, with am LM7809 and some capacitors, and a Valvecaster in front of it (looks like the Ruby Tuby but not exactly the same schematic).  I left out the battery and power it with a decent wallwat and it's working, plugged direclty in a 12 inch guitar speaker cabinet. I'm at the moment still trying out some different cap values to see how that changes the sound. It's great fun, and thanks again for all your help!

BRgds,

Bert

[mth5044]

Hi,

i took out the battery to make sure the power would be from the wall wart. The polarity was mentioned on the thing itself, + on the inside, - outside, looked o.k. to me.

Anyway thanks for your replies, I think I'll go for a regulated power supply but  will take a look a the LM7809/7812 also.

BRgds,

Bert

Don't ya want center negative?