Little gem mkII problems please help!

[fredy]

i thought that was wierd getting volts to the ground. I have the black probe on the sleeve, where ground is connected. IT is also attached to the -ve terminal of the battery.

And the voltages on each amp are pretty much the same give or a take a point of a volt.

[fredy]

Nope sorry, you were right, for the last set of reading i di have a probe on the battery. Whoops sorry. When i get back from work i will try again. Thanks for all of you help with this, i do aprreciate it.

[fredy]

i have some pics now.

[CLAAS]

Sorry,Made
Too much of Zinn...
I thing:Did it again.
Hopefully Thilo

[fredy]

hi claas, sorry, but i have no idea what you are on about.

Here are my new voltages after the cap change:
the battery is at 9.1v
at +ive termanal of battery 8.09

transistor

tip = 0.0
ground = o.08
9v = 8.08

+ amp

1 = 1.30
2 = 0.0
3 = 0.0
4 = 0.0
5 = 4.6
6 = 7.96
7 = 4.02
8 = 1.29

- amp

1 = 1.28
2 = 0/0
3 = 0.0
4 = 0.0
5 = 3.85
6 = 7.93
7 = 3.98
8 = 1.29

Is it getting better?

[mdh]

Well, one thing is clear from your pictures, and that is that I'm not surprised you're not hearing any bass with a 220pF (picofarad = 10^-12 farads) cap in the audio path.  That little ceramic cap labeled "221" is in the position that should be occupied by a 220nF (nanofarad = 10^-9 farads).  I suspect that the other ceramic cap I see is about 3 orders of magnitude too small as well, but that's a guess based on its physical size.  A 220nF cap will be labeled "224" and a 50nF cap will be labeled "503" (or more likely you'll come across 47nF caps labeled "473").

I think the voltages are credible, but I'm not really sure what to expect on the output pins of the amps (pins 5) or the gain control pins (1 & 8 ), and I'm not certain what we should see on the source of the FET (that's the pin connected to ground through the 10k resistor).  Another thing, though, is that it looks like the battery is going to continue to take a beating, because even if there's no short (and I don't think being pulled down by 1V under load is evidence of a short), the circuit probably still draws too much current for sustainable battery power (note that ROG recommends running it off of a 9V wall wart rather than a battery; I just didn't want to suggest that while it seemed that you were shorting the power supply, lest you start frying wall warts).

Honestly, if I were you, I'd start over with an original Little Gem or a Ruby.  I'm a little dubious on the utility of bridging LM386s, especially to drive a 2 or 3 inch speaker.  On top of that, I think the LM386-1 chips you have are probably least capable of running in a bridged configuartion without overheating.  You can probably find a verified vero layout for the Ruby or something similar in the layouts gallery, and get an easier start for yourself in this addictive hobby.

Edited for uninentional smileys.

[fredy]

i changes the caps now and the volume is better but still not great. The input cap had an "m" after the 224, does that mean anything? Here is a sound clip of the amp.
link

[mdh]

The M most likely means 20% tolerance, which is to say that the capacitance is guaranteed to be within 20% of the nominal value.  It's nothing to worry about, 20% is pretty typical for capacitors.

The link to the clip doesn't work... could you try again, and make sure the link doesn't have an ellipsis (...) in it?  Or make a tinyurl link and post that?

[fredy]

sorry,, should work now.

[mdh]

Um, sounds fine to me.  What's your complaint about the sound?

[fredy]

well, to start with, it is very quiet. Its no where near the volume of my marhsall 1w micro stack. Also, with pins 1 and 8 disconnected, it is inaudiblely quiet. secondly, the camera makes it sound good, its not quite like that in real life. I will try and get another, more accurate recording of it later. Thanks for all of your help by the way. I really do appreciate it!

[mdh]

I haven't played with a Marshall Micro Stack -- are you sure that the speakers are comparable?  Speakers really make a big difference in volume with these little circuits.  A Smokey amp (single 386) can sound huge through a proper speaker cabinet.

[fredy]

tbh, i just have a cheap generic 3 inch speaker. I will try and get hold of another jack socket so i can hook it up to my cab, but i doubt it will be able to push a 4x12.

[Trogg]

tbh, i just have a cheap generic 3 inch speaker. I will try and get hold of another jack socket so i can hook it up to my cab, but i doubt it will be able to push a 4x12.

Have faith in the little 386, running them at 12v lets them punch out nearly a full watt of transistorised grunt.
my LG MKII is so loud I'm going to add a tonestack to try and take some of the sting outta it
running off 9v the 386 doesnt develop quite as much output, 1/2 watt, but this is still enough for the original little gem/smokey to run a 4x12
for anything up to 4 hours off a 9v battery

[fredy]

i just tried this amp with a greenback, sounds much better. Not much gain though and still quieter than you would expect from a 1w amp, but things are better and the greenback is a quiet speaker as far as i know. After about an hour or 2 of playing through it the battery went dead (well, 6v, sound went all fuzzy and mushy). I swaped in a nice new fresh one to find the amp farted at me! Lots of puts, odd distortion that does not correspond to what is being played and finally, "burrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr rrrrrrrrrrrrrr"...

perhaps there 9v is enough to cause some arcing across the strips where my soldering went bad? although that wouldn't be constant. hmmmmm... i'm stuck again!

[fredy]

anyone have any ideas about the problem? what would make it do that?

[fredy]

could it be the 0.5w resistors? i wouldn't have thought it would matter with the ones going to earth, but how about the one parrallel to the speaker?

[mdh]

Actually, from your pics the 10 ohm and 10k resistors appear to be 1/4W, not 1/2W, while the 1.5M appears to be a 1/2W device.  I don't really know how much power should be dissipated by each of the resistors, though I suspect that it's negligible for the 1k and 1.5M since the signals in that part of the circuit should be very small.  The 10 ohm resistor is part of what's called a Zobel network, and if there's a fault somewhere in that part of the circuit, I think it's possible that it could be responsible for the oscillation.  I really don't know, though, I'm getting a bit out of my depth here.

I want to reiterate my suggestion that you set this circuit aside for the moment at least, and build one of the single LM386 amps such as the Little Gem or Ruby from an established layout.  From your board pics, it appears that you need some more practice soldering anyway.  I would also suggest that in your next build you use sockets for your IC and any transistors.  This will make it easier to correct problems with pinout (at least for transistors), and will make it easy to swap out a part if you think you might have damaged it.  I don't really have any idea what's causing your oscillations, but again judging from your pics, there could be cold solders or solder bridges anywhere on that board.  There's also  a very real possibility that some of the components are just flat out damaged, because early on in the history of the build, *something* was passing a whole lot of current.  I don't really know to what extent each of these factors may contribute to oscillations that appear to be dependent on the supply voltage.

[fredy]

i hear what you are saying, and i would love to start all over again, but i just don't have the money at the moment and i would really really love to fix this thing. Unfortunately, i have worn my iron out and need to buy a new one, i have been using a 40w  and i have a 50w psu which doesn't get used, would 50w be too much? it will be used for all sorts from amps, guitars, fx, at home, everything.

Thanks for everything, i will continue work with this untill i have melted it to a puddle, while i am doing that i would love to learn more on the theory side of things. Do you know of any good resources on the web that could help?

[mdh]

I understand how that is when you get into something new and you don't have the money to fix your initial mistakes, but you're also really excited about what you're going to learn, or what new things you're going to be able to do, so it's hard to get your mind off of it.  I'm just concerned that you could work yourself up to a level of frustration where you'll throw up your hands and give up on the whole thing.

Unfortunately, i have worn my iron out and need to buy a new one, i have been using a 40w

What is your evidence that you have worn out your iron?  Are we talking about a safety issue like a frayed or otherwise damaged cord?  Is it not heating up?  Is it possible that it is heating up, but you think that it isn't because it isn't melting solder very well due to a dirty tip?  What brand of iron is it/where did you get it/etc.?

and i have a 50w psu which doesn't get used, would 50w be too much?  it will be used for all sorts from amps, guitars, fx, at home, everything.

You have a 50W power supply, is that what you're saying?  And you're wondering what you can use it for?  If this is what you're asking, there's a lot that we would need to know about this power supply in order to advise you as to the circumstances in which you should use it.  Power per se isn't really the main issue.  A power supply won't "pump" more power into a circuit than the circuit would pull on its own, but it could be unsuitable for all kinds of reasons.  At the most basic level, what is its intended use?  Does it provide DC or AC at its output?  What voltage(s) does it supply?  Is it regulated?  Are the outputs protected against short circuit or overload?  Do you depend on it to run any particular piece of equipment that you already own?

Thanks for everything, i will continue work with this untill i have melted it to a puddle, while i am doing that i would love to learn more on the theory side of things. Do you know of any good resources on the web that could help?

No problem.  I've obtained enough free advice around here myself, so it's nice to give back.  Finding good *and* free resources on the web for learning about electronics is something of a challenge.  One place to look is the wiki, and in particular the Tech Tips section of the wiki.  Also, for particular components and circuit elements, you can often learn a fair amount by reading articles on wikipedia.  Another resource I know of is Lessons in Electric Circuits.  I'm sure there are other references to be found in the FAQ and the wiki.