Little Gem MK2 - Problem

[Delvius]

Hello,

It is my first attempt to build something myself completely. Normally I only modified or repaired stuff but now I wanted to try building something and failed  .

The Problem is: When I switch it on and plug in the Guitar it is silent at first which is fine but when I try to play something only a buzz sound appears with a certain frequenzy. It sounds like a cap sends a signal through the speaker and the guitar sound is almost not audible, only very very silent and sometimes gone completely. I do not know what might be the reason for this ... I checked it many times, rebuild the AMP 3 times now with 2 different layouts and it never worked, always the same problem. I think my layout is fine but ... well then it should work so there must be something wrong.

I hope somebody could help me :/. I'm in despair and it pisses me off that I'm to stupid to build such an easy AMP .

Edit:
I measured some voltages and found out that both LM386 pin 5 outputs have ~+3V and which makes ~0,1-0,2V at the speaker. What could be the reason for this? Where is the problem? Damn it...

EDIT: I just realize that i wrote the lables in German.
Eingang = Input
Masse = Ground

First Layout:



For the second try I used isolated cables to connect the parts to make sure nothing connects where it should not. So anything that is longer than 1 dot distance is connected by cable, there are no solder lanes.

[brett]

Hi
just a guess... but I think each LM386 needs a large cap between the output and the speaker (e.g. 220uF).
Otherwise you'll get a DC bias and general mayhem.
cheers

[vendettav]

are you sure you grounded everything???

also may be give the voltages? i guess there's some technical problem. by that i mean the problem is not in the layout

[Delvius]

That you for the adivces .
Everything that needs to be grounded is grounded. Checked.
About additional 220uF caps, how should I put these in?
Like This:   LS+ Out  -||+   LS+ and LS- Out +||- LS- ?

And now some Voltages:
IC1:=2,4 grounded ; IC2:=3,4 grounded
MPF102 Drain: +5,81V
IC1 pin 6: +5,76V
IC2 pin 6: +5,76V
100uF Cap: +5,74V
LS+: +2,73V
LS-: +2,73V
IC1 pin 3: 0V (Ground as reference, with the 220nF cap as reference it has -3,11V)
IC2 pin 2: 0V (Ground as reference, with the 220nF cap as reference it has -3,11V)

Also the battery drain is quite high. And I went to sleep with the buzzing ptoblem and now as I checked again there is nothing. Completely silent. No sound at all :/. I think it wants to provoke me...

Also some pics of the actual work:

[Gurner]

Firstly ...layouts hurt my eyes...if you want more input from folks on here, best to post a schematic.

I'm assuming you've built to this schem...



My first obvious question is why have you only got 5.74V across the 100uf cap ...it should be 9V! (your battery should be rather warm!)

If you are using a 9V battery, then something is loading it down badly (which would explain why your battery is draining quick)  first remove your two socketed LM386s to see if the voltage across the 100uf cap goes back up to near 9V, if it does, replace one at a time to see which one is causing the problem...then see if the problem moves by spaping LM386 posiition.

If  the vopltage doesn't go back nearer 9V when you remove the LM386s, then next remove the fet & then measure the voltage across the 100uf cap again....if it goes do back up to 9V, then your probelm is to do with the fet etc.

You've got to establish what's loading that 9V down...once you've got the general area, then sorting this will be easy.

[Delvius]

Thanks a lot for the suggestions. Sounds like a good plan, working on it now .
About the schematic, I thought there is only one for the Little Gem MK2 so I just posted the layouts because there could have been the problem, too.

Without the LM386s the voltage at the cap is a bit above 9V which should be fine.
Then i replaced both with other LM386s and nothing changed :/. I checked the Voltage at the Cap again and it is ~8,22V and sinking pretty fast (about 0,02V/s), still no sound at all.

I use LM386L but these should be fine I think.

Any other ideas?

[Gurner]

You're partially assuming that only those who have built the little Gem MKII can help you fix it - I've never built one, nor had I even seen the schematic prior to searching for it after I saw your thread. This is why posting the schematic is important...there may be other like me ...but who can't be bother to seek out the schematic....& it's the schematic that's key here, so best to post it with your problem.

Ok, so now the voltage is fine across the 100uf cap, can you post the voltages on all the LM386 pins?

[Delvius]

Ok, I will remind that.
Checking at the moment.

Without the ICs:
IC1: Everything 0V and pin 6 has +9,32V.
IC2: Same.

With the ICs:
IC1
1: +1,325V
2: 0V
3: +5mV
4: 0V
5: +2,75V
6: +5,98V dropping
7: +2,7V
8: +1,288V

IC2
Same but lower because of discharge. And the 9V Block is not warm.


Edit: Now I know why it went silent. As I changed the output loudspeaker i cut of the connection to it. The buzzing is back again. Yay...
Another obersvation while measuring: The buzzing frequency increases with Gain and while the Buzz appears when the Speakers get about +1,58V -> Buzz -> 1,28V -> Increasing -> 1,58V -> Buzz and so on. Hmm... Sounds like a false connected cap, but where?

And now the voltage at 5 and 7 are the same. (~1,3V)

Edit2: The buzzing also appears if I play a tone on the guitar.

Edit3: Damn it, I check it again and again and I'm still clueless ...

Edit4: I found out that my problem might be called motorboating. put put put sounds (The buzzing).

[Gurner]

I think there's interpretation problem here becuase earlier you said....

"Without the LM386s the voltage at the cap is a bit above 9V which should be fine.Then i replaced both with other LM386s and nothing changed :/."

Which I'd understood as the 9V was holding up, but I've just revisted your post and can now see what you mean.

So if I'm understanding you correctly, with the LM386s in their sockets, the voltage across that 100uf cap is dropping from 9V down to about 5.75 volts. This is becuase something in or around those LM386s is loading (pulling too much current) from your 9V  supply rail.

What happens if you remove one LM386 at a time (remove one ...observe, put it back, remove the other observe)

[Delvius]

If I remove one LM386 the "put-put" buzzing is gone. When I insert it back again and remove the other one it is the same.

[Gurner]

If I remove one LM386 the "put-put" buzzing is gone. When I insert it back again and remove the other one it is the same.

That's not what I meant by observe! (as irritating as the buzzing is, ignore it for now) if we're to get anywhere here, we need to focus on why the voltage is sagging - so so again with each LM386 removed (one at a time) measure the voltage across the 100uf.

[vendettav]

correct me if im wrong but I guess the lug 6 has to have the current of the battery not any different?

also by put put. dya mean like a metronome kind of little clicky thingy? half audible cause if it is so than it sounds to me as a shorted ground somewhere. (if i remember correctly that's what was the reason for that sound in my Noisy cricket build)

[Delvius]

Without any LM386: 8,41V
IC1 inserted: 7,16V dropping
IC2 inserted: 4,16V dropping

and now more confused than ever.

About the sound, it really acts like a metronome. But it is more than half audible, not loud but good to hear. What do you mean by shorted ground?

[Gurner]

Firstly, is your battery fresh?

Next, it looks like your problem is around the IC2 area.

if you want to prove all is well with the IC1 circuit, do this (with IC2 removed)...

IC1 pin5-> 220uf cap (or similar....whatever you have) -> speaker positve terminal ....the speaker's negative terminal moves to your ground.

If the battery is healthy the voltage across the 100uf should not drop too much

(essentially, with tthe config I'm proposing, you've a std non bridged audio amp)

[Delvius]

The IC1 circuit works . What i just found out is that I increase the gain if I turn the pot against the clock. This should not be like this, should it?

Edit: Gain Pot removed, and reinserted IC2 and connected it like the schematics. Well, still not working correct... But it is getting closer . Thanks you a lot Gurner, at least I got my hope and enthusiam back now  .
I was about to give up on it but now I'm

[Gurner]

Good IC1 is working...the gain pot working incorrectly is easily sorted - just swap the wires on the outer lugs around.

Now you have to focus on IC2 circuit, to keep this simple, I'd do the same for IC2....

IC1 pin5-> 220uf cap (or similar....whatever you have) -> a speaker positve terminal ....a speaker's negative terminal moves to your ground.


IC2 pin5-> 220uf cap (or similar....whatever you have) -> a speaker positve terminal ....a speaker's negative terminal moves to your ground.

If you you don't have access to two louspeakers, then just move the +ve speaker connection between ICs (the important thing is that you must have a large electrolytic cap connected between ICx pin 5 & the +ve speaker connection...esle you'll have 4.5V straight across your loudspeaker, which probably won't kill it, but your LM386 aint gonna be happy at all)

Ok, things to try next...

try the LM386 that worked in IC1s position...into IC2's socket  (it may well be IC2 is bust)...check for that sagging voltage across the 100uf cap again.

[Delvius]

Checked circuit 2 alone. Works better than before, but this feedback loop or whatever it is appears when I play something on the guitar.
So the error must definitaly be here .

Edit: Just changed the 220nF and the 47nF caps, did not solve the problem.

Could it be that the LM386L is not bridgeable while the LM386N is? They seem to be the same...

Edit 2: Very interesting stuff. Circuit 2 alone with 6,28V on the cap (Battery slowly gives up xD). When I play a tone the voltage drops to 2,89V and the buzzing appears.

Edit 3: @#$% Yeah! The MK2 still does not work but now I know why . All thanks to you Gurner! And of course thank you vendettav, you made me check the ground and because of this is recabled it a bit so it is easier changeable now!

The LM386L seems to be not bridgeable! Out of curiosity I connected the pins in circuit 2 like the ones in circuit 1 (2,4 Ground, and Signal into 3) and, oh magic, it worked.
Now might there be another way to bridge 2 LM386L? Both with 2,4 ground and 3 Signal? Because I want the 1W version because 0,5W is not loud enough.
Well anyways, I'm going to get the LM386N now...

So if anyone wonders why the Little Gem MK2 does not work, check the IC! The LM386L does not work bridged like in the schematics!

Edit 4: Shame on me. I recableded it for the Little Gem MK2 circuit again. And now it works!? It's not like I'm unhappy but I do not know what is going on... I mean it's the same as it was before when it did not work... I checked it no false connections back then... whatever. It works now .

[Delvius]

Another problem came up :/.
How do I wire an additional output jack? And how about a headphone jack? Can I realize both with just one jack?
I tried it with a stereo jack but I can not get it to work.

[twabelljr]

http://runoffgroove.com/faq.html
If you are using a mono jack to connect to an external speaker, connect the jack like the speaker is shown in the picture (output to tip, sleeve to ground), then wire your headphone jack as shown. The speaker jack will be disconnected when headphones are plugged in.