Help with my second build - LITTLE GEM!

[Korwedge]

My first build was the NPN Boost. I'm still tweaking with that one, but it basically works.

I built the Little Gem on my breadboard and hooked it up to a 6 inch speaker, and it worked, and I was able to get some really cool sounds out of it! I have a few problems, and I was hoping I could get some advice.

1. At the lowest volume, and with the 1&8 pins of the IC disconnected (gain), there is still distortion, especially when I strum harder. It sounds really cool, but I have found no way to set this up so there is a clean sound. The speaker I am using is a 3-cone speaker for a car stereo. Could this be my problem?

2. Can I put a switch in between pins 1 & 8 and the 5k pot, so the gain option can be switched on and off? I figured I could and this would add even more versatility to the amp. This could also be a footswitch. The gain sounds so cool on this little amp, and the range is so huge!

3. The quietest volume (still with distortion), when the 25ohm rheostat is turned all the way down, is still guite loud, and I can't get it any quieter. Should I change the value of the rheostat, or does this mean something is wired incorrectly? I am using 9V DC adapter. I can turn the volume down on my Strat, but you know how poorly the volume knobs work.

4. The rheostat functions backwards, no matter how I configure the pin connections. The 5k pot works in the correct clockwise direction.

5. Cabinet ideas for the little gem? I saw someone put this in a mini lunchbox, which seems like a good idea, just curious if anyone has any other ideas for a 6 inch speaker Little Gem to be mounted in?

THANKS for any help! I'll post some pics once I get this done. This is turning out to be a GREAT second project, especially since all the pedals I build next will be tested through this Little Gem instead of my real amp, so I won't have to worry about blowing anything up!

[twabelljr]

     I think if you use a higher voltage supply you will have more clean headroom. Check the datasheet for your IC and see what the maximum voltage is. I've seen many posts recommending a 12 volt supply for this reason.

[Korwedge]

I'm using a 9 volt AC adapter right now. and I tried a 9 volt battery. Both allow for plenty of volume, but I do have the distortion sound on all the time.

The IC I am using is one I picked up from Radio Shack: LM386N 400mW
It says the supply voltage is 15V!

I ordered two different 386 chips from SmallBear, does anyone know if those are 9volt?

Thanks everyone for your help! I just pulled out all of the components from the breadboard and rebreadboarded it, same issues. I'm getting good at reading these schematics!

[petemoore]

I'm using a 9 volt AC adapter right now. and I tried a 9 volt battery. Both allow for plenty of volume, but I do have the distortion sound on all the time.
  Yupp try 12vdc. oh your using AC ? that's wierd, I thought that ran on DC.
The IC I am using is one I picked up from Radio Shack: LM386N 400mW
It says the supply voltage is 15V!

I ordered two different 386 chips from SmallBear, does anyone know if those are 9volt? With the chip #'s you ordered and a little google search you can get the DATA Sheets tells all about the chip...I don't know, cant' tell.

Thanks everyone for your help! I just pulled out all of the components from the breadboard and rebreadboarded it, same issues. I'm getting good at reading these schematics!

[Joecool85]

The reason for the distortion is that if you are using a car speaker it is 4ohms, this chip is designed for 8 and 16ohm loads.  On top of that, the little gem is setup so that all of the guitar's sound goes into the circuit and then the volume is controled at the end.  This means that it will distort regardless and if you turn down the volume it just distorts more quietly.  Turn down your guitar's volume and it should clean up nicely.  More voltage will help headroom to an extent also.

[bancika]

I built both Little Gem MK2 and Ruby, during testing with two 4ohm ~4inch speakers I salvaged from old desktop speakers I could never get rid of fizzy distortion. But when plugged into 12" greenback or through line out -> speaker sim -> PC my LG2 is pristine clean (with pins 1 and 8 open). I suggest you to try different speakers, and if you can't find any better build it without speaker and connect it to computer. Here's mine LG2 "head" It's about 3"x1.5"x1"
http://i24.photobucket.com/albums/c22/bancika/lg1.jpg
http://i24.photobucket.com/albums/c22/bancika/lg2.jpg

[Joecool85]

Odds are those greenbacks are 8ohm, that will help a lot.

[bancika]

Yeah it's 8ohm, but those two shitty speakers I've got are wired in series giving 8ohms and they distort no matter what.

[Joecool85]

Ah, didn't know you had two wired in series.  Missed that part. 

Regardless, most of the distortion is caused from having too much signal coming in, either put a pot on the in, or just turn down your guitar.

[bancika]

I don't know, maybe. Although those speakers are rated 5W each, and small 386 can't deliver 0.5W

[Joecool85]

Yeah, but we've already discussed that in here.  It's about the speakers efficiency more than it is wattage handling.  I hooked mine up to a 0.5w 2" speaker, and then to a 10" 50w dean markley cab.  The 10" 50w speaker was easily 3 times as loud, and way better quality.

[Korwedge]

The speaker I'm using is an Audiovox 3-con 6"  4/8 ohms compatible. I just tried an 8ohm speaker from my stereo and still the same distortion problem I'm thinking this means that the signal from my Strat is just too strong to run into this circuit, or I need to change the voltage of the power supply from 9 volt to something higher? The IC from Radio Shack does say 15V. I've been using 9volt DC, as that's what the schematic says, but perhaps the actual voltage needed depends on which IC you use, correct?

The IC I have on order: NJM386D - 4V-18V
and the LM386d - Nom. Supp (V) = 6
Hopefully one of these will solve my distortion problems!

Another problem still, is the 5k pot for gain seems to have very little effect on gain until it gets to the end of it's rotation and the gain kicks in full force. Almost like the pot is acting like a switch. The gain sounds really nice, but it is either on or off. I put some resistors at pins 1 and 8 and this had no effect.

Thanks again for all the help here....

[Joecool85]

Sounds like you have a log pot instead of linear for the gain.  Also, the 15volts is a maximum, I wouldn't run the radio shack one with more than 12v, I find 9v to be fine for them though.

[Korwedge]

Nope, the 5k pot is linear taper. I was hoping that using a 9 Volt IC would solve my two problems, but are you saying that you power the 15 volt IC from Radio Shack, and you get no problems with distortion?

[Joecool85]

I am saying exactly that.  However, the only way for me to get clean sound is to turn off the OD on my little gem (turn down gain knob) and turn down the volume some on my guitar and it cleans right up.  My gain control works quite well though, not just at the top of the turn.

[petemoore]

  Hard to SFSure....but it seems like good possiblity the Gain Knob you are using is just the 'other taper' and ranges through the possible gain settings, just not 'smoothly'...

[TheBigMan]

An idea I looked at before building an LG MKII was to build a Ruby, but with a DPDT switch to bypass the input buffer and volume pot.  Instead of a jumper wire for TB it would have the 0.01uF input cap from the the Little Gem.  I haven't actually tried this but it should give the best aspects of both circuits in one box.  I play Strats mostly so I don't get any OD at all from a Ruby even at full bore, it's like a much quieter Twin.  The LG is like a plexi-era Marshall and even with the Strat I can't get it 100% clean.

[Korwedge]

Ok, I tried the two 386 IC s from Small Bear, and they sound esctly the same!

When it should sound clean, it sounds distorted. I've rebuilt the circuit several times on my breadboard, and still the same problem

Any suggestions?

[bancika]

Which speakers are you using?

[Korwedge]

I am getting the same problem no matter which speakers I try. I have tried 8ohm computer and stereo speakers, and 4/8 ohm car speakers.