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Little Gem Guitar Amp Questions Answered

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I recently went to build the Little Gem guitar amp from runoffgroove.com. Its a great little amp from what i have read. I made the basic heart of it so far. I have the four caps, resistor, IC socket, and its all connected. I think basically all thats on the breadboard for the PCB layout here: http://runoffgroove.com/littlegem.html(scroll down).

I'm confused on where to put the two pots and connect power and all that. I don't have the gain pot yet, but i do have the 25 ohm rheostat. The rheostat has three prongs, would i use the middle leg for the "out" part and the other two for the other part? I can't seem to get it.
Also, is there a common ground between the input from the guitar, the 9v battery, volume pot, and output?

Help please!


Update:
I found a layout on Flickr that explains it, i think. See the third picture.

24 Replies

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stevegossett (author)2012-08-09

This is a nice little circuit and does well with my cigar box guitar projects. I even made for my iPod but factored in a jack for a 9v power supply.

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John Smith (author)2008-12-08

On the gain pot, would it be better to use a linear taper or audio taper one?

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tashiandmo (author)John Smith2008-12-15

I've built two of these (my first ever electronics project), and used an audio taper for the first one. It works fine, but the gain is spread over the dial more evenly on the second one I made with a linear taper. Audio taper is much better for volume control, but I think the gain in this amp doesn't change the volume as much as it changes the character of sound.

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gmoon (author)2008-12-14

Ha. This just popped up. It shows on my browser as a 220MDF which is OK (220uF.) If this is indeed your shopping cart you shouldn't have made it public. Delete your post ASAP...

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gmoon (author)gmoon2008-12-14

Oh, and the voltage ratings are all OK.

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John Smith (author)gmoon2008-12-14

Ok, and why shoudln't i have showed you my cart...? Either way I'm pretty sure those are the right things.

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gmoon (author)John Smith2008-12-14

Well, I shouldn't have been able to view your cart, if the online merchant was setup in a secure manner... If viewable, it might be modifiable.

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John Smith (author)gmoon2008-12-14

Oh, ha, no. I copied and pasted the information into my Google Documents thing, then made it public.
Either way I removed the links, so now its just a list. Here's the new link:
http://docs.google.com/Doc?id=dgmsm6nq_12hqxpb2fc

Tell me if you see any links.

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gmoon (author)John Smith2008-12-15

Clever of you to copy it elsewhere. But are there links on that page? Yeah, there are existing links to Allelectronics (I didn't click on any...)

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John Smith (author)gmoon2008-12-15

ok, its fixed. I removed all of the links and put a link at the top to my website where it has the page on it. Thanks there! Oh, and I save all of my invoices and stuff to Google Documents. From any computer with gmail i can look at them. Man i love gmail.

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John Smith (author)2008-12-10

Pics of the final project:
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp014.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp015.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp016.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp017.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp018.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp019.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp020.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp021.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp022.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp023.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp024.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp025.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp026.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp027.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp028.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp029.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp030.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp031.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp032.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp033.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp034.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp035.jpg
http://i219.photobucket.com/albums/cc317/Altctrller/grandmaandamp036.jpg

There should be enough close pics there to tell what i did wrong (?).

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gmoon (author)John Smith2008-12-11

It's a bit difficult to troubleshoot from photos alone. I've done a cursory comparison of your point-to-point wiring and the layout, and it looks OK. Except I can't see if the 100uF cap is grounded.

-- Bad components: you desoldered the caps from other devices. It's possible you damaged one or more.

The most crucial components are the 0.01 and 220 uf caps and the 386. Assuming the input and output are wired correctly (as is power), you should be getting some sound with those components alone.

The 100uF cap is a power filter cap, and not crucial. The resistor / .047 cap roll off high frequency and needs to be there for the amp to sound good, but the amp should still make sound without them.

If any of those caps were shorted when you desoldered or soldered them--the amp won't function.

-- Cold solder joints: this is a common problem with builds.

-- Power supply polarity: If you hooked up the battery wrong (even for a second), the 386 might be fried.

Thoughts:

-- Is the case metal or plastic? If metal, then insulate the circuit board.

-- The input jack: looks like the orange lead is wired to the circular collar of the jack. That's the shaft of the plug, therefore it should be ground, not signal (assuming orange lead is the input.) If the case is metal, and anything else is grounded, that's a problem.

If the case is plastic, wiring the input with reverse polarity would still work well enough to hear sound. But it would be noisy.

-- How many


Troubleshooting

-- Recheck your wiring carefully. It's not uncommon to mis-wire projects.

-- Is there any sound at all? Place your finger on the input lead--there should be some hum or other noise from the speaker.

-- Test the power pins on the 386 with a VOM to be sure the IC is getting juice.

-- Simplify. The circuit doesn't need the rheostat. Remove it and wire the speaker directly. If that works, then the rheostat was wired incorrectly.

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John Smith (author)gmoon2008-12-13

Well, I just checked...the 100 uF cap wasnt grounded. How that happened, i don't know. But i guess i have to replace the LM386 now, what about the other components? Are they ok, or do i need to completely redo the board? Thanks, I wouldve never caught that.

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John Smith (author)John Smith2008-12-13

I switched the input wires(they were wrong) and i added that ground to the cap. So now i just need to replace the LM386? or what else do i need to replace? Now the LM386 still gets hot, so it needs to be fixed. Still no sound except when i turn it on or off.

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gmoon (author)John Smith2008-12-13

Possibly. It's tough to troubleshoot from photos only, like I said. If one of the caps is shorted (like the 0.047 or the 220uF), that could be the cause. It's easy to melt through the cap dielectric layers if you use too much heat, causing a short circuit. Do you have one of those solderless breadboards? That's the easiest way to test parts. And to be sure everything works before you solder...

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John Smith (author)gmoon2008-12-13

Where's a good, easy to order from online site that i can get all these things from? Radioshack is making me mad at 2.99 for two capacitors... I do have a solderless breadboard, but i figured all the things worked...and...and...yeah.

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gmoon (author)John Smith2008-12-13

Part suppliers (thanks to NachoM who referenced this link a few minutes ago):
https://www.instructables.com/community/10-Best-Electronics-Suppliers/

RS is a rip-off--but with the extra shipping charges from the other suppliers, I usually wait until I have $20 or more of parts to order... If it's a part or two, I still use RS.

I've ordered from about 5 of those suppliers; Sparkfun has the lowest shipping charges, but stocks only a relatively few components.

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John Smith (author)gmoon2008-12-13

Nice. I found the LM386 at BG Micro I think. Couldn't figure out how to use Digikey and stuff.
Is this it?
http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=10500

And what capacitors would i need? Electrolytic and...tantalum or ceramic?

Tips please!

Thanks tons, btw!!!

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gmoon (author)John Smith2008-12-14

You can use ceramic (but it's not the best choice for audio signals.) Something like mylar, polyester, polypro, etc., is better, here's a selection.

A ceramic would be OK for the 0.047 cap, since it's not directly in the signal path.

You'll need electrolytics for the larger ones (100uF, 220uF.) Allelectronics has those.

They've got the LM386, too. I'm not stuck on Allelectronics, but they have decent selection.


Check your layout carefully before you plug the 386 in, to be sure there are no unintended solder "bridges" or other shorts.

As for me, if I'm ordering parts I always get one or two extra, just in case I "brick" the first one...and for future projects.

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gmoon (author)2008-12-09

Hi John (got your pm.)

-- Ground
Yes, all the ground symbols connect together, ultimately to the negative terminal of the battery.

-- Rheostat
The rheostat forms a voltage divider. Connect one side to the circuit output, and the other side to the ground. The "divided voltage" is the center tab of the rheostat (the "wiper") and that connects to the speaker.

Connect the other speaker terminal to the ground.

Normally in an amp the volume control would be a voltage divider wired at the input. In this case, the rheostat is a different type of volume control--an attenuator. Using a attenuator lets the amp run "hot" all the time (distortion-- bad in stereos, good in guitar amps), and subtracts volume from the output. But a normal pot can't be used here--even the little LM386 can generate enough current to fry a 1/4 watt pot. Hence the need for a rheostat (most low-resistance "pots" are rheostats by their nature, anyway.)

-- An audio taper pot is probably referable for the gain pot. Connect the center and one of the outer pot terminals to the circuit (pins 1 and 8.) Per the schematic, you can connect the unused pot terminal to the center wiper terminal. It's usually good practice to do so--if the pot malfunctions (gets dirty, which is common) then the resistance can never exceed the maximum value of the pot.

Actually, it doesn't so much matter here, since an open connection (near infinite resistance) is the normal low gain setting anyway...

-- Both layout drawing look pretty instructive. Note all the common connections to the left-side power strip (the ground.)

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John Smith (author)gmoon2008-12-09

Thanks! I'm not sure what you mean in your description of using the audio taper pot, though. And what is the difference between the pots(audio and linear taper)? I'm pretty far on it so far, i made a decent speaker enclosure for the miniature speaker i have(3watts). I'm working on putting the amp part into a Rolodex box, and mounting it on top of the speaker box. I already have the pots, knobs, on/off switch, and input installed. I think all thats left is to wire it up, insert the LM386 and then some other minor things. I'll try to get some pics up tomorrow. I think the rolodex case looks pretty cool myself...

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gmoon (author)John Smith2008-12-09

Audio loudness is a log function. That just means that an audio taper pot changes resistance very gradually on one side, and very abruptly on the other side. Of course, I can't guarantee that an audio taper is right for the gain loop--the LM386 datasheet doesn't say. Suffice to say that either type will work, but when appropriate the audio taper will be easier to adjust.

Our perception of volume is that it's linear; when it is not. A 10-watt amp is only twice as loud as a 1-watt amp. So a 10-watt amp turned down to half-as-loud can't be 5 watts (that's linear.) It's closer to 7 watts. The audio taper takes that into account. Here's a good PDF that explains the differences, and shows how to modify the response of a linear pot with a resistor or two...

As far as wiring the gain pot, just look at your schematic (pic 2.) It shows one side connected to pin 8, and the other side together with the center wiper is connected to pin 1. Again, you don't have to connect the "unused" side--the pot center is the important connection (tying in the unused side is just good design practice.)

Yeah, post a pic or two...

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gmoon (author)gmoon2008-12-09

Err, preferable, not referable...

And the drawing with the ground strip is the third one...

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Sandisk1duo (author)2008-12-06

for the potentiometer, the center "pin" and one outer pin are used to give an adjustable output

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