Little Gem Guitar Amp Questions

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!

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

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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.
John Smith (author) 8 years ago
On the gain pot, would it be better to use a linear taper or audio taper one?
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.
gmoon8 years ago
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.)
John Smith (author)  gmoon8 years ago
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...
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...
gmoon gmoon8 years ago
Err, preferable, not referable...

And the drawing with the ground strip is the third one...
Sandisk1duo8 years ago
for the potentiometer, the center "pin" and one outer pin are used to give an adjustable output