Making first simple amp-

[betweenthees]

hello everyone. As the thread label states im building a simple amplifier. To the amp guys out there this is going to be dissapointing-ly simple but here goes anyway.

The idea for the amp is to have a "pocket amp" that runs exclusivly off a 9v battery. This has practical applications for me. when i do repairs we dont always meet somewhere that has a real amp. this pocket amp would allow a quick "it works" check for a skeptical customer.

Now that ive convinced myself to make this...

This amp will utilize the lm386 Low Voltage Audio Power Amplifier IC chip. It is a primitive chip probally more so used on speaker phones or ansering machines. there are better choices of chips for this but i happen to have half a dozen of these.

here is the data sheet ive been looking at.

www.national.com/ds/LM/LM386.pdf

if you scroll to page 5 on the data sheet (or the pic links below) you will see some options for scematics to follow. I am not familiar with the "Low Distortion Power Wienbridge Oscillator" or the "Square Wave Oscillator" and am looking towards the bass booster or a slightly modified ver of the gain=200/50 models.

some helpful pics
www.syntax.com.tw/proddata/IC/IC-LM386.JPG
mycaramplifiers.com/wp-content/uploads/2009/03/lm386-50db-amp.jpg

I was thinking of using the "gain=50 model" but acualy have a 10k pot controling the gain from 20-200. another 10k pot will be placed on the Vin (singal +)
ill be using a what i call a"hobby board"
www.uchobby.com/Graphics/SpecialBoard1.jpg
probally cut down to a mere 3x2 inches.

So there is my crazy idea i came up while my house has been burried in 3 feet of snow the last 2 days. i would love to here you opinions and exspecially your ideas. still looking for a speaker size and also a case build format.

Thanks for readin!

[JohnH]

Sound like fun! If you want another example, check this out:

runoffgroove.com/ruby.html

Which usies a 386 driven by a JFET

Runoffgroove is a great site for guitar electronics ideas

John

[newey]

Here's a video of a guy who bread-boarded one. Not a lot of info but you can hear how it sounds. (There's a commercial or two first):

tv.boingboing.net/2007/12/13/mark-makes-a-mini-am.html

In googling about, I also saw a headphone amp using an Altoids tin as an enclosure, so these can be made pretty small (of course, no speaker in that one). The headphone amp used 2 9V batteries to utilize the full 18V power of the chip. As you can imagine, once you put 2 9V batteries in an Altoids tin, the circuit has to be pretty small- looked like about a 1½" by 2½" piece of perfboard.

[sumgai]

adgb,

Does this thing have to have a speaker built in to it, or will it be an amp only, and depend on an external speaker?







sumgai

[betweenthees]

JohnH -- i realy like the idea of using the mpf102 jfet . i think im going to incorperate that into what im doing. Great link you posted, thanks!

Newey-- nice little vid. very similar to what im making, but i think he used the min. parts schematic from the datasheet. I beivlive im using a 386L which has a max volatage of 12v. JohnH's link said to try 8xaa batteries to acompish this- Im still contemplating 9v-12v.
You realy got my brain thinking when u suggested the altoid container. My plan is to have a perminate speaker with the amp, so the altoid container might be too small, but something similar to it may work great. (or if i can find a large altoid container)

sumgai-- The speaker will be permentaly connected to the amp. It will an isolated unit. the only input will be from passive with no preamps or speakers.

Thanks for your replys-and ideas- i should get the rest of my parts early this week.

[betweenthees]

had a question you guys might be albe to answer for me---

Looking at the rudy schematic runoffgroove.com/ruby.html you will see the mpf102 near the left side of the drawing. Im trying to figure out the 3 lugs on that peice. heres what im thinking....

The "in" coming from the left is the "gate"
on the right side going down is the "source"
right side going up to the 9V+ is the "drain"

that is my educated guess on how to wire that peice in. Please correct me if im wrong.

im having a tough time understading how the transistors work and also what they do.

edit/add:: quick noob question:
does 1M5 resistor mean a 1,500,000 ohm?
does the 3k9 mean a 3,900 ohm?

[JohnH]

I think you are right in what you say about the transistor, and also the resistors.

The transisor is a Junction Field Effect Transistor, a JFET. There are many differnt types of JFET, but they all work on similar principles, and are analogous to valves in the way they respond, which make them quite interesting for guitar circuits. THe short explantion is that the gate is the input, and it is very high impedance so does not load what it is connected to. The JFET responds such that the current that can flow from source to drain is dependednt on the voltage between source and gate. If the gate is (for an MPF102) about 3V below that of the source, the current will be cut off and the drain voltage rises up towards the + supply. As gate voltge rises from there, more current flows from drain to source, dropping the drain voltage. Depending on resistors at source and drain, a voltage gain can be created. The advantages of this type of device for us is the high input impedance, also, if it is pushed, the resultiant clipping can be quite nice and valve-like.

EDIT I just had a closer look at Ruby cicuit. In this case the JFET is just acting as a buffer, to give you a high impedance input. See how it goes, but if it needs more gain, a JFET can be configured to do so

John

[betweenthees]

This little amp ended up taking over 5 hours to acualy peice together. I have only tested lightly so far, but everything is working well. I followed the Ruby diagram for then most part changing very little. The components barley fit inside the altoid container (thats again for that idea newey). Ill prob get a "loberster claw" ruber band and wrap that around it just to make sure it stays closed, even though it does close fine without it.
This little guy has a surprising loud output. i need to test it on something more then a 3" hobby speaker but im impressed with what i thought would be more of a toy.

Thanks for your help with setting things up and getting a working schematic. If you have any questions on how i did things or what i used feel free to ask.



I dont know how to resize pics so i only posted this one-
pics from the rest of the project can be found here
s745.photobucket.com/albums/xx99/moyer20279/

[D2o]

Nice job!

I tried to make a cMoy once ... I found everything too small to work with. My hat's off to you.

How does it work? er, how does it sound?

Cheers,
D2o

[newey]

Looking good! But where's the speaker?

[betweenthees]

to quote myself....

Feb 8, 2010 14:57:55 GMT -5 betweenthees said:

The speaker will be permentaly connected to the amp. It will an isolated unit. the only input will be from passive with no preamps or speakers.

i must sound like a polotian, for i never acually atached a speaker to the amp! I have some ideas on making another altoid container into a speaker box, but i did lie: its not going to have a spaeker perminatly attached.

It just has a 1/4" in and 1/4" output jacks on it.

add:: can u resize images on this forumn using the code or some other way?

[newey]

E2E-

I posted before you had added the text, when you still had a bunch of pix up. I don't know if using the IMG resize works or not, never tried it. The best way to resize things is to do it in Photobucket (if that's what you're using).

Glad the project worked out for you. It may sound pretty good into a 10" speaker if it'll drive one.

[sumgai]

adgb,

No, sorry to say, the tag does not accept any parameters at all, let alone give you a way to control the size. If only it did, us Admins here would have a much easier life, eh newey? ;D

You'll have to post it at the "proper" size on Photobucket, and then link directly from here. We ask that you keep the width at 800 pixels or less, but the height can be whatever you need/wish.

HTH


Oh, almost forgot.... how much money did you spend on all the parts and pieces of this project?





sumgai

[betweenthees]

most of the parts i had purchased in bulk ( like resistors, caps, the cuicuit baords, lm386, 8 pin ic socket). So maybe 5$ for everything on the baord.
The black knobs on the pots were from an old bass, so no charge there (funny story, i traded a guy a case of beer for a guild pilot bass that needed major work)
The 1/4 jacks were from something else i salvaged from parts
The pots used were 1$ each.
the altoids were 1.79 in the candy isle
the switch might have been 2$
the battery was like 3$

so the pocket amp can be constructed for under 15$.

im running into one problem so far- the volume and gain pots are sooo close together that sometimes they "bump" and the signal is grounded out. im thinking about using a seperator to hold them apart and fill the gap (maybe 1mm wide) with hot glue. Still open for suggestions on this though.

add:: i figured how to resize using the photo bucket site. pics wider then 800 make the entire thread hard to view on my browser (stretches it out to fit the pic in) so im learning a little bit of everything on the guitarnuts this week