8 Watt Guitar Amp Help

I've been fartin around with electric and acoustic guitars for about the past 5 years. Only now am i getting decent (by no means good, but better, i guess...).
I've been wanting to make a small amp with a small speaker for some time now.
I saw this circuit the other day. Its VERY simple.

It says to reverse the values of R1 and R3 for use as a guitar amp.
there's some other stuff too.

I was going to see if i could pick the stuff up at radioshack tomorrow, and maybe make it an after-work project for the week. Maybe cut up some wood and make a nice box for it.

1)Do you think these parts would be available at radioshack?
and what do i need to look for if i cant find the exact IC?

2)Any special soldering tips?

3)What kind of pot do i need to add, and where, for a volume knob?

4)Is there any extra things you think that needs to be added?

5)Are those triangle things grounds? as in 12v negative? And what about the other wire from the guitar?

6)Also, what ohmage speaker do i need to use?
What am i looking for in a speaker?

7)Will 8 AA batteries do the job?

8)Do i need to add like a fuse or something somewhere? Going straight into the IC...i dont know.

If anyone knows of simpler or better designs, let me know. I seem to really [insert proper four-letter word here] up electronic things.

I think thats all the questions i have here. To respond you can just go like..
1)i think that....

like that. Hopefully that will make it easier to answer all my ignorant questions.

Thank you!


(sorry if i seem like a a$$ here. Its 1:35am, I've been looking this stuff up for two hours, and i dont feel like i got any further....)

And since i dont have a suitable picture, have yourself a look at my LEGO steam engine:

(after the third try of trying to post the video with unsuccessful results, i realized that i was posting the embed code int eh URL place...need....sleep....)

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gmoon9 years ago
A couple thoughts, to go with NM's.

-- Looks like a discontinued component, so one of the substitutes Nacho lists will be easier to find.

-- Here's the datasheet for the LM383. There's good info here, if you dig for it.

Look at the Electrical Characteristics table, the Output Power section. The Typ column (typical) shows the output wattage for different parameters.

Characteristics at 13.2V (lowest voltage listed, but higher than your 12V):

VS = 13.2V (Supply Voltage)
f = 1 kHz (input test Frequency)
RL = 4ohm (Load Resistance--the speaker)
THD = 10% (Total Harmonic Distortion)

Note that the lower the load (4 ohms is less load than 2 ohms), the lower the power output:

4.7 watts for 4 ohm speaker
7.2 watts for 2 ohm speaker

So an 8 ohm speaker will reduce the power output. The LM383 can handle a load as low as 1.6 ohm (but try to find a 1.6 ohm speaker...)

Increase your VS to 16V, and the 4 ohm load gets you 7 watts output.

-- You'll get somewhat more wattage if you don't mind a higher THD (usually OK for guitar amps.) The Distortion vs Output Power chart shows the trade-off. Note that the chart tops-out at 10% distortion (THD), but theoretically it extends above the chart.

It charts the 4 ohm and 2 ohm RL curves (2 ohm load is to the right.) An 8 ohm load curve would chart left of the 4 ohm curve. Again-- left on the chart is less power output.

-- Playing with the values of R1/R3 determines how much output signal is returned to the inverting input. That's called negative feedback.

It's a little complicated, but in a nutshell, the more of the output signal you feedback into the inverting input, the quieter and less distorted the amp becomes. Since the two inputs (+IN, -IN) are opposite (phase), feeding the same signal into both would result in zero output.
John Smith (author)  gmoon9 years ago
Ha, that's a LOT more complicated than i thought it was. also, how come, in the schematic, the pins are labeled, 1, 2, 3, 4, 8? Or is the 8 supposed to be a 5? hmmm Here's what i have on my list: [everything on the parts list] 220ohm pot(volume, will be added RIGHT before the speaker) pc board 8 or 10 AA battery pack 1 ohm resistor-this and the cap below were suggested to stop oscillation? .2uf disc cap- two 4 ohm speakers (possibly an 8ohm too. depends on what i can find. Will car stereo speakers work?) maybe a knob for the volume. a switch 1/4" mono jack does this sound good to you? awesome tube amp, btw Jason
Or is the 8 supposed to be a 5?

Yeah, looks like a typo.

BTW, the schematic on the datasheet is identical to the 8 Watt Audio Amp link (with the exception of the cap and 1 ohm resistor, which you already noticed.)

Re: the volume POT on the output--
The specs call for 2 or 4 ohm load, so a 220 ohm pot will only work in a tiny bit of it's range, like less than 30 ohms. If using a 8 ohm load decreases the output so much, what will 220 ohm load do? (hint, it's very quiet.)

Plus you're drawing 5-8 watts through the POT when it's near max, and most are rated 1/2 or 1/4 watt. If you can find a 25ohm rheostat, that could work--but it needs to be a 10 watt model.

There is some advantage to using a rheostat on the output--the amp gain can be maxed out and distorted, but you can back down on the rheostat. It gets you more "bite" at lower volumes.

Two other options for the volume:

-- Do as NachoM suggested, and wire it as a gain control.
-- Wire it to the input as a voltage divider (one end to the incoming guitar signal, the other end to GND, and the center to C1 / Audio In. Something like 10K up to 100K would do.)

-- Two 4 ohm car speakers would be fine. Mix and match speakers, if you have any old computer speakers, etc., and maybe you'll like what you hear.

A lot of bass response comes from the speaker cabinet, so cabinet / no cabinet will sound different.

1 ohm resistor-this and the cap below were suggested to stop oscillation?
.2uf disc cap-

Right--good eye, that's on the datasheet. Might not matter for a guitar amp, since you're not trying to minimize distortion.

On the other hand, if you push the amp really hard to get distortion, then oscillation might be a problem... Might be worth ordering the parts.

awesome tube amp, btw
John Smith (author)  gmoon9 years ago
alright, going to guitar lessons, be back. i'll respond fully once i'm back. i'm going to try to pick up some of the parts. thanks! Jason
John Smith (author)  John Smith9 years ago
ok, no ICs at radioshack. well, not the ones i needed.

and, i think i'm going to make a simple headphone amp type thing. like where i can plug in my headphones and play guitar that way.

I still want to make the 8 watt amp, just not now.

I was going to make a small amp as per instructions on:

I got this stuff at radioshack:
25ohm 3 watt Rheostat (probably cant be used on this, but i can use it somewhere)
LM386 audio amplifier IC(it says 400mw on it? but the amp in the plans is 1 watt?)
LED(i can work it into the plans somehow)
1/8" headohone jack
1/4" guitar jack
small PC board
8 pin IC socket

i think thats it. I have a ton of caps and stuff, though. I'll have to look and see if i have the right ones for this project.

Will that IC work for a small audio amp? I only need to take the raw guitar signal and amp it up to power my small headphones.

Sure, the LM386 is great for lotsa projects. You can make a headphone amp, but it'll drive small practice amps, too.

Make Mag had a crackerbox amp project a while back (and it uses the 25 ohm rheostat!)

The Ruby amp is a good project (this one uses an FET transistor as well.) The same site has the simpler Little Gem, which looks like the origin of the Make crackerbox (the schematic is identical.) Also uses the rheostat...

The Little Gem page has a two IC version, that's a little "hotter..." Regardless of the wattage, all these projects are great prep for building the larger LM383 / TDA2002 amplifier...
gmoon gmoon9 years ago
Here's another great link: http://www.beavisaudio.com/

Check out the Build your own Noisy Cricket PDF...
And the YA386A (Yet Another 386 Amp.)
John Smith (author)  gmoon9 years ago
I ended up tearing apart an old "spy" microphone thing. The idea is that you plug in headphones, and you can hear things from far away. Well...it has a small circuit in it, i tore it out, added a switch and a 1/4" jack in place of the mic element, and boxed it up. Shockingly, it WORKS. I can hear it pretty clear, although theres a decent amount of static on the front and back pickups, but you can switch to useing both, and the static isnt bad. It has niiicccee distortion when you turn it up(you have to use the volume on the guitar, however). It turned out good. i think i'm gonna do the Ruby amp. maybe. I cant belive it, but i have a mpf102 transistor in my parts box. And i have the right 386 IC. And some of the caps. Thanks alot for the links. Is there any way i could run two or more of these amps in series or parallel to run a larger speaker? Jason
Ah, the modded "spy" amp sounds cool...

Is there any way i could run two or more of these amps in series or parallel to run a larger speaker?

Look at that YA386A mentioned above--it's two 386 amps (apparently an N4 version, which must be higher voltage / wattage.) There's no schematic, but check out the wiring: the input is routed to the +in for one 386, and to the -in for the other 386.

That would cause the outputs of each amp to be 180 degrees out-of-phase; each an inverted duplicate of the other. The outputs are connected to opposite sides of the speaker, which is a push-pull configuration.

Push-pull amps are theoretically much more efficient than their single-ended counterparts....
John Smith (author)  John Smith9 years ago
maybe the crackerbox amp... i have time to decide.
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