DIY guitar amp: This easy??

Factual Questions
1

Hey musically inclined dopers, I have a question for ya…

I recently was loaned an old junker electric guitar for trying to learn to play with. I’m doing okay, just taking the very first tentative step, but I’ve run into a problem. Basically, since it’s an electric guitar, the sound is really weak and hard to hear clearly.

“Well go get an amp!” Yes, that would be the right thing to do… if the IRS hadn’t sodomized me to the tune of $3000 this year. I don’t have an extra $20 to spend on McDonalds, much less $150+ to blow on even a used guitar amp.

However, I do have a boom-box… and I just looked on the back, and it has a small phono jack labeled “mic.” If I buy a cheapie cord and a big-phono to small-phono jack adapter, and plug my guitar into the back of my boom-box, will it just work, just like that?? Isn’t there some worry about matching impeadances? Most mics are 4 or 8 ohm, and I don’t have a clue what the guitar is.
I’ve also been considering building myself a small headphone amp that I can plug the guitar and a pair of headphones into and listen to the guitar that way. I’m a fair hack with a soldering iron and I understand basic electronics and amps. I’ve found a fairly nice design at http://www.headwize.com/projects/guitar_prj.htm - anyone think this is worth my while?
-Ben

2

Microphones are not 4 or 8 ohms impedance, you’re thinking of speakers. A typical dynamic mic input should work fine for a guitar.

There are lots of DIY guitar amps but unless you build a tube amp you probably won’t get much better sound than you will from the boom box.

3

Get yourself a little “Rockman” type portable amp. These are little boxes the size of a Walkman, with a guitar jack and a headphone jack. There are multiple advantages to a Rockman. They have extra inputs so you can input a second channel to play along with a CD, or with other Rockmans. They also have preamp, simple EQ, reverb, overdrive, etc. settings. I’ve heard of some musicians who like the Rockman sound so much they use them as inputs to their real amps.
You can plug them into external speakers, or into headphones. Just be careful with headphones, you can easily damage your hearing through cumulative exposure, these suckers are LOUD! I have one myself, I attach it to my belt, then put the headphones on my head, but not over my ears, then play along to records. It’s loud enough to be heard clearly even when the headphones are about an inch from my ears. And no more complaints from the neighbors about noise.
But hey, sometimes it’s FUN to annoy the neighbors. When I first started playing, I had the fortune to pick up a Marshall 50 for cheap. I used to practice out in a barn, about 2 miles from the nearest neighbors. One day, after a Hendrix-inspired feedback fest, I got a phone call asking what all that noise was. When I explained, my neighbor said, “it sounded like the WORLD was coming to an end!”

4

There is was, deep in the trenches, fighting ignorance tooth and nail (or at least watching while others did so, and applauding their efforts…) when I saw the OP!

“Can I plug an electric guitar into the mike jack on a boom-box?”

My day had come! This was my moment to shine!

I have an electric guitar (recently purchased), and a boom-box! The CD player in the boom-box has been broken for about a year, so I never use it anymore… so I wouldn’t mind possibly blowing out the speakers; and I can’t think of any way in which the guitar might be harmed! By Golly, I’m gonna plug that sucker into the boom-box, turn it on, and crunch out a few bars of “Purple Haze” to see what happens, then report my findings here…

Uh oh! Problem: Astrogirl is sleeping, and the bed is directly under the shelf where the boom-box is located… there’s no way I can get to it without waking her. [sup]Side issue: even if I could get to the boom-box without waking her, my proposed experiment, if successful, might very well wake Astrogirl anyway… but some sacrifices have to be made in the fight against ignorance![/sup]

Girding my loins (well, scratching my testicles anyways…), I hassled Astrogirl out of bed (she had to go home soon, so I would have had to wake her in a few minutes regardless)…

Ignoring Astrogirl’s somewhat bleery inquiries (“Whey? Mo hey?” Translation: “Why? What are you doin’?”) I grabbed the guitar and a cord, and stood on the bed…

I threw the guitar strap over my head (slung way down low, like in the Foreigner song), plugged the cord into the guitar, and reached up to plug it into the boom-box…

And was crushed!

The male end of the cord was WAY too big to fit into the female jack in the boom-box (story of my life!!:smiley: )!! I have no adapter to make it fit (got one that goes the other way, but that doesn’t help!).

DAMN! Foiled again!! My experiment was a flop, and Astrogirl is less than pleased (she could have slept for another tem minutes or so…)!

I tried ModernRonin2! No luck! But I bet it would work (if it didn’t blow out the speakers in the boom-box!)…

5

CRAP!!! That should have been “There I was,”

6

My local pawn shop has a couple of battered but serviceable Crate practice amps for 10-15 bucks. Check your pawn shop and see what they got. Maybe even swap 'em something you got but don’t need for the amp.

And as long as you got blood, you can get 10-15 bucks. Besides, I don’t know what you’re going to play, but you can’t play the blues on a guitar (or amp) that hasn’t been in a pawn shop, according to B.B. King.

b.

7

Following up to my own post after a long, hard night of digging up info from the web…

Can you do what I proposed? Yes. It won’t sound as good
as it should, but you can do it.

Basically, guitars are about 1000 ohm impeadance. The mic input on most boomboxes, computer sound cards, etc… is about 100 ohms. The rule of thumb is, a low impeadance output can drive (often several) high impeadance inputs, but not vice-versa. When you do, distortion and once in a great while frying of the input can occur. Guitars are a little “hotter” than mic input levels, but not as hot as line levels. So you’re kinda screwed regardless.

It’s fairly simple to construct a guitar pre-amp out of an op-amp and some resistors and caps to feed a correct impeadance signal to a LINE level input on a stereo, computer, etc. There are a fair number of circuit schematics diagrams out on the net. Unfortunatly I don’t have a line-in, just a mic-in, so these don’t do me much good. Hooking up an amplified signal to an internally amplified mic-in stands a good chance of blowing up my speakers, the little amp on a chip inside the mic jack, or both.

So I guess I’m getting a cable and a big-phono to small-phono converter and giving it a try. Even distorted sound will be better than the lack of sound I get trying
to play the thing acoustically. I’ll also haunt the pawn
shops for an amp - thanks, good suggestion there.
-Ben

8

I used to run my guitar into my boom-box before I bought an amp. It won’t sound great but beggars can’t be choosers.
You just need a 1/4 to 1/8 adapter which you can buy at radioshack for like $3.00.

9

FWIW, when I was a kid, I figured out that the phono input for my stereo would work reasonably well for this ( OK, it was actually a little JCPenney’s all-in-one unit, so I actually had to take the back off and wire a jack into it, but the idea’s the same). If you have a stereo receiver with a phono input, give it a try (though I do advise moderation on the volume).

OTOH, there are any number of small headphone-only or small-speaker practice amps you can buy for less than $50 new, or less than $30 used (in some cases, even less). I have a little Danelectro Honeytone that sounds almost as good as the the little Peavey Backstage 20 that was my first “real” amp (minus the spring reverb).