Cigar Box Guitars: building tips, music and the cigar box culture

Simple amps

I'm working on building a simple battery powered mini-amp to play my CBG's through. Inspired by the creations of nation member smojo, which he displayed at the recent UK fest in Birmingham, I'm experimenting with designs based on an LM386 integrated circuit. These tiny chips allow the construction of some amazingly compact yet great sounding amps (some of smojo's elegant little amps are shoehorned into very small tins).

I've found a lot of useful information at the websites of Beavis Audio (http://www.beavisaudio.com - see the "Smokey" and "Cricket amps") and runoffgrove (http://www.runoffgroove.com - info on "Little Gem" and "Ruby" amps). I've also investigated Maplin's LM386-based 1w mini amp and I downloaded the LM386 datasheet from National Semiconductor. However there are still a few things I'm struggling to figure out (either because they aren't well covered by the sources I've found or, more likely, because my electronics knowledge isn't sufficient). I wonder if any nation members have worked with these circuits and if so whether they can provide answers.

Firstly, some designs have a capacitor wired between pin 7 of the LM386 and ground (in fact the only design I can see where it's omitted is the Little Gem). The only description or explanation I can find is on the LM386 data sheet where pin 7 is described as "bypass". Any idea what they mean by "bypass" and what effect it might have on the sound of an amp if this is connected to ground via different capacitance values?

Also, I note some designs have a capacitor on the input (eg. Little Gem) while others don't (eg. Smokey and Maplin 1w). My first build doesn't have an input cap (partly because it was based on a Maplin PCB for convenience) and it seems OK. Can you suggest any reasons either in favour of or against adding an input cap?

Tags: amps, lm386, mini-amp, ruby, smokey

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Replies to This Discussion

Permalink Reply by Michael Gajeski on October 21, 2009 at 3:36pm: you could always do some experiments with different valued caps on a breadboard without having to solder anything.; ▶ Reply to This

Permalink Reply by David Lloyd on October 22, 2009 at 5:23am: Hi,
When I try to see the photo I get
"You need to be a friend of Junk Box Instruments to do that" - Just call me Billy-no-mates(LOL).
Good luck with the LM386 - Have tried various circuits and found the smokey schematic sounded as good as the more complicated versions.

Regards,
David; ▶ Reply to This

Permalink Reply by Mark aka. Junk Box Instruments on October 22, 2009 at 10:10am: Sorry the pic wasn't accessible - I've changed my account settings so hopefully my pics ae now visible to all.

I think you're probably right about the simple approach, the LM386 seems to be a robust little package which needs very little to turn it into a workable amp. The aspect of amp design that's causing me most problem at the moment is not the electronics (which I think I've figured out) but the enclosures. Tin cans are neat but I'm finding they can have some unfortunate vibration modes which cause annoying buzzes at higher volumes. I've found ways of sorting the problem but I think I'll also be experimenting with alterantive boxes.

David Lloyd said:
Hi,
When I try to see the photo I get
"You need to be a friend of Junk Box Instruments to do that" - Just call me Billy-no-mates(LOL).
Good luck with the LM386 - Have tried various circuits and found the smokey schematic sounded as good as the more complicated versions.
Regards,
David; ▶ Reply to This

Permalink Reply by J. K. on October 22, 2009 at 10:20am: Check out the magic 8ball on my page. If you can find it, get the "mystic" 8-ball kind. It has sort of threads around the equator of the ball that makes disassembly easier(I think. Don't have the "magic" brand here)
http://www.cigarboxnation.com/photo/albums/magic-8ball-amp

The end result doesn't rattle at all and the window bit works excellently as an opening for a speaker.

Basically you can use just about anything. I was thinking of using a flower pot or a glass jar to make one.; ▶ Reply to This

Permalink Reply by HOGS GRUNT on October 22, 2009 at 10:23am: Just make friends with him like I did.

David Lloyd said:
Hi,
When I try to see the photo I get
"You need to be a friend of Junk Box Instruments to do that" - Just call me Billy-no-mates(LOL).
Good luck with the LM386 - Have tried various circuits and found the smokey schematic sounded as good as the more complicated versions.
Regards,
David; ▶ Reply to This

Permalink Reply by David Lloyd on October 22, 2009 at 10:33am: Hi Hoggs Grunt,

A very good suggestion that is now done.

A very nice amp. Do you find the 9v battery lasts OK with the 7w output?

Regards,
David; ▶ Reply to This

Permalink Reply by Mark aka. Junk Box Instruments on October 22, 2009 at 11:07am: Actually battery life was something I had doubts about, but the first battery seems to be lasting OK so far. Only time will tell for sure.

David Lloyd said:

A very nice amp. Do you find the 9v battery lasts OK with the 7w output?

Regards,
David; ▶ Reply to This

Permalink Reply by Michael Gajeski on October 22, 2009 at 9:48pm: 8 watt amp

has anyone tried this? a friend of mine found it on the yahoo cigar box forum, but the pinout of the lm386 doesn't match up with it. is it a waste of parts?; ▶ Reply to This

Permalink Reply by Jkevn on October 22, 2009 at 11:06pm: http://www.national.com/ds/LM/LM386.pdf
That diagram you posted looks like a variation of the lm386 amps, except the diagram is mis-labeled. 12v into pin 8 will let the smoke out, and it wont work without its smoke. 12v is the max voltage on a 386 chip (most variants), I dont see it putting out 8 watts, the N-4 variant only puts out 1000mw@16v.

These chips are less than a dollar each, you get a dollar's worth of fun playing it loud for a minute and watching it smoke....

Michael Gajeski said:
8 watt amp

has anyone tried this? a friend of mine found it on the yahoo cigar box forum, but the pinout of the lm386 doesn't match up with it. is it a waste of parts?; ▶ Reply to This

Permalink Reply by Michael Gajeski on October 22, 2009 at 11:53pm: Lmao... He was talking like he had actually built one until I showed him the data sheet... then the story changed.

Jkevn said:
http://www.national.com/ds/LM/LM386.pdf
That diagram you posted looks like a variation of the lm386 amps, except the diagram is mis-labeled. 12v into pin 8 will let the smoke out, and it wont work without its smoke. 12v is the max voltage on a 386 chip (most variants), I dont see it putting out 8 watts, the N-4 variant only puts out 1000mw@16v.

These chips are less than a dollar each, you get a dollar's worth of fun playing it loud for a minute and watching it smoke....

Michael Gajeski said:
8 watt amp

has anyone tried this? a friend of mine found it on the yahoo cigar box forum, but the pinout of the lm386 doesn't match up with it. is it a waste of parts?; ▶ Reply to This

Permalink Reply by Todd Plummer on November 23, 2009 at 1:21pm: Just a thought. If you are really just looking for a practice amp... I use this one:
http://www.voxamps.com/amplug/
I then plug in headphones or plug it into my home stereo's 'Line In.' I can play it through just about anything...
It sounds great and is really compact. But, if you are trying to build a small amp to fit into a similar Cigar Box, you may be better off buying a phono amplifier and wiring in some pots and a jack from Radio Shack...; ▶ Reply to This

Permalink Reply by tad adams on November 23, 2009 at 1:51pm: ive just made one with one of these, http://cgi.ebay.co.uk/MAKE-YOUR-OWN-CIGAR-BOX-TIN-CHOC-BOX-GUITAR-A..., you just connect your vol/tone to the output jack ,make a hole for the speaker in your guitar box,add a 9v battery and away you go.Its not the loudest but its certainly louder than an acoustic guitar; ▶ Reply to This