New Guy with an interesting design idea

[Bajaguy]

Howdy! I'm new here, and found the forum while researching an idea for a bigger portable amp. I have been an electronics tech for a grass roots company for the last 14 years, and I've been fixing pedals and amplifiers for a few of my musicain friends for what seems like forever...

There's a group of guys that I've hung out with for years that have been bitten by the cigar box guitar bug. They have built several 2, 3, and 4 string CBG's that sound awesome! They commonly play in the smoking area at work, which doesn't have access to power. Several have bought small 9V powered mini amps, but most of them have horrible tone and very small output. I built a couple of Noisy Crickets that were converted to cigar box amps, but the tone of those amps is horrible and not what anyone considers usefull. I built a MKII with the bridged LM386 chips and it was a flop with the group as well.

My goal has been to design and build a small portable amp that's battery powered and has clean headroom with enough volume that they can play for at least an hour without recharge. I'd like to add in a SLA battery with a charging circuit that would allow the amp to be played off of the charging circuit without loss from the battery, yet keep the battery charged and ready for any excursion. The ability to charge the battery from a cigarette lighter in a car would also be very usefull.

The enclosure would be similar to a 10" x 10" x 5" cigar box built from 1/4" western red cedar, and I'd like to have the option of using 5", 6", or 8" speakers.

I've been paging through the portable amplifiers that grace the pages here, and I'm sure there's a chip amp that will be perfect for this, but I'd like to get a few opinions on what would be the better route for this set up. I have a umble preamp that I built as a pedal that I'd like to use for the preamp section on this, but it's not a design requirement.

Anyways, nice to meet everyone, and I'm interested in any ideas that may come up.

Thanks,

Baja

[mickmad]

Hello there, welcome to the forum!

The whole cigar box guitar thing is just amazing! I never heard of this kind of diy guitars and it's very interesting! When I got some time I'll definitely go to my local flea market and look for a nice cigar box to do one myself!

By the way, maybe I got something for you: I got a Marshall M2-S which is a mini Marshall amp, powered by a 9 volt battery, with clean and distorted channel, and that thing ROCKS! It's very small, but powerful, and when the battery is at the end of its life, the distortion channel degenerates to a very cool fuzz tone!

I was going to tell you that there's no schematic of it, but I just found it!

http://guitar-dreamer.blogspot.it/2010/06/modding-marshall-ms-4ms-2-micro.html

This guy talks about a little mod he has made to its mini marshall, and was kind enough to add the whole schematic of the mini amp too! Hope you'll like it

edit: just found out this topic on the forum: http://www.ssguitar.com/index.php?topic=1596.0 with schematic, give it a look because there is an alternate version of that amp with a different op amp, the original op amp used in it is very hard to find

[J M Fahey]

The big problem in such amps is the battery itself.
6 to 9 V, no matter what they say, is not loud enough.
Chips designed for such low voltages are meant for radios or cassette players or portable Nintendo/GameBoy/PSP, etc.
Now if you get 12V, whether Lithium/Lead Gel/NiMh/etc. the horizon opens immensely because we enter Car Amp territory.
You can easily get from 4W RMS to almost 20W which is *loud*.
But usually it's not light, unless you invest in highest technology batteries.
Your choice.

PS: and LM386 is crummy, it's the *worst* 6 to 9V amplifier.
It's so popular only because it's *so* simple.

[Roly]

Welcome!

Acoustic bass guitar uses water jug and two strings
http://hackaday.com/2013/04/07/acoustic-bass-guitar-uses-water-jug-and-two-strings/#more-97413

In a world populated by multi-thousand dollar guitars and amps I love this back-to-the-roots stuff.  I built a few "hobo instruments" called monochords from garbage scrounge, tin can, lump of wood, bit of wire, even a giant bass one, and even had one of them used on stage too - kind of slide-banjo-dobro sound.

As I'm sure you are aware running or recharging from a suitable wall wart wouldn't be a problem, but recharging batteries like SLA's from a car system can be a bit of a challenge because the voltages are so similar.  Overall I think the electronics are the least of your problems, particularly as you are experienced in the art.

IMO the most serious limitation of small low powered amps is the speaker.  I have often amazed people by hooking up some flea powered crud box to a big cab to demonstrate just how compromised "practice" class amps are by the small case and speaker, so I would consider an 8-inch unit absolutely the smallest speaker to consider, and a 10 would be much better, but really, the physics is against you in a small enclosure and you are never going to get good sound in a tiny cabinet.

You might be able to get away with a small driver if you can add something like fold out wings that form a bigger baffle or horn-like structure around it.

Running from batteries also means that speaker efficiency is very important and high dB/W is vital - the better the speaker is the less power you have to generate and store.

I've seen a small amp built using a car door speaker in the side of a plastic jerry can with a couple of 6 volt gel cells in the bottom, and being enclosed sounded fairly reasonable (but I'm sure could be improved).  Very easy to carry.

I hope you will keep us posted on your progress.   :dbtu:

[Bajaguy]

Thanks guys, I'm tickled that I found a forum like this!

That mini marshall is cool, and it one of the ones they play out there alot, along with the mini Orange. They just don't make enough noise is the big problem, but they have tones of cool tones.

I'm thinking more of something in the 10 - 20 watt range that runs a 6" or an 8", with a umble tone control and a clean boost circuit for those days that ya just gotta be clean and loud. It's also got to be able to handle a pedal if they need it. I spent a bunch of hours last night going over some of the chip amps availible, and there's a ton that will run on 12V. I really don't want to build a differential supply if I don't have to, but I'm not afraid of it either. Anyone got a preference or a suggestion?

Baja

[J M Fahey]

10 AA NiMh batteries will give you *very good* 12V , and 2700mAH (2.7 AH) at very low weight and reasonable price.

There are more modern options (Lithium, LiPo, etc.) being discussed in other Forums but they run U$ 100/125 a pack, *plus* have very expensive shipping, on safety concerns.
A plane was almost brought down when one of those exploded and a cargo compartment caught fire, and it already happened quite a few times, so .... getting a pack f them would border U$200 , I think it's too much.

If you can carry more weight, a gelled lead 12V 4.5AH costs around U$20/25 .

This battery thing has been discussed in:
http://www.ssguitar.com/index.php?topic=2780.msg20357#msg20357
FWIW you'll see there my biamplified 30W "Callejero", run from a 12V 7AH "Alarm" type battery.

[Bajaguy]

I read that thread, your amps are very nice! Are you running the tweetr and 10 on separate controls? It's an interesting concept, and very close indeed to what I'm looking for in this design.

I'm looking at both of those chips now, thanks for the leg up!

Baja

[J M Fahey]

 :cheesy: Thanks.
Yes, I had to add a separate (unabelled) volume control for the TDA2003 teweter amplifier, because it was louder than the 10".
Since I was using an active crossover, it was easier than adding the typical L-Pad .
And in a huge, cavernous hall (many of my customers play at Tube Stations), you just put it on 10 and cut through the noise.
Incoming/outgoing trains are *very* noisy, plus all the door hissing (compressed air), etc., you need every dB possible to fight that.

[Roly]

Twenty watts!  Holy Himmel.  How big a venue/crowd are you serving here?  Do these guys work in a barbed wire factory that runs during lunch time?  Lot's of guys here play clubs and pubs, noisy venues, with 15 watt amps and they are plenty loud enough.

Loudspeaker parameters and loudness
http://www.ssguitar.com/index.php?topic=707.msg4933#msg4933

And...

Speaker sens. (1dB/W@1m)	Required power (watts)
104db	1
101db	2
98db	3
95db	6
92db	12
89db	24
86db	48
83db	96
80db	191
77db	382

Listener range 8 feet, desired listener SPL 80dB, amp headroom 15dB

[joecool85]

Twenty watts!  Holy Himmel.  How big a venue/crowd are you serving here?  Do these guys work in a barbed wire factory that runs during lunch time?  Lot's of guys here play clubs and pubs, noisy venues, with 15 watt amps and they are plenty loud enough.

Loudspeaker parameters and loudness
http://www.ssguitar.com/index.php?topic=707.msg4933#msg4933

And...

Speaker sens. (1dB/W@1m)	Required power (watts)
104db	1
101db	2
98db	3
95db	6
92db	12
89db	24
86db	48
83db	96
80db	191
77db	382

Listener range 8 feet, desired listener SPL 80dB, amp headroom 15dB

Great chart!  I amp going to share this on the Watts vs Volume thread.

[Bajaguy]

I look at it like the horsepower rating in your car. You don't NEED 400 horsepower to drive back and forth to work, but you'd be glad to have it if you take your car to the dragstrip :dbtu:

I always have a tendency to overbuild, not saying that I couldn't get the job done with something way smaller. Still, the idea of having a clean 20 Watts through a nice fat 8 in a portable amp that looks like a cigar box would rank right up there high on the cool scale :tu:

I looked through a few amplifier kits online the last few days to see if I could buy the building blocks cheap enough, rather than having to proto them. I'm looking seriously at a bridged TDA2050 or TDA2030 to get the job done, but i'm not sold until I've rocked one in the box to see if it's the right fit.

Baja

[Roly]

Great chart!  I amp going to share this on the Watts vs Volume thread.

Fine, but look around first Joe, there are better.


@Bajaguy - my point, to continue your parallel, is that you need less (average) power if you don't drive around with the handbrake on.  The difference between an 89dB/W speaker and a 98dB/W is the difference between the battery going flat before lunch is over, and going for hours with awesome SPL reserve - using the same battery and chip.

[J M Fahey]

bridged TDA2050 or TDA2030 to get the job done,

If you have only 12V, these are not a good choice, original TDA2003 (or 2005, which is 2 x 2003 in the same chip) or all other "car amps" are better.

[Bajaguy]

@Bajaguy - my point, to continue your parallel, is that you need less (average) power if you don't drive around with the handbrake on.  The difference between an 89dB/W speaker and a 98dB/W is the difference between the battery going flat before lunch is over, and going for hours with awesome SPL reserve - using the same battery and chip.

I understand your point completely.

Baja

[Bajaguy]

bridged TDA2050 or TDA2030 to get the job done,

If you have only 12V, these are not a good choice, original TDA2003 (or 2005, which is 2 x 2003 in the same chip) or all other "car amps" are better.

You are completely right, sometimes I think that without dislexia I'd have no fun.... :duh

Thanks for the correction, those are the two chips I looked into.

Baja