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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: LM386, Ruby, Smokey, amps, mini-amp

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PS. I just discovered the strand "Simple Underground Electronics" - rather than re-post everything there I posted a link flagging up that I've put some questions here. If people feel it's unhelpful to have separate strands then I could delete this and try to move it over there.
If you are going for an amp kit and have been looking at Maplins why not check this out:-

http://www.maplin.co.uk/Module.aspx?ModuleNo=10502

By the time you have bought all the bits needed you might just as well go for something like this and you will get more power output.
Just a thought so tell me to shut up if the info is not what you are looking for.
I'm putting a few links for amp kits here - thats the easy way. As far as lm386 based amps and the small cap @ pin 7, it helps to isolate the high-gain stage of the IC from noise from the power supply etc. when the output load is heavy. The "ruby" calls for 100n, but values up to about 4.7uF may work better depending on the power supply used....battery versus wall charger.

http://store.qkits.com/moreinfo.cfm/FK617
http://www.goldmine-elec-products.com/prodinfo.asp?number=C6238

My favorite "cheap" method has been using old computer speakers to make amps. Everythings already put together, you just have to put it in a cooler box....most are 12v, but speakers that connect to a computer's USB port are powered by 5v, and will work off of 9v.

http://www.premierguitar.com/Magazine/Issue/2007/Dec/The_5_Travelin...

http://buildinggadgets.com/index_key.htm
The Bypass capacitor removes "AC" noise or filters any alternating current from the DC portion of the circuit, making it more stable and eliminating hum.
I've looked at those kits and might give them a try later on. For the moment the LM386-based ones suit my purpose - among the advantages they have are that their low power consumption makes them suited to battery power and there's already a fair knowledge base out there in terms of a variety of designs by people who've used them as guitar amps. I've also found that 1w amps can be perfectly loud enough for the situations I have in mind.

HOGS GRUNT said:
If you are going for an amp kit and have been looking at Maplins why not check this out:-
http://www.maplin.co.uk/Module.aspx?ModuleNo=10502
By the time you have bought all the bits needed you might just as well go for something like this and you will get more power output.
Just a thought so tell me to shut up if the info is not what you are looking for.
Thanks for that, I think I might be on my way to understanding pin 7 now. As I'm using a battery supply I was assuming there wouldn't be much noise from it - am I missing anything there? I presume there could be other sources of noise in the general environment. I also read on one of those links that a capacitative link to ground on pin 7 helps avoid oscillation at high gain.

Jkevn said:
I'm putting a few links for amp kits here - thats the easy way. As far as lm386 based amps and the small cap @ pin 7, it helps to isolate the high-gain stage of the IC from noise from the power supply etc. when the output load is heavy. The "ruby" calls for 100n, but values up to about 4.7uF may work better depending on the power supply used....battery versus wall charger.
I did the ruby amp as a preamp in my Oliva box. Works well but get the right components. Tho will work, the right components help the gain and should keep interference down. Mine picks up FM signals when the volume is 0.

Ground the strings and bridge and anything else that might pickup signal.
Anything taking electrical current and using it to provide "work" is gonna create noise, even on a minute scale like this. Those capacitors, input and at pin 7, try to shed unwanted electrical thuds and thumps to ground where they don't:
a: get amplified into bigger thuds and thumps
b: begin the endless oscillation death-spiral.

Are you using a gain pot or strapping out pins 1 & 8?
I'm using a gain pot - seems to be the best way to allow some adjustment of the sound/distortion. I'm wondering about the capacitance in this: The datasheet seems quite clear about the need to put a capacitor between pins 1 and 8 if you're changing the gain (either with or without a resistor in series), but I notice the "Little Gem" and "Ruby" just have a pot there.

Jkevn said:
Anything taking electrical current and using it to provide "work" is gonna create noise, even on a minute scale like this. Those capacitors, input and at pin 7, try to shed unwanted electrical thuds and thumps to ground where they don't:
a: get amplified into bigger thuds and thumps
b: begin the endless oscillation death-spiral.

Are you using a gain pot or strapping out pins 1 & 8?
Hi,

I have built a few mini amps that I have added to CBGs as on board amp/speakers. I have tried several ones and found the smokey schematic works as well as the more complex designs. Without the overdrive function the LM386 amp sound is really weak, but with it the sound is ok for bedroom playing. With the smokey amp if you want a cleaner sound you have the volume turned down more.
http://www.bitsbox.co.uk/kits/projectkits.html sells a 1W amp kit based on the LM386 for £4.49 and the seller is a top bloke - very helpful and usually next day delivery.
For a stronger clean sound the £5.99 Maplin 3W mono amp kit is good http://www.maplin.co.uk/Module.aspx?ModuleNo=2904 and works fine with a 9v battery (no overdrive circuit though). For simplicity Maplins sells a 3.5W Universal Amplifier Module for £6.99 you just need to solder a speaker and a 9v battery http://www.maplin.co.uk/Module.aspx?ModuleNo=37734.

The main improvement you can make whatever schematic you use is to use bigger speakers rather than the 500mW paper speakers - I prefer the mylar ones you can get off ebay such as http://cgi.ebay.co.uk/Mylar-Cone-Speaker-2W-77mm-Round-Alarm-Sounde...

One useful thing I did was to get a breadboard (solderless prototyping board) off bitsbox so I could have a play around with different circuits and still be able to reuse the components.

Good luck,
David
Re: the 7w TDA2003-based Maplin kit suggested by HOGS GRUNT - I've now given this a go and I think it might have the potential to be a usable compact guitar amp. It certainly packs plenty of volume while still running comfortably off a 9v battery. And by adjusting resistance values I seem to be able to crank the gain up and get some nice distortion. However I still need to play around a bit to get the altered components just right because, if I push the gain too far, it sometimes goes into oscillation.

Re: The 3w Maplin kit - it sounds interesting for clean sounds (which might suit my piezo-fitted tin can banjo) but it looks to me as if there isn't a neat way to get it to distort nicely. Unlike the chips in the other simple amps the TDA7267 at the heart of this one doesn't seem to have any way to alter it's gain.

It seems to me the great advantage with the LM386 chip is that other people with much greater expertise than me have done a lot of work on creating practical circuits based on it.


HOGS GRUNT said:
If you are going for an amp kit and have been looking at Maplins why not check this out:-
http://www.maplin.co.uk/Module.aspx?ModuleNo=10502
By the time you have bought all the bits needed you might just as well go for something like this and you will get more power output.
Just a thought so tell me to shut up if the info is not what you are looking for.
Just completed my first amp - the "Choc-O-Cube". The internals are a modifies 7w TDA2003-based Maplin kit and the enclosure is an old cholocate tin.

http://www.cigarboxnation.com/photo/the-chococube?context=album&...

Next project is to try to fit an LM386 circuit into a much smaller tin.

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