portable power supply?

[kvb]

I would like to make a portable power supply for my MkII 1w practice amp, so that it is a self contained amp - just plug in the guitar and go.
I think I would like to use a 12v lantern battery - unless someone has a better idea.

I think I could run 12v to the 386s, and I could use a voltage divider to get 9v to the jfet buffer.
Or, I could power a voltage regulator with the 12v and just run 9v to the power input as it is.

Am I on the right track?

Comments and suggestions will be appreciated. 

[Seljer]

you could probably set up the JFETs to work off 12V without too much trouble

[Ardric]

Yup, just run everything off of +12V.  The FET will have a little more headroom.

Check the mAh rating of that lantern battery.  They're usually pretty crappy old drycell technology and don't last very long for the size/weight.  I've gotten great results with an 8-pack of AA alkaline cells running a 4W amp.  Rechargables work well here too.  Don't use a regulator unless you absolutely have to -- they eat up a lot of juice all on their own, and you can't afford that in a battery operated device.

LM386's at 12V should probably not be given a speaker load less than 8 ohms.

If you want to be really loud, consider boxing up your preamp, a bridged car stereo power amp and a motorcycle battery. 

[Gilles C]

When I decided to replace the 8 D batteries of my Portable Peavy Solo Amp, I got lazy, and bought a 12V/2.2A recheargable battery and a  12V/500ma Smart Charger from this company.

http://www.batteriesamerica.com/newpage4.htm

This battery system works so well that I thought I could buy another battery to make a portable power supply with it, and use the same charger for it. I found it cheaper to buy such a battery than a lot of recheargable batteries in the AA, C or D format.

Gilles

[grapefruit]

I second what Giles said about using sealed lead acid batteries. I use Panasonic 7.2Ah Batteries, and a plug pack style charger designed for this type of battery. If you know where to get them the Panasonic batteries are cheap and a lot better than the "cheap" brands. (I'm in Australia). You can get smaller, lower capacity batteries too. I got these so I could go camping...

This is for 240V, but so you get the idea this is the charger I use...
http://www.electusdistribution.com.au/productView.asp?ID=1804&CATID=18&keywords=&SPECIAL=&form=CAT&SUBCATID=190

And this is the battery, though I don't buy them from these guys...
http://www.electusdistribution.com.au/productView.asp?ID=4605&CATID=18&keywords=&SPECIAL=&form=CAT&SUBCATID=190



For 12V operation I recommend the National LM4950 Amplifier IC. If you use it in bridge mono mode you can get about 7W from it before it clips. I haven't used it for guitar yet but when I was doing a power test it clipped quite nicely. Not as sudden as most transistor amps I've power tested. I can't remember the idle current draw but it was low.

You could run the JFet buffer direct from 12V. Depending on the circuit you may have to change some resistor values. It may be handy to include a 9V reg though so you can power external effects.

Stew.

[kvb]

thanks for the info and links.  I'll certainly post whatever solution I end up using - when I get it figured out.

[syndromet]

I have a stereo Fetzer ruby that run on a 12v, 10A motorcycle battery. I have been using it for about 30 hours now, and still haven't need to recharge it. I also find it to sound cleaner than all other batterys I'we used.

[kvb]

Did you have to make any modifications to the circuit, or did you just hook up the 12v?

[petemoore]

  These private security installers, one after the other, because each time we 'got a better deal', would leave 'the other guys' 11v and 14v batteries for me when they installed the new system.
  I've been using these two batteries for a long time. They stay charged a long time too.
  Most effect circuits would be perfectly happy @12vdc, provided all the capacitors are rated for at least 12 v...which means 16v caps...and a DMM test to the supply to be certain it's below say..14v to be on the safe side.

[kvb]

Well, I got a 12v battery and completed the build of a self contained Gem MkII.

It's a little silly and cool at the same time.

I built a frame around an old 4" speaker enclosure. I put the battery on the side and used the space above for the amp.
It's got a cool copper control panel; indicator light, input, and gain switch on the front; speaker jack and power switch on the back.
It's got a leather handle with forged copper thingydoos to hold it tight on both ends.
There are extra terminals exposed so I can recharge the battery without opening it up. All it needs now is some rubber feet.

The silly part is that, for only 1w, it is slightly large. And it's a bit heavy - Heavy Metal, that is . . . hyuh hyuh hyuh

[RaceDriver205]

Definately use a rechargeable battery.
Also, have a look at putting a battery indicator in the amp (i.e green led for full, orange for middle, and red led for empty).
Its a really useful and fairly simple feature to add. Look for the 'state-of-charge' graph for the battery you want to use. It gives you the voltage versus how much the battery has discharged.

[petemoore]

grapefruit, I have two batteries here that look alot like what's in the picture provided in your link.
  They came from alarm systems, when we got a better deal from a different alarm company, they installed their new battery to the old system..I asked and recieved the old batteries.
  I call 'em 'battery-bricks', I ran one to too low/high a voltage I guess, it doesn't seem to want to hold charge or charge to the same voltage levels it once did.
  I surmized It must have been low voltage that damaged it, it provided good voltage for a long time, when I measured the 11.4 volt unit after that cycle it was down to under 7V IIRC, and charging with trickle charger/lots of voltage check tests, showed it didn't want to take a charge.
  Available power ranged for greater than 6 months, it hadn't been tested for most of the latter part of that time, not until a voltage starved circuit sound prompted me to test it.
  I don't know...should find out what the recommended cycle is, I start charging the working unit when it drops to around 8V.

[grapefruit]

Peter,
I often work on portable PA systems. Most of them use two 12V 4.5Ah batteries in series. On one particular model there's a relay power switching circuit that cuts the power to the amp when the battery voltage gets much below 24V. I cant remember the exact voltage but I don't think it's very good to drain the battery too much.

For a portable guitar amp (or an amp for camping!) it might be better to have a CHARGE ME! LED to indicate when the battery gets low, because it's a PITA when the relay cuts out.

Often when the batteries are knackered you can hear stuff moving around inside when you shake them.

Stew.