converting small practice amp to battery power

[flotrain]

Hello!

Is it possible to convert a low wattage mini amp like the Marshall MG10CD (10 watt) to battery power?  I don't mean a car battery and inverter-- I mean bypassing the main wall power and supplying the correct operating power directly to the circuit board from a rechargeable battery pack.  What is range of internal voltage through the circuit of small ss amps like this?  Cheers and thanks for the help!   8|

[Roly]

Please locate the circuit for your amp and post it here, then we can all see what might need to be done.

[Enzo]

ANything is possible.   But even as simple as this little amp is, it still runs on split supplies.  That is not straightforward from a single battery pack.  You could use a pair of battery packs or make up a dual voltage pack.

The preamp runs on a pair of 15v supplies, while the power amp IC uses whatever the main voltage is, probably about 20v.  You could change it to run the entire thing off 15v, accepting that the power output would be lessened.   That way you could eliminate just about the entire power supply circuit.

DO a search on this forum for the MG10CD, the attachment control tells me the file is already here somewhere.    Otherwise I'll have to save a renamed file to post.

[armstrom]

http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG10CD.pdf

There's the schematic. As was already said, this amp is not a good candidate for battery conversion due to the split rails. You could possibly get away with a pair of large Lithium Ion cells (like those used in RC vehicles) but you would need to take care where you inject the voltage. Or just use an inverter

[flotrain]

Thanks Roly, Enzo and Armstrong!  Question answered  :dbtu:

[flotrain]

Hello armstrong and Enzo.  I'm considering Enzo's idea about running the whole thing off 15 volt rechargeable (I don't need full output power for my purposes).
Would this battery work?  How much "play time" could I expect?  Here are the battery specs.. again thanks much for your help! 

Specifications:

    * Li-Ion Size: 4 x 18650 (cylindrical)
    * Capacity: 2600 mAh
    * Chemistry: Lithium Ion (Li-Ion)
    * Working Voltage: 14.8v
    * Peak Voltage: 16.8v
    * Cut off voltage: 11V
    * Max Charging Current: 2 Amp(1C)
    * Max Discharging Current: 5 Amp (2.5c)
    * Included Qty: 1


Dimensions and Weight

    * Length: 2.9"
    * Width: 2.5"
    * Height: 0.8"
    * Weight: 6 Oz


Features:

    * No memory effect
    * Longer storage life then NiMH Batteries
    * Light weight and higher energy density than any rechargeable battery
    * Installed IC chip will prevent the battery pack from over charge and over discharge. It helps protect battery chemistry integrity and prolongs battery life.
    * Installed polyswitch will cut off power if battery discharging current more than 5 Amp.
    * Perfect for building up battery pack for RC air, car, and robot which requires compact size and lighter weight
    * The battery module is wrap by PVC shrink and with 20 G wire

[armstrom]

Keep in mind you will need two batteries wired up as a dual rail voltage supply. In other words, you connect the negative of one battery to the positive of another to give you a full 30V potential difference (29.6 with the batteries you listed) Then you tap your ground connection into that connection between the battery terminals. That gives you +14.6V from one battery and -14.6V from the other (relative to your chosen ground). One thing to keep in mind is that 2600mAh is not much capacity. It might do fine if you keep the volume down but I wouldn't count on more than an hour or two of run time. If that. 2600mAh = 2.6 Amps of current continuously for 1 hour. I'm not sure how efficient the TDA2030 is but it could easily draw 2.5-3A of current when playing loud. The rest of the circuit has current draw as well but that power amp is your biggest contributor by far. You might want to look for some larger batteries if you don't want to have to recharge it all the time.

Speaking of recharging, you need to keep in mind that any Lithium Ion batteries you buy will require a special  charger. Since you're going to need to charge two batteries at once you should look into special chargers for RC batteries. This will allow you to get faster charge times as well as take precautions to ensure the batteries are charged safely and maintain a long life. NEVER charge Li-Ion batteries with a charger that isn't designed for them.

[armstrom]

Just as food for thought. Why not just build yourself a small practice amp? In the long run it would be cheaper and far more rewarding. Several years back I built a practice amp called the Hurricane (it won an award on this very site! ) The whole amp ran off a single 12V supply and could put out about 15W. It had a nice 6" weber speaker in it and sounded great. You can get all the details here: http://www.ssguitar.com/index.php?topic=1237.msg7705#msg7705

It was a fun project. The power amp cost a whopping $10 in kit form and I assembled it in less than an hour. I built the preamp from a proven design using parts from radio shack. The speaker was probably the most expensive part of that build. You could search your local pawn shops to find a small practice amp that you're willing to gut (assuming you don't want to gut your Marshall ) and reuse the cabinet and speaker.

Just putting it out there.
-Matt

[Kaz Kylheku]

Here is a Texas Instrument paper about obtaining a +/- 12 dual polarity supply from a 5V DC input, using the UCC3803.

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua288&fileType=pdf

Building the circuit described in this paper might not exactly be a good beginner project, though.

It should be straightforward to modify it to +/15V rails and a different input voltage to match the available battery.

[Enzo]

That sort of approach might work, but the example in the paper only provides 100ma output current.

[flotrain]

Thanks for all the info and insight Matt... the Hurricane is cool!  I'm curious about the "Big Muff tone control".  Do you mean you used the same circuit for the tone control as the tone control on a Big Muff π pedal?  I put together a Ruby type amp in a box with a 5" speaker- 9 volt battery powered.. just using plans from the web.  That amp sounds great but if I can get a little more output plus gain/ tone control like the little Marshall I'm messing with that would be sweet!   Battery power is essential for my purpose... that's why I'm trying to learn what is possible with rechargeable batteries.  Any other thoughts/ suggestions much appreciated... 
Joe

... just saw your posts Kaz and Enzo—  thanks!  yes that looks a bit out of my league... beginner.

[Roly]

Just to reinforce the point about treating Li-Ion batteries properly.  All batteries tend to be a multi-media hazard because they store energy in somewhat unfriendly chemical reactions, but the Lithium family have a particularly nasty bite if mistreated - they can go off like incendiary devices.  Search youTube and you will find many examples.


The most common way to get more power from an amp is to use higher voltage rails, but it's not the only way.

In cars there is a practical limitation of the car battery voltage.  One way around this is to use lower impedance speakers, 4 and 2 ohms to get more power from the same voltage swing.

But if you drive both ends of your loudspeaker in opposite directions you effectively double the available drive voltage, and this is called "bridge connection".  A stereo chip amp can be employed driven by a phase inverter to provide more power output than is normally available on a limited supply voltage.

There is no such thing as a free lunch - every watt out has to come from somewhere, and all things considered you are lucky if you get a watt out for every two the amp consumes.  The only way to win is to use a speaker and enclosure that is as efficient as possible.

Batteries are normally rated in amp/hours, for example a battery rated at 20A/Hr will deliver 1 amp for 20 hours, 2 amps for 10 hours, or 20 amps for one hour.  (Strictly speaking this isn't true, the higher the current the less efficient the battery becomes, but you get the idea)

If we know the voltage of the battery, say 12, then we can get an idea of the energy storage in terms of watt/hours; so a 20A/Hr 12 volts battery nominally stores 12 * 20 = 240 watt/Hrs; again, the faster you use it, the less you get.

Batteries are roughly 2/3rds efficient - you have to put 50% more in recharging than you'll get back, and again, the faster you do it the worse it gets.  Typical figures are 14 hour charge for 10 hour discharge.

I'm with @armstrom - I think you would be better off gutting an old amp and building something specific to suit your needs in the case.

HTH

[armstrom]

Another benefit to building an amp based off a single 12V rail is that you can use cheap sealed lead acid batteries to power it. These are VERY easy to charge, cheap, and available in many different sizes. They are pretty much small car batteries. Yes, they're heavy but that's the penalty for lots of capacity with older (but cheaper and easier to implement) technology. The batteries used in computer UPS systems or home alarm system backups would work perfectly. Just search amazon for "sealed lead acid" and you will find a ton of options.

Another thing I failed to mention is that you have to also be careful when discharging Li-Ion batteries. If you drain too much power from them you can damage the battery such that it will no longer take a charge. This means you either have to build a sophisticated power management system that keeps track of the current into and out of the battery and shuts your amp down when it thinks the battery is depleted, or you have to just try to play it safe. "playing it safe" is what RC helicopter/plane pilots do. They know how long they can typically fly on a fully charged battery so they set an egg timer and clip it to their controller. When the time is up they stop flying because continuing to do so may damage the battery.

[J M Fahey]

Just a couple practical examples, of my "Callejero" amps, powered by 12V 7AH Gel Cell Batteries.
1) Biamplified 30W (TDA2005 + TDA2003) into a 10" speaker and a LeSon Tweeter. *VERY* loud and clean



Same:



That said, I have converted lots of amps like yours by using *two* 12V Gell Cells, to have the dual rail; only to make the weight bearable I used the smaller 4.5AH ones.
They are also available in 2.4 and 1.2AH sizes, although surprisingly they are more expensive.
So if you want to go the least effort route, that's one way.

[Kaz Kylheku]

That sort of approach might work, but the example in the paper only provides 100ma output current.

I didn't even notice that. That is pitiful compared even to linear regulators like 78XX with no heatsink on them. Switching regulators are supposed to provide more current, since the pass transistor is fully on or off.

Maybe the authors were just going after "proof of concept" to show the establishment of voltage, and so they didn't use "beefy" external components.

In any case, going to read that thing in more detail; it's interesting.