Portable 12V for an audio amplifier- ideas?

[echelon]

I wonder if any of the kind members of this forum could direct me to a circuit that would allow me to run a cheap/basic small guitar amplifier from a 12V lead-acid battery, in addition to retaining the ability to run from mains when not being carried about?

(Equally, if this isn't an appropriate place/time for me to ask this, please accept my apologies, and ignore appropriately).

That aside: presumably this exact type of requirement has been thought about many times by others, including people here, and tried/tested solutions have already been applied.

I'm suspecting that a circuit using the LM317 might be a good basis to start from, but I don't really mind what components might be recommended.

My overall plans for this included:

- the observation that there's plenty of space in the cabinet for a small (say 7Ah) lead-acid battery;

- that the amp chassis has plenty of space for an IEC shrouded plug, to replace the captive mains lead and strain relief, for operation in temporary portable mode;

- the hope that the existing 12-0-12 mains transformer would be sufficient to supply enough voltage/current to an added charging circuit

So: would a relay be too old-school, from the perspective of toggling the DC source supply (I was thinking of having it such that AC-off would result in the hold-on coil de-energising as default, and therefore the battery-only option wouldn't be wasting current on the relay). Unless this could all be done with steering diodes? Or just a cunning combination of DPCO switches?

And trickle charging...also needed to be constant-voltage, for a lead-acid?

As I said at para2, if this isn't something I shouldn't be asking about, I do apologise- it's just that I do trust the excellent experience and opinion of the members of this brilliant forum.

[Herald1360]

First question:

What DC rails does the amplifier actually run from? Only 12V would suggest an output limited to a few watts.

If it runs from >12V you will need some sort of DC-DC converter, not just a regulator which can only output at most 2V less than its input voltage.

You may be better using a 12V to mains inverter, but a 7AH battery would not last very long if the amp delivers more than about 5W output. A large leisure (caravan) type battery (60-90AH) might be more appropriate.

[julie_m]

One way would be to use an inverter. They aren't very expensive, Maplin often have them on special offer. Note that some amplifiers are (acoustically) noisy when running from an inverter; it's not harmful, but could be annoying. I built an inverter into a battery carrier, which was soon christened "the cordless extension lead"!

Another way would be to design and build a specialised battery amp. Two TDA2030s bridged and driving each of a pair of 4 ohm speakers can make a surprising amount of sound. The idea is, the speaker sits between the outputs of the two amplifiers, both at 6 V under no signal. When one is driving towards the + rail, the other is driving an equal amount towards the - rail. So you can get effectively double the voltage swing (or almost, anyway, because you've got twice the output impedance in series with each speaker).

Lead-acid batteries are easy to charge. Just don't give sealed ones too much current (10 hours' worth is plenty). Vented ones are even more forgiving; any excess energy goes into separating water into hydrogen and oxygen, and you just top them up when the cells start looking dry.

[echelon]

Will check and confirm the suppy rail voltages. I don't think it's actually running from a symmetrical supply, and yes, it is only a few watts.

It is a small guitar practice amplifier/speaker combination, used by my son.

Previous to this idea (in a moment of lazyness) I'd bought from Maplins a small amplified speaker, which runs ineffectively from a PP3/6F22, but it is a bit of a plastic montrosity as far as regards it's sound. Unsurprisingly it has no bass capabilities, and resonates like a tin mug. I suspect it'll have something like an LM386 in it.

The type of wooden-cased 'combo amplifier' that I'm thinking of trying to modify has much more presence and depth of tone, and is much better in every respect than the 9V wonder/blunder. The intention would be for this modified-to-portable to be able to run for only a limited duration from battery, not for hours on end.

Thanks for the response so far- I do take the point re the voltage drop via the regulator, and the idea regarding the invertor is interesting...reminds me of an oscillator I made 20yrs ago to replace the 15V HT battery in my trusty old Avo 8 for the higher Ohms ranges, it used the main 1.5V D-cell (what did we call them before American lables took over? Was it HP2? Because I remember HP7s were AAs, and HP11s were C-cells. Or am I getting confused?)

[edit]Oh, and I can get hold of bigger Ah batteries without any problems, up to the max physical space available, if required[/edit]

[Silicon]

My first thought was a bridge mode amplifier. The Sanyo LA4440 ic contains two amplifier stages and the datasheet shows a simple bridge circuit.

The quiescent current is about 100mA which seems high for battery operation.
If you are going to distort the guitar sound with fuzz, etc then an amplifier with a lower quiescent current (and therefore crossover distortion) might be acceptable.

[Refugee]

Measure it with a volt meter on mains and get back to us.
There might be a simple solution with an SLA battery.

[Ed_Dinning]

Hi, a few simple thoughts and calculations first.

If the amp supply rail is 12v or pretty close, and there is no negative rail, then feed it directly from the battery through suitable fuses and polarised connectors. Note that if you use blocking diodes then there will be a 1v loss of voltage.
A protected relay/ diode combination will be better, or a straight "crowbar" diode.

What power do you want to run?
If it is 5W from the amp then you will need about 15W input power without further inefficiencies from inverters etc.

15W @ 12V is 1.25A, so a 7Ah battery will theoretically last for 5.6 hours.
Bearing in mind that average music output will be less than the peak of 5W it should last longer.

Ed

Note that if you use blocking diodes then there will be a 1v loss of voltage.

Shottkeys would drop a lot less.

[OscarFoxtrot]

depending on where it's to be used, using an inverter - which generates mains voltage - may be a safety issue.

[Boom]

Originally Posted by merlinmaxwell

Shottkeys would drop a lot less.

Why not just put a diode across the power rail with a fuse in series to the supply? Zero voltage loss then.

[dave cox]

If you need more that 12V DC, then a DC-DC (buck) convertor will be the best option and will be much more efficient (>90% is possible) than an invertor supplying 240V ...

dc

[Mike Brett]

If you can get hold of a copy there is a complete circuit and build instructions for a portable power supply in the book, Practical Projects by Dr George Brown. I built this power supply over four years ago and has served me very well on camping trips. It has an internal charger and uses an LM723 and a 2N3055. Copyright laws prevent me from printing the circuit but I can thoroughly recommend this circuit.
Mike