Power supply question

[SurreyNick]

I want to build a 12V battery-powered amp employing a TDA2005 and would like some advice about the power supply.

I propose using a Ritar RT1250 12v 5.0Ah SLA battery to power the amp but I'd also like the option to run the amp from the mains using an AC-DC adapter.  I'd also like to be able to re-charge the battery without having to remove it from the amp cabinet.

I'm guessing it would be inadvisable to draw power from the adapter via the battery i.e. connect the battery in series with the adapter because the current would eventually damage the battery, so what would be the simplest way to achieve my goals?

[Enzo]

Use the battery at all times, but you can connect the charger while using the amp to restore the battery charge.  In other words, you don't need to use an adaptor that can power the whole amp during operation.   Think of the battery as more like a giant filter cap.

[SurreyNick]

I see, and it's an attractive idea, but won't this just slightly extend the time the amp will be operable?

I estimate the amp will draw 2.24 amps with a 20W 4 ohm speaker, giving about 2 hours use on battery only.  The charger I have is rated at 600mA 0.6A so I'm assuming it won't provide enough current to power the amp in operation or keep the battery topped up.

Am I right?

[Enzo]

Put an ammeter between amp and battery and watch the actual power consumption.  A 20 watt amplifier does not sit there making 20 watts steadily.  It makes the 20 watts for the instant of each kick drum hit, or maybe a bass note pluck.  But mostly it uses a lot less.  Like your 100 horse power car.  It puts out 100hp when stomping on the gas pedal to accelerate, but driving down the interstate at 70mph uses 10-12 hp.  Ever notice the highway speed gas mileage is so much better than in town driving?  No accelerating from zero on the interstate.


Likewise, your power adaptor has to handle the whole 20 watts it it is alone, and most will choke trying to do that.  Every kick drum hit will gag the poor amp.  The battery will sit there slowly charging, while keeping up the power level for those every so often short peaks.

I am assuming you want to play music with this, rather than a steady sine wave.

I know this from the hard way.  I was "repairing" a Crate Taxi battery powered amp.  I assumed it could run on adaptor alone, so I spent hours trying to "fix" the voltage regulator and charging circuits for voltage drop.  I discovered the adaptor was never made to work that way, the battery was required.  The adaptor of course can charge the battery as it sits, and if used while playing will keep the battery charged up.  You will spend less time purely charging than if you used no adaptor while playing.

Now, if you want to run without the battery, you need a real 12v power supply, one capable of powering the whole 20 watt amp needs.  A little adaptor won't handle it.

You won't need to remove the battery from the amp, it can charge from your external supply however you decide to do it.

Just for reference, I attach a Crate schematic, battery connects to J5, and power switch across J3.

[SurreyNick]

Ah, that makes sense.

Yes, it's a guitar amp I'd like to construct using up bits and pieces I have kicking around.  My initial idea is a mono guitar amp employing a TDA2005 20W bridged amplifier with a pre-amp stage based on the MXR Distortion Plus using a RC4558, driving a single Jensen Chicago 8/20 MOD Speaker (8 inch, 4 ohm, 20 watts) and the whole thing powered by a Ritar RT1250 12v 5.0Ah SLA battery.

By and large 2 hours playing time will be enough anyway, so I like your proposal

[g1]

Those batteries are pretty cheap.  For $10 more you can get a 9AH which is only 2.5 inches longer and will almost double your playing time.

[SurreyNick]

Definitely an option.  I have the 5Ah already so I'll start off with that, but taking your suggestion I'll build the cabinet so that if the amp works well I can upgrade to the 9Ah in the future.
Thanks

[Enzo]

Or even a pair of the original battery type.