battery based audio amplifier

i want to built a 20 watt battery operated audio amplifier. with fully charged battery it should work for at least 4 hours. i have built amplifier using lm1875, now i want to connect battery to it. can anyone advice which battery to use, i have 6v 4.5Ah battery can that be used?

i also want to step up battery voltage to 20v for lm1875, which IC to use?

Here are raw calculations (assuming speaker impedance of 8 ohms):

20W into 8 ohms...

Yields 12.6 VAC at 1.58 A.

That's 36 V peak-to-peak. Your power supply needs to be 36V, if you want to use a single supply.

Have you considered using two LM1875, in bridge arrangement, with an 18V supply?
It will be the equivalent of one 1875 with a power supply of 36 V.

To obtain 36V (or 18V) from your 6V battery you can use a boost converter. It will be a job constructing one. At maximum power it must draw 3.3 A from the battery.

Your speaker will not be using that amount of power full time, of course. A massive smoothing capacitor may get you through brief moments of high volume.

thank you for your reply. i have built lm1875 amplifier which uses -18v GND +18v supply. tested amplifier using center tapped transformer.

i can get +18v from battery using lm2577 but how to get -ve voltage from battery?
also how to calculate in how much time battery would be discharged?

devil6600 said:

thank you for your reply. i have built lm1875 amplifier which uses -18v GND +18v supply. tested amplifier using center tapped transformer.

i can get +18v from battery using lm2577 but how to get -ve voltage from battery?

Click to expand...

You can use a buck-boost converter to obtain a negative polarity from the battery. It will give you -18V from +6V.

With the +18V coming from the LM2577, you will get the bipolar supply you plan.

The buckboost may need a different IC. (The LM2577 is described as a boost converter control IC.)

You might prefer to do it all in one circuit. Run the 6V through a full H-bridge to get AC square waves, then through a step-up transformer with center tap. (18-0-18). From there you use diodes and capacitors to obtain the +18/-18 V bipolar supply.

also how to calculate in how much time battery would be discharged?

Click to expand...

In raw math, to make the battery last 4 hrs as you plan...

You will draw 6 W average. (Since your battery is 6V, 4.5 A-hrs. That makes 27 watt-hrs.)

6W into an 8 ohm speaker is 6.9 V at .87 A. That should fill a room. The peaks may or may not extend to the rails of a +18/-18 supply.

Now if you decide to draw 12 W average, then your battery will last 2 hrs.
Then, 12 W into an 8 ohm speaker is 9.8 V at 1.2 A.

Or if you decide to draw 24 W average, your battery will last 1 hr.
24 W into an 8 ohm speaker is 13.9 V at 1.7 A.

It all depends on (a) how much volume you want from the speaker, and (b) how long you will operate it, and (c) how much distortion you can tolerate, and (c) how much power the battery can provide.

devil6600 said:

thank you for your reply. i have built lm1875 amplifier which uses -18v GND +18v supply. tested amplifier using center tapped transformer.

i can get +18v from battery using lm2577 but how to get -ve voltage from battery?

Click to expand...

You use two batteries in series. The middle point is your system 0V.

Do you need 20W? Will two x 15W be ok? If so, look on ebay for TA2024 amplifier. It will develop 2 x 15W in 4 ohms with a single 12V supply.

I have one put into a small metal case with a three section LiPo battery (11.1V). Very impressive.