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I want to design a car audio amplifier. I am going to use 4 x Amplifier ICs like TDA7294(Datasheet) for 4 Channels. The problem here is that with 12v input voltage these ICs output maximum 20W. I am thinking of using step-up converter to boost the voltage to 20-30v to power the amplifiers. Will there be any problems going that way? If yes what are other solutions?

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  • \$\begingroup\$ You have to filter the DC very well not to have any noise from the step-up in it. It may not only be hearable but also make the amplifier swing with/against it and overheat (or limit the output power.) But, short note: 4x20W are enough for any car. Even when your exhaust is broken. If you still need more, the usual, and much simpler way is to configure the outputs into a bridge configuration, with the speaker between two outputs. That way you also get rid of the output capacitator. \$\endgroup\$ – Janka Apr 9 '17 at 18:03
  • \$\begingroup\$ My car's speakers are rated at 60W RMS and the rear speakers at over 80W RMS. I would like to make most of them using that kind of circuits. Are there other solutions if not with boost converters? \$\endgroup\$ – Christos Mitsis Apr 9 '17 at 18:06
  • \$\begingroup\$ Oh, and about the bridge configuration, see page 13 of the datasheet. \$\endgroup\$ – Janka Apr 9 '17 at 18:09
  • \$\begingroup\$ @ChristosMitsis your speakers being rated for a power doesn't mean you "make the most of them" only at that power. Instead, you can be quite sure that it'd be the most power you can put into the chassis' without gross misbehaviour. Louder doesn't always sound better – and that especially applies to speakers in small rooms \$\endgroup\$ – Marcus Müller Apr 9 '17 at 18:16
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    \$\begingroup\$ for example, datasheet p7 fig13 says that for 20W Pout, you'd very likely get more than 20W of waste heat. That's 80W for four of these amplifiers. That is a very bright incandescent light bulb heating the inside your amplifier, and you need to get that heat out before you turn the amplifiers to toast. \$\endgroup\$ – Marcus Müller Apr 9 '17 at 18:23
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I'll go for it: Of course it is possible to design a step-up converter; let's, however, give the numbers a quick spin:

  • Max assumable audio power that you (see comments) aim for: 2·80 W + 2·60 W = 300 W
  • Class AB wild guess efficiency estimate: 50%
  • Necessary output power of your converter: 600W
  • Cost of that converter, size, cooling, weight: meeeeeep, not desirable.

In fact, whereas websites like TI.com usually have a good calculator for supply designs, a 12V in, 24 V @25A (==600W) out design is nothing they can offer - you'll have to design your own regulator for that, with beeefy switches. Oh, and make sure you have a > 50A capable fuse and cabling to that step-up converter!

I agree with Jaka, 20W per speaker should really be plenty for something that is designed to make it loud on the inside of the car, not the inside of a whole club. If you really aim for powers higher than that, I'd go for one of the hundreds modern amplifier chips that were designed for exactly the automotive market, and work directly of the 10 V – 14 V input range, at high efficiencies.

Buying a single quad-channel Class D amplifier IC and redesigning your amplifier will be cheaper, less hot, easier to design & implement, more automotive-safe (big step-up converter coils do not like being subject to constant motor-induced vibrations) and smaller than a design based on four Class AB TDA7294.

It really depends, but for a circuit that comes close to 95% efficiency (like, let's say, the TPA3245) under >10W loads, using a step up-converter becomes easier, because a) you don't have to provide power just for it to be converted to heat and b) because the class D architecture itself is pretty much immune to switching noise (as the output stage is needed to filter the class D switching noise, anyway). Still, I'd avoid it and, worst case, use four TPA3245 (or similar), one for each speaker, at raw battery voltage.

Most of these ICs can be used in a configuration where the FET output stages can be paralleled before the LC filters. For examples, see TPA3245 datasheet.

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