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I'm thinking of making a portable Bluetooth speaker using the TDA7492P module. I have some speaker and tweeter combos from a cheap old 5.1 home theater system (cjc 690r).

I want to be able to connect any different set of speakers to the amplifier, not just mine. What happens if I put a 100 W speaker or more to one channel if it's rated for 50 W?

I want to power the amplifier with 18650 cells and I've been thinking of getting a 4s or a 6s protection board. Can I use 5 W speakers if I have 22 V or vice versa having 14 V with a 50 W speaker?

Is such a portable audio player even a good idea with this board?

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Loudspeaker power is a recurring question. I'm not gonna answer your question exactly, rather try to put you on the right track.

We perceive sound power on a logarithmic scale. It's the way our ears are built. This means 2x the power is not "twice as loud"... rather it is "a bit louder". However, it will still need 2x more batteries (heavier, more expensive...) and a 2x more powerful amp!

Acoustic power is expressed in dB. This is also logarithmic. Adding 10dB means 10x more power. 3dB is 2x more power (approximately).

Also, loudspeakers have efficiency/sensitivity ratings. For example, one loudspeaker might output 85dB at 1W 1m, which means if you feed it 1W then the sound pressure at 1m will be 85dB. Another speaker might do 91dB at 1W 1m, that's 6dB more, so it makes just as much noise with 1/4 the power.

So the "watts" number on speakers is half marketing. Regarding power output, it is meaningless unless you know the efficiency/sensitivity rating too.

Also, most people think they need humongous amounts of watts. This is again marketing. Put an oscilloscope on the output of your amp and check it out. In my living room, I have 92dB et 1W 1m speakers. It's a large room. When playing loud, the amp delivers sometimes 20-30W peaks, but average power is tiny, less than 1W... because 92dB is REALLY LOUD !!

For example, I sit 3m away from the speakers. At 1W power, the speaker puts out 92dB at 1m. Since 3m is 3x further, power is reduced by 3^2=9 or 9.5 dB. However there are two speakers, each using 1W, and they are correlated, so +6dB. So there's 92-9.5+6=88.5dB average on my couch.

90dB is a gas-powered lawnmower.

Most of the times, I'm listening way, way below 1W.

For portable speakers, you'll put them on a table, very close to yourself. Proximity makes it louder. With small inefficient speakers, you won't need more than 10-20W peak, 1W average. Probably much less.

So:

  • Use a class D amp, which is efficient at low power. This will extend your battery life greatly.
  • Try not to use inefficient speakers. This is a compromise, as efficiency in the bass needs size and weight. So, consider the acceptable dimensions and weight for your speaker, and pick one with reasonable sensitivity.
  • Do not expect huge bass, or extra loud sound, from tiny speakers. After all, the amount of air they can move is: (membrane surface area * displacement). A 38cm woofer with 1cm displacement moves 1134 cm3 of air, a bit more than 1 liter. A 5cm driver with 5mm displacement on a bluetooth speaker moves 6 cm3 of air. About 200x less. There is a difference...
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You may want to choose another IC as this one only output 25W+25W. http://www.st.com/en/audio-ics/tda7492p.html

  1. For class D design, once you fix the LC filter at the output; you must get back the same speaker impedance for all future use. Refer to page 17 of the datasheet. 8ohm requires L=33µH C=220 nF. If not, you may get either a boost at high frequency or roll off at 18kHz.

  2. I doubt it will be portable of you're bringing high wattage speaker moving around.

  3. 2 pieces of 18650 will get you around 19Wh. 6 = 57Wh. If your amp uses 50W, it may last you slightly longer than 1 hour.

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SPEAKER

A speaker is a mechanical element. The speaker membrane may move within certain limits and the coils can handle a limited amount of power. You would normaly chose a speaker that can handle more power than you deliver to it, to make sure you do not blow up the speaker. So if you have a 30W amplifier you should use a speaker rated with least 30W. I'd recommend to use higher ratings for the speaker (30% higher or something like this). This will give a) safety and b) driving a speaker close to its limit will distort the sound

AMPLIFIER

The maximum power an amplifier may deliver in theory is simple maths. You probably heard of

R = U / I and P = U * I One possible combination of these two is P = U²/R. U: amp supply voltage R: speaker resistance

The amps have a power rating too. Whatever value is lower (amp power rating or U²/R) will be the maximum power that could be delivered to your speakers. The amplified signal will get distorted if you get close to the maximum output power of the amp.

Q&A

Can I use 5W speakers if I have 22V

you didnt specify if the speaker is a R=2/4/8 ohms speaker. But the maximum power is (22V)²/R, what translates to 240W/120W/60W. You shouldnt use a 5W speaker here.

14V with a 50W speaker

  • (14V)² / 2 ohms = 98W
  • (14V)² / 4 ohms = 49W
  • (14V)² / 8 ohms = 25W

power the amplifier with 18650 cells

Lets assume you use 4 Ohm speakers. to deliver 50+50 W you will need U = sqrt(P*R) = 14V. This will be 4 cells in a row.

Note that you will draw about (50W*2 / 14V) = 7A current. At 2500mAh/cell you will be able to okay music for about 20 minutes for every 4 cells you use. This is not a lot.

I am actually building a speaker like this as well currently. check amazon for bluetooth connectors and speaker amps. Im using a car battery for my project.

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  • \$\begingroup\$ Thanks for this really good explination. 7A seems a lot for some dinky little 18650s. I think the protection circuit will come in at about 5A. Is there another portable power solution for a 50+50W setup? What about a 4s2p? Also I'm gonna check these speakers out and reconsider my options. \$\endgroup\$
    – user158048
    Jul 26, 2017 at 8:42
  • \$\begingroup\$ Please take peufeus info into consideration! Some really really good points in there! I, personally, use a 30W+30W+60W amplifier board (setero setup in mids/highs + sub). And boy 30W are a lot for mids/highs! A car battery is the power supply I use. Its big, heavy and may not be turned for transport/operation. It also delivers ~12V only, so you cant really reach 2x50 Watts @ 4 ohms. but it lasts quite long! I haven't done any field testing yet. But a 72Ah battery should give ~10 hours of play time at full power (that you will never reach, see peufeu's answer) \$\endgroup\$ Jul 26, 2017 at 10:48
  • \$\begingroup\$ Hold your horses, you don't drive a heater here, that 50W/7A are peak values not average load. As long as you don't enjoy sinewaves at maximum Volume, you should be able to get plenty of time out of your Batteries. Also the protection should be fine when you add a capacitor in parallel to mitigate the peaks. Anyway, no one stops you from using a 4S2P configuration and have even more runtime. \$\endgroup\$
    – wiebel
    Dec 22, 2018 at 10:45
  • \$\begingroup\$ I replaced the car battery with a motorcycle battery, weights 20kg less, may be flipped on its head without having battery acid running out. Capacity is about 12Ah and with a boost converter up to 17V (my primary goal is to use the speaker for dancing outdoors) the system can run for several hours on high volume. If i will ever build something like this again i would stick with the motorcycle battery. // I also use a small 12V stereo audio amplifier with bluetooth for my 3-way speakers at home -> totally enough for the 6 ohm speakers, i can hear the glass in my cupboard on higher volumes \$\endgroup\$ Jan 2, 2019 at 16:38
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People are talking about a different amplifier. The datasheet for the TDA7492P rev6 IC says it is a bridged class-D type. It shows a graph of supply vs output power and it produces 15W to 18W per channel with low distortion into each 8 ohm speaker when the supply is the 20V max shown on the graph. Maybe it gets too hot with a higher supply or with 4 ohm speakers. It produces 25W per channel with very high distortion into 8 ohm speakers with the 20v supply. ebay lies and says 50W per channel.

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