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How to drive TDA8954 from batteries?  (WARNING: Long and boring backstory.)
How to drive TDA8954 from batteries?  (WARNING: Long and boring backstory.)
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Old 4th December 2017, 01:46 PM   #21
kodabmx is offline kodabmx  Canada
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Quote:
Originally Posted by ICG View Post
I already wrote it twice, it isn't 800W! At 36V with 8 Ohm speakers it's 200W peak with both channels driven and losses, which means around 50W rms.

Even if it was 800W, that would be peak only. And that means, it would last at least 4 times longer with music of 6dB crest factor, IF it's all the time full power.

36V / 8R = 4.5A = 162W continuous. Both channels driven 324W continuous = 650W peak. Less than 800W continuous but still a lot of power to source from a battery.

I measure peak as 2X continuous power. Anything more is PMPO (pretty meaningless power output). I also measure power output driven at 30Hz full power sine.

Also, without a boost converter, you won't be able to use the full capacity of a battery. By the time you get to 50% discharge the voltage is only 11V or so. With the above load you still get about 1 hour run time from a 60Ah battery, assuming 90%+ effiency from the power supply.

Last edited by kodabmx; 4th December 2017 at 01:57 PM.
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Old 4th December 2017, 09:23 PM   #22
infinia is offline infinia  United States
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Re battery powered PA
After a long and expensive journey (probably not boring too bad they rarely report back on their mistakes along the way) youll end up easily carrying a couple of these.
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Old 5th December 2017, 11:52 AM   #23
ICG is offline ICG  Germany
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Location: I had a Déjà Moo - I've seen that BS before!
Quote:
Originally Posted by kodabmx View Post
36V / 8R = 4.5A = 162W continuous. Both channels driven 324W continuous = 650W peak. Less than 800W continuous but still a lot of power to source from a battery.

I measure peak as 2X continuous power. Anything more is PMPO (pretty meaningless power output). I also measure power output driven at 30Hz full power sine.
I'm honestly sorry but that's wrong on so many levels. The amp is not able to deliver 650W at all! Look at the datasheet (PDF), it's way below that figure! 420W on 4 ohm (or 8 ohm bridged) is the maximum and you reach that only with the maximum supply voltage of 41V. Just because there is one Voltage and one Impedane does not mean you can simply multiply or divide it and your result is correct - the amp got a limitation on how much current it can deliver. Besides that (and this is much more important!), the 36V are for both halfwaves, you can't just ignore it and only listen to the half of it or only amplify half of it, you need both, positive and negative side, which means it got a voltage swing which is only the half of that! Yes, repeat that calculation with 18V. You end up nowhere near your numbers, I won't calculate that again for you, do the math yourself. And to assume what you are playing got a crest factor of only 3dB means you'd have compressed-over-every-edge noise which would be unbearable. 6dB crest factor is already compressed very much. You can calculate the rms and peak power of an amp that way but that's no base for an actual power use or runtime calculation.

Quote:
Originally Posted by kodabmx View Post
Also, without a boost converter, you won't be able to use the full capacity of a battery. By the time you get to 50% discharge the voltage is only 11V or so. With the above load you still get about 1 hour run time from a 60Ah battery, assuming 90%+ effiency from the power supply.
That is ofcourse completely correct, I'm glad we have some points we can agree upon.
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Last edited by ICG; 5th December 2017 at 11:55 AM.
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Old 5th December 2017, 04:00 PM   #24
kodabmx is offline kodabmx  Canada
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I was calculating the maximum theoretical power. Ohm's law doesn't lie. I misunderstood that you mean the amp running from +-36V rails, not outputting 36V into the load.


The datasheet doesn't specify if the rated power is continuous or peak. I'll assume it's peak. It also doesn't give a SE spec for 8R load, so I assumed you are going to use BTL mode. That's 840W output for a stereo pair. Regardless, powering this for any significant period of time from a battery will be struggles.
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