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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 16th November 2017, 06:46 PM   #11
ICG is offline ICG  Germany
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Quote:
Originally Posted by daveError View Post
The speakers I'm using are only rated at 250W, but I read somewhere that you should get an amp that can drive like one and a half times what your speakers are rated, hence a 400-ish watt amp. Something about it being easier for the amp to drive that way. Better to have slight overkill than underpower, right?
Yes, that's right, better to have more power than driving into clipping. But you are saying speakers, which would imply you want to use the amplifier in stereo (2 channel) mode. That would mean you won't get that power figure, the TDA8954 can only deliver 2x210W and that is at +/-41V and - this is also very important - at 4 Ohm. If you have a typical PA speaker, it will be likely 8 Ohm, which halves the power figure.

Assuming the 8 Ohm speakers, that gives following options:
  • Buy a 2nd amp and use both in bridged mode, which gives you the power you're aiming at with a voltage of ca. +/-36V. 3 12V batteries for each voltage rail are needed, that will be bulky, heavy, expensive, depending on what batteries you'll buy.
  • Buy or build 4 Ohm speakers, they will draw more power from the amp. You need to use around 40V on each rail, which might require to build your own special batteries from single cells. The loading electronics, protection (overload, deep discharge) and capacity will be likely a challenge. I'd say that's a lot of effort for the power, remember you need two of each (because of the two rail voltages).
  • Buy or build a different amplifier which only needs one rail voltage, which is much easier and cheaper to realize. Generally such a system is not recommended to be built with an amp that needs symmetrical voltages.

Quote:
Originally Posted by daveError View Post
Anyway, I know the amp chip can run on 12v, so would it be worth it, just for testing, to take, suppose, two of the cheap-cycle(tm) batteries wired in series as suggested, provide the amp with +/- 12v by wiring them in series, and just see what happens?
The datasheet (PDF) says it starts working at 12,5V, in reality it might start at a bit different voltage. The usual car batteries provide more than 12V fully charged but you cannot use the full capacity of it. And it will be a whole lot less power, probably around 20-30W/4 Ohm, half of that on 8 Ohm. If you have the batteries laying around, go for it. If you have to buy them, I'd scratch that though.

Maybe you could describe your speakers and the use you have in mind? That would help a lot to search for a better solution. I mean, open air or a room, do you have to carry it or drive with a bike, car etc? How loud does it have to be? How many listeners will be there? Background music? Party-level? Disco?
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Old 16th November 2017, 11:35 PM   #12
kodabmx is offline kodabmx  Canada
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Get a big 12V battery or connect a bunch of 18650 cells and use one of these...


HP-6 Car Amplifier / 12V Boost Converter Board / Switching Power Supply Board | eBay
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Old 17th November 2017, 01:24 AM   #13
ICG is offline ICG  Germany
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That DC-DC converter is really tempting.

They got 4 different versions:

Quote:
Model Name Mains Outputs the output voltage on the car
Type A 2 groups ± 66V (12V input) ± 77V (14V input)
B type 2 group ± 42V (12V input) ± 49V (14V input)
C type 2 group ± 30V (12V input) ± 35V (14V input)
D type 2 group ± 21V (12V input) ± 25V (14V input)
Type B is too much, the converter is not regulated, it would exceed the maximum voltage of the TDA8954 (+/-42.5 V), type C would provide usable voltages. Unregulated usually means higher ripple. I don't know how much ripple and noise it brings or how the amp reacts on it.

At +/-35V of the type C one the TDA8954 got 2x150W/4 Ohm or 75 on 8 Ohm, which seems a bit low. Adding a second amp and bridging both would bring the output up to 300W/8 ohm but that's way more than one of the converters can handle, a second 12V Boost Converter is then needed too.

It's getting quite expensive, 50 bucks + customs (I assume, don't know about that in the US or additional taxes) and two amps, that's getting close to 200 bucks and there's no enclosure, wires, plugs and connectors in it and neither are the battery(s).
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Old 17th November 2017, 04:06 PM   #14
kodabmx is offline kodabmx  Canada
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Yup. And you just might find a class D car amplifier on sale that costs less than this build. But what fun is that?


As far as ripple I wouldn't worry about it. It will be in the 10's - 100's of kHz and easy to filter with a small LC or RC filter.


There's also this kind: +-48V Audiophile Digital Power Amplifier Board 350Wx2 IRS2092 HIFI 2Ch Class D AMP | eBay

Using that, and the boost module would cost $125CAD not including the large deep cycle SLA...




And an actual car amp. The 3000W rating is horses*it since 12VX70A=840W but it gives you an idea...



Boss Armor AR3000D 3000 Watt Mono Car Audio Class D Power Amplifier Amp + Remote | eBay

Last edited by kodabmx; 17th November 2017 at 04:14 PM.
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Old 19th November 2017, 01:54 AM   #15
PRR is offline PRR  United States
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Quote:
Originally Posted by ICG View Post
No, your calculation is way off....
I have no idea which of my calculations is "way off"; or how bridging matters.

He thinks he wants two channels at 400 Watts. Ignoring efficiency, this is 800 Watts demand at maximum output. For two-12V and two-24V options (assuming that voltage suits the amp, whatever the amp is), this must be at-least:

800W/(12V+12V) = 800W/24V = 33 Amperes
800W/(24V+24V) = 800W/48V = 17 Amperes

These will be VERY heavy drains on any battery I would call "mobile".

I just got a 1000W inverter to turn 12VDC into 120VAC (they must exist for 240V lands). I can run power-tools from my car/truck. Assuming I do not saw/drill steady, the 80 Amp peaks are covered by the 50AH battery so the car's 40A alternator about keeps-up. I suspect this thing will comfortably power a 2*200W audio amplifier (plain wall-plug, not car-sound or bare-board). The 3dB "short" should not ruin the gig. Larger inverters are available, but unless duty cycle is low (in speech/music it may be), might be too much for many car's electrical systems.
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Old 2nd December 2017, 06:44 AM   #16
ICG is offline ICG  Germany
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Originally Posted by PRR View Post
I have no idea which of my calculations is "way off"; or how bridging matters.

He thinks he wants two channels at 400 Watts. Ignoring efficiency, this is 800 Watts demand at maximum output. For two-12V and two-24V options (assuming that voltage suits the amp, whatever the amp is), this must be at-least:

800W/(12V+12V) = 800W/24V = 33 Amperes
800W/(24V+24V) = 800W/48V = 17 Amperes

These will be VERY heavy drains on any battery I would call "mobile".
Well, firstly, he only got ONE amplifier. So, no 800W, only 400. But it's not even close to that either because..

..Secondly, you don't seem to understand that the rail supply voltage determines how much power the amplifier can put through. You cannot simply divide/multiply the maximum power ratings to get the actual power output. That is because Ohm's law, it still applies. On lower than maximum rail voltages you get - surprise - lower power output. Look at the available voltages: 12V, 24V, 36V and 48V, via multiple batteries or the (unregulated) DC converter. Only two of these voltages are effectively usable and therefore the amp will have much lower power already. Look at the datasheet, it will tell you how much output you can expect at which voltages. (or look at my former post)

Thirdly, the output of the amplifier is also dependent on the impedance. Maximum output specs are only reached at the specified impedance. He got higher impedance, so there will not be much power left at the output. How much? Read my post up there again.

So there we determined your calculation is off by at least factor 4 to >20. And yes, that is what I'd call 'way off'.

Quote:
Originally Posted by PRR View Post
I just got a 1000W inverter to turn 12VDC into 120VAC (they must exist for 240V lands). I can run power-tools [...] I suspect this thing will comfortably power a 2*200W audio amplifier (plain wall-plug, not car-sound or bare-board). The 3dB "short" should not ruin the gig. Larger inverters are available, but unless duty cycle is low (in speech/music it may be), might be too much for many car's electrical systems.
That does not help. Because the amp runs neither with 220 nor 112V AC, it runs with symmetrical (+/-) rail voltages. So you have to add another power supply to work with the 110 or 220/240V AC and provides the needed rail voltages. Cheap non-regulated inverters work fine with crude power tools which draw a mostly steady current, the huge voltage variation with of their output with current fluctuations makes it very difficult for a lot of amplifiers to cope with.

It would be an option though to use such an inverter and use a cheap PA amplifier (i.e. an used iNuke 3000). That might be a solution for the situation but it's not a solution for the TDA8954 amplifier though - which means, he can't use anything he got so far.
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Old 2nd December 2017, 03:04 PM   #17
kodabmx is offline kodabmx  Canada
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Cheap non-regulated inverters don't like reactive loading. They rarely work with anything that isn't powered by a SMPS. If he's running a power tool I suspect it's at least stepped sine, if not pure sine.

In any case trying to get 800W from 12V is going to be a challenge. Even if it was 100% efficient, that would pull 67A from a 12V source.

That means the average 12V car battery would be dead within an hour.


As mentioned a gas generator would be a better solution, as would be using standard AC powered amplifiers.

Last edited by kodabmx; 2nd December 2017 at 03:07 PM.
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Old 2nd December 2017, 07:15 PM   #18
ICG is offline ICG  Germany
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Originally Posted by kodabmx View Post
In any case trying to get 800W from 12V is going to be a challenge. Even if it was 100% efficient, that would pull 67A from a 12V source.
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.

Quote:
Originally Posted by kodabmx View Post
That means the average 12V car battery would be dead within an hour.
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.

Quote:
Originally Posted by kodabmx View Post
As mentioned a gas generator would be a better solution, as would be using standard AC powered amplifiers.
Yes, I agree on that. Especally the run time is not an issue anymore with a generator. The question is if a generator is acceptable in weight, dimensions and noise. Without some feedback of the thread starter it's only guessing. If a generator is okay I presume a PA amplifier would also be okay and in that case I'd just get a used PA amp. That, ofcourse, is not a solution for this amp. On the other hand, it would be most likely cheaper and better power-wise and in terms of safety and relyability.
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Old 3rd December 2017, 05:05 AM   #19
PRR is offline PRR  United States
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Originally Posted by ICG View Post
......you don't seem to understand that the rail supply voltage determines how much power the amplifier can put through.... . ... ....
I am an illiterate ignoramus. I am sorry to bother you.
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Old 3rd December 2017, 03:02 PM   #20
ICG is offline ICG  Germany
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I am an illiterate ignoramus. I am sorry to bother you.
It's never too late to learn something.
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