Hey guys. I'm trying to decide on batteries to power a TPA3118 in PBTL mono mode. I'm looking at THESE ones from Sanwu on ebay. It says 1x 60w. My plan is to supply something like 12 volts to the amp and drive an 8ohm speaker. I found a little calculator online (since i don't know the math myself), and if that is correct, that should put me somewhere around 18 watts?
I'm thinking Li-Ion batteries, but deciding on the quantity is difficult without an actual understanding of the math involved. Is there a way to estimate how long this amp can run (at a decently high volume) based on the battery pack's mAh rating?
If the speaker is an important part of this calculation: I have't decided on one. It will be a 5" or 5.25" round, 8 ohms, and probably a "full-range" type if i can get away with it. Does the speaker wattage rating make much of a difference so long as it is over what the amp can supply?
Thanks for any advice you can offer!
I'm thinking Li-Ion batteries, but deciding on the quantity is difficult without an actual understanding of the math involved. Is there a way to estimate how long this amp can run (at a decently high volume) based on the battery pack's mAh rating?
If the speaker is an important part of this calculation: I have't decided on one. It will be a 5" or 5.25" round, 8 ohms, and probably a "full-range" type if i can get away with it. Does the speaker wattage rating make much of a difference so long as it is over what the amp can supply?
Thanks for any advice you can offer!
There are ways to estimate, but all of them are very inaccurate. The best way is to measure current during actual usage, and calculating. Time=capacity/current.
The maximal undistorted power output will be much lower than 18 W, about 8 W, but this is not very important regarding to your question.
The maximal undistorted power output will be much lower than 18 W, about 8 W, but this is not very important regarding to your question.
Headphone amps have it easy, as you can mostly calculate quiescent consumption as final as headphones take usually only a fraction of that number (for every 200 mWh only 1.5mWh goes to the headphones). Speakers have it different. I'd say measure quiescent consumption of your amp and average draw at your preferred volume level. Compare that to the battery capacity and you should be good to go
Thanks guys. It sounds like i should just build the thing, power it up temporarily with a 12v power supply, and buy or borrow something that can measure the watt draw.
Concerning the speaker choice: I know the physical size and ohm i want (8), but what other factors should i look out for? Does the speaker's wattage rating or magnet size have an impact on battery life and/or sound? Or is it just important that i choose a speaker that is rated at least as high as the amp's watt rating?
Concerning the speaker choice: I know the physical size and ohm i want (8), but what other factors should i look out for? Does the speaker's wattage rating or magnet size have an impact on battery life and/or sound? Or is it just important that i choose a speaker that is rated at least as high as the amp's watt rating?
Measure current (Ampere), not watt! You can do this with a multimeter, the cheapest costs about 5 dollars...
Depending on your music listening habits, the acoustical environment, speaker efficiency, playing time, etc... you will need 1...20 Ah capacity. (20 Ah assuming you will realize you need much more power, and choose 2x4 ohm double coil speaker, tie both to a bridged amp, and you want to run the amplifier all night long heavily overdriven.)
You can start with a small accumulator and extend it by others paralleled if it doesn't run long enough. You don't need to know the exact capacity in advance.
Speaker design is a rather complex task. If you want the most sound pressure for the longest time with the same battery, then you need a high efficiency speaker, but it will cost more, so there is always a tradeoff. And many others regarding mass, quality, size, etc...
I have no clue how you determined you need 8 ohm...
Magnet is important, but not the size, rather the strength. And the length of the voice coil wire. Together with resistance. And with xmax.... Basically every Thielle-Small parameters are important. And the enclosure. And the placement.
Tell what do you want to use for, and you will get advices, but don't expect telling how to design it generally, because you wouldn't understand it.
Depending on your music listening habits, the acoustical environment, speaker efficiency, playing time, etc... you will need 1...20 Ah capacity. (20 Ah assuming you will realize you need much more power, and choose 2x4 ohm double coil speaker, tie both to a bridged amp, and you want to run the amplifier all night long heavily overdriven.)
You can start with a small accumulator and extend it by others paralleled if it doesn't run long enough. You don't need to know the exact capacity in advance.
Speaker design is a rather complex task. If you want the most sound pressure for the longest time with the same battery, then you need a high efficiency speaker, but it will cost more, so there is always a tradeoff. And many others regarding mass, quality, size, etc...
I have no clue how you determined you need 8 ohm...
Magnet is important, but not the size, rather the strength. And the length of the voice coil wire. Together with resistance. And with xmax.... Basically every Thielle-Small parameters are important. And the enclosure. And the placement.
Tell what do you want to use for, and you will get advices, but don't expect telling how to design it generally, because you wouldn't understand it.
Pafl, thanks. Below is an example of the amps that i build. Obviously it's a bit of a novelty. In the past i was doing these with low power 9v amps and whatever used speaker i could find at Goodwill. I built an internal charger for the 6 NiMH batteries that powered it. The sound quality was obviously not great, but the battery life was simply amazing. They made good camping amps for this reason.
Now i'm trying to up the quality and see how good i can make one of these beer keg amps without breaking the bank. I'm moving up to a 12v amp, adding bluetooth capability, moving to Li-Ion batteries, and purchasing a new speaker. With that said, battery life is still a very high priority. I was thinking an 8 ohm speaker for this reason. I also chose a class-D amp and Li-Ion batteries for this reason. I intend to switch the bluetooth as well, so in situations where battery longevity is a concern (like a long camping trip), i can just use the 3.5mm jack and not have the bluetooth module suck any power.
So, if you could tell me what specifications to look for so that i can choose an efficient speaker i would appreciate it. I was thinking i would use a 2-way coaxial speaker since it's a mono application. If you have a better idea, please let me know. I can fit a 5" or 5.25" speaker. Once i have all the pieces, i'll put it all together and power it temporarily with a 12v power supply and use a multimeter to measure current, as you suggested. Thanks for all your help!
Now i'm trying to up the quality and see how good i can make one of these beer keg amps without breaking the bank. I'm moving up to a 12v amp, adding bluetooth capability, moving to Li-Ion batteries, and purchasing a new speaker. With that said, battery life is still a very high priority. I was thinking an 8 ohm speaker for this reason. I also chose a class-D amp and Li-Ion batteries for this reason. I intend to switch the bluetooth as well, so in situations where battery longevity is a concern (like a long camping trip), i can just use the 3.5mm jack and not have the bluetooth module suck any power.
So, if you could tell me what specifications to look for so that i can choose an efficient speaker i would appreciate it. I was thinking i would use a 2-way coaxial speaker since it's a mono application. If you have a better idea, please let me know. I can fit a 5" or 5.25" speaker. Once i have all the pieces, i'll put it all together and power it temporarily with a 12v power supply and use a multimeter to measure current, as you suggested. Thanks for all your help!
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Jackstraw,
Now I understand better your goal, but some details are unclear:
- how typically you use it? Crank up volume to max no matter how it distorts the music? Or only as far as it is not distorted much? Or there is significant headroom most times, only for short times you use its max power?
- how important is bass? Not important, you need only loud music without real bass? Or bass quality is more important than loudness? (This is important for selecting speaker.)
- Can you mount a separate tweeter somewhere, or the speaker must be fullrange/coaxial?
Getting back to amplifier and battery and impedance:
Idle consumption is typically very important. TPA3118 is quite good in this respect compared to similar amps (~50W 2ohm), but if you use it at 8 ohm, 8W, then the ratio of output power/idle loss becomes worse. You can decrease idle loss significantly by halving supply voltage (according to datasheet it operates from 4.5V), and at the same time you can get the same output power at 2 ohms load. By using only 3 diodes and a switch you can arrange 2 battery packs in a way that they can give for example 7.2V 4Ah in one state and 14.4V 2Ah in other state of the switch. This way you can choose any time between long time running and high power, as the loudness demand dictates. Additionally in this arrangement balancing between the two packs happens automatically when operating at low voltage, and still you can charge them whenever you want with a 17V stabilized, current limited charger. And at lower setting the amplifier will shut down before serious overdischarge may damage cells.
If you still need more playtime at high loudness, then you must search for a sensitive speaker. Bass sensitivity (for a given size and impedance) is determined mostly by Bxl product (given in Tesla*meters which is the same as Newton/Amper, so simply gives the force arisen for 1 A current), this is given on correct datasheets of speakers. You should choose the one with bigger Bxl. But typically high Bxl is achieved by more copper wire on the voice coil, that makes it heavy, and this reduces the sensitivity gain at higher freq. This is why you also have to take attention to moving mass parameter (given in gramms). Too high value results in lower loudness at middle/high freq, but too low value is also not very good, some mass is required to keep resonance freq not too high. There are many programs and excel tables made for calculating freq response of speaker boxes based on the mentioned parameters, but Im not at home, I can't attach one. You can compare the possible speakers and find the one with the highest SoundPressureLevel in your box (can) at the freq domain you want to reproduce.
Now I understand better your goal, but some details are unclear:
- how typically you use it? Crank up volume to max no matter how it distorts the music? Or only as far as it is not distorted much? Or there is significant headroom most times, only for short times you use its max power?
- how important is bass? Not important, you need only loud music without real bass? Or bass quality is more important than loudness? (This is important for selecting speaker.)
- Can you mount a separate tweeter somewhere, or the speaker must be fullrange/coaxial?
Getting back to amplifier and battery and impedance:
Idle consumption is typically very important. TPA3118 is quite good in this respect compared to similar amps (~50W 2ohm), but if you use it at 8 ohm, 8W, then the ratio of output power/idle loss becomes worse. You can decrease idle loss significantly by halving supply voltage (according to datasheet it operates from 4.5V), and at the same time you can get the same output power at 2 ohms load. By using only 3 diodes and a switch you can arrange 2 battery packs in a way that they can give for example 7.2V 4Ah in one state and 14.4V 2Ah in other state of the switch. This way you can choose any time between long time running and high power, as the loudness demand dictates. Additionally in this arrangement balancing between the two packs happens automatically when operating at low voltage, and still you can charge them whenever you want with a 17V stabilized, current limited charger. And at lower setting the amplifier will shut down before serious overdischarge may damage cells.
If you still need more playtime at high loudness, then you must search for a sensitive speaker. Bass sensitivity (for a given size and impedance) is determined mostly by Bxl product (given in Tesla*meters which is the same as Newton/Amper, so simply gives the force arisen for 1 A current), this is given on correct datasheets of speakers. You should choose the one with bigger Bxl. But typically high Bxl is achieved by more copper wire on the voice coil, that makes it heavy, and this reduces the sensitivity gain at higher freq. This is why you also have to take attention to moving mass parameter (given in gramms). Too high value results in lower loudness at middle/high freq, but too low value is also not very good, some mass is required to keep resonance freq not too high. There are many programs and excel tables made for calculating freq response of speaker boxes based on the mentioned parameters, but Im not at home, I can't attach one. You can compare the possible speakers and find the one with the highest SoundPressureLevel in your box (can) at the freq domain you want to reproduce.
Pafl, thanks!
"- how typically you use it? Crank up volume to max no matter how it distorts the music? Or only as far as it is not distorted much? Or there is significant headroom most times, only for short times you use its max power?"
I can't stand distortion, so it never gets turned up loud enough to do that. With the current amps, that's only a problem if the battery starts to run low. Otherwise, with the phone/device's volume at around 75-80%, i can turn the amp volume all the way up with no distortion. There are some exceptions, like songs that have extreme bass.
"- how important is bass? Not important, you need only loud music without real bass? Or bass quality is more important than loudness? (This is important for selecting speaker.)"
I think for typical use (in my kitchen or garage), a balanced sound is more important than loudness. It's camping and back yard party use that loudness becomes important. I would say that i would prefer quality sound over loudness.
"- Can you mount a separate tweeter somewhere, or the speaker must be fullrange/coaxial? "
There's nowhere to mount a tweeter. I'll need to go full range / coaxial
I'm intrigued by the dual battery pack idea. I've been pretty intimidated by the Li-Ion batteries and their need for balanced charging and over/under charge protection. My NiMH packs just needed a circuit to detect voltage slope to prevent overcharge and that was it. Well, i actually put a thermister on a few of them as a back-up stop in case the voltage slope was missed. Anyway, can you explain more about how the 2 battery packs arranged like you describe will balance each other?
Thanks again for all the great info!
"- how typically you use it? Crank up volume to max no matter how it distorts the music? Or only as far as it is not distorted much? Or there is significant headroom most times, only for short times you use its max power?"
I can't stand distortion, so it never gets turned up loud enough to do that. With the current amps, that's only a problem if the battery starts to run low. Otherwise, with the phone/device's volume at around 75-80%, i can turn the amp volume all the way up with no distortion. There are some exceptions, like songs that have extreme bass.
"- how important is bass? Not important, you need only loud music without real bass? Or bass quality is more important than loudness? (This is important for selecting speaker.)"
I think for typical use (in my kitchen or garage), a balanced sound is more important than loudness. It's camping and back yard party use that loudness becomes important. I would say that i would prefer quality sound over loudness.
"- Can you mount a separate tweeter somewhere, or the speaker must be fullrange/coaxial? "
There's nowhere to mount a tweeter. I'll need to go full range / coaxial
I'm intrigued by the dual battery pack idea. I've been pretty intimidated by the Li-Ion batteries and their need for balanced charging and over/under charge protection. My NiMH packs just needed a circuit to detect voltage slope to prevent overcharge and that was it. Well, i actually put a thermister on a few of them as a back-up stop in case the voltage slope was missed. Anyway, can you explain more about how the 2 battery packs arranged like you describe will balance each other?
Thanks again for all the great info!
You're welcome!
Balancing means making voltages equal (theoretically the charges should've been equal, but this can not be achieved directly). In low voltage mode a pack thats voltage is somewhat higher will provide higher part of the load current, as long as the voltages don't become equal again.
Within a pack the 2 cells are not balanced yet, but there are many matched pairs available that don't require further balancing...
SW can also be a voltage controlled switch, fed from output through a peak rectifier, this way it can switch automatically to high voltage mode when needed by high music peak voltage.
Balancing means making voltages equal (theoretically the charges should've been equal, but this can not be achieved directly). In low voltage mode a pack thats voltage is somewhat higher will provide higher part of the load current, as long as the voltages don't become equal again.
Within a pack the 2 cells are not balanced yet, but there are many matched pairs available that don't require further balancing...
SW can also be a voltage controlled switch, fed from output through a peak rectifier, this way it can switch automatically to high voltage mode when needed by high music peak voltage.
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