help with a SIMPLE 36v7a battery powered class D portable sound system

[budget minded]

this is the THIRD forum i've joined trying to get help designing a simple class D sound system after drawing nothing but trolls who, if anything, sought to UNDERMINE my efforts instead of offering any actual help. i'm hoping the members here can help me solve a few issues that you shouldn't need to do ANY calculations for. my understanding of electronic circuits is VERY basic, so i need some expert help double checking my planned design.

it will start with 3 @ 12v7a batteries in series for a 36v circuit as class D amps like high voltages as you probably already know if you dabble in audio circuits. i'd like to use lighter lithium-ion rechargeables, but they're just too expensive for this system that still requires almost everything including some custom fabrication.

1. i'm planning on using a 2x100w class D amp for the mains **broken link removed** and would like to use this 1x400w amp for subwoofers that cross over at 70Hz/24dB per octave **broken link removed** . do you see any issue with that setup and those batteries? lithium ion batteries would have a much lower amperage rating, and i've seen them driving fairly high powered, but less efficient class A/B amps. i don't know how to calculate power requirements.

2. i wanted to buffer the batteries with a 1 farad car audio capacitor, but those aren't rated for 36v. other capacitors i've seen are more expensive with lower farad ratings. car audio caps aren't just cheap, but also come with mounting brackets need for installation. i don't know why i couldn't use car caps because even though this will be a high voltage system, it will only be 7 amps. i wondered if maybe i could run 3 caps in parallel to, in essence, divide the 36v by three to get 3x 12v. what i understand about caps is that you add their values in parallel like they're 1 cap and that a bunch of smaller caps will be faster acting than 1 big cap, but not how it relates to volts and amps, just farads. i figure a cap buffer would really help seeing that i'll be using such low amperage smaller batteries, but needing high wattage.

3. what sized fuse should i be using in this system? i figure if the batteries are rated at 7 amps, then i should get a fuse, more likely a circuit breaker that i can also use as an on/off switch, rated at 8 amps so the circuit doesn't impede battery output, but doesn't let it exceed it either. these kinds of questions are very hard to look up online, probably because i'm not using the right technical jargon to ask.

4. if the circuit is broken by a switch, then i should be able to directly tap each battery's terminals to charge them without having to pull them out, right? as there's no circuit, they can't interact, as i understand it.

5. similarly, could i directly tap individual batteries at 12v to power low draw items like line level EQ/crossovers and still maintain 36v across the main circuit? it would seem to me like that shouldn't be an issue

6. how would i integrate a voltage meter? i would have thought i would just insert in series with the circuit and it would directly measure the voltage (36v meters are harder to come by) but in reading specs on various, usually digital ones rated to even 100v, it looks like they all require ANOTHER 30v power supply which doesn't make much sense. isn't there such a thing as a self powered meter like the ones used in car audio caps? back to question two, what about maybe running 3 caps, all in series with individual batteries, and using those as 3 x 12v power meters instead of trying to figure out how to get another 30v circuit to power a single 36v meter.

thank you in advance to anyone that can shed some light on these questions that should be ridiculously easy for you, but that are above my head and very frustrating to try and solve with spam engines. if i can get a handle on these little issues, wiring everything up should be a piece of cake. i understand enough about electronics to be able to handle that without a problem, but system load rating? that's a whole other level of technical. i truly appreciate any help that will guide me to building this system right and SAFELY.

 

[alec_t]

Your batteries aren't 12V 7A, they're 12V 7Ah. That means they can provide (in theory) 7 Amps for 1 hour, or 14A for half an hour, etc. You could probably get over 100A briefly, with a near-short-circuit.
The linked 400W amplifier needs a 9.8A supply, so a 10A fuse should do for that (max current is drawn on signal peaks so is unlikely to be sustained).
I'd forget about the car capacitors: a lead acid battery behaves much like a capacitor anyway, and the power supply should have built-in decoupling capacitors.

do you see any issue with that setup and those batteries?

Battery voltage = 3 x 12.8 = 38.4V when charged. 100W amp supply max voltage rating = 36V .
A cheap DMM works for ages from a 9V battery and can easily read 36V.

 

[ronsimpson]

i wanted to buffer the batteries with a 1 farad car audio capacitor, but those aren't rated for 36v

i wondered if maybe i could run 3 caps in parallel to, in essence, divide the 36v by three to get 3x 12v

Yes run three capacitors in series. Put a cap across each battery.

how would i integrate a voltage meter?

On the market there are some rally nice meters.
**broken link removed**

 

[ronsimpson]

5. similarly, could i directly tap individual batteries at 12v to power low draw items like line level EQ/crossovers and still maintain 36v across the main circuit? it would seem to me like that shouldn't be an issue

I like to have a "ground" with + and - supplies. So I have done two batteries to get +12V, 0 , -12V.
You will need to AC couple between "items". AC couple is simply putting a capacitor between items. There probably already is a capacitor in place. You will need to check the cap's voltage ratings.

If the crossovers are running from the center battery and the power amp is off the three batteries that should work. You will run down the center battery first which is not good. Hopefully there will not be much current pull from the EQ.

 

[audioguru]

Sure Electronics and you did not read the datasheet for the TDA7498 "100W" amplifier IC or they are lying about its power. Its datasheet says it provides 100 Whats at a horrible-sounding 10% distortion into a 6 ohm load (where will you find a 6 ohm speaker?) or 80 Whats at 10% distortion into an 8 ohm speaker. Its graph shows clipping at only 38 Watts into 8 ohms then 1% audible distortion at about 68 Watts.

The subwoofer amplifier has no manual and no part number for its IC but its "400 Whats" or 300 Watts of power is rated when its power supply is 44V and its load is 3 ohms (where will you find a 3 ohm speaker?). You might get 100 Watts into 8 ohms.

 

[tcmtech]

If it was me I would use a simple buck converter to bring the 36-volt power line down to 12 volts for the other gear.

After that, I would not worry about the big capacitors too much being that the amplifier units appear to have a pretty good amount of capacitors built into them to begin with.

 

[budget minded]

well, i FINALLY heard back from PE, and the tech there told me there's no way 36v 7.2 AMP HOURS is enough to run the system as designed for more than an hour. my only two options, are to build a REALLY heavy system around 12v batteries using heavy car amps, or invest hundreds of dollars i don't have into li-poly batteries.

i'm going to start with just one of these and my mains, then add another as i start collecting subs until i can afford a proper li-poly battery system.

Specifications: Power handling: 2 x 50 watts @ 8 ohms • Minimum impedance: 4 ohms • THD: 0.1% @ 8 ohms @ 1 watt • Recommended power supply: 10V - 26V • Requires 1A - 4A • Stereo wiring harness included • Dimensions: 2-3/4" L x 2-5/16" W x 7/8" H.

**broken link removed**

for phase 1, i've decided to just go with a pair of cheap "50 wpc" modules (30wpc realistic) and a pair of 12v batteries as by the time i have enough money to buy $300+ in batteries, another $500 for 4 woofers (+6dB) and boxes, $150 for a trailer, not counting having to have a couple sheets of aluminum fabbed so the 14" w each boxes can fit in a barely 28.5" wide, if memory serves me right wheelbase and all of the accessories i'd need, the riding season will be long gone. i'm not redesigning the trailer around a couple big and heavy batteries + amps where i planned to mount everything to the underside, and i don't have an extra $300 for 15 li-poly packs.

i'll just have to build everything in stages.

never heard of a buck converter.

for a given class D amplifier that you could use 4 batteries in series for higher voltage, or series/parallel for higher amperage, which would be better, especially for run times? i would THINK it wouldn't make a difference as you're still using the same power reserves, but class D amps seem to work better with higher voltages. i would aim for higher voltage.

Its datasheet says it provides 100 Whats at a horrible-sounding 10% distortion into a 6 ohm load

actually, a lot of home speakers are 6 ohm. i used to have a pair of 6 ohm JBLs. the mains i'll be using though are proel FLASH8Ps rated at 96dB and i'm pretty sure they're 8 ohms.originally, i planned on building a 2 way system around 4 x 12" high efficiency woofers with $70 each 96dB air motion tweeters, but getting the proels wasn't just a cheaper "quick start", they would allow me to mount them at ear level on tripods as true "full range" satellites, or run a starter system using them alone which is the direction i'm taking now. i have an opportunity to make some extra income to help build this system, but that won't be for another month when i want the system built yesterday.

i wasn't expecting to get full rated power from these modules. i think i read in the reviews that a 1x200w module is closer to 1x150w. i'll be happy with whatever CLEAN output i can get from this system which will be designed for SQ over SPL. regardless of actual wattages an amp puts out, this system should blow a boombox away and that's all you really NEED for a party.

i got an AMAZING deal on the proels (celestion 8" woofer and titanium tweeter) which street at $225 each and paid just $100 for the pair at MCM who sold out fast. i kind of wish now i'd bought 2 pair. they sound really nice, at least at low volumes for PA speakers and are really heavy actually for plastic boxes. the drivers must have huge magnets

Your batteries aren't 12V 7A, they're 12V 7Ah. That means they can provide (in theory) 7 Amps for 1 hour

OK, but is there a way to convert volts and amp hours into amps and watts? an amp hour rating would be stretched when a load uses less watts would it not?

If the crossovers are running from the center battery and the power amp is off the three batteries that should work. You will run down the center battery first which is not good. Hopefully there will not be much current pull from the EQ.

i never even wanted to use active crossovers to begin with but nice, cheap, and simple inline passive RCA crossovers, but the trolls in the other forum lost their mind over the elegantly simple solution, but i'm convinced they were doing everything they could to undermine me at every turn eg. arguing that 4 x 12" subs at 200w would be "inaudible" like subs were designed not to work without offering any ideas for improvement other than compromising SQ in a way that's totally unacceptable to me, and i think decided to troll me for doing my own thing as well as pointing out well documented facts on the topic they just didn't want to hear & decided to shoot the messenger.

i was hoping between class D's 80%+ efficiency and using a 96dB system that i could get away with just 3 smaller, cheaper, and not insanely heavy lead acid batteries. so much for wide eyed optimism. if i want to do this right, it looks like i'm going to need at least a dozen, fifteen would be better, 11.1v x 3.5ah li-polys to get the most of whatever a 1x400w module can put out for a useable run time. there are no cheap meals.

BTW thanks everyone so much!

as a form of repayment, here's a heads up on the most bang for the buck i could find in lipoly batteries...
Rhino 3700mAh 3S 11.1v 25C Lipoly Pack

Capacity : 3700mAh
Constant discharge: 25C
Burst rate: 37C (15sec)
Configuration : 3S 11.1v
Pack size: 160x53x21mm
Weight : 327g
Discharge plug: 5.5mm Gold Connector
$19.50 each @ https://www.hobbyking.com/hobbyking...h_3S_11_1v_25C_Lipoly_Pack_US_Warehouse_.html

4 or 5 x 3 wide rows of these at 33.3v at 14.8 or 18.5 amp hours should hopefully be enough for a 2x100w + 1x400w system, if a very expensive one.

i wonder why generic packs at battery specialist sites actually cost a lot more. lower volume sales?

 

[alec_t]

OK, but is there a way to convert volts and amp hours into amps and watts? an amp hour rating would be stretched when a load uses less watts would it not?

You trade Watts for hours running time, since Hours = Volts x AmpHrs / Watts

 

[budget minded]

You trade Watts for hours running time, since Hours = Volts x AmpHrs / Watts

right, but how do i calculate run times for the amps? you're saying volts times amp hours over watts as a fraction?

oh do i ever i hate fractions. LOL it's only been what... 40 years since i've used them.

 

[audioguru]

i'm going to start with just one of these and my mains, then add another as i start collecting subs until i can afford a proper li-poly battery system.

Specifications: Power handling: 2 x 50 watts @ 8 ohms •

for phase 1, i've decided to just go with a pair of cheap "50 wpc" modules (30wpc realistic)

I am glad to see that you looked at the reviews and the datasheet and saw that the IC produces 50 Whats but only 30 Watts. Actually, the datasheet graph shows it beginning to clip at just 19 Watts.

 

[audioguru]

for a given class D amplifier that you could use 4 batteries in series for higher voltage, or series/parallel for higher amperage, which would be better, especially for run times? i would THINK it wouldn't make a difference as you're still using the same power reserves, but class D amps seem to work better with higher voltages. i would aim for higher voltage.

If an amplifier is allowed to have a higher power supply voltage then Ohm's Law says its maximum input and output powers are increased. If amplifiers are allowed to be paralleled then Ohm's law says the output power is more if the load impedance is less (example: one amplifier produces 100W into 8 ohms, two paralleled amplifiers produce 200W into 4 ohms which is the same as each amplifier producing 100W into 8 ohms).

here's a heads up on the most bang for the buck i could find in lipoly batteries...
Rhino 3700mAh 3S 11.1v 25C Lipoly Pack

On the hobby forums you will read that many people say that Hobby King sells cheap poor quality Chinese batteries at very low prices, some work and some don't, some last for many recharges and some fail after a few recharges.

 

[alec_t]

you're saying volts times amp hours over watts as a fraction?

Yes.
Example:
Say you have two nominally 12V batteries each rated 7Ah, and your amplifier requires 50W on average.
Hours run time (theoretical, if you run the batteries flat, which you shouldn't do unless you want to kill them quickly) = 2 x 12V x 7Ah / 50W = 3.36 hours. In reality you could probably reckon on not much more than 2 hours.

 

[bountyhunter]

FYI: battery A-hr ratings are typically for discahrge rates of c/10 (ten hour discharge) and loads that drain them significantly faster reduce the A-hr rating by a lot. How much, battery maker should have curves.

Li-poly is the chemistry that account for more battery fires than all the others put together. Consider that when selecting where you buy batteries.

 

[audioguru]

The Battery University shows the mAh graph of a "light duty" Panasonic 18650 Lithium ion cell rated at 3200mAh. Its capacity is more than its rating at C/5, is the same as its capacity at C/2 and is 0.7 times its rating at 2c.
a Panasonic "heavy duty" 18650 cell is shown with a rating of 1950mAh. Its capacity is almost the same as its rating at any discharge up to 10C.

Li-Po batteries for electric model airplanes have very high maximum continuous current ratings as high as 70C. Their mAh capacity is the same for any allowed discharge current.

 

[DirtyLude]

If you are looking for battery packs larger than those RC hobby batteries, you might want to check electric bicycle forums or dealers. I know my brother has a bunch of rechargeables including one huge lipoly 36V brick about the size of a large car battery.

 

[tcmtech]

You are aware that no one here has any idea what you are building let alone why and what for. For all we know you're trying to put some subs in a trunk to pull behind your bicycle.

If we did there is a good chance more suggestion would be available on how to do whatever it is you want to do.

 

[bountyhunter]

If you are looking for battery packs larger than those RC hobby batteries, you might want to check electric bicycle forums or dealers. I know my brother has a bunch of rechargeables including one huge lipoly 36V brick about the size of a large car battery.

There are also Li battery vendors for motorcycle applications such as Shorai that make 12V/15 A-hr batteries. Good quality, small size but pricey. They use Lithium Iron Phosphate type.

 

[budget minded]

I am glad to see that you looked at the reviews and the datasheet and saw that the IC produces 50 Whats but only 30 Watts. Actually, the datasheet graph shows it beginning to clip at just 19 Watts.

i look more at reviews than specs. they speak plain english, like me.

overspecking amps is nothing new. the original sonic impact, which i used to own, was rated at 15wpc, but most people agree it was really 8-10wpc. that's OK, even that was decent as my first portable system used that, with just 8 AAA batteries and 85dB 70Hz rated 5 1/4" 2 way speakers. i'm not trying to SPL dragrace here, just put together a nice sounding system that's much better than a boombox, and class D is both efficient and light compared to the more common class A/B. i can live with whatever limitations this system will have. that's why i've made speaker efficiency an issue.

On the hobby forums you will read that many people say that Hobby King sells cheap poor quality Chinese batteries at very low prices, some work and some don't, some last for many recharges and some fail after a few recharges.

OK, i did NOT know that. it's infuriating that li-poly batteries are so insanely expensive when they probably cost 50 cents to make. a SINGLE 11.1v 5 amp hours tenergy battery is a whopping $80! i could more than double that capacity with rhinos which seemed to have 5 star ratings, but might be sketchy. do you know of a cheaper way? i wouldn't mind making my own batteries, but that seems even more expensive with even lower amp hour ratings.

Li-poly is the chemistry that account for more battery fires than all the others put together. Consider that when selecting where you buy batteries.

now see, the propaganda i was reading at one site rated li-poly as safer than li-ion.

it's like there's no way to do this except with 100 pounds worth of lead acid and another 30+ pounds of overpriced car amps. ARRRRRRRRRRRGH!!!!!!!!!!

The Battery University shows the mAh graph of a "light duty" Panasonic 18650 Lithium ion cell rated at 3200mAh.

OK... that's a pretty high rating for such a little battery that should be rated 3v, right? that's the li-ion standard isn't it? a long time ago i tried looking into D cell sized li-ions which could be $10 for a battery rated 800 milli-amp hours which is terrible for such an expensive battery. i mist have been looking in all the wrong places because i don't remember ever seeing a single 3.2ah cell. working 3v at a time, it would be a lot easier to balance voltages with amp hours. when i have time, i'll go look those up as it sounds like a potential way to squeeze a lot of RELIABLE current out of a lightweight affordable system. i need to rest my brain after trying to soak these replies in. LOL

If you are looking for battery packs larger than those RC hobby batteries

from what i've seen of larger packs, at least at universal battery stores online is that those are even more expensive for a given capacity than overpriced R/C batteries. it's really annoying that such sellers don't sort by capacity.

There are also Li battery vendors for motorcycle applications such as Shorai that make 12V/15 A-hr batteries. Good quality, small size but pricey. They use Lithium Iron Phosphate type.

yes, $300 for battery that has only half the voltage i need IS pricey, like all of the generic higher voltage battery packs i've seen at battery sites. $300 or more is a lot for what is in essence a glorified boombox battery.

For all we know you're trying to put some subs in a trunk to pull behind your bicycle.

that's EXACTLY what i'm trying to do, with sealed subs at that, and for some reason, those two things seem to drive people out of their effing minds and fill them with murderous hate and rally against me in thug posses that only trigger my "bring it, but you are target #1 instigator" anti-phobia.

i try to be honest about that of which i'm ignorant and give props and gratitude to anyone that helps me learn whatever i need to know. when it comes to the things i DO know, i'm happy to help whoever until they disrespect me. advanced electronics is one of those things i'm totally ignorant of, but i understand basic acoustic principles just fine.

the only difference from your theory is that i intend to use a pair of 2 @ 1.23 cubic feet each sealed sub boxes and not a suitcase. i'm trying, in my own little way, to fight back against car systems playing LOUSY, anger based (read: gangsta crap & modern "spitting") music and promoting eco-friendly cycling. i guess that's a SERIOUS threat to people who size compensate with big truck systems. they just can't bear the idea of a bicycle being cooler than them. it challenges their social climbing d-bag world view and make them congregate against the threat to their status quo.

(i'm ALWAYS bucking against the status quo because it's always wrong and dictated by fidiots, but that's another story. NOTHING makes enemies as much as honesty and/or being right)

now that my "dirty little secret" (?????????????????!!!!!!!!!!!) is out, THIS is what i'm trying to improve on


in my experience, people on the street react more favorably to a system like that (maybe in part because i ONLY do upbeat & funky music) more than car systems. playful music is more inviting than music that tries to convey social dominance through bragging and and an adversarial tone. that's just not my thing at all, and i find such music impossible to dance to. i like music whose vibe radiates "c'mon! let's have fun!" without all that stupid butt ego crap & hostility. that's probably what always gets me in trouble in forums... my "eradicate all social climbing instigators without mercy" instinct.

OH! this site's video linking feature is WAY COOL!

here's an INCREDIBLY rare groove that used to be a sweat inducing favorite

634 people IN THE WHOLE WORLD have heard this?!!! ARRRRRRRGH! i miss the 80's so much! this used to be on the radio all the time in NY.

i also think it's because riding a bike makes a person less isolated and easy to relate to. regardless, ONE of the run ins i had with that modest system was an extra busty MILF freaking on my leg so hard it was a challenge to keep my bike from tipping over. such interactions are impossible with a car system.

"here in my car i feel safest of all. i can lock all my doors. it's the only way to go"
uh, no gary, it isn't! not by a longshot!

again... many thanks to EVERYONE kind enough to educate me. i really do appreciate it. your ability to translate my ignorance back into practical knowledge is a valuable resource. i hope no-one feels disrespected by anything i've said as none of it was directed at anyone here, but if you do, well you've got a guilty conscience and that's your fault, not mine. LOL

 

[budget minded]

2 nagging questions still remain, which is the preferable target to aim for with batteries, amp hours or volts?

and, is overvamping an amp module as dangerous as overvolting it? i thought i read somewhere that a circuit can only flow the amount of current it was designed for, as far as amps are concerned.

i'm guessing that a buck converter (didn't do my homework there) is a sort of potentiometer for voltage compensation judging by the context of that reply. if it's an issue, then, a buck converter would solve the problem, but i'm HOPING that the panasonic "AAA" li-on cells someone mentioned are the most cost effective solution as those would make containing voltages and amp hours a piece of cake.

the first time i started looking into this, i did a lot of research into li-ion battery holders

 

[audioguru]

2 nagging questions still remain, which is the preferable target to aim for with batteries, amp hours or volts?
and, is overvamping an amp module as dangerous as overvolting it? i thought i read somewhere that a circuit can only flow the amount of current it was designed for, as far as amps are concerned.

Amp hours are not how many amps of current a battery can provide. Instead it is the duration that a battery can provide a certain amount of current. Some battery cells can provide not much current for a long time but not provide a high current and other battery cells can provide a high current for a reasonable length of time but not provide a low current as long as you think.

An amplifier uses only as much current as it needs, it does not take all the current available. A car battery can provide 800A to start a car engine when its oil is thick on a cold day but its radio 14W amplifiers use the maximum of 2A that they always use (28W/14V= 2A) where the additional 14W is producing heat.

i'm guessing that a buck converter (didn't do my homework there) is a sort of potentiometer for voltage compensation judging by the context of that reply.

No, a potentiometer is a resistor that throws away power by producing heat. A buck converter converts a high voltage to a lower voltage with only a small loss of power. But your amplifiers produce a certain power when powered by a certain voltage. If you reduce the voltage fed to an amplifier then its maximum output power is a lot less.

i'm HOPING that the panasonic "AAA" li-on cells someone mentioned are the most cost effective solution as those would make containing voltages and amp hours a piece of cake.

A Lithium battery cell averages 3.7V during a discharge. You need 6 of them in series to produce about 22.5V to power two 30W into 8 ohms BTL class-D amplifiers. The maximum battery current will be about (60W/22.5V) x 1.2= 3.2A. Each battery cell has a mAh rating of about 0.8Ah (I could not find its actual spec's) and the string of 7 cells in series also have a 0.8Ah rating. Then the battery might last for 0.8Ah/3.2A= 15 minutes but probably only for 10 minutes at that high current if the amplifiers play continuously at full power. Luckily the average power of loud music is about 1/10th the maximum so the battery will last for 100 minutes when playing loud music. You will hear more distortion and will need to turn down the volume a few times as the battery voltage runs down. You need a larger battery that has a higher mAh rating to play for a longer duration.

We mentioned 18650 Li-ion cells (18mm diameter x 650mm long) that are much larger than an AAA cell and a little larger than an AA cell. An old laptop battery used them. A battery made with 7 cells would produce loud music on the class-D amplifiers for about 5.8 hours per charge.