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12v Speaker build - Lots of questions!

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TheORig View Drop Down
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    Posted: 29 March 2020 at 3:53pm
Hi everyone,

Looking for some advice and answers to a number of questions. Or to be pointed in the direction of any existing threads that could help. Thanks in advance!

I'm looking to understand how to build a battery powered party / PA speaker.

Using either a 10" or 12" Driver with a 1" compression driver + horn. Class D amps and Lifepo4 batteries. However I have several questions about the configuration, wiring, power and crossover requirements of a build like this. 

As an example say the 10" driver has a sensitivity of 97dB, 250W RMS and the 1" driver has a sensitivity of 109dB 85W RMS. Both 8 Ohms.

Would you use one 2 channel amplifier to power both speakers?
Giving each speaker the same power output. 
Would you be limited by the lowest RMS, i.e. the Amp couldn't be above 85W RMS per channel. 
Due to sensitivity would the woofer/mid be lacking in power output?

Is it better to Bi Amp the speakers?
E.g. have a 100W RMS class d amp for the 10" and a 20W RMS amp for the tweeter?

If I Bi amp, how is the power divided from the battery? Do I use a distribution block and does the battery require a higher voltage to power 2x 12v amps?

As the speakers are both 8Ohm do I need an 8Ohm amplifier? As most class d amp i've found are 4 Ohm. Or do I simply need a 4Ohm amplifier that outputs 2x the power?

Also how do I correctly crossover the speakers as I believe you cant use capacitors on a battery powered (DC) circuit?

And finally is there a way to effectively add a low pass and high pass filter to the woofer? One for the correct crossover to the tweeter and the other to stop the speaker from over excursion of really low frequencies.

Apologies there are lots of questions i'm just trying build my knowledge and understanding during this lockdown. 

Any advice or answerers for any of the questions is greatly appreciated.

Thanks,
Theo!!!Smile




Edited by TheORig - 29 March 2020 at 3:55pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 29 March 2020 at 11:41pm
Originally posted by TheORig TheORig wrote:

As an example say the 10" driver has a sensitivity of 97dB, 250W RMS and the 1" driver has a sensitivity of 109dB 85W RMS. Both 8 Ohms.

Would you use one 2 channel amplifier to power both speakers? Yes, if there's just one of each driver.
Giving each speaker the same power output. No, you can turn down one channel to give the HF less power
Would you be limited by the lowest RMS, i.e. the Amp couldn't be above 85W RMS per channel. No, you can use amps with much higher output capabilities than the drivers RMS value, as long as you limit the power somehow
Due to sensitivity would the woofer/mid be lacking in power output?

Is it better to Bi Amp the speakers? yes but this can be done with one 2 channel amp
E.g. have a 100W RMS class d amp for the 10" and a 20W RMS amp for the tweeter? this is also possible, you may find you need to bridge two channels of your bass amp to get the power you need, then it makes sense for your HF to be run from a second smaller amp. 4 channel amps that allow you to bridge pairs of channel are another good option

If I Bi amp, how is the power divided from the battery? you just make parallel connections between any 12v equipment you use. Do I use a distribution block A small fuse box is a good way to safely distribute the live cables and a block or stud for the negatives and does the battery require a higher voltage to power 2x 12v amps? no

As the speakers are both 8Ohm do I need an 8Ohm amplifier? As most class d amp i've found are 4 Ohm. Amps are not 8ohm or 4ohm etc. they have a minimum rating that you should not go below Or do I simply need a 4Ohm amplifier that outputs 2x the power? approximately, yes

Also how do I correctly crossover the speakers as I believe you cant use capacitors on a battery powered (DC) circuit? use a 12v active crossover, some car amps have them built in although they tend to be limited in function.

And finally is there a way to effectively add a low pass and high pass filter to the woofer? One for the correct crossover to the tweeter and the other to stop the speaker from over excursion of really low frequencies. One way is with a 3 way active crossover and not using the output from the lowest output, check the limitations of filters though. Generally 12v systems (outside of cars) cut the low end of the bass higher than other systems in order to gain efficiency and run time.

Apologies there are lots of questions i'm just trying build my knowledge and understanding during this lockdown. 

Any advice or answerers for any of the questions is greatly appreciated.

Thanks,
Theo!!!Smile


Keep pushing on, things are gonna get better.
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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2020 at 2:20pm
On crossovers:
Fully featured analogue car audio crossovers that power from 12v do exist, Alpine and Kenwood make them amongst others. However they have some disadvantages for 12v mobile rig use -
- high cost, expect to pay £200 for a good one
-large size. They are about the same size as a car amplifier.
-weight. Usually in an extruded metal chassis that is needlessly heavy.

You can use a MiniDSP digital unit. They power natively from 12v and have most of the features you would need. Very small and lightweight. However again, best part of £200. And have limited output capability, sometimes necessitating the use of analogue line level boosters on the outputs.

I've had good success using fixed analogue crossover boards from Xkitz and KMTech. The KMTech boards need a small boost converter unit to give them +/-15v from +12v. 



You will note these are 3 way crossovers. You would use the lowest highpass filter to cut your bass off at a battery-friendly frequency, like 50Hz. So you would run your bass bin from the "Mid" output and your mid-highs from the "Hi" output. 

I haven't yet found a board that has a fundamental highpass filter in addition to three crossover filters. You could possibly use two or more boards together to achieve this. Car amps also usually do not have an adjustable fundamental highpass. Maybe they cut at 10 or 15Hz but that is too low for our purposes.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote imageoven Quote  Post ReplyReply Direct Link To This Post Posted: 30 March 2020 at 3:11pm
 https://m.thomann.de/gb/the_t.racks_dsp_4x4_mini.htm

These 12v active DSP look like they could be useful and are cheap (from another thread, currently below) but I have no experience of them though.
Keep pushing on, things are gonna get better.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TheORig Quote  Post ReplyReply Direct Link To This Post Posted: 28 April 2020 at 2:28pm
Thanks for taking the time to write all these answers! Really helpful information Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Natedub Quote  Post ReplyReply Direct Link To This Post Posted: 28 April 2020 at 9:17pm
The thomann or sure electronics dsp will give you an easy to tune crossover and eq etc. The Thomann unit is much easier to setup than the sure electronics unit that requires a certain amount of knowledge of sigma studio. 

As for the volume/sensitivity difference between drivers, you can simply turn the gain up or down in the dsp on the individual channels as required to get a balanced ratio between mid bass and tweeter.

Check out the sanwu tda7498e class d amp boards. They are cheap, sound very good for what they are and very efficient. They can do nearly 100w/channel at 4 ohms. They run on between 24v dc and 40vdc. You can either use a 24v lithium battery pack or use a dc dc boost converter to step up from 12v to 24v+. DC boost converters can introduce switching noise however (hiss) so better to run straight off batteries. Tpa3116 boards are another option but no where need as much oomph and sound to bright/hissy for my liking.

Regards
Nath
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 30 April 2020 at 8:20pm
Originally posted by TheORig TheORig wrote:

Hi everyone,

Looking for some advice and answers

Also how do I correctly crossover the speakers as I believe you cant use capacitors on a battery powered (DC) circuit?



Thanks,
Theo!!!Smile




Speaker crossovers with capacitors can be used on any amplifier output - The audio output is AC as otherwise there would be no sound.
Anything else is fake news - Don't go there!

Plenty of amplifiers use Switch mode Power supplies. They can have advantages on DC supplied systems, like maintaining a constant power output available while the voltage in the battery drops.
Decent ones should not increase noise levels in a system.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 01 May 2020 at 3:40pm
That's a good point about switching converters as amp power supplies - they are a very good idea, because they will maintain the power output of your rig as the batteries drain.

On my system I use a variety of batteries from 12v to 24v including lithiums and lead acids. These feed a boost regulator from Wondom/Sure that steps up +11-25v to regulated +48v at up to 10 amps (480 watts). This way I can get my maximum rated output power until the battery drops below 11v under load, at which point the converter cuts out. 
If I had a 12-series lithium pack made up to give me about +48v, then it would actually be 50.4v at full charge, which would be great. But at full discharge it would have dropped to just 36v, which would mean I lose about half my output power (power is proportional to voltage squared). No good!
 
Some car amps (really good ones) have such a boost regulator in them. Quite a lot though, just have an unregulated boost converter, which simply steps up the input voltage by some fixed factor and will still lose power as the battery drains. So from that point of view, a boost regulator and amp board are potentially a better choice.
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