[DIY Build Log] Mobile PA 12V/230V 2.1 Class D Amp Build

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Define "more power" in dB. It's always a tradeoff in price, weight, frequency range.

So if you trade 2dB more "power" for 10kg more weight, i wouldn't do. (Always keep in mind that 3dB are barely noticeable)

Thats true of course, so I will probably not get significantly more power at the same size of sub woofer I guess...
Before making any decision on a new sub I will first see how the MTH-30 does with the new amp, I will probably also get a measuring microphone to get some real numbers.

German forums are know to be notorious for this hyping of cheap, simple constructions, if you criticise any high praised construction, you get attacked from all sides, even if you are right. Every single 4" loudspeaker in some wood panel is lifted to high end spheres by these simple minds.

I've noticed that aswell, hence the reason I am here :D

The JM 2x12" will work down to 45Hz open air I suppose, you can tune it (in a limited region) a little deeper or higher. A clean, simple construction. 45 Hz is really low for outdoor PA on a budget, as for example 30 Hz need extreme power and cone area.
The Thomann chassis are very good, compared to the same quality from Brands like Visaton, they cost not even a quarter. They find their limits in excursion and max. input power, but in a region you will not drive by any battery.
Instead of 2x12" in one box, you will get exactly the same result with two 1x12" boxes, just before you ask. Before building, running a simulation will tell you how long the ports have to be and what to expect. I think you understand that you can trade some low bass for SPL and vice versa with the tune of your construction.
The Thomann 15LB075-UW4 mentioned by “doctormord” is on my list for low frequency cheap PA chassis, other 15” are more often made for mid bass. It´s price is ridiculous low and 4 ohm make it ideal for battery use (two in 2 ohm configuration get the most out of typical PBTL amps.

I would build the JM-sub212, but if its really only a 3db increase that might not be worth it. Maybe I should consider going a bit bigger to double 15LB075-UW4 @ 2ohm? That would be bigger but a significant boost in SPL right? I would rather have more SPL than going super low, I like punchy.

What kind if db increase can I expect with two 15LB075-UW4 @ 2ohm on the TPA3255-1CH-480W 3e-audio module? Is the module even powerful enough to properly drive those?

EDIT: I've found some interesting plans that both have >100db/w/m and aren't to big, as long as they aren't to wide the length is okay.

MN-238
MT-215B
 
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The first one is interesting, primarily it is a reflex construction, but with both drivers pressurizing the open chamber, there is some secondary effect. The speaker cones see the air load and some speed transformation will happen, as the opening is smaller than the cone areas.. How strong this effect is, would be easiest be analysed by separating the cabinet in the middle. A little like a bass reflex ripole...

The second construction is a reflex construction that wastes volume, as the short, small horn does not do anything usefull for low frequencys. It may direct the high frequency (you don´t want to hear from this chassis) in one direction.
Looks good, is useless. Better take the whole volume as resonator and place the speakers on the front. Or build smaller.
 
Maybe you should consider some general rules:
A reflex cabinet is larger than a closed one, but has more efficiency.
A real horn gives more SPL, but is much larger. Small horns don´t do sub bass.
Band pass can give very high SPL, but the design for good sound is very complicated and driver dependent. Usually it does not sound “dry”. A delay for the other speakers might make things better (Only with DSP).

If you take the right chassis and tune a reflex cabinet well, it is a very good compromise considering efficiency, sound impact, frequency range, volume and ease of build.
 
3dB from 4R -> 2R, 3dB from doubling come area, and maybe some 1-2 dB due to better beaming.

Beside that, I use a an adaptive subsonic filter which shifts the lower cutoff to higher frequency when driving the chassis hard. This gives more kick at high volume at the cost of low frequency which mainly consumes only energy in a free-air situation.
 
Such an "adaptive subsonic filter" in some DSP´s can be made with the "SPL dependent equalizer". Hardly the same. The Behringer DSP Amps have it. It is not only useful for PA, but also for home cinema. The lowest frequency just rises to a fixed level, while higher bass still gets louder. Prevents the voice coil from bumping into the magnet, especially with cheaper chassis.
 
TS: What size and weight limits do you have as the 2 options above are the size of small fridges and are IMO not very portable, but more movable.
The MT-215B is more a kick-bin then a sub-woofer.

A portable setup is battery and power limited; so I would make sure to use all I've got.
The biggest impact (trade off) on portability and output will be the sub-woofer(s): from 1 sealed 12" @2 Ohm (car) sub to 2x 4 Ohm horn loaded subs like the SS15.
Problem is that there are no (not many?) 2Ohm pa sub-woofer drivers; 4Ohm is already rare.

A 2 ohm (car) sub in a 1/4 wavelength horn would be nice.

Car sub drivers are often dual coil 2x2Ohm or 2x4Ohm; a 2x4Ohm model wired in parallel will be 2Ohm.
To bad Dayton Ultimax drivers are 2x2Ohm.

A JBL 1204D is about €50 second hand but is not very lightweight; about 5.2kg but does well in a 30liter sealed box and is weather/water proof. (all plastic cone)

If anyone knows a better 2ohm, low(er) weight, high output sub driver.......
 
Such an "adaptive subsonic filter" in some DSP´s can be made with the "SPL dependent equalizer". Hardly the same. The Behringer DSP Amps have it. It is not only useful for PA, but also for home cinema. The lowest frequency just rises to a fixed level, while higher bass still gets louder. Prevents the voice coil from bumping into the magnet, especially with cheaper chassis.

Correct, i had implemented that with two 2nd Order highpass filters in series, as SigmaDSP doesn't provide an index-selectable n-th-Order filter. The curve is then selected via index-number controlled by the volumepot (ADC) so it doesn't track the signal directly. (Which reminds me on testing a variable Q-filter with external input controlled by some RMS-envelope-generator.. ) Thanks for the input. :Pirate:

A JBL 1204D is about €50 second hand but is not very lightweight; about 5.2kg but does well in a 30liter sealed box and is weather/water proof. (all plastic cone)

If anyone knows a better 2ohm, low(er) weight, high output sub driver.......

As the datasheet already states, the chassis is not very effective when used outside of a car with the enclosures shown. Sub-90dB may be a show-stopper, when you can have 98-102dB with a proper PA-chassis in something like 80-100 liters. I haven't simulated the JBL in a bigger enclosure yet.
 
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Update!

Many of the key components have arrived.

mZheOUb.jpg


I have ordered another one of the DC/DC booster just to be sure, one for each amp.


EIZmzJf.jpg



The DSP is working flawlessly, getting the drivers right for the Free USBi programmer was a bit tricky but all in all I'm super satisfied with Sigma DSP and Sigma Studio. Its easy enough to understand for DSP beginners and offers live parameter changes and A/B listening. I'm now working on controlling the parameters with Arduino using the script and library mentioned in my earlier posts.

With lots of help from a forum member I've also started to design my own bass reflex enclosure for dual super low price mivoc WS 1565 C-4 15".
qXVwnb8.png

The design and more information can be found here:
->MPA-BR-215<-

In the next steps I will be ordering all the parts from Thomann which will include the 19" enclosure and all the connectors. But since I'm also building that Sub now, am a student and also moving places now, the budget is running a bit dry so the next steps might take a bit.
 
Nice parts, look good. The DSP seems to be much more value than the Sure/Wondom.
Even symmetric outputs with decent OP-amps.

The sub looks impressive too. The bracing is perfect. No flabby panels at all with just a little more material. Should keep losses from the enclosure to a minimum.
 
Very nice, are the parts on the right a screen and controls?
Do you have an indication of the weight of the woofers?

I like the design of the subwoofer box. Any weight indication?
You could make it a lot lighter in weight by using 12mm and extra bracing instead of 19mm and if you go for okoume plywood instead of birch you end up with 5/7th the weight of birch.
 
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There is always a way of doing loudspeaker lighter and more expensive by using other materials. If you take 16mm instead of 19mm you have to use even tighter bracing. So, what you take away at one point has to be compensated for somewhere else. Or you pay a lot of money...
At the end of the day you have to make a decission, so you take what you feel comfortable with, another person might take someting else. a little bit "belly" deciding.
This sub should not only sound good, but also use limited energy, say be efficent. If the box is too weak, the rear pressure will bend the panels and you have less output from the reflex port. This is no design for 2x 1000 Wrms woofer, but two 15" more in the 200-300 W range. If you have unlimited amplifier power, you can make good LF from any funny or flabby construction, just equalize and crank up the pot. High power and extreme excursion will fix any mediocre design.
For limited space, "normal" input power and "conventional" drivers, a solid, quite conventionel reflex construction like this still has it´s place. There is never the final, best construction anyone agrees to, too many variables are in play. Anyway, this one will sound way above it´s price class and make some serious noise.
 
Well, im using 12mm plywood for my 15" bassspeakers (90l) with doubled up frontbaffle and one cross brace. Doing so, i'm at barely 17kg per Box. As these aren't intended to work below 60Hz, they work like so. Linearity is not a goal so they're tuned for 80Hz at 104dB/1W/1m with a 6dB peak.


"The more lower, the more thickness needed". :D


@Philipp,


i highly recommend to glue down the bigger components on these DCDC-converters if you want to use them "off-road". Especially the inductor is prone to stress the solder joints alot.
 
Very nice, are the parts on the right a screen and controls?
Do you have an indication of the weight of the woofers?

I like the design of the subwoofer box. Any weight indication?
You could make it a lot lighter in weight by using 12mm and extra bracing instead of 19mm and if you go for okoume plywood instead of birch you end up with 5/7th the weight of birch.

Jep, that's a 128x64 LCD screen and a board with navigation buttons that will be panel-mounted to the front.

The box will be made out of 3.2m² 19mm MDF, with a density of roughly 700kg/m³ that will be 42kg of MDF. The woofers weigh 5.3kg each so a total of 53kg for the sub.

My MTH-30 weighed 45kg total (had a built in amp and stuff) and it was pretty manageable, so I don't think the weight will be an issue.

i highly recommend to glue down the bigger components on these DCDC-converters if you want to use them "off-road". Especially the inductor is prone to stress the solder joints alot.

Wise words! Siliconed down all the components, material fatigue can be a b*tch.

You should investigate cabinet lamination using a soft layer in the middle. Rather than making the walls stiff, it absorbs energy which reduces resonance Q, so it doesn't mater if the walls are thin. Acoustical Plywood

I imagine that's going to be quite pricey and hard to find in Germany.
 
This sub is going to run far under 200Hz.
This very nice acoustical plywood does not work too well in the low range. While it can be an advantage in cabinets for, say, two way speakers running a cone up to 2000 Hz, it will not do much at 80 Hz.
Then, there seems to be some missunderstanding: In a vented cabinet you do not want to absorb any sound, as you need it badly at the port to increase the low end.
Think of a ball you throw at a hard floor, it will contain much of its energy and roll on. Take the same throw on a well damped, soft carpet. The energy will be transferred to the ffibers and the ball loose it´s momentum.
So you need a hard cabinet for a vented sub. The higher frequency is removed by the active x-over anyway.
What about people putting some fiber stuff on the walls of a vented bass cabinet? If the x-over is something like a passive 12 dB/oct for 400 Hz, this makes sense, as a lot of resonating, high frequency is still there. As the fiber, like RockWool, does not take away much energy at low frequency´s, it works well higher up. On the other side, if you have an active 18 or 24 dB/oct x-over set to 100 Hz, you dont have anything to absorb and only waste work and money.
If you completely fill a vented sub cabinet with RockWool, you turn it into an aperiodical damped construction and loose all sound energy from the port.
 
Minor edit to post requested by OP.

I would not use MDF for outdoors as it soaks up water like a sponge and then loses its strength quickly.

IMO you should use an exterior grade plywood (which is made with a waterproof glue in between the layers) and use Polyurethane glue (Action) instead of regular PVA wood glue.
 
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The reason is rather simple, I'm getting the material from my local hardware store. They will cut all my plates for free, square and with sub Millimeter precision. For 18mm MDF I pay 13.45€/m² and for plywood it's 30.85€/m² (I need around 3.2m²). I am going for cheap on this sub. The chassis are 30€ each...

My MTH-30 is made from the same hardware store MDF screwed and glued together with Spax and water resistant wood glue and painted with Warnex paint. That thing was dropped, rolled down a ditch, was standing in puddles overnight, was rained and puked on, people jumped on top of it. And it doesn't have a dent in it and didn't suck up water.

For the price difference I'm going for MDF, it might not be as good as plywood but its cheaper and works in my experience.

I will be gluing it together with polyurethane glue this time, I got this one "Soudal Pro 45P".
 
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