Battery-powered portable boombox project

[andreasmaaan]

Hi all,

 

I'm considering plans for a new DIY project that's a variation on the portable boombox concept, hopefully with a more linear frequency response due to higher quality drivers and equalisation and (possibly) digital crossover using MiniDSP.

 

The other major difference would be that the system would be modular, i.e. each driver would be housed in its own box, as would the mimiDSP/battery/PS/amp, and all would slot and fix together making them usable as either a portable boombox, or, at home, a 3 component (2 x speakers + 1 x amp/crossover/psu) system.

 

It won't be dirt cheap, but I do want to keep costs down and try to achieve similar sound quality to a similarly priced consumer hifi system, so around 300-350€  (I'm in Germany these days, hence the Euro) or a bit more if I go for a more complex design.

 

Similar projects have been written up extensively online as 'DIY boomboxes', but the main differences I'm considering in my build are:

- the addition of a MiniDSP chip (or two) to improve the frequency response and (possibly) to replace the passive crossover with an active crossover

- the use of modular boxes, allowing the system to be utilised as separate components

 

I've chosen the PC165F because they are coaxial (making them easy to deal with) and relatively sensitive (91.9db @ 2.83V/1m), yet (according to Focal at least) have a reasonably linear frequency response and decent dispersion characteristics, as well as sufficient bass extension. On the other hand, I'm concerned about the high resonant frequency (70hz) and the high magnitude of the resonance peak (over 50 Ohm), especially given this will be powered off a cheap battery. So at this stage I'm also open to other driver suggestions.

 

Input will be stereo RCA (or 3.5mm jack) into MiniDSP.

 

From there, I am considering the following options:

 

(1) MiniDSP as EQ (only) / 12V lead acid battery

 

The MiniDSP  would be used simply to create a high pass (maybe 40hz) filter, compensate for baffle step, and iron out frequency response problems in the mid and (particularly) high range.

 

I would house the coaxial drivers in well constructed 10-15 litre MDF boxes, and use either sealed boxes or more likely ported boxes, tuned (hopefully) to around 50hz. The MiniDSP would provide a quite steep high pass filter at 40hz-ish, and some EQ to compensate for baffle step and iron out irregularities in the response.

 

This is the concept in a microsoft paint drawing:

 

The basic equipment I have in mind for this is as follows:

• 1 x MiniDSP
• 2 x Focal PC165F
• 1 x Hifimediy T1-M TK2050 
• 1 x 12V UPS lead acid battery
• 1 x switching PSU

 

With this configuration, I have these questions:

 

Power

 

Is it a good idea to use a 12V UPS lead acid battery for something like this? The basic reason I have it in mind is that it's cheap (around 10-15€) and easily replaceable. Are there better ideas for the battery?

 

If I do use such a battery, how do I set up the system to: (1) run off the PSU when plugged in and switched on, (2) charge the battery when plugged in and switched off, and (3) run off the battery when unplugged?

 

I'm an electronics novice but a patient learner. Is this something I can realistically and safely achieve?

 

Is there a simpler way of doing this that I'm missing? For example, is there a ready-made battery/psu that I could choose that would recharge and provide power to the system at the same time, so that I don't need the PSU or any kind of switch?

 

The miniDSP also needs to draw a small amount of power from somewhere. Where would this come from and how would I make it work?

 

Speakers

 

These drivers have slightly odd parameters as I mentioned before:

 

 

Will that very high magnitude resonance peak be a problem for a relatively low powered amp like the Hifimediy T1-M running off a 12V battery?

 

The amp is 57w into 4Ohm with a 24V DC supply, but where the resonance peak is it would be running off a 12V DC supply into probably at least 40 Ohm (I am assuming that the heavily damped ported box will move the resonance up a bit from 70hz and reduce its magnitude somewhat).

 

My understanding of how this amp works is that it might struggle to produce much power in this frequency region, is that right?

 

Boxes

 

The driver parameters suggest a ported box will work better, and since these are going to be midwoofers, ports would make sense from the point of view of efficiency. Can anyone see a problem with using a 10-15L box with damping material, with a 5cm diameter port tuned to quite a bit below the Fs (hopefully 50hz), and a digital high pass filter just below the port tuning frequency?

 

(2) Extra amp and sub

 

This would involve adding an additional monoblock chip amp and a small high-efficiency sub, which would most likely be housed in the central unit with the battery and amps etc. This would change my power supply needs, but I'm not sure I understand how. Assuming I found an efficient D-class monoblack 50w or 100w change amp, how would I calculate my power requirements? Would there be any chance of sticking with the 12V lead acid battery or would I simply need to abandon this idea?

 

And does anyone have any suggestions for a decent, small (ideally 8 inch) and efficient subwoofer and amp for this arrangement? 

 

(3) MiniDSP as crossover

 

This design idea would be very similar to (1), but in this case the miniDSP would split the input between the woofers and tweeters and replace the passive crossover in the driver unit. There would be an additional Hifimediy T1M amp giving a total of 4 channels.

 

My question here would again be about the battery and PSU. If I were now powering two Hifimediy T1-M's off one PSU or battery, how would that change my power requirements?

 

Would I need a more powerful battery? If so, how powerful would it need to be, and does anyone have any idea what I might use?

 

(4) Extra amp and sub / 2 x MiniDSP

 

This would be the most complex setup and would involve housing an additional sub in the central unit, adding a MiniDSP to create a 2.1-way crossover, and adding an additional sub amp.

 

My question about power supply / battery is similar to my question about the extra amp in design (2) and (3). How would I calculate my power requirements? Are there any suggestions for batteries / PSUs?

 

(5) 2 x sub / 2 x MiniDSP

 

This is basically like design (3), but there are 2 subs instead of one and an additional stereo amp (rather than a mono amp) to power them. The subs would either both sit in the central unit or would have their own boxes, ideally also not much bigger than 10-15L in size. I guess my questions about this design are basically the same as for design (3).

 

Many thanks in advance.

[Swarm]

You can get fairly compact car audio stuff now if you havent thought about it already

[Petit-fou-fou]

I wouldn't use the MiniDSP stuff, although it is only a boombox so the quality might not be so important. Personally for a lower cost project I'd still go for better DSP, what about Xilica XM units? Much better performance and sound and only slightly more expensive.

[andreasmaaan]

8 hours ago, Swarm said:

You can get fairly compact car audio stuff now if you havent thought about it already

 

Thanks Swarm. Yeh, the drivers are from Focal's car audio range. With the amps etc., sticking with DIY boards to save money.

[andreasmaaan]

8 hours ago, Petit-fou-fou said:

I wouldn't use the MiniDSP stuff, although it is only a boombox so the quality might not be so important. Personally for a lower cost project I'd still go for better DSP, what about Xilica XM units? Much better performance and sound and only slightly more expensive.

 

Yeh, I know miniDSP might not be the market leader for SQ but I already happen to have a couple lying around and they are very cheap and functional.

[Swarm]

8 hours ago, andreasmaaan said:

 

Thanks Swarm. Yeh, the drivers are from Focal's car audio range. With the amps etc., sticking with DIY boards to save money.

Oh ok. do you have all the parts already ? 

[andreasmaaan]

5 minutes ago, Swarm said:

Oh ok. do you have all the parts already ? 

 

No none of them except the miniDSP yet.. If you've got interesting ideas for any of the other components, let me know

[Stevesie]

Not sure if it suits your purposes, but Sure do a board with built in lithium battery charging http://store3.sure-electronics.com/aa-ja32472-1751

[andreasmaaan]

14 hours ago, Stevesie said:

Not sure if it suits your purposes, but Sure do a board with built in lithium battery charging http://store3.sure-electronics.com/aa-ja32472-1751

 

Thanks, that looks interesting!

[andreasmaaan]

I thought I'd do a little update since the designs are changing and starting to take shape, and also ask a few more puzzling questions about power supply / batteries etc 

 

So, I've scrapped the Focal coaxials in favour of a 4 inch pair of W4-937SF from Tang Band, specs here.

 

These are obviously much smaller and a little less efficient, but the TS parameters indicate they are much better suited to a very small vented box, which is great for this compact project. I also decided that with the larger 6.5" drivers I was kind of pushing the portability thing in terms of size and weight, but wasn't yet into the SPL ballpark of a system for occasions any bigger than picnics and BBQs. Best to keep it small (...and maybe add a little sub later...).

 

So, I'll be happy with a more compact unit and lower SPL (but hopefully better SQ) using 2 x Tang Band W4-937SF in separate small 1cm thick MDF vented boxes.

 

Modelling indicates that a (tiny!) box size around 1 to 1.5 litres will work best with each of these. I'm planning to use a port with a diameter of 2cm and a length of 5-10cm depending on what the testing shows. The modelling gives me a 7-8cm long port in a 1.5 litre box for a tuning frequency of around 80hz. A standard alignment would put the tuning frequency at closer to 100hz, but subjectively I think that those 20hz between 80 and 100 can mean the difference between a speaker that sounds like it has bass and one that doesn't. So I'll see if I can get it sounding ok with a tuning frequency of around 80hz and a steep HPF at 60-70hz to protect the woofers below the tuning frequency. I'll also be able to adjust the EQ from the cutoff point up using the miniDSP, so hopefully I'll manage to get flat-ish bass down to 100hz, and maybe a little lower, at the cost of some efficiency.

 

Having worked out the basic driver and box designs, as well as digital EQ (miniDSP) and probably amps (think I will go with the one of the Hifimediy options), I'm just left with questions about power supply / batteries etc.

 

I can see two ways of doing this:

 

1) Rechargeable battery // Switching PSU // recharger // switch between battery, charger and PSU

 

2) rechargeable battery // recharger // switch (for on and off, and maybe to switch the charger off??) // no PSU

 

This is where I really have no idea. All I know is that battery power is important cos it needs to be portable, and that I want to keep it cheap, simple and safe.

 

So I'd love some suggestions about what batteries to use, whether a PSU and a switch are necessary, and what kind of recharger to use, etc...  

 

Keep in mind that the battery will need to power a small class D chip amp, as well as a miniDSP.

 

Of the two amps I have in mind, one is not quite as powerful as I think it should be (only 8w into 8Ohm @ 12V or around 30w @ 24V), nor as well-suited to 8+ Ohm loads, but requires only 12V DC to run. The other one is much better suited to 8+ Ohm loads and puts out 2 x 50w @ 8Ohm when sufficiently powered, but needs at least 22V DC to operate. So to use this amp, I'd need a 24 or 30V battery.

 

Here are the specs for both options: lower power // higher power

 

Thanks again for your help! 

[andreasmaaan]

Hmm, having researched a but, I guess I am basically trying to make something like this, but ideally one that runs at 24V rather than 12V (and not necessarily lead-acid, just whatever is affordable and works well).

 

[Stevesie]

Could you use lithium packs/charger from cordless tools? Much more expensive, but much lighter than lead acid, and can be easily found in higher voltages. 

[andreasmaaan]

5 hours ago, Stevesie said:

Could you use lithium packs/charger from cordless tools? Much more expensive, but much lighter than lead acid, and can be easily found in higher voltages. 

 

Thanks, great idea

 

Have done some googling (e.g. this) and it seems that people are also putting together their own lithium batteries of various voltage ratings. I guess I would need e.g. 2 x 4S4P cells (32 batteries) to get just over 32V and over 20 amp hours. Will have to look into the specs of the amps etc. but seems that with some research it would be possible to get a configuration together that fulfils my power needs for not too much money (maybe 24-32 cells @ around 3€ each). Then it would be a case of finding the right PCB for the battery.

 

Do you know if it would be possible have this unit corrected directly to the AC via a charger while drawing power from it? In other words could the battery power the amplifier and DSP while plugged into the wall?

 

[andreasmaaan]

And another couple of questions actually if anyone knows the answer.

 

Could 2 or more 12V lead acid batteries be used to power the system, i.e. would it be possible to get higher DC voltages from more than one lead acid battery?

 

And could a lead acid battery safely be plugged into the charger while also discharging to the load?