View Full Version : Another festival boombox packpack
Hi all.
A friend of mine has asked me for help to build a festival boombox backpack. He is determined that it has to use full-range drivers and no tweeters and be very cheap. He will use an Amp6 amp and have it running on a single 7,2Ah SLA.
I've looked around to find some cheap small full-range speakers, and mainly looked for speakers in the size of 3"-4" since I assume that these will not require much speaker cabinet volume. I also looked at their SPL, and it seems that around 88dB is reasonable in that size- and pricerange. (He does not care about a completely flat response-curve at all...loudness > quality) And because of the required loudness, I've looked for 4ohm speakers.
I found these Monacors (~$23): http://www.monacor.dk/produkter/hoejttalerenheder-hifi-4/vnr/100680/
and according to WinISD's (not Pro) database they have the following specs:
Qts: 0,55
Qms: 2,82
Vas: 1,5 L
Qes: 0,68
Fs: 120 Hz
SPL: 88 dB
Re: 3,3 ohm
Pe: 20 W
Le: 0,1
BL: 0,00
Xmas: 0,00
Dia: 7,6 cm
Z: 4 ohm
Sd: 31 cm^2
I have tried to make WinISD Pro calculate a cabinet size and vent size for me, but this is very new to me, so I don't know what 'curve' to aim for. I've read several times in Saturnus' posts that a Qtc of 1,2 is the best for outdoor use, so I've tried to match that in the calculations in the image below.
I made both a vented and closed version, and they are VERY different, and it clearly seems that vented is much much better than closed, but it just seems suspicious...I'm not sure that I have done it correctly. (And I can't find any Qtc on the vented cabinet, although I'm quite sure that I've read alot about Qtc for the Boominator (you probably know it) which IS vented..so I'm confused!)
It recommends a vent size of 3,9cm diameter and 7,27cm long.
Is there someone who can help me confirm that the vented version is correct and good for this use? Or maybe someone knows another fullrange 3"-4" 4ohm speaker which is better suited?
Thanks for your help!
It would in my opinion be a terrible injustice to connect the amp6b with such speakers. It's like having a Ferrari to pull a manure wagon.
I know you are right about them not doing justice to the Amp6, but then...it will last until next year and then he can upgrade. But besides that, do you think my calculations are completely wrong? Btw, I have tried to convince him to use the Monacor SP-60/4, but since they require a tweeter he is not happy about that unfortunately. :(
Screw him. Tell him to either invest the bare minimum it takes to make an acceptable sounding system, or go buy a DKK399 ghettoblaster in elgiganten (electronic wholesale store). I'd not build anything for anyone if I couldn't vouch for the quality myself.
Hehe you got a point. But I also think that he values that it is homebuild, and looking forward to assemble it all. But I can understand from you that those speakers are no good at all. He actually has a budget of 1000DKK so the money is there for the SP-60/4 and tweeters......but lets say that the speakers could be twice as expensive as the previously posted, do you think it would be possible to make a system with full-range speakers? My main problem is to find some OK ones that will not require a lot of room (and play loud of course).
jesperstald
12-May-11, 09:40
If sound quality isnt the goal but loudness is, why not just get a couple of cheap piezo's with the SP60? Looks like its hard to find full-rangers with descent sensitivity....
Or get these tweeters. They're a perfect match for direct crossing to an sp60/4 with just a cap. No level adjustment required.
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=275-025
Thank you for your warning against the fullrangers...I managed to convince him because of it. So now it is the Monacor SP-60/4 with a pair of the Dayton tweeters which I found in Europe here: http://www.europe-audio.com/Product.asp?Product_ID=7640
I have a question though, which is also relevant to my own small boombox with the same specs... Should the two speaker chambers be connected to eachother?
And is it very important that the capacitors are metal-film? (I bought 4,7uF electrolytic for myself...)
Metal film capacitor? You mean MKT?, they're vastly better at only fairly little extra cost.
Are you going for closed or reflex box. Both should be about 4l, the reflex box should have a 1" diameter tube, 5cm long. In a closed box you can couple the chambers, in a reflex box you definitely shouldn't.
The reason being if there are different signals in the 2 speakers cabinet tuning will be affect if the boxes are coupled as the drivers each sees a larger cabinet volume. In a closed box that just results in a lower tuning with a lower Qb, so no a big deal. In a reflex box it will affect the port tuning as well, and that can be problematic.
Okay thank you! I was very confused about the coupling of the chambers. I use a tube with the exact dimensions as you suggest. Although the cabinet is actually only 3,6L (when you exclude the driver volume) since you suggested that volume over at diyaudio.com, but when I fiddled around in WinISD it does not seem to make that much of a difference.
I thought they were called metal-film, but I will look for MKT's then, thanks :)
All capacitor are in principle metal film. The quality and type is designated by the dielectric instead. That can be paper in vegetable oil, an electrolyte (a fluid gel), or a polymer like mylar, polypropylene etc.
The reason being if there are different signals in the 2 speakers cabinet tuning will be affect if the boxes are coupled as the drivers each sees a larger cabinet volume. In a closed box that just results in a lower tuning with a lower Qb, so no a big deal. In a reflex box it will affect the port tuning as well, and that can be problematic.
Ahh that makes sense. When you say "so not a big deal", does that mean that it would still be a bit better to separate them?
Ahh that makes sense. When you say "so not a big deal", does that mean that it would still be a bit better to separate them?
Yes. It does.
About the crossover... I read that if you only put a capacitor on the tweeter, the bass-speaker will still try to play the high frequencies, but can't and therefore transform it into heat and steals the signal from the tweeter. Therefore the guy advises to put a resistor on the bass like this. This should also be more efficient that not doing it. What do you think?
http://www.valveamplifierrepairs.co.uk/revampblog/wp-content/uploads/2011/03/Xover-2.bmp
The whole document is listed here:
http://www.valveamplifierrepairs.co.uk/revampblog/
It's not a resistor, that's an inductor. And no, you shouldn't and there will not be any loss of sound/heat. The VC is already an inductor, that's why speakers rise in impedance.
If you do insert an inductor, ie. make a true passive XO, please note that you will get a lower output.
Thanks for that! There's so much stuff I would have done wrong without your input :)
Okay so my friend made the boombox backpack as mentioned, but I'm puzzled about it's performance. What he did was to build a box with ~3L enclosures for each speaker (a bit under what you recommended), and used a 4.7uF MKT capacitor for the crossover of the Dayton ND16FA-6 tweeters. He also decided on a Lepai TA2020A+ amp instead because he wanted build-in volume control and power-switch. (I hope it is still OK to continue this thread?)
After it has played for several nights I have the impression that:
1. It lasts not close to 20 hours of playtime on a 9Ah SLA. More like 6-7 hours. I suspect that our 'smart' charger doesn't charge the battery to it's full potential. When it had finished charging it, I measured the voltage to be 13.07V, which sound quite low? The charger is an OptiMate 4. http://www.tecmate.com/optimate4_detail.php . I'm also speculating that it uses battery on trying to play the very low frequencies. I can see the cone move a lot when playing loud, but not hearing any of the deep bass that it looks like it is trying to produce. Is this because of the small speaker chambers? Or maybe because some of the wood used is a bit thin and cheap? I think it's 9mm, but bad quality.
2. The sound is okay at lower volume, but becomes harsh at higher volume. I have read more about crossovers, but it seems like 4.7uF is in the 5-6kHz range of a 6ohm tweeter, so I don't quite understand it. And it plays OK at lower volumes I think.
3. It distorts quite a bit when playing loud, but it helps somewhat to set 0-125Hz at -12dB on the players (iPhone 4) EQ, but it still distorts. I just can't figure out why this happens. V-Bro said in another thread that it is very difficult to get the SP-60/4 to distort, but it is a very easy task for this box unfortunately. And I suppose it is not because of high input signal to the amp since it plays fine at lower volumes (turned down on amp, not iPhone)?
4. I don't think the tweeters are playing loud enough...I have to put my ear pretty close to the tweeter to identify it from the SP-60/4. This I have absolutely no clue about. Maybe it's just my ears being fooled or something, but I would have thought that the tweeters were quite easy to hear since they (in theory) should play all the high frequencies.
5. It does not sound like all the top frequencies are played...it sort of sound like it stops playing frequencies over 10-12kHz (very rough guess)...it seems to lack something. Based on this and point 4. I'm afraid that I have not understood correctly how to add the capacitor. As it is now, it is put on the tweeters + terminal, such that it goes:
amp+ ----- SP-60+ ------ cap ----- tweeter+
amp- ----- SP-60- ----------------- tweeter-
Isn't this the correct setup?
I hope some of you can help me out on this, because I am going to finish my own box (with better wood and ~4L enclosure, but otherwise identical) which I build for a girl camp, and i don't want it to sound bad :/
First of all it is off course a crap piece of amplifier, sorry to say.... Probably the gain isn't set high enough, but you wouldn't wanna raise it with such crappy components, all you'll gain then is noise...
Then if it is a closed box try what happens if you gently press the cone down and release it fast, if it comes up slowly you have a good seal on the box, if it returns fast the box is leaking like a basket. Then the panel thickness should be fine at 9mm, as long as it is not a very large panel....
Your connections of the drivers also seem like everything is placed in series?? Can you make some pics or a better drawing of this??
It also can't hurt to take a smaller input capacitor on the channels, like 0,47uF to ensure it won't have to swing on frequencies it doesn't play anyway... This also saves power...
That charger looks a bit too beefy for a 9ah battery, did you match the charge currents? If it is an 'automatic' charger that can deliver higher than the max. current given for charging the battery you are on thin ice!
Yes I know it is not a good amp, but I still thought it would deliver roughly the same amount of volume without distorting as the Amp6, since it is the same chip...I hope the problem lies elsewhere.
I don't know about the gain, but it has a 'mp3' minijack input on the back which is where we plug in the iPhone cable. Input sensitivity is rated at 200mV if that helps!
I'll ask him to try the push-cone-in trick!
The input capacitor you mention, should that be soldered directly on the input signal somehow? Like opening the amp and put it where the 'mp3' port is on it's + or how? It would be very nice to have it block those frequencies by default!
Well the charger was recommended by the store for this exact purpose. It is automatic in some way and tests the battery to charge it optimally, but only up to 0.8A when it is flat, then drops to 0.6 and 0.2 in the end to finish off. It is also supposed to recover damaged batteries which it seems to do fine. After it has run a recover program it looks like the battery is good again, according to it's own tests.
I have attached a drawing of the conenctions. And thank you very much for the quick answer!!https://lh5.googleusercontent.com/-RVFl7Mlz7JA/Tez1UwwN7sI/AAAAAAAAABE/7L1dmdFvXlE/Boombox%252520chart.jpg
Connections look ok...
The capacitor that is in line with the signal on the board inside the amp, first thing that the signal will 'see' can be swapped for a smaller value...
Ok thanks I can manage that! Do you think that will get rid of the distortion or is there something else I should look into also?
Mayeb a stupid question but I would like to be sure...there is no difference between left and right channel in this diagram right? Because it might be connected as the right channel when I think about it.https://lh3.googleusercontent.com/-kHW3KX6PszI/Tez5u3__uAI/AAAAAAAAABI/zl0YvwAhe_s/Boombox%252520chart%252520Alternate.jpg
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