Building my first boombox

[V-bro]

Off course filtering all upper frequencies out of the woofer characteristic that the tweeter is already playing will increase the sound quality. Also don't forget that in the overlapping part the output of the whole system will be raised for quite a bit, hence my advise to listen well and have a few spare values at hand that can make a pretty quick satisfying sound balance. The thing is the phase won't be 100% the same on both drivers in the overlapping area, that's what I meant to say a few times with the multi-dimensional 'math' involved... It's by far the easiest to go for a satisfying result by experimenting than to try to figure it all out completely before even starting. You'll learn to know how to improve and that's where I know with these drivers this point will be achieved fairly simple...I actually guarantee....

Then after a long listen and after the mental break in period you can start to ponder in to the world of 2nd order filtering and learn there is actually both improvement and degradation. More of the first than the latter in my opinion, but this may be proven wrong one day, who knows...

Dynaudio used to swear by 1st order filters that were at a first glance rather looking like 4th order due to very complex impedance correction networks, both towards the drivers and towards the amplifier output. Not a bad idea in itself, nothing works more ideal than to create a situation in which parts behave like how it is explained in school books. And with great results!

All I want to say is there is no golden rule, everybody works with their own theories and methodes and several approaches may work and even give similar results. Given you have at least basic knowledge of the matter. Just use your head....

[amp6erik]

Thank you again, V-Bro!

Haha, you always manages to make me eager to find out more about what you have written. I basically spend half the night yesterday researching the whole internet about this

Anyway, I've decided to go with a capacitor of 4.7 µF and a series resistor of 3.3 ohms to each tweeter a beginning. I'll probably buy some more caps, but I'll use this as the "standard value" until I have found some other combination that hopefully will play (even) better! I'm thinking of refining the boombox next year with a real improved filter
Though my house is just a few meters away from the ELFA store in Tallinn, I would really appreciate if somebody could take a look here and find the two parts that I need. I've done some searching, but I don't really know if I've found the correct parts.
Here are they:
Capacitor 4.7 µF (I think it is MKP?)
Resister 3.3 Ohm (Although a carbon film resister..?)

Hmm.. Looking forward for your help
Erik

[krilli]

(I just wanted to point out that you can use digital EQs to find the crossover values, and then later buy parts that correspond to the EQ curves you create. Probably you have one stereo analog output on your computer - you could then use one channel as the tweeter channel and the other as the midwoofer channel ... Put some nice freeware VST EQs on the channels and play with the slopes until it sounds right. If you pick the right EQs and tune them within limits, you can then recalculate and recreate that slope with analog components ... Might be better than guesswork, especially if guessing costs time and money with hardware.)

[V-bro]

Yes, but the digital EQ won't give you any values, just a guidline to tweak towards perhaps.... It is a helpful tool in combination to measurement equipment and a simulation software program indeed...

That capacitor looks fine, probably MKT, but that is perfect quality. The resistor seems a bit underpowered though?? Doesn't say...

[krilli]

Yes, but the digital EQ won't give you any values, just a guidline to tweak towards perhaps....

Sure it does, we just use a parametric EQ which gives us a precise Q value and -3dB point, and calculate the corresponding component values back from there ...

[amp6erik]

Thanks guys! I have to say that the trick to use a computer to get the values sounds really smart and simple to me, not mention logical when I am thinking about it for a while. One thing that I do not get though; let's say that I connect one tweeter to one of the channels and one woofer to the other channel as you said, Krilli (correct me if I am wrong). Won't the amp get an unbalanced resistance - literary (don't know the word) - to power? Which could damaged the amp?

Perfect, in that case I'll go with that series of capacitor. When you say that the resister seems to be underpowered, what does that mean? I continued my search for a resistor and find a bigger variety when I clicked on "Metal Film Resistors, Axial Connections" from this link and then chose 3.3 Ohm in resistance.
I would appreciate if somebody could take a look and see if I can use one of them or not.

Erik

[tlaine]

Hi guys!

I'm working with amsaarin on this project and wanted to share a couple of pictures and remarks on our progress so far.

All pics can be found here: http://koti.kapsi.fi/~tl/blaster/img/

Here are all our components set up and working. I mainly have two points that turned out to be interesting.



First, making any kind of box for the mid/bass speaker will do wonders. We were really surprised that if you event put your hands around the speaker that is lying on the table, you'll hear the lower tones much better, let alone carve a hole in any cardboard box and put the speaker there. The difference is ridiculous and we didn't expect it to have nearly as big an impact as it did.

Secondly, the breadboard turned out to be quite useful tool for testing the filters. However, since our speakers are not in any kind of proper housing, it turned out we cannot hear any difference when using the resistors (maybe the "blaster box" made the SP60 too loud). We have 2.2, 3.3 and 4.7 ohm 5 watt power resistors, but are probably useless until we have the actual box ready.

Also a funny remark: When we first listened with bare SP60s, we thought that the tweeters might have gone to waste as the SP60s sounded so good alone. However, after listening with the whole setup and disconnecting the tweeters on the fly, we could really notice the positive impact they made to the sound range. The saying "you won't miss it untill you lose it" made its case yet again :-)

Here is a picture of our cardboard prototype of the box.



Ikea furniture came in handy as I finally had a good reason to assemble the shelf I had lying around in the storage room... just to get the packaging cardboard.

First, we made the blueprints (sorry, they are in finnish). The measurements are based on 9mm plywood for all walls. The sides are reserved for the speakers and those spaces are connected by an air tunnel that you can see in back middle.

The center pillar is divided to two parts. Front part, which is visible and actually covered by the front panel, will be the input device compartment. We'll make a hatch to the top board through which you can drop the input device (phone/mp3 player). We will also pad the compartment so that the devices won't break even if the blaster is taking hits. Stereo input plug along with enough cabling will be in this compartment.

The back side of the pillar will hold the amplifier and the battery. The battery will be strapped to the walls and bottom. There will be some kind of shelf or other mechanism to hold the amplifier in the top part of the compartment. We haven't yet decided how to do that. Perhaps we just use plywood to split the compartment.

Our aim was to keep the box design as simple as possible while still separating the speaker compartment from the device and electronics compartment. After selecting the design in the blueprints, the sizes mainly came from the battery sizes we wanted to support. We searched for most common sizes for 7A batteries (just in case we wanted to use one instead of a smaller one), and set that as the size of the battery compartment. That determined the depth for the device compartment, height of the air tunnel, and the width of the whole speaker box.

I'm going to make a more detailed picture of the box and include all the sizes there. I'll post it to this thread once it's ready.

We should get the pre-cut plywood pieces tomorrow. Let's see if we can get something done by the evening :-)

[tlaine]

When you say that the resister seems to be underpowered, what does that mean?

We were pondering the same thing last Friday.

The thing is that the resistors you usually see on circuit boards, like the one you linked, don't have to withstand high power throughput. In this setup you apparently need a power resistor (wire resistor?) instead of a "normal" resistor. You can check this component picture to see that they are actually several times larger physically than the usual resistors.



The resistance is just 2.2, 3.3 and 4.7 ohm but if I understood correctly, they can handle 5 watt power throughput.

As I have next to no understanding of electronics, I don't know what will happen with the "normal" resistors but I'd guess they would just burn like a fuse. Also, I'm not sure if it actually makes any difference here in which order you put the resistor, the capacitor and the speaker. Help anyone? :-)

Here is the actual 5W 4.7ohm resistor we bought: http://www.yeint.fi/index.php?main=2...roductID=16153

[V-bro]

Hey, good work!! You'll be sureprised about how a wooden box sounds compared to the cardboard!

And Id take MOX (metal oxide film) resistors in at least 2 watt power handling. The wirewound resistors are often used in commercial speakers, but not the best quality... The MOX resistors are usually blue and indeed pretty large (about the size of a kidney bean)

http://nl.farnell.com/multicomp/mor0...10r/dp/1357898

[tlaine]

Hi,

After a short hiatus, here's a short status update on the project: We made the critical deadline which was the Finnish midsummer last Friday!

First test run in the casing: http://www.youtube.com/watch?v=uEXj2n1CVcE

The blaster is now fully operational but still lacks some finishing touches: paint, back panel locking, and carrying mechanism. They are currently replaced by a black marker, meters upon meters of duct tape, and a firm two-handed grip.

During midsummer we managed to blast for a good 30 hours in two and a half days. We eventually run out of time instead of battery (7,2Ah), so we don't know how long it would actually last. Around 20% of the time we used near-maximum volume we could get with different audio sources.

Here's a reference photo from Saturday 2AM on how to enjoy the results:

photo-712369.jpg

... and if that wasn't clear enough, here's a video:

http://www.youtube.com/watch?v=Ya8-M8lHIh0



Next up: finishing touches.

P.S. I started to write down our schematics for the box. Here's the latest working draft: http://koti.kapsi.fi/~tl/blaster/img...g_draft_2).svg. I'll try to make also one for all the electronics.