Boominator light-edition

[amp6erik]

Hey everybody!

It feels like I have read basically every thread here now, so I guess it is time to do something practical of all that reading. I am thinking of building a boombox - surprisingly enough - and I would need some help to see if what I have planned to build is a good combination or not. I also have some questions.
First of all, I planed to build the famous Boominator, but though my budget did not allowed that I thought that I maybe could "cut it in half", building a "light edition". My thought was to use two HP10W and two tweeters instead of four of each. It was still a little bit over budget unfortunately, but I would still manage. But now I saw that ljuda.se, where I was planning to order my elements, was out of stock of the element, and they expected to be in stock again in august..
So I began to search for other possibilities and this is what I found out:

I'm thinking to buy two Monacor SP-60/4 and two Monacor DT-25N that I have seen V-Bro recommend a lot.
It seems like it will suit my criteria quite good. Now, to some questions:
- I was trying to use ljudia.se's software to calculate how many litres the box should have. With two SP-60s in the same box, it says that I should have a box that is 11.09 litres big calculated with a qtc that is 0.707. I read somewhere that a qtc that is 0.707 is optimal in a middle size room and not outside. And therefore should you should write ~1.0 instead in the qtc (if I remember it correct...). But when I insert a qtc that is 1 instead of 0.707, and now it says the box should be 3.79 litres. Is that correct, or do I do myself to a laughingstock at the moment? )
- The SP-60 speakers is on 4ohm, if I understood it right, 4ohm is the best for the amp? The tweeter on the other hand is 8ohm. But according to what I have read here, it doesn't really matter. I have also realized that I need some kind of resistance between the tweeter and the amp to filter our the low frequencies. Is that correct, and what kind of resistance should I use?
- Do I have to (or is it recommended) to use a crossover (or maybe the name is frequency crossover?) to the woofer? In that case, is it easier to buy a two-way crossover so I can use it both with the woofer and the tweeter?
- Will it sound good? Will I have some bass at all?
- Will this setup suit the amp in a good way? Or can I get more bang for the buck maybe?

I am really sorry if the questions are so called "noob questions" but I have tried to find the answers in the forum before I posted, and I have probably found them, but I didn't really understand them I guess.

Thank you all in advance!
Erik

Edit: Sorry for the misleading name of the thread, "Boominator very light edition" does not really suit its contents. I accidentally pressed the Enter button before, and now I can't change the name. That's why.

No worries Erik, I fixed it for you...
Scratchy

[amp6erik]

I really hope that I didn't do anything stupid a second ago, I ordered the speakers without your expertise guys..
I will build the boombox together with a friend in Sweden - I live at the moment abroad - and I am going there in a couple of days. So I had to order the speakers now if I wanted them to arrive before/while I'm going to be there.
Instead of buying the Monacor DT-25N as I had planned, I bought the Monacor DT-99 because they had it in stock. According to what I read in the forum, the DT-99 should work as well.

That's all I had to say. Please answer my questions in previous post if you know the answers and have the time
Erik

[V-bro]

That is an excellent tweeter, no worries. Yes you will have bass, plenty I'd even say. Qtc of 1.0. well higher Qtc will indeed initially decrease the size of the enclosure quite a bit, but you will get a raise in output just above the resonant frequency of the driver. This is pleasant and very welcome for outdoors use, definitely. Just as long as it doesn't cost too much of the linearity, like what happens with car audio drivers in boomboxes. I would not go under 3 liters, around 4 is the best value for the SP-60 in a boombox. Don't go over 5 liters whatever you are planning.

So make it 8 liters...

You don't really need more of a filter than a cap for the tweeter. Anything else but a really well designed filter is only a risk of messing it up. Especially when you are using a 4 ohm woofer and 8 ohm tweeter, this takes totally different values....

The resistor in series is for the tweeter to lower it's output a little to match the woofer output, apply to taste....

I would say there is hardly more bang for the buck available at the size. The SP60 is a quality driver and very hard to beat.

[jesperstald]

As im using the EXACT same drivers for my miniblaster, i thought i might aswell move my questions to this thread.

For the DT-25N, a 12 db/octave (2nd order) high pass filter is recommended. As a 2nd order filter only takes an additional coil, it seems pretty straight forward to calculate. But how do you take that the 8 ohm tweeter needs to run along with a 4 ohm woofer into account?

If calculating this for a 2nd order filter is too complicated and too likely to mess things up, how do you take it into account for just a 1st order filter?

[gnome]

Actually they are never taken into account, the high pass and low pass filters are calculated in isolation. BTW that is what makes filter design so hard because frustratingly every component interacts with every other component including the voice coils, and it's not even something you could hope to calculate, even modelling with software is a bit of a crap shoot, which is why active crossovers are so desirable and there is a huge following of single driver designs, just so that people don't have to mess with passive crossovers. Now there are of course speakers with passive crossovers that sound great so there's no need to get too antsy about this, second order filters are pretty straight forward and there is no problem with mixing first and second order crossovers it might even be desirable, the only real problem is that you might have to match levels for the woofer and tweeter because of their differing efficiencies, but it is easily done with a couple of resistors.

[V-bro]

The hard part is that it is three dimentional math and there are so many exceptions and non linearities. That's why there is still need to measure and prove our theories. Even by the most experienced persons. If it would be easy to calculate then there would be excellent software that you would be able to go blind on....

12db/oct, shifts the phase even more, possible need to swap polarity of the tweeter, otherwise the interaction with the woofer will become funky and the marriage will be crap.

12db/oct is recommended for use near the resonant frequency, With the SP60 it is not necessary to cross that low. Crossing above 3K will go fine with a 6db/oct filter of just a single cap...

As for the 4/8 ohm differences please refer to the tables posted here:
http://www.41hz.com/forums/showthrea...r+values+table

As said you need to separately look for the desired values for the woofer and tweeter. Do keep in mind these are ballpark values and an experienced listener can tweak it further from there. Or if you want to go serious you can install measurement software and buy some (pretty affordable) measurement hardware. This used to cost several thousand about ten years ago, now for maybe €250 you can play along with the big speaker brands....

[Saturnus]

Experience also counts for a lot. For example if you see a rising response on the drivers near the intended xo frequency then an experienced person will know that instead of trying to cancel this with fancy notch filters, it's much easier to just shift the xo frequency for that driver (up for a tweeter and down for a woofer). Furthermore, reading the response graph would also tell you the optimum filter to use for a driver with a rising response. If the peaking happens over a full octave or more then a butterworth filter is desirable. If it happens over half an octave or less then a linkwitz-riley filter is desirable.

An experienced person will also know how to use impedance curves to his/her advantage. For example in the above example the SP60/4 is as everyone should know only a nominally 4 ohms driver. At the intended xo of around 3-4Khz the actual impedance is 6-8ohms which means that if you don't make an impedance correction it will actually be a perfect match for directly filtering to an 6-8ohms tweeter. There's a hint hidden here in that filters should be calculated on the actual impedance at the xo, not the nominal impedance.

I could go on forever about small observations like this but I'll stop here as we are to keep some trade secrets

[jesperstald]

The hard part is that it is three dimentional math and there are so many exceptions and non linearities. That's why there is still need to measure and prove our theories. Even by the most experienced persons. If it would be easy to calculate then there would be excellent software that you would be able to go blind on....

12db/oct, shifts the phase even more, possible need to swap polarity of the tweeter, otherwise the interaction with the woofer will become funky and the marriage will be crap.

Yeah i kinda had the feeling that it wouldn't be straightforward at all :P

12db/oct is recommended for use near the resonant frequency, With the SP60 it is not necessary to cross that low. Crossing above 3K will go fine with a 6db/oct filter of just a single cap...

That makes a lot of sense Hurray for simpler design!

At the intended xo of around 3-4Khz the actual impedance is 6-8ohms which means that if you don't make an impedance correction it will actually be a perfect match for directly filtering to an 6-8ohms tweeter. There's a hint hidden here in that filters should be calculated on the actual impedance at the xo, not the nominal impedance.

Monacor unfortunately only supplys a freq.curve for the 8 ohm version of the SP-60's.

The imp. is around 12 ohm at 3 khz. Can i assume that the 4-ohm version would have an identical curve with just half the values, so that it would be 6 ohm at 3 khz? if so, your estimation of the driver having 6-8 ohms at around 3-4 khz would be pretty spot on :P

Also, the DT-25N has an imp. of around 6 ohm's at 3 khz aswell:


Assuming 6 ohm at 3 khz, i use this calculator and get a cap value of 8.8 mf. Would this cap of 8.2 mf be fine?

[V-bro]

You are definitely on the right track and soak up the information well.

I would keep the cap value on the safe side, also you don't want to overlap too much in the most sensitive region of the human hearing curve. Here's also where the decent filter would have the edge, because the SP60 does go up to 5K pretty well, but many tweeters do it better down to 2.5K. Oh well, compromises.... I would take a couple of 2.2 and 3.3uF caps (NOT mF by the way!!) and then you can first cross really high, then parallel one or the other value and see what happens...

This kind of experimenting increases your experience in what sounds best...

And yes, the SP60/4 has almost the same curve and the impedance curve is very identical, only half the height over the whole range....

[jesperstald]

You are definitely on the right track and soak up the information well.

I would keep the cap value on the safe side, also you don't want to overlap too much in the most sensitive region of the human hearing curve. Here's also where the decent filter would have the edge, because the SP60 does go up to 5K pretty well, but many tweeters do it better down to 2.5K. Oh well, compromises.... I would take a couple of 2.2 and 3.3uF caps (NOT mF by the way!!) and then you can first cross really high, then parallel one or the other value and see what happens...

This kind of experimenting increases your experience in what sounds best...

And yes, the SP60/4 has almost the same curve and the impedance curve is very identical, only half the height over the whole range....

Sounds like the EXACT right way to do it. There's really no reason why i shouldn't mess about with it a bunch, to get it all optimized. Now i only wonder about cap choice; is it important to get caps designed for filtering, or will just about any (electrolytic)cap do? And don't worry, i meant microfarrad

Fx what about this one: Bipolar Rough foil capacitor 3.3uF 100V, or is 100 v overkill?