DIY build- Briefcase Speaker

[MFPT]

I've been taking to bringing my music with me when I go outside to study, but it's a pain lugging my speakers and wires around. Thusly, I designed a powered speaker I intend to build. It's very much a budget opportunity build, seeing as I don't have the funds to be buying expensive components. It will be built into an existing briefcase so that I can easily carry it around, and all wires/plugs will be self-contained.
Assets:

-Small Class-T amp, rated at about 20watts
-Full-range mids -ovals 3"x5"
-1/2 dome tweeters

Of course this does not include wires/building materials/solder etc.

I feel confident in building this single-cabinet speaker up to the point of crossing over. These drivers have a generous frequency overlap range of 6000Hz (4000-10000Hz). They're both rated at 8ohms and have almost identical SPL ratings. I think this means that they will balance well (neither too bass nor treble emphasis.) Is this correct?
Now for the Questions:
Where is a good source for concise information on crossovers? Do I absolutely need one? How do I build one? What would happen if I were to wire these speakers w/o attention to crossover? I'm very much an amateur in speaker-building and I've only ever put together simple single-driver speakers.

I'm hoping to build this setup while on break from University in December/January. Would anyone be interested in a writeup? I'll be sure to follow up with a report on my degree of success.

Thanks and all the best,
MFPT

 

[BoredSysAdmin]

You mean something like this:
Portable Amplifier and Speaker Box Mod by MetkuMods - Because you love your hardware!
??

 

[MFPT]

something similar...

Thanks for your reply
That's similar to my idea, but it's more flashy and of a higher budget. Mine will be contained within a briefcase. My biggest question is about crossovers, and the project writeup you reference (thanks, btw) doesn't say much about them.

As I wrote in my first post, my questions about crossovers are:

Do I absolutely need one?
How do I build one?
What would happen if I were to wire these speakers in parallel w/o attention to crossover?

Thanks,
MFPT

 

[Swerd]

Thanks for your reply
That's similar to my idea, but it's more flashy and of a higher budget. Mine will be contained within a briefcase. My biggest question is about crossovers, and the project writeup you reference (thanks, btw) doesn't say much about them.

As I wrote in my first post, my questions about crossovers are:

Do I absolutely need one?
How do I build one?
What would happen if I were to wire these speakers in parallel w/o attention to crossover?

Have a look at this design, the Bandit that uses a 4" Tang-Band driver. It works without a tweeter, but still includes a filter needed to smooth out the sound. Instead of crossover, think of the filter as a built-in passive equalizer.

I think the Bandit write-up should answer most of your questions.

 

[Swerd]

Assets:

-Small Class-T amp, rated at about 20watts
-Full-range mids -ovals 3"x5"
-1/2 dome tweeters

Maybe I misunderstood your original post. Have you already bought the 3×5" oval driver and the ½" tweeter?

I feel confident in building this single-cabinet speaker up to the point of crossing over. These drivers have a generous frequency overlap range of 6000Hz (4000-10000Hz). They're both rated at 8ohms and have almost identical SPL ratings. I think this means that they will balance well (neither too bass nor treble emphasis.) Is this correct?

The frequency overlap range of 4000-10000 Hz is useless information as stated. To design a crossover you must have SPL vs. frequency and impedance vs. frequency curves measured while each driver is actually mounted in the cabinets you intend for them. This will be different from the manufacturers "raw" specs.

The 8 ohms impedance and almost identical SPL ratings suggest they may balance well, but you really need to see how the cabinet changes things. It will.

Crossovers…
Do I absolutely need one?
How do I build one?
What would happen if I were to wire these speakers in parallel w/o attention to crossover?

Yes, you need a crossover if you have two or more drivers that aren't identical.

If you wired these speakers in parallel without a crossover, you would soon destroy the tweeter (especially a small inexpensive one). Before the tweeter died, the frequency range where both drivers were overlapping would have a very uneven sound, as the two drivers would add and cancel sound at different frequencies. Any tweeter must always have at least one capacitor in series with it to block low frequencies that can fry it. You have to learn by measurement, or by the manufacturer's specs., what that low limit is.

To design a crossover, you must learn how high that oval driver can go without starting to make what is known as break-up noise. This is likely to be much lower than you may believe. Then you must also know how low can your tweeter can go, while mounted in the cabinet, without distorting. Then you can estimate a crossover frequency, and go about designing the filters for smooth roll-off curves.

You said you wanted to keep this simple and inexpensive. I'm not sure these goals are compatible with designing your own crossover.

That's why I suggested a design, like the Bandit, where someone else has already done that work.

 

[MFPT]

perhaps I need to simplify

I see what you mean, Swerd.

I think it's probably best that I forget about the two-way system for now. While I want to research and do the work involved in making a hi-fi two-way system, it's not practical for me right now. I have decent full-range drivers that would give me satisfactory performance by themselves.
I'm effectively replacing what a set of small harmon/kardon computer speakers have been providing for me. I bring them around with me to provide portable music. (They're simply powered single-driver desktop speakers.) I believe it will not be difficult to improve upon them.
Since I will have already made the enclosure, perhaps at a later time I can do the proper research & calculations and upgrade to a properly crossover'd two-way system.
The woofer and tweeter I named I do not currently own. They're positively-reviewed sale items on PE. I added them to my order for the amp because they cost less than my shipping. Plus, they will be good learning tools, even if not immediately applicable to the project.

Thanks much for the good information. As I don't want to hog forum space for information that is readily available: is there an 'audio textbook' of sorts or technical .pdf file that would teach me the basics on creating hi-fi speakers and tell me how to make all the measurements and calculations I need to make good sound? It seems that the basics would not be too complicated if they're all detailed in one place.

A a side note, I noticed you're from Gaithersburg, MD. That is also where I am from, up on the North side near Goshen.
All the best,
MFPT

 

[MFPT]

this brings me a thought: it seems, at least theoretically, that two identical tweeters wired in series would yield a similar effect as having a resistor and a tweeter wired in series. Granted, a resistor provides a more consistent resistance, but it seems this would be practical.

 

[Swerd]

I think it's probably best that I forget about the two-way system for now. While I want to research and do the work involved in making a hi-fi two-way system, it's not practical for me right now. I have decent full-range drivers that would give me satisfactory performance by themselves.

I found another full-range small speaker design that might be adaptable for you. Zaph|Audio HiVi B3S single-driver

It's a long read, but it might be useful for you to see his thinking as he designed it. He illustrates with lots of measurements and computer predicted responses. It will also establish what you do and don't understand about speaker design – as a starting point.

I'm effectively replacing what a set of small harmon/kardon computer speakers have been providing for me. I bring them around with me to provide portable music. (They're simply powered single-driver desktop speakers.) I believe it will not be difficult to improve upon them.

Since I will have already made the enclosure, perhaps at a later time I can do the proper research & calculations and upgrade to a properly crossover'd two-way system.

The woofer and tweeter I named I do not currently own. They're positively-reviewed sale items on PE. I added them to my order for the amp because they cost less than my shipping. Plus, they will be good learning tools, even if not immediately applicable to the project.

Your basic idea of using a T-amp and a single full-range driver is a good one. It should be doable. The two designs I've mentioned both may be a bit large and heavy if portable is your main goal. But there are ways to make them lighter.

Designing a cabinet for bass response is more straight forward than getting a crossover right for a 2-way speaker. It's a matter of knowing three measured parameters (Thiele/Small parameters) for the driver. They are the resonance frequency (Fs), volume of compliance (Vas), and total system Q (Qts). With those 3 values, you can plug them into a number of web sites that have calculators that calculate the volume and bass tuning of a box appropriate for your driver. Such as http://www.mh-audio.nl/spk_calc.asp#mnu_drivers.

I know of a book which explains all this, Speaker Building 201 by Ray Alden. I have a copy. There are many web sites that talk about this, but right now I can't think of one that really explains it well. Try google and wikipedia for starters.

A a side note, I noticed you're from Gaithersburg, MD. That is also where I am from, up on the North side near Goshen.

We are very close by. Keep in touch by PM if you are interested in DIY. It's my favorite topic.

I got into this a number of years ago as a way to interest my son who was then in high school. There was a science project involving something we named "the plywood doghouse", a comparison of sealed vs. ported speakers. I had fun helping. After I met a few very good DIYers in the local area, I realized I could learn lots from them. I also decided I was better at building established and proven recipes rather than buying all the gear, teaching myself how to use it all, and designing my own. A good crossover can make even inexpensive drivers sound good together. But it is also very easy to design a crossover that "sucks the life out" of speakers. It is not a simple topic.

this brings me a thought: it seems, at least theoretically, that two identical tweeters wired in series would yield a similar effect as having a resistor and a tweeter wired in series. Granted, a resistor provides a more consistent resistance, but it seems this would be practical.

A good question, hmm, let me think (as I stall for time)…

A simple resistor is cheaper…

Two speaker drivers in series is usually avoided, but never say never. There must be examples of it, but I can't think of one off the top of my head.

The voice coil of a driver has both impedance (the AC version of resistance) and inductance because it is a coiled wire, which acts as a low-pass filter with an AC signal. So the 2nd tweeter might see a different signal than the 1st in the series, and that signal might vary depending on how the 1st voice coil modifies it. It isn't impossible, but it does introduce another variable. Unless it provides a clear benefit, why bother?