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Going to build a pair of loudspeaker monitors!, Help with circuits and stuff...
krafty
post Jul 4 2012, 02:17
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Hey guys,

Time to upgrade my little PC audio outputs.

I am planning to build up, DIY way, a pair of small loudspeaker monitors.

Currently, I have:

- A pair of these 4 inch mid-bass, 50W RMS 4 Ohms:




- And a pair of these 4 inch, 40W RMS 4 Ohms... that looks just like this one (with a little tweeter built-in):




These are very good quality speakers in the market here and I was also planning to acquire a separate tweeter for the monitor.


What I wanted is advice on design and circuitry for the setup. I have lurked on a website called http://www.humblehomemadehifi.com/ and there are some pretty good projects there but I am not sure if those big wire coils and huge capacitors are needed, as shown in their monitor models. It would be good to have a circuit to take care of frequencies like theirs but something simpler. Any suggestions?

And what about design... What would it be the best possible box/case design. Is MDF-wood good enough to make those?

What I am looking for is basically the best circuit to go with those speakers I presented and if I should seal the case or make it "opened".

Thanks for suggestions

This post has been edited by krafty: Jul 4 2012, 02:18
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Speedskater
post Jul 4 2012, 13:39
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The site that I would go to for lots of DIY speaker building information is:

DIY Audio forum loudspeakers
http://www.diyaudio.com/forums/loudspeakers/


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Kevin
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DVDdoug
post Jul 6 2012, 14:15
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You might want to get your hands on a basic speaker building book. Or, read everything you can find on the Net. I have a couple of speaker building books, but they are out-of-print. Parts Express has one that looks reasonable. ePanorama.net has links to quite a few helpful websites. It's easy to build a basic speaker, but it's not easy to build a good speaker (or a proper monitor).

At the low/mid price range, you can generally build something that's better than you can buy for the same price. But, most of us could not home-build a system to match the performance of speakers/monitors that cost thousands of dollars. We don't have the test equipment, nor do we do we have the time & money to experiment with various driver combinations and cabinet designs... We can't build our own drivers, and we don't have the clout to get design changes from driver manufacturers.

QUOTE
but I am not sure if those big wire coils and huge capacitors are needed, as shown in their monitor models. It would be good to have a circuit to take care of frequencies like theirs...
A minimum 2-way crossover network should have a capacitor and inductor for the woofer AND for the tweeter (4 components). Inductors in series block high frequencies and pass low frequencies. Capacitors in series block low frequencies and pass high frequencies. In parallel (with the load) these components essentially "short-out" the signal, so an inductor in parallel across the tweeter reduces the bass to the tweeter, etc. A a properly designed crossover "maintains" the impedance, so that an 8-Ohm tweeter and 8-Ohm woofer result in an 8-Ohm speaker. Without a crossover, wiring the two speakers (drivers) in parallel would result in a 4-Ohm speaker system (and it would sound terrible, and you'd blow the tweeter).

Some cheap speakers simply have a capacitor to block the bass from the tweeter, and nothing to keep the high frequencies from the woofer. If you don't block the bass from the tweeter, you will probably burn-out the tweeter. But, a speaker system with only a capacitor crossover can hardly be called a "monitor". wink.gif

For your first design, you might want to buy a crossover, rather than build one. I've built several speaker systems, and I've NEVER built a crossover from scratch, although I might have ordered one that was custom assembled for my requested crossover frequency.

Sometimes, there are additional crossover components to smooth-out the response of a particular speaker design. Unless you are copying a design exacty, with the exact same components and exact same cabinet, you should not copy a more complex crossover.

It's fairly common to add some resistors to reduce tweeter output (but only if there is a problem with highs overpowering the woofer output) If the sound is "harsh", that's someting you can experiment with. It's unusual to reduce woofer output, and it's something you should generally avoid. If you are getting too much woofer and not enough high-end, it's better to change to a more sensitive tweeter (or maybe adjust the box design to "tone-down" the bass).

QUOTE
...but something simpler.
The simplest way to build a speaker is to get a 2-way or 3-way automotive speaker and stick it in a sealed box. (Most of these will have simple capacitor-crossovers.) This might be the way to go on your first project...


QUOTE
And what about design... What would it be the best possible box/case design. Is MDF-wood good enough to make those?
MDF is fine. Many good speakers/monitors use MDF, and it's fairly easy to work with. But of course it's ugly in it's natural state. A couple of easy solutions are to use a "woodgrain kit" to create a "faux wood finish", or to "bondo" the cracks & defects and paint it, or on my last few builds, I've covered the box with leather-look vinyl (similar to what's done on guitar amps). 3/4" (~ 19mm) MDF is standard here in the U.S. Since a stiff-solid cabinet is important for good performance, it usually doesn't make sense to go thinner. Large cabinets often need thicker (or doubled-layer) cabinets and/or internal bracing. (If you were building a large subwoofer say 15" or 18", MDF can be very heavy and you might go with plywood because it's lighter.)

QUOTE
... and if I should seal the case or make it "opened".
That depends on your woofer. It's usually "safest" to go with a sealed box, and to some extent the larger the better. Bass performance depends on interaction between cabinet design and particular woofer. A poor (design (mis-match between the woofer and cabinet) can result in either wimpy bass or boomy one-note bass. As a general rule, you can get more bass from a ported design, but smoother deeper bass from a sealed design. But, you can build a very-good design either way. (There's no way to get "feel it in your chest" bass with a 4" woofer.) If you just randomly build a box, you won't get the best performance available from your drivers and most-likely you'll get poor performance. It gets VERY complicated, but if you have the Thiele-Small Parameters for your woofer, there is software available. WinISD is FREE.

If you don't have the Thiele-Small parameters, it's generally safest to build a sealed box, although you can experiment with adding different ports, and simply block the hole if it doesn't work out.

Good monitors and high-end sepakers are designed using software to start, and then the prototypes are tested and tweaked for the best actual performance. You can get a calibrated microphone and some test software for a few hundred dollars, but most of us don't have anechoic chambers to test in. wink.gif And, it doesn't make sense to spend more on test equipment than you spend on the speaker...

This post has been edited by DVDdoug: Jul 6 2012, 14:32
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Arnold B. Kruege...
post Jul 7 2012, 16:32
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QUOTE (krafty @ Jul 3 2012, 21:17) *
Hey guys,

Time to upgrade my little PC audio outputs.

I am planning to build up, DIY way, a pair of small loudspeaker monitors.

Currently, I have:

- A pair of these 4 inch mid-bass, 50W RMS 4 Ohms:




- And a pair of these 4 inch, 40W RMS 4 Ohms... that looks just like this one (with a little tweeter built-in):




These are very good quality speakers in the market here and I was also planning to acquire a separate tweeter for the monitor.


What I wanted is advice on design and circuitry for the setup.


The process of designing speakers seems simple enough:

(1) Obtain frequency response information about the drivers

(2) Design a crossover that optimizes the system response given the driver response information that you have.

(3) Build and enjoy.

The problem is that there are a few possible rough spots along the way.

(1) Reliable frequency response information may not currently exist or at least it isn't available to you.

(2) Nobody tells people this much, but there are drivers that just should never be in the same system. One obvious example would be a tweeter that is less efficient than the woofer.

(3) Custom designed crossovers are only as good as their designers, and off-the-shelf or cookbook designs are a route to mediocre reasponse at best unless you are tremendously lucky.

(4) Modelling a crossover and the drivers involves solving simultaneous differential equations with at least two unknowns, and maybe a dozen dependent variables, and that's just the linear part.


QUOTE
I have lurked on a website called http://www.humblehomemadehifi.com/ and there are some pretty good projects there but I am not sure if those big wire coils and huge capacitors are needed, as shown in their monitor models. It would be good to have a circuit to take care of frequencies like theirs but something simpler. Any suggestions?


You may be sensing it, so I will tell you outright, this page is a giant red flag:

Red Flag! Turn back! Go another way!

Executive summary: The site author majors in minors.

His basic crossover design may be good or not, but its implementation is pure snake oil.

This part sounds right:

QUOTE (humblehomemadehifi.com)
First of all loudspeaker drivers don't have ruler flat frequency and impedance curves. The load that a driver presents to an amplifier consists of a complex electrical impedance, a combination of resistance and both capacitive and inductive reactance. A loudspeaker driver does not have a constant resistance across its frequency range. Instead, the voice coil is inductive, the driver has mechanical resonances, the enclosure changes the driver's electrical and mechanical characteristics, etc. (source: http://en.wikipedia.org/wiki/Loudspeaker). For example a nominal 8 ohm midwoofer can have an impedance curve with peaks of over 50 ohms and a dip down to 6 ohms. So that makes all static charts, tables and online calculators completely useless.

So the only way to design a loudspeaker crossover correctly is to take very accurate measurements of each of the driver's frequency and impedance curves with their corresponding acoustic and electrical phase response, in their final enclosures and after they are fully burnt-in. Also waterfall plots must be made to see if there are any parts of the frequency spectrum that take a little too long to decay. And add in a few off-axis frequency measurements to check baffle edge diffraction problems and some distortion measurements for spotting any other nasties. And don't forget to measure the output from the port either - especially with a two-way system, there can be quite some midrange energy radiating from the bass reflex port!


The above is all fine and good, but unfortunately he seems to drop the topic right there. People have made careers by properly executing what's suggested by those two paragraphs, while many others have tried to follow them and spent inordinate amounts of time and money and ended up with junk, especially the time part. If you want good sound in the near future, buy some good cheap mini-monitors.

If you read and understand what the author said above, you should understand by now that since his drivers and your drivers are vastly different, what you seem to be wanting to do is probably going to fail to produce anything close to ideal or even good results.

QUOTE
And what about design... What would it be the best possible box/case design. Is MDF-wood good enough to make those?


MDF put together right can be fine. However, if you chose box material like the author chooses coils and capacitors, you'd probably be talking about foamed platinum. ;-)


QUOTE
What I am looking for is basically the best circuit to go with those speakers I presented and if I should seal the case or make it "opened".


If it would help, I'll do a line by line explanation of the two paragraphs from the site you mentioned that I quoted above, and explain how it means that you seem to have 0.1% or less of the clue you need to be successful at what you want to do. Meanwhile, you might just want to read them over and over again until their obvious meaning sinks in.
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krafty
post Jul 9 2012, 02:18
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Ok, I've seen a lot of information here, and am going to read it carefully line by line. Thanks for all replies.
I want to mention that I've been to that diyaudio.com website and perceived the people there are not very objective... I mean, they love pharaonic ideas and seemed to me that those are people who spend a thousand dollars for the sake of their intuition, and those drivers I have were about to be mocked - even showing them a local online shop they didn't find "any recommended" speaker to their taste. Thanks here I found more objective straight line answers.

I don't expect that these drivers will be the best thing in the world, but I am sure they will be better than what I have now. Believe, what I have now is really crap can't handle more than 4W RMS. Would like to thank Arnold for the comment on the Humble Hifi, indeed he takes the wrong path in that text, thanks for pointing that out.

As for the MDF comment, it is true, it is heavy. But I have worked with plywood and it is a pain because it cracks very easy. I was thinking about pinewood, it's the cheapest, but they are literally termite food. Still that would be only a remote possibility - being that way any one recomends pinewood? It's very easy to work with it and the tools.

Is it sensible to just ignore the "tweeted" woofer and just go with the woofer plus a separate pair of small tweeters?

This post has been edited by krafty: Jul 9 2012, 02:20
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