Enter the Dayton 2-Way Bookshelf Project (aka the Budget Bookshelf
Project). I had heard some great things about the Dayton
1 1/8" silk dome tweeter and that it compared very favorable
with a Morel
tweeter costing around $45. The Dayton tweeter cost $15.50 making me
want to try it out. If it tanked, so what?
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Dayton LoudSpeaker 1-1/8" Silk Dome Tweeter
(# 275-070)
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Dayton LoudSpeaker 5ΒΌ Shielded Woofer
(# 295-300)
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But what woofer to match it up with? Then the project idea hit me...
Set a maximum price limit of $100 for the speakers (in enclosures) with
good quality crossovers. All things considered, I decided to go with the
Dayton
295-300 5 1/4" woofer. In retrospect, I think I made a pretty
good choice, although woofers in this price range aren't all that
impressive. The 295-300 has excellent bass extension for such a small
driver. Also, it's very clean bass. The midrange however is merely
adequate and the rolloff of the driver isn't exactly picturesque. But
this driver cost a whopping $13.50. I think for the prices we're talking
here, we have to put all things in perspective and wait for the final
product before we judge any of the parts.
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My feeling about a loudspeaker must be designed for a room and with
a purpose. You don't shop randomly, right? No, you go to the store
with a particular amplifier, room, and placement in mind before you
shop. You might decide you want a tower or a bookshelf, but at least
you know before you start buying. Same thing with a loudspeaker... and
in this case I wanted to build a small bookshelf speaker that would
sound good in a small room (in other words, plan on a fair amount of
room gain and sacrificing efficiency for clean, accurate sound.)
Finally, to meet the spousal 'eye-roll factor' (S.E.R.F.) I decided
that a friend of mine would receive these speakers as a christmas
gift. A hundred bucks was just about the right price for this gift.
Now I had to deliver.
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Then, as usual, reality reared its ugly head when I realized
that I couldn't even begin designing a speaker without any measuring
equipment. Dayton's
tends to be pretty reliable and I figured their published T/S specs probably
were too... but I wasn't taking any chances, so I built my own woofer tester
(hence my woofer tester software was born). That solved my box design
concerns, but what about the crossover? That's when I added the radioshack
SPL tester to the rig (it was sitting on the desk anyway) and that's why the
frequency response
curve generator was added.
| Anyway... one thing always led to another; and after a
ton of programming, a lot of questions (helpfully answered by Paul @ Parts Express
who will will just call "da man" for short) and some good,
old fashioned hard work, the Budget Bookshelf Project came to life.
The bottom line is that for under $100/pr in attractive enclosures,
the Dayton Budget Bookshelf Project is a great sounding set of
speakers. Clean midrange, excellent high end and surprisingly deep,
tight bass. Would this project sound better with some Vifa PL18's and
3" ribbon tweeters? Sure.. but those drivers would cost
>$400/pr alone.. the crossovers probably another $50-100/each.
But.. that's the NEXT project ;) For the price of these drivers,
crossovers and enclosures, and given their excellent performance in
view of the cost... I'll be building this project again. |
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The Crossover
To select a crossover point, the first thing I did was to
test the woofer's T/S parameters and to build an enclosure based on the
measured T/S parameters and my anticipated crossover point of 2500-3000hz.
I used this to determine the width of the baffle and the location of the
woofer (making sure I put it in a place where the tweeter could be mounted
less than 1 wavelength from the woofer, center to center, and
approximately 1 wavelength from each edge for reasons I'll explain later.)
This resulted in a .48 ft^3 vented enclosure tuned to
47.5hz. As it turned out, the box measured at 48.33 hz, so I guess I did
ok in that respect. I then mounted the woofer in the box and began testing
the frequency response both on-axis and off-axis. The off-axis -6db point
for the driver was around 3.25 khz, so I built my first test crossover
based on the book values for each driver using the impedance I measured
for each driver at 3250 hz (13.5 ohms for the woofer and 6.35 for the
tweeter). I used a linkwitz-Riley 2nd order for each. Lo and behold! It
sounded pretty ugly. Between the baffle
step diffraction of the cabinet and the fleshiness of the upper
midrange of the woofer, this thing didn't sound pretty -- the design was
augmenting the worst aspect of the whole design, the upper midrange of an
inexpensive woofer.
On the plus side, the bass was clean, tight, and as deep
as I've ever heard from such an inexpensive woofer; and the high end was
impressive. When you open the box, the 275-070 has the appearance and feel
of a quality tweeter. When you put it IN a box, it has the sound of one.
This definitely gets my 5-star award for one of the best values in DIY
speakers. Accordingly, the high end in my enclosure was surprisingly crisp
and accurate. Nevertheless, the gain of the frequencies where the woofer
performed the worst, i.e., the midrange, had resulted in a pair of
speakers that would make your ears hurt (the midrange was overly
emphasized and quite muddy... Mariah Carey sounded like she had a pillow
stuffer over her face and she was screaming into it.)
Not being one for underkill... I then tried a crossover
point of 1850hz (I didn't want to go any lower because the tweeter's
rolloff is not very steep and the Fs is around 925hz for the driver i
tested). This actually sounded quite nice... so nice, in fact, that I
almost went with it except for some nagging peaking I could hear. After
about a full day of tweaking the components and retesting the frequency
response after each tweak, I found that the peaking was due to the high
crossover Q of the woofer when i used an 18 awg inductor (trying to keep
costs down). Alternately, i could go with a larger inductor and a lower Q,
but then there was a nasty null around the crossover (which appears to
have been a standing wave between the back of the woofer and the rear of
the enclosure. I tried to dampen the wave with stuffing, but then the Fs
of the enclosure would begin to rise as the null was removed.. nasty
catch-22.) So, I tried raising the crossover point. First I went to 2700hz
(which sounded great) and then i tried 2250hz (which also sounded great.)
The difference between the two was a +2hump in response of the woofer for
the higher crossover vs. about a +3db hump for the tweeter for the lower
crossover (baffle step diffraction.) I went with the lower crossover and
added the Lpad to even things out a bit. There's still a slight hump in
the tweeter response at around 3khz, but it doesn't seem to have any
negative affect on the overall sound quality. Hence I went with 2250hz.
Finally, the zobel was added at the end for two reasons.
First, the rise in response of the woofer was causing some moderate
peaking above crossover and giving me trouble getting the rolloff the way
I wanted (and overall 4th order acoustical filter.) Also, by using the big
cap in the zobel (about 4 bucks) i was able to use a smaller gauge
inductor for the lowpass and achieve the overall DCR I needed to get the Q
i was shooting for (basically to save money and eliminate a small peak in
the woofer response around 1500hz.). Also, the baffle step diffraction
causes some rising response up to about 1.2khz but is not compensated for
-- these speakers are meant to be used in small/mid sized rooms on stands
close to a wall or on a bookshelf where they will naturally compensate for
baffle step because of room gain. By attenuating the highpass i was able
to get a nice smooth curve up to about 9.5khz... after which the response
measures flat with a slight dip at 15khz which is inherent in the driver
(and just about every 1 1/8" tweeter I've ever measured.).
Here's the schematic for the final version
System Impedance (very amplifier friendly)
Notice that the "hump" is skewed somewhat to the left of the
crossover point. This is due to the higher impedance of the woofer
relative to the tweeter, so the maximum system impedance that results from
the crossover occurs where the lowpass section resonates. The highpass
resonates at around 2500hz.
| Internal dimensions: .48 cubic feet, W: 7.5", H:
12", D: 9.5" (this overvolumes the cabinet slightly to account
for the port and speaker volume, as well as a 1/2" birch plywood
'h-brace' mounted horizontally between the woofer and tweeter to support
the side f/r baffles. All construction is 3/4" mdf with 1/2"
quarter round edges. The front is 4 coats of hi-gloss black enamel spray
paint. The edge veneer is 3/4" white-birch banding and the top,
sides, and back are red-oak veneer. All wood surfaces have 6 coats of
minwax wood-oil. The vent is center-mounted on the rear of the enclosure
and measures 1.5" diameter x 2.75" long. The crossover is
mounted on the bottom of the inside of the enclosure and the top, rear,
and sides have egg-crate acoustical foam lining. |
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Final Comments
When you design a project like this, it's hard to separate
your ears from your mind -- you want it to sound good and every little bump
and dip in the frequency response graphs seems to weigh on your mind. Going
by the opinions of the people who have listened to the speaker, I'd have to
say it was a success. I built this as a christmas gift for a friend and my
wife said she was sorry to see them leave the house -- she wants a set too!
My 12 year old son thought they sounded great... and at least two visiting
neighbors (who didn't know I had designed and built the speakers) inquired
about where I got them and how much they cost.
MANY THANKS!!! to Wayne J. of Maryland for submitting this great project. You can see many of the incredible projects Wayne (also known on the technical discussion board as "The Reverend") on his website speakerbuilder.net Wayne is a "regular" on the board and countless speaker building enthusiasts have gained from his knowledge and experience building speakers. We appreciate your efforts!!!
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Copyright 2000 Parts Express
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