The speakers: The new 6.5" co-axial bookshelf speakers offered through Best Buy for $45.99.
Objective 1: Measure, test and evaluate.
Objective 2: Design an upgrade to maximize their performance potential.
Objective 3: Offer the upgrade at the lowest possible price point without giving up performance.
First thoughts: Not a bad looking pair of speakers. They were well packaged. They have good looking binding posts too. But, knocking on the side of the cabinet revealed some serious hollowness.
First listening impressions: They sound better than most speakers out there in the $50 a pair price range. You get a lot for your money. However, that hollowness is pretty prominent if played very hard. The resonation of the box really colors the vocal range. There is also a peaky resonance in the upper vocal range that is very annoying. Vocals seemed very veiled as well. Highs seem smeared and muted too. These are no high-end contender or giant killer, but there is potential.
Taking an initial frequency response measurement showed them to be within a +/-3db range but pretty rough.
These are the response curves taken on each speaker. There is a little variance in the tweeters response between the two as shown. The peak in the 840Hz region might be responsible for the upper vocal edginess. What's worse is the near 6db dip that follows. The tweeters response is a little ragged too, but workable.
Next I ran a spectral decay to see if any of those response peaks were the result of a resonance problem.
It looks as if there's a little excess energy in that 840Hz peak. This will require attenuating. The rest looks pretty clean.
Next is an impedance sweep.
No issues here. The tuning frequency is 57.9Hz and the impedance minimum is 5.45 ohms.
Taking a look inside the box allows other problems to come to light, and no surprise at all was the very cheap components.
The woofer circuit used a single laminated Iron core inductor with what looks like 18 gauge wire.
The tweeter circuit used a pair of small electrolytic capacitors coupled to make one value. It also used a small (maybe 20 gauge), air core inductor, and an 8 ohm sand cast resistor shorted by a polyswitch.
The box resonance problems are now very apparent as well.
In order to create the curved sides, the MDF was slot cut to allow it to flex. My measurements show the MDF was .65" thick. The slotted grooves were cut 1/2" deep. Those are quarters standing up in the slotted grooves. This left only .15" of MDF (and vinyl covering).
There is one brace centered between the drivers and the port, but this did not help the large open area behind the drivers.
So this left two areas of major improvement to focus on. The enclosure problem and the network.
First lets look at the crossover. I designed a new network that uses second order electrical slopes on each driver. I also had to use a notch filter to attenuate the peaked area centered at 840Hz.
The crossover point is right at 2kHz. I would have pushed it a little higher but the woofer had some break-up just before 3kHz that needed to be avoided.
The new spectral decay shows to be cleaner as well, especially in the 840Hz range.
The new impedance response shows the effect of the more corrective network.
Further mods to the box, and damping also lowered the tuning frequency to 51.27Hz.
Parts used in the crossover upgrade:
In the new tweeter network there is .22mH inductor in parallel to the tweeter. With the 16 gauge inductor that was used for testing, the response looked better if a one ohm to one half ohm resistor was used in line with the inductor.
To reduce the crossover cost and number of parts used (no resistor needed) a creative solution was found in the stock crossover.
There is a higher DCR small gauge (maybe 20 gauge) air core inductor that is a .30mH value. This can be easily removed from the old network and reused. Adjusting the stock inductor to the correctly needed .22mH value involved de-winding it by 23 turns.
For capacitors we chose a high quality, poly cap from Erse. It is similar to and with a slight edge in performance to an Axon or Solen poly cap but is less expensive.
We then by-passed them with a small .1uF Gen. 2 Sonicap. The by-pass cap gives a very noticeable increase in resolution and detail, with less smearing effect and cleaner space between notes. The .1uF Gen. 2 Sonicaps are only $2.70 each and clearly worth the cost. There are six of these used in the upgrade kit.
The cap value used in the notch filter (in the woofer circuit) is a 56uF value. This can be an expensive component. An Axon 56uF poly cap is $14.17 each. The less expensive Erse 56uF value is $10.79. We chose to use a 47uF electrolytic ($1.00) by-passed with an 8.2uF Erse poly cap ($2.45), then by-passed with the .1uF Gen. 2 Sonicap ($2.70) for a total cost of only $6.15. It also sounds better than using a single poly cap value (due to the additional Sonicap).
Inductors pictured below are the Alpha Core foil inductors. Due to the increase in the cost of Copper since this upgrade was developed we had to either go to a less expensive inductor or increase the price of this upgrade kit. So we choose to go with the Erse, high purity, OFC, wire wound inductors.
Resistors used are high quality (but inexpensive) Lynk brand resistors.
High purity 16 gauge OFC wire, 4% Silver solder from Vampire wire, and heat shrink is also included in the kit. Also shown on the right side of the picture are the re-used inductors.
On to box mods:
The box needed a lot of help. Those slotted grooves had to be filled. There are several products that could be used for this, but we chose to use Durham's rock hard water putty.
This product dries hard as the name implies, but the real pluses are that it is mixed with water, has no smell, has no fumes, and you can stick your hands in it to smear it around without harm. It easily rinses off when finished too.
It was also low cost. The 4-pound can was only $5.49.
I mixed it a little thin and spooned it in to the areas around the port. I then wiped it around with the spoon and tilted and shook the box to get it down into all the hard to reach grooves.
I did all of one side of each box and let it dry for several hours then flipped the box over to do the other side.
After giving the second side time to dry, I then went back over each side with a second coat as some of it had settled down into the grooves and did not completely fill them in. After a second coat to each side the enclosure already felt much more solid.
I used 3/4 of that 4 pound can of water putty.
Now the enclosure was solid and had more strength but still had some resonance to it. More needed to be done, but first I would need to mount all those crossover parts into that small space.
The solution that I came up with helped solve another problem. I was able to mount the components in a way that also stiffened the box. To do this I mounted the crossover to a 1/2" piece of MDF.
The piece of MDF was measured to be the same length as there was from the top of the box down to the horizontal brace that is located just below the drivers and above the port.
So once installed it will also be acting as an additional brace from the horizontal brace to the top of the box.
Pictured on the left is the side of the piece of MDF that the woofer network is mounted on. This was all that would fit on one side of the 7.75" tall and 3" wide piece of MDF.
Flipping the MDF over, the tweeter network was mounted on the other side.
Liquid nails and plastic zip ties were used to mount the crossover. These materials are not included in the kit. You'll have to pick them up at a local hardware store.
Next came mounting the network into the box and connecting it to the binding posts. This was fairly easy.
First it was connected to the binding posts in the rear. Then I coated the brace and top of the enclosure with plenty of Liquid Nails. The tightly fitting crossover board was then slid into place. I then let it dry overnight.
Once dry I could then move forward with adding the rest of the damping material needed to control those wall resonances.
First small pieces of fiberglass insulation was stuffed in behind the network to fill the space between it and the back wall.
Then the No Rez damping material was cut and applied to the side walls.
This made a huge difference. The walls now had a solid and well dampened thud to them instead of the ringing sound of a thin piece of wood.
See info on No Rez.
No Rez comes in a 24" by 27" sheet. It cost $38 a sheet. The cost of one sheet of it will blow the budget for the upgrade, but it is an absolute necessity.
So I decided to do something that I don't normally do. I am going to sell a partial sheet of No Rez.
It requires about 1/3 of a sheet. I am going to slice off 1/3 of a sheet of No Rez and add in only $13. to the cost of the kit. It is pre-cut in one direction. You only have to slice off pieces in one direction leaving no waste. In the end there was one small piece left over that was cut in half and added to the front edge of the top of the enclosure.
I then stuffed more fiberglass insulation around the crossover to minimize any surface reflections from it.
The final result is a fairly smooth response, a well dampened enclosure, and a very good sounding pair of speakers. I know of nothing for under $150 a pair that will touch these.
Total kit cost for all parts, wire, solder, heat shrink, No Rez, and wiring diagram is $95.
So the total cost for an upgraded pair is $45.99 for the speakers, $5.49 for the rock hard water putty, $95 for the upgrade, some sales tax, some small materials cost. For this you get a weekend of fun, and you wind up with a very serious little speaker that can complete easily with a lot of very expensive speakers out there, and embarrass a bunch more too.
Even though these will beat up on a lot of speakers, I'd still have to say that they are still not quite to the performance level of our A/V-1 kit.