Boston Acoustics HD8 – build quality impressions & DIY woofer surround repair

[momaka]

Boston Acoustics fans, this thread might not be for you.
A few years back (~2018 time frame), I picked up a set of free JVC speakers from someone on Craigslist that no one seemed to be interested in. The guy also had a single Boston Acoustics HD8 speaker – yes, just one cabinet. He said he had no idea where he got this one from or why it's only by itself. But like the JVC speakers, it was going to get tossed at the end of that week… so I came to collect, as usual. And that's all I know about this HD8. After all, I figured why not take it and see what's inside… show it to the world, if nothing else.

On the outside, it looked in pretty decent condition. The front cover had a bit of staining from unknown liquid that I couldn't get out, but it's actually not as bad as the picture above shows.




Next is a picture of the name/model plate, as shown below. Made in USA and the box has good weight to it.


Removing the front cover revealed rotten foam surround on the woofer driver. Of course, problems like that are to be expected on old speakers – especially on freebies.


The woofer is 8” (20 cm) diameter with a polypropylene cone. The soft dome tweeter is 1” (2.54 cm) diameter and actually looks like good quality stuff. Sticker above shows this:


And below is a closer look at the (now) 30-year-old woofer driver! Yes, production date on the woofer says “06/14/91”. It wasn't quite 30 years old when I found it, but not like that makes any difference for the foam surround – it was still quite rotten when I took the pictures below.




In my opinion, the woofer driver is the only weak point on these speakers, as you will see later. Overall, it was built quite on the cheap side. The size of the magnet is disappointing at only 2 & ¾ “ (7 cm) diameter. Even if the motor has very tight gap tolerances, I don't think this magnet is strong enough for the 1” voice coil to produce good low-end bass (50 Hz and under.) Moreover, the cabinet is sealed type, which means the woofer driver will have to work even harder at very low bass frequencies, as the cabinet will present higher and higher load pressure. Also, the motor is not magnetically shielded, as far as I can tell. On the plus side, at least the motor bottom plate is “bumped” for extending the voice coil travel distance slightly. The basket is also decent, made of fairly thick steel with good folds to keep it sturdy. Under the speaker rim, there is a foam gasket as well, which should keep air from leaking around the speaker (this is something that cheap speakers always skip out on.)

Anyways, moving onto the soft dome tweeter…




It's actually has a good weight to it and seems to be of much better quality. What I find amusing is that its magnet is almost as big as the woofer's, measuring at 2.25” (5.7 cm) diameter.


Next, a look inside the speaker cabinet/box itself with the drivers removed.


That's very good construction, IMO. Holes are cut nice and clean. The front wall is made from decently thick particle board. There's also a good amount of poly-fill baffle inside for a sealed box. And although not visible in the picture, there are many big triangular pieces glued on the corners inside the box to make it stronger and reduce self-resonance.

Finally, here's a look at the crossover network:




Again, IMO those are some pretty good quality components right there. The woofer crossover has a nice big air-core inductor and a large cap to limit the high frequencies going into it. Meanwhile, the tweeter has the smaller series cap (the 4.7 uF one, I think), along with a 1.5-Ohm 10W resistor and another air-core inductor to remove any low frequencies from going into the tweeter.

So overall, this is a really well-built speaker, aside from the underwhelming woofer driver. But because the woofer plays such an important role (particularly in the bass department), I didn't really see a point in doing a proper surround repair (i.e. buying replacement foam surround online.) I was already pretty sure this speaker would lack a lot of low-end bass. On that note, a pair of these speakers might actually sound pretty decent with a separate subwoofer. Unfortunately, I didn't get a pair of these – only a lonely single speaker. So there's not much I can do with it. The only two things I can think of are either to use this speaker as a center channel in a home theater or just convert it into a portable/blutooth speaker as a fun project. The former probably won't happen, since I don't really watch TV and I already have a system for that. The latter… maybe I can see it happening (if I get really bored.) But in that case, I probably don't need a properly repaired speaker for such project anyways.

I also entertained the idea of giving it away for free… but I just couldn't see anyone who would want a single speaker like this. I'm sure the dome tweeter and X-over network will sell easily online… but that would leave me with a cabinet and broken driver to junk. Therefore, I figured it might be better to fix it cheaply / in DIY fashion, so that at least it's not sitting around broken. This would also allow me to hear how the speaker sounds overall. And if it really does sound good, I can always undo my crappy DIY surround repair and redo it properly. So off to that part next.

[momaka]

Boston Acoustics HD8 – DIY woofer surround repair

Part 2: making a cheap DIY paper surround

As some of you might remember, this is not my first rodeo with a DIY paper surround repair… or my second, for that matter. (The first were these Realistic Nova-18 speakers and the second was a loose Cerwin Vega VSW-10 driver from a set of 4, just to test out the driver.)

Such paper surrounds are clearly not a stellar repair in any way, but it does allow the speakers to be operational again. Actually, in the case of the cheap-o Realistic Nova-18: I've been using them with the crappy DIY paper surrounds for 7 years now, and they are not showing any signs of going bad. Sure I don't play them every day… or even once a week, for that matter. But the occasional times I do play them (once a month, give or take), I regularly crank the hell out of them. I even bottomed out the cone once or twice. Still working OK, though. If anything, the paper surround on those has gotten “better” – that is, it makes a lot less noise now, since it's had a chance to “break in”. I also made a second surround for another of the CV drivers (not pictured here), and on that one I experimented with latex paint. The results from this were very positive (oh, and the CV drivers are not ruined in any way, BTW – all these paper surrounds are easily undo-able.)

So I figured the Boston HD8 above is another good candidate for the DIY paper surround fix. (On that note, the CV drivers are not, as they do deserve a proper repair… but I promise I will redeem them some day soon. ) Here are my steps to doing such a repair.

Step 1: Measurements and making a “former” for the surround

First take measurements of the woofer's outer basket rim diameter, inner basket rim diameter, outer speaker cone diameter, and cone glue surface thickness. After that, find large enough piece of cardboard. Using a compass (or other similar tool of choice), draw these diameters/measurements as circles on the cardboard. Then find a bendable round object (like a hose or cable) to lay down onto the cardboard where the surround “bump” would coincide on the actual speaker driver. In my case, I found some clear ½” (~13 mm) PVC tubing in the garage that had just the right diameter for this particular surround (actually it was a little too thick, but more on that later.) I've also used computer cables for this before… so the choices aren't restricted. Lastly, tie down the hose/cable/round object to the cardboard former, so that it doesn't move. The final result of this preparation should look similar to the picture below:

(Or here is the one I used for the Realistic Nova-18.)

Step 2: Make the DIY paper surround

Lay some wet paper on the former that was just created, as shown:


In my case above, I used Bounty kitchen paper towels, since the paper is nice and soft, as well as slightly stretchable. Of course, feel free to experiment with any other kind of paper here. I've already tried regular PC printer paper with the Nova-18 speakers. While it does work, I don't like it or recommend it – there is too much noise from the surround until the paper “breaks in” (may take months or a couple of years of regular use.) I also don't recommend toiler paper, as that will likely fall apart quickly… unless it's really cheap unbleached TP that's about the same texture and strength as newspaper. Still, I won't recommend that either. On a similar note, newspaper should also work… but IME will have the same problems as PC printer paper. So kitchen paper towel really is the best IMO, provided its layers don't separate easily (had that happen with one brand, and it made for a poor surround that needed re-gluing multiple times… but that was on a more recent speaker repair that I need to post.)
Also, give yourself extra paper on both sides of the surround where it will be glued to the speaker cone and the speaker basket. You can always cut away the excess later, but may not have an easy time adding more material if it's too short.

Step 3 (optional, but highly recommended): Paint the paper surround

I don't have pictures for this step (and only the final result), because I didn't paint the surround until after I installed it onto the woofer driver.
Up to this point, I have only tried regular household latex paint. Both interior and exterior paint should work… though the more “rubber-like” the paint is, the better it will be. For that reason, I do recommend “glossy” or “semi-glossy” paint, as it appears to be slightly more stretchable.

Again, while painting is not a necessary step, it really helps to dampen noise from the paper surround, since dried latex paint by itself is fairly flexible (maybe not as much as rubber, but certainly not hard like plastic either.) It also makes the paper surround more durable.

Now, there are two options here: 1) paint the surround right away as soon as it is placed on the DIY former (recommended) or 2) paint the surround after it is installed/glued on the speaker (not recommended, but that's what I did with the Boston HD8 woofer driver here, just to try this method too.) The reason I don't recommend painting the surround after it is installed on the speaker is because it's hard not to make a mess and the surround will very easily loose shape after it is removed from the former (therefore making the installation on the speaker driver much harder.)

Also, painting the (wet) paper after it is put on the DIY former will actually make the paint soak into it. This is a good thing, as it will allow the surround to retain its form when removed from the former. Just be careful not to over-apply too much paint, as that can make the surround stick on the former (making it hard to remove) or make it too “dense”. On that note, it also helps if the cardboard former has a smooth finish (such as packaging from soda & beer boxes, or similar) instead of more “raw” cardboard like mine above.

Step 4: Install/glue surround onto the speaker driver

This step is pretty much the same as doing the repair with regular surround replacements. The only difference is that some trimming/cutting may be required. In particular, it's best to make the paper surround edges slightly larger and then just cut away the excess paper in order to make it fit on the speaker. Also, feel free to glue whichever piece/side you want (i.e. you can either glue the cone to the surround first or the surround to the basket first.) With proper replacement surrounds, the recommended method is to usually glue the surround to the cone… but with my DIY paper surrounds, I often do it the other way around, just because the DIY surround doesn't always come out as one piece (for the 10” surrounds on the Nova-18 and CV drivers, I built multiple “halves” that then had to be spliced them together on the speaker while gluing at the same time.)

With that said, here is how the Boston HD8 woofer driver turned out:


Again, I'm actually a bit dissatisfied about not painting the paper surround before gluing it onto the speaker, as it made the task much harder and I ended up not centering the cone 100% correctly. Although the voice coil does not rub anywhere in the motor for the whole travel of the cone inwards or outwards, I can make the VC rub a little if I press the speaker cone on the lower side… but not if I press it on the upper side. It does still work correctly, though, without rubbing if moving on it's own from the magnet (tested in open-air with low frequencies ~10-20 Hz for high excursion.)

That being said, here are the results after painting the surround with some black latex paint.




It's definitely not the cleanest-looking job… though not bad considering I did it in a bit of a hurry. Most importantly, the surround works as it should without making clicking noises when moving, thanks to the latex paint and the fact that paper towels are softer. Moreover, the paint really sealed the surround well, making it fairly air-tight - I can press on the speaker cone (when it's inside the cabinet) and feel that no air is escaping from it… which matters here, since this is a sealed/closed box design. Actually, I'm surprised how well the speaker holds pressure. With the speaker loose out of the box, it acts springy when tapped, just as it should. But inside the box, if pressed in hard, it takes about two seconds to come back up. Good to go!

[momaka]

Boston Acoustics HD8 – DIY woofer surround repair results and comaprisons

Part 3: Results / Sound

In terms of sound reproduction, I was spot-on right (about my hunch) of how I though this speaker would sound: good midrange and highs, but very lacking in bass, due to weak motor (small magnet) on the woofer driver. In particular, the bass roll-off starts to get real around 70 Hz. By the time it reaches 60 Hz, it's noticeably weaker. And below 50 Hz, there's hardly anything. But otherwise, the sealed box does quite well in providing a linear bass drop off. There are no harsh “boomy” notes or other weird phenomenon. And in terms of midrange and high frequencies, both the woofer and 1” soft dome tweeter work very well together without over-emphasizing anything too much in the mid-range or high frequencies.

So overall, the Boston HD8 would make for a decent set of speakers, minus low-end bass. With an additional subwoofer, a setup like this might even sound respectable.

Now just for fun, I did a comparison test with my other speakers, playing them all side by side. All in all, the HD8 sounded similar to my Sony SS-MB215, only with slightly less detail in the treble and a lot less bass. (The SS-MB215 are no Hi-Fi speakers either, but overall decently “balanced” for everyday use.) Actually, my Sony SS-MB215's also used to have a lot less bass, but I added a port on them, and that really opened them up. Since the SS-MB215's use 8” woofers too, I thought it would be interesting to see the difference between the drivers here. Below you can see a picture of the SS-MB215's motor stacked against the HD8's.



Ignoring the shielding magnet on the SS-MB215, the HD8's magnet is almost ¼” to 3/8” (~10 mm) smaller in diameter. Otherwise the rest of the driver construction is pretty similar between the two (1” voice coils measuring about 8 Ohms DC resistance and light polypropylene cones.)

So when I tested the HD8 woofer in its cabinet, it didn't surprise me that it lacked punch when it came to bass. However, that spawned another idea: what if I tried the HD8 woofer in the (now ported) Sony SS-MB215 speaker cabinets?
Well, it was a good idea… but the HD8 woofer didn't fit in the hole of the Sony speaker box by a mm or two. This didn't stop me, though!



As terrible as that may look, it was good enough for a test. Actually, it revealed that the HD8 woofer also isn't too bad in terms of bass, once placed in a larger ported enclosure. There was reasonable output down to about 50-ish Hz with the HD8 woofer and good output above 60 Hz… fairly linear too. In comparison, my SS-MB215 woofers go down only a little further – good output to about 40 Hz, some output to 35 Hz, and very quick drop-off below that, regardless of the port length. But this is to be expected – neither of these speakers have very big motors. So the HD8's smaller magnet will definitely loose some dB's at the lower frequencies. Also worth mentioning is that I tested both of these with the bass adjustment on my tone controls maxed out (about +10 dB.)

And that pretty much concludes the repair & tests. The HD8 works for the time being.
Now the next question is whether or not to make the HD8 speaker box ported. I haven't decided on that yet, as the box looks fairly nice and I don't want to make it uglier with my “handiwork”. On the other hand, that HD8 woofer motor really needs all the help it can get, and a port will certainly do that. Until I decide, though, back to storage it goes… unless anyone here wants a lone HD8 speaker for any reason or can suggest a better use? Let me know.