Baby Advent II Haxx

[nittpickens]

Hi there. Most people on this board seem to be interested in building speaker enclosures from raw MDF or plywood, whether the designs are their own or those of more experienced designers on the forum. I am going to try posting a hack to a commonly available commercial speaker. Not all of us have the skills, equipment, or workshop to build our own enclosures. In my own case, I live in an apartment in Brooklyn, and have had to build enclosures out on the balcony with hand-held power tools, dodging complaints (or worse) from the neighbors. It's been much easier to use pre-existing enclosures. Empty cabinets from Parts Express or Madisound have their appeal, but they still require driver and port openings to be cut. This post will discuss "zero sawdust" upgrades to the Baby Advent II speaker. The goal is twofold: people in my workshop-challenged situation may find a way to get good DIY sound, and others may get some information that they can use in new builds.

I ended up with an Advent Baby II after talking to my sister about my DIY audio enhancement and design projects. She had some of these speakers and I decided to use them for a rebuild project, after giving her some loaners.

The Advent Baby II is a sealed design with a 6.5" paper woofer and a cheap mylar tweeter. International Jensen bought the renowned Advent name back in the 80s or so, and started mass-producing speakers. The enclosure is maybe 0.4 cu ft, made out of decently sealed 1/2" fiberboard, with a shallow profile. The crossover is ignorantly designed. It's a first order x-over with the components chosen from a textbook. Combined with the woofer roll-off and mylar tweet, the system has a muddy sound.

OTOH this speaker is commonly and cheaply available. I've seen it on Ebay for $40/pair and think many people will be able to find it easilyfor $40 or less. As such it's an interesting rebuild candidate.

I'll proceed with two approaches:
1) modify the drivers and crossover in the existing speaker system, trying to improve the sound ($5 option)
2) replace drivers and crossover with upgrades

I can understand if people don't want to read about hacks to existing craptastic speakers. If moderators want to shut down this thread, please go ahead and do so. I will try to post some useful observations from the redisign process within the next day or two, including some measurements.

[servicetech]

I'd at least replace the tweeters and add a zobel network to the woofer. I like the simple series crossovers, how about a 6db/octave network? Keeps the cost reasonable while dramatically improving performance. I won't sound like some of the $200/pr designs posted on here, but you can keep the total cost under $100/pr

You will probably have to saw the tweeter hole unless you can find a decent tweeter to fir the hole exactly.

[nittpickens]

Thanks, servicetech. It wasn't my intention to start a thread here and then disappear, but life intervened. I did plan to try a couple of different modifications as an experiment, including a tweeter replacement, as well as moving on to a full woofer and tweeter replacement.

First, I measured and modeled the existing drivers. They are, unsurprisingly, not that great. First, the woofer has something like a 5 ohm nominal impedance, with a nasty looking peak at 9k, above -3db. It does have a nice low Fs making for decent bass response in this cabinet.

The tweeter has no Fs peak that Speaker Workshop can find. On other sites more knowledgeable persons have associated this effect with high harmonic distortion.

Taken together with the textbook 1st order crossover, the modeled frequency response shows a broad dip in the treble response, reflected in the muddy sound of the original speakers. I didn't measure the in-room response at this time, figuring I would A/B compare a modified speaker later.

My first idea is to coat the OEM woofer with a couple coats of thinned Mod Podge, to smooth out the resonant peaks and roll off the treble response. Then, switch the first order crossover to a 2nd-order crossover on just the tweeter, letting the woofer roll off on its own. To do that, unsolder the inductor lead from the terminal cup + and rotate it over to the T+ lead. Move the woofer + wire to where the inductor lead was. Solder a jumper from the old W+ hole to the terminal cup -.

With these modifications, the modeled frequency response improves quite a bit. There is still a lot of roughness in the treble response, including a phase-induced null. An earlier model with one coat of Mod Podge looked better, with no nulls. Listening to the modified speaker, it had much better clarity and definition. Unfortunately, the better clarity also revealed the higher order distortion problems in these drivers. The speaker sounded pretty gritty. A/B comparison with the original speaker showed the grittiness is there in the original speaker, but masked by the overall muddy sound.

Worse, at this point I realized I had damaged the foam surround on the woofer, so I stopped taking measurements and decided to move on to replace the drivers.

In conclusion, you can improve the sound of the Advent Baby II by coating the woofer with Mod Podge and changing the crossover topology using the existing components. But you still don't end up with a good sounding speaker, and the age of the foam surround in the woofer means it might be easily damaged in process. Of interest, the Mod Podge coating can be measured to do exactly what is expected, lower any resonant peaks and increase treble rolloff.

I apologize in advance if I can't get the attached images to show up in the right place in this post.

I do a question for the board. I'm having trouble getting equivalent levels in my tweeter and woofer measurements. My tweeter measurements typically come in several db lower than my woofer measurements when they should come in higher. I think this might have something to do with using a Trends Audio TA-10 (T-Amp's brother) as my measurement amp. Does anyone have any ideas about this?

I've got some new drivers that fit the holes in the cabinet and will be trying them out over the next few weeks.