Here are 3 simple modifications I made to a pair of Jamo Concert Series 93 II bookshelf speakers that give them a much more balanced and more neutral sound with better bass extension better vocal response. I’ll do a complete teardown of the speakers, sketch out the schematic for the crossover and go through each of the modifications one by and show how each modification produces a measurable improvement in the sound. If you’re up to the task, feel free to take on this project with your own Jamos, or if you’re just curious about what’s inside these little speakers, then read on!
So I picked up my Jamo C 93 IIs on sale from Crutchfield as a open box deal and did no-rush shipping so I think I only paid a little over $220 for the pair. I’m not sure what prompted the purchase, they aren’t one of the more popular speakers on the market but for the most part people who have bought them do like them. I thought they looked really cool and also thought just maybe they would lend themselves to some simple upgrades to make them sound better. Plus, coming from someone who’s spent most of his life designing and building speakers, there is something super intriguing to me about buying commercial speakers and figuring our what makes them work. The immediacy of the process is amazing too, to go from the day of purchase to fully-built speakers on my doorstep in 4 days is something I could get used to. I mean I love spending hot weekends in my garage, breathing MDF dust just as much as the next guy, but this takes the simplicity of purchasing/owning speakers to a whole new level.
Which is why I couldn’t leave well enough alone and within the first week of getting these speakers, decided to do a complete teardown and started working out what modifications might be possible. My initial intention wasn’t really to change anything, but to just inspect what we were working with and see if there was some performance left on the table, so to speak, due to the commercial drive to mass-produce cheap speakers. Some sound elements of a speaker are design choices, not technically limited by cost, but perhaps driven by the need to give a speaker that showroom sound, a little extra bass, a touch more treble, anything to give it the edge and make the consumer say, yeah this one sounds better. But once you get it home, the added bass and treble end up being too much for day to day listening, and without the A/B comparison to any other speaker, the speaker ends up becoming is own worst enemy always trying to out-do itself. Let’s see if we can fix that.
First Modification – The Crossover
The first thing I started to dive into was the crossover. See here a complete schematic of what we are working with. I was overall impressed with the complexity of the crossover. It contains a total of 11 elements comprising iron and air-core inductors, electrolytic and poly caps and sandcast resistors. I would say the quality of the components is just acceptable at this price-point and none of my modifications made any attempt to improve the quality of the parts. That of course is still an option down the road, though it comes at significant additional cost. But if you want to purchase better parts of the same values, like resistors, the cost is minimal, move up to better caps, that will cost a little more, and better inductors would cost even more. Initially I am just going to adjust the crossover a touch without swapping out any of the major parts.
The low-pass section is a quite nice, it’s a just simple 2nd order filter but it has some decent baffle loss compensation built into it. It’s got a big 1.3 mH inductor and large shunt 33 uF electrolytic cap. Shelving starts at about 200 Hz and with the inductor alone would be about 3 dB down at 400 Hz and 6 dB down at 800 Hz. Introducing the 33 uF cap brings up the response at 800 Hz by about 3 dB and adds a corner to the response at about 1300 Hz, which then transitions to the overall filter to a 12 dB/octave slope at this point. The series RLC notch filter (8.2 ohms + 1.2 mH + 22 uF) is centered at about 1000 Hz and provides a 4 dB notch that’s just a few hundred Hz wide. This cleans up some peaking in the driver response and also helps out with the final baffle step compensation network. This puts the crossover frequency to the tweeter at right around 1.9 kHz, which is a good spot for most woofers, considering it’s about 6″ in diameter and the tweeter is a full 1″ soft dome. Pushing the crossover lower can put stresses on the tweeter and can make vocals sound nasally while pushing it higher can lead to poor directivity and more interference from cone break-up modes. Jamo does not provide a specified crossover frequency in their cutsheet nor do they mention the filter slopes.
I played around a little bit with just the low-pass section, adjusting the notch filter and changing the shunt cap value, but after all my fiddling I decided that there wasn’t anything drastically wrong with where the response was sitting and decided just to leave it alone. I think Jamo did a decent job here getting the baffle step right, setting the crossover point just low enough, the notch filter prevents some peaking around 1 kHz and enables the woofer to blends nicely to the tweeter without any massive suckouts. The final response ends up being more 3rd order acoustic once you add in the natural roll-off of the driver. So I was happy with not changing anything here. Maybe a future upgrade might be swap the 22 uF and 33 uF electrolytics for some poly caps just because they’re pretty cheap and make for a better-looking crossover and you never know, maybe to some people it will even sound better. You could also swap the iron-core inductor for a lower-gauge air core.
So let’s break down the tweeter section: we’ve got a single 1.5 ohm series sandcast resistor feeding a 7.5 uF poly cap and 0.18 mH air core inductor creating a nice 2nd order filter. This is almost textbook, but with a smaller inductor value which increases the Q and creates a sharper roll-off with a tighter corner at the cut-off frequency. Pretty standard adjustment that I tend to do as well. So the only red flag I see at this point with the tweeter is that itty, bitty, teeny, weeny 1.5 ohm resistor. You know that soft dome tweeter is more efficient that the woofer, we know the woofer has nearly 4 dB of baffle step compensation, so there’s no way a single 1.5 ohm resistor is going to provide enough attenuation to level match the woofer and the tweeter. That tweeter is going to be bright! And that is clearly by design, because it’s so easy to make that resistor anything the designer wants at zero cost. Maybe the guy hand-tuning the final design at Jamo was in a heavily damped room, maybe the speakers were tuned off-axis, maybe they really wanted to compete in the showroom by giving that upper end some sparkle. I mean when you’re up against the likes of Klipsch and B&W at Best Buy, seriously, these won’t win an A/B standoff with your average listener if the treble is deadlocked with the mids and bass. Even if that’s where it should be. Or maybe that’s just the Jamo signature. As you can see here I adjusted everything from 0 ohms to 8 ohms which provides a 15 dB spread in tweeter level and eventually settled on 3.3 ohms as the ideal spot from both a measurement perspective and from my own listening even though the treble is still just above flat. I like a touch more brightness over flat if I can choose it. So let’s double the 1.5 ohm series attenuating resistor to 3.3 ohms and move onto the last crossover tweak.