#62 Jamo Sub 200 modding

For background / context: https://mehlau.net/audio/multisub_geddes/

I had been off-and-on "hunting" for a twin, for my Jamo Sub 200 that i bought and revived years ago. Partially for SPL reasons, partially for the multi-sub approach mentioned above, and most recently, for actually sitting my couch on top of the subs.

At long last, a couple months back i finally found one locally, and in matching black finish. And with the 5.1 Jamo speakers it came in a set with, to boot! Those might get reused / repurposed later, (i'm thinking, in a manner similar to one of my previous projects).

But back to the task at hand - since the original sub is driven from my trusty(?) old Studio Konnekt 48, and that only has one (assignable) low-passed output, plus in order to simplify the wiring, and not wanting to use any signal splitters, i figured it couldn't hurt to daisy-chain the two subs, while adding a balanced in & out (wired as a "loopback"). A schematic might illustrate (no pun intended) the idea slightly better.

Balanced signal comes into the XLR female (left end), with added ground-lift switch ("just in case"), goes into a differential-to-single-ended opamp; signal gets tapped off through the 47k resistor, and goes into the input summing stage of the stock electronics, while also going into a buffer opamp stage, and out into the male XLR, wired as an impedance-balanced output. Power gets tapped from the stock electronics as well, so four wires should suffice.

Since i only needed two of these, and it's only a matter of a literal handful of parts, i went with manual DIY on both little circuit boards. The layout was intended to require the least amount of scoring, and as straight ones as feasible. With that in mind, the sacrifice of having to run a single short wire as an "extra", was more than reasonable. Also, in order to need to drill fewer holes, i went with SMD electrolytics for the output DC-blocking caps, because reasons. Ok, i had a handful of scavenged Panasonics, that i trust enough for the purpose, so there.

A whopping 8x3cm of a board (3.5x1.5in). I didn't bother including any mounting screw holes, because the two XLRs should be more than sufficient anchoring for such a small weight. Speaking of XLRs, they get mounted from the other side of the board, in order to allow soldering the connections (without too much contorting and whatnot).

Due to limited imagination capabilities, i actually "overshot" a couple of the cuts around the opamp connections, buuuut that's nothing a couple small pieces of used solder-wick can't fix. Hashtag recycling, am i right?

Right then - first part, drilling the holes for the XLR connectors. Started off with a 2.5mm pilot hole, went up to the 10mm that my little cordless drill can take, and then enlarged the holes with a cheapo step-drill up to the required 22mm. I went up to 20mm from the outside, then did the 20-to-22mm step from the inside (after having to temporarily detach the transformer from the amp panel), in order to have the inside edges deburred / chamfered as well.

Test-fit looks promising. Slight niggle though - looks like i'll have to nip off two opposite corners of the boards, so as to be able to mark the centers of the two outermost mounting screw holes for the XLR's. And, as it turns out, to clear the plastic bobbin of the transformer as well. Nothing a few seconds with a dremel can't fix.

Done and done. Now, on to the actual electronics, for a change. I did my best to match the relevant pairs of resistors for the balanced receiver, as well as the output section, in order to maximize the CMRR. Partly for that reason, I ended up with through-hole capacitors for the input RF filtering, because they were easier to measure and match. Other than that, the boards are pretty much as designed (see above).

And whaddya know, they pretty much fit as intended. Granted, i had to slightly "oval out" one of the male XLR mounting screw holes, but nothing too dramatic. And i managed to find some machine screws (possibly M2.5) that seemed to bite just about well enough inside the mounting "ears" of the connectors.

Looking even better, all wired in and connected. A quick DC-test revealed a short-circuit between the negative rail and one of the opamp inputs on one of the two boards, but no damage was done, and it was easily remedied. For the power connection, i even managed to use a pre-existing but unused hole in the stock PCB, to use as a makeshift strain relief / cable guide of sorts.

Some brief testing revealed a few issues - one subwoofer considerably quieter (which turned out to be an opamp input accidentally making contact with the groundplane on one of the add-on boards, quickly remedied), and extraneous noises at higher volumes. The source of the latter was (or at least might've been) two-fold, the newly-added grey power-cable rattling against the inside of the enclosure, or air escaping via the unused XLR connectors (not being airtight). I cable-tied some foam around the grey cables to cushion them when contacting the wall, but there wasn't much to be done about sealing the XLR's. Well, not until i figured a quick & dirty solution, namely an XLR female "dummy plug", to seal up the free male XLR. The other three would be populated by the signal source and the daisy-chain cable, respectively.

Speaking of testing, the initial run was done with the input (female) XLR pin 1 left floating, but that turned out to result in considerable levels of hum. Connecting that to the add-on board groundplane cured the issue straight away. Come to think of it, that should've been more or less obvious, given that these are double-insulated (ie. non-earthed) devices. Someday i might end up going through with my initial plan of adding a ground-lift switch (even though i'm now uncertain of its usefulness), but since i'm out of panel-mount toggle-switches, that'll have to stay hardwired for now. Regardless, no added hum is present with the two subs daisy-chained, so hooray for that.