GordonW
07-07-2004, 08:52 PM
Well, I'm embarking into uncharted territory, in a very small way.
A customer of mine at work, asked me to build him a center channel to match his home theatre system, which currently has JBL L100T mains, JBL L20T side speakers, and JBL L20T rear speakers. However, he's been using some other center, due to the fact that JBL really never made a center channel for this series (naturally, as there WAS no home-theatre, or at least nothing that used a center channel, in the mid 1980s!)...
I suggested that we take the "guts" from some L20Ts, and make an MTM out of it. He brought me the parts from a pair of L20Ts he found on Ebay (damaged cabinets, but perfect drivers), and I got started tonight.
I first took a baseline measurement in LMS, then I tried a few things, with the woofers and a test cabinet... which led to the following measurement, with two of the 115H-1 woofers and one 035Ti tweeter in an MTM arrangement, woofers wired in series. Wow... this thing worked better, sooner than I thought it would. With the stock tweeter crossover (which acts like a third-order acoustic highpass at about 3KHz), and a modified version of the woofer crossover (adjusting the values for a 12 ohm woofer system, instead of the stock 6 ohm setup), it started to draw curves in LMS, that I swear, I doubt, by listening, ANYONE could tell from a stock L20T, without some serious concentration.
Here's a snapshot of the last graph we took- the dark color (purple) line is the stock L20T, and the bright green line (despite saying "tweeter only", we forgot to change the descriptor from an earlier test) is the prototype MTM arrangement as described above. I'm stoked- it's FLATTER than stock, while having almost IDENTICAL "effective tonal balance" contours. Unless something radical happens when I transfer the system to the final cabinet (which I plan to build, to match the cosmetics of his oak L100Ts), it should kick some serious butt!
Now, I'm planning, if possible, to try attaching some bucking magnets to the drivers (for a modicum of video shielding)... I know those will change the frequency response a tiny bit, but I don't expect to see anything, I can't compensate for. That said, I think it's remarkable, that this result happened SO QUICKLY. As someone said... sometimes it's better to be lucky than good! :D
A customer of mine at work, asked me to build him a center channel to match his home theatre system, which currently has JBL L100T mains, JBL L20T side speakers, and JBL L20T rear speakers. However, he's been using some other center, due to the fact that JBL really never made a center channel for this series (naturally, as there WAS no home-theatre, or at least nothing that used a center channel, in the mid 1980s!)...
I suggested that we take the "guts" from some L20Ts, and make an MTM out of it. He brought me the parts from a pair of L20Ts he found on Ebay (damaged cabinets, but perfect drivers), and I got started tonight.
I first took a baseline measurement in LMS, then I tried a few things, with the woofers and a test cabinet... which led to the following measurement, with two of the 115H-1 woofers and one 035Ti tweeter in an MTM arrangement, woofers wired in series. Wow... this thing worked better, sooner than I thought it would. With the stock tweeter crossover (which acts like a third-order acoustic highpass at about 3KHz), and a modified version of the woofer crossover (adjusting the values for a 12 ohm woofer system, instead of the stock 6 ohm setup), it started to draw curves in LMS, that I swear, I doubt, by listening, ANYONE could tell from a stock L20T, without some serious concentration.
Here's a snapshot of the last graph we took- the dark color (purple) line is the stock L20T, and the bright green line (despite saying "tweeter only", we forgot to change the descriptor from an earlier test) is the prototype MTM arrangement as described above. I'm stoked- it's FLATTER than stock, while having almost IDENTICAL "effective tonal balance" contours. Unless something radical happens when I transfer the system to the final cabinet (which I plan to build, to match the cosmetics of his oak L100Ts), it should kick some serious butt!
Now, I'm planning, if possible, to try attaching some bucking magnets to the drivers (for a modicum of video shielding)... I know those will change the frequency response a tiny bit, but I don't expect to see anything, I can't compensate for. That said, I think it's remarkable, that this result happened SO QUICKLY. As someone said... sometimes it's better to be lucky than good! :D