Klipsch Chorus II

[Ty_Bower]

I picked up a set of Chorus II a while back. There's more than couple references in this hijacked thread:
http://diytube.com/phpBB2/viewtopic.php ... c&start=37

One had a badly damaged passive radiator, so I fabricated a plug-in port tube replacement for it.



I had hoped to properly tune the port to match (or exceed) the performance of the factory PR, but things got real busy this summer. Only recently have I had the time time to spend on these speakers. I've built an impedance testing jig and plotted the curves for the Chorus II. The next images show the impedance plot for the left and right speakers, both with the passive radiator installed. I believe the trick is to trim the length of the port until the resonant peaks in the impedance curve (seen at approximately 29 Hz and 68 Hz in the plots below) are of equal magnitude. You can see that with the PRs installed, the tuning is correct. Next I need to mount my port assembly into one of the cabinets, and determine how far off it is. I intentionally left the port long, so I expect the port tuning frequency will be too low.

[SteveH]

Interesting stuff. I would be interested to hear then vs my Chorus I. FWIW they have two ports at the bottom; but they are not tuned; just two holes drilled in the baffle....

[Ty_Bower]

I've never listened to a set of Chorus I, but it's my understanding that the "ports" are too short. If you do the math, the port frequency is somewhere upwards of 60 Hz. It should be closer to the Fs of the driver, somewhere in the mid 30 Hz or so. The net result is that the first gen Chorus ought to have a bump of a few dB somewhere around deep C, and then roll off quickly after that. The Chorus II shouldn't exhibit that bump, but should be flatter and go down just a little farther. We're only talking about a couple dB across 5 keys on the piano here, maybe a half octave at most.

For the Chorus I, it would be crazy easy to cut a PVC or cardboard tube to the "correct" length and then wedge it into the hole. I don't have the equation handy, or I'd calculate the length right now. For testing purposes, install the tube with the long end hanging outside the cabinet - only the length is important, not where the port is located. If you didn't like it, you could always yank it back out. Maybe find a tube that's a hair smaller than the hole, and wrap tape around the end to make a snug fit.

I took some impedance measurements with my port contraption installed. The port I used is 3.88" ID by 4.55" long. I expect it is longer than required, and the tuning frequency will be lower than optimal. As you can see from the impedance curve, my expectations are fulfilled. The resonant peak caused by the port is a bit too low. It should be above 20 Hz, but instead falls below 20 Hz. The net result will be a low frequency rolloff a little sooner than expected (due to the lack of contribution from the port) but the rate will be lower. I wish I had a good calibrated mic for taking FR measurements. Here's the PR vs. port charts:



Here's the dimensions and a good image of the port I used. I'll try to develop the motivation to pull the port out, and cut it off at the first notch mark to see if the measurements are closer to ideal.

[Ty_Bower]

I was having so much fun measuring the impedance curves of my speakers, I took some measurements of my Paradigm Mini Mk3 and my Dynaco A25. The Mini Mk3 is interesting in that it shows good tuning of its port, and it doesn't exhibit the autoformer impedance weirdness of the Klipsch. The A25 is interesting in that there is no port resonance peak evident - it's an aperiodic enclosure, and the port does not resonate.

[SteveH]

Ty,
Super interesting stuff. I wonder if I should plug one port and try to install a tube in the other, or just try with two tubes ?
Here's a pic of how the ports look fwiw:

[Ty_Bower]

Ty,
Super interesting stuff. I wonder if I should plug one port and try to install a tube in the other, or just try with two tubes?

If you can make it work, you're better off with two ports. The greater the cross sectional area of the port(s), the lower the velocity of the air within the port. This reduces port noise (chuffing, etc).

Assuming the volume of the Chorus is approximately 5 cubic feet, and guessing your port holes are about 4 inches diameter, then two ports about 7 inches long should tune the enclosure somewhere just under 38 Hz. That should be about right for the K-48 woofer. You want to make sure there is nothing within a port diameter of the end of the port tube - in other words, the end of the tube should be a minimum of 4 inches away from the back, sides, or any drivers inside the box.

I'll guess the stock "ports" are about 1 inch long (the thickness of the MDF). That should tune the box to about 62 Hz, and put a ~5 dB bump around 77 Hz. The -3 dB point of the stock Chorus I should be somewhere around 53 Hz. If you lengthen the ports, it should eliminate the bump and lower the -3 dB point to about 43 Hz.

FYI, I'm using WinISD Pro to do the box calculations. There's also some related discussions on the Klipsch forums. This thread is one; others can be found with a little searching:
http://forums.klipsch.com/forums/p/5532 ... spx#534472

Nice looking listening room, by the way. What are the two monoblocks front and center?

[SteveH]

Ty,
Thanks for the help ! That'll be my project tomorrow...I'll try to score some PVC that should fit.
The monoblocks are PP807's that I built a loong time ago. They were really brite for some reason; and that mixed with the Chorus was a very bad combo. I wired 'em up in triode mode a couple of months ago; and they've been singing ever since...Its a real nice combo.
I'll let ya know how things turn out tomorrow...

Thanks again