Crossover mods for the AR4x

[speaker dave]

My local vintage audio dealer had a nice pair of AR4x's a few months back. Grilles and cabinets were in great shape and the drivers looked unmolested, but the tweeters were dead. Thanks to JKent and Zilch I now have new tweeters (from opposite coasts) and rather than put the system back together stock I want to see what I can do to improve the crossover network.

I'm hoping this will be educational to those curious about network design, and along the way we'll see if we can bring the performance of an AR4x up to date. I'm going to stick with crossover mods that are internal and retain the stock outward appearance.

A note to the usual suspects: I'd like to keep this on-topic rather than have it break down to the usual debate and end up relegated to you-know-where, please!

This is my first run with the Holm measuring system mentioned in the kitchen. So far it is a very handy FFT (with MLS) measuring system. Better than the PC RTA system I've been using, it gives true phase and lets you time window in a number of ways. Very cool.

The first curve is the system at 3 angles, 30 degrees left, 0 degrees and 30 degrees right. This is from about 1 meter away with the grille off (grille on was surprisingly similar). I've seen other curves on the web and they are similar. Response is level from 50 Hz or so up to a "significant" bump at 1000 to 1500, followed by a 2kHz dip and finally a significant bump above 10k. I was hoping that the 2k hole was cabinet edge related because then some discrete damping under the grille could improve it. The fact that it doesn't change with lateral shift suggests it is inherent in the tweeter.

David

[speaker dave]

This curve shows the individual parts, the woofer and tweeter. The woofer curve (orange) looks great and has a clean rolloff even with a simple network of inductor only. AR obviously worked hard to get a woofer with a smooth roll-off rather than the typical peak. I'll leave the woofer network stock and concentrate on the tweeter. For the tweeter the 2k dip is much more obvious here in the individual sections curve.

It's easy to see where the 1200Hz peak comes from. For 2/3 of an octave the woofer and tweeter both have full strength. Overlap between units is not a good thing.

The dashed curves are the phase curves for the two units (orange dashed = woofer, green dashed = tweeter). The actual phase curve of the units doesn't matter but the realative phase between the units is very important, especially in the crossover range where they both have about the same energy. The system response is the vector sum of the two sections so if there phase curves are close, their outputs add fully. If there phase curves approach 180 degrees apart then they will cancel rather than add.

In our case the curves are within 30 degrees of each other in the 1 to 2k region so we will get full addition, i.e. a bump. If we could somehow spread the phases we might getl less bump but that isn't a good solution because at other vertical listening angles they would come back into phase.

I'm guessing the original designers struggled with a compromise between the bump at 1200 and the dip at 2k. Pulling down 1200Hz would make the range above sag lower. This is compounded with a first order network (series cap) where the unit may roll off as intended, but at resonance where the driver impedance bumps up, the voltage after the cap bumps up as well. A higher order network would give more degrees of freedom hence better control of shape (we'll try that later).

David

[speaker dave]

This shows the individual curves again plotted with the combined response of the whole system.

Again you can see where the individual curves overlapped excessivley the summed response shows the midrange peak.

David