[Crossover mods for the AR4x]

Very nice descriptive comparisons. Once the schematics are up I may give it a shot!

[Crossover mods for the AR4x]

First off, I don't want to sound critical of what was a very successful design for a long period of time.

We can only speculate while looking at the system several decades later. We do know that the AR design preference at the time was to optimize the individual driver and crossover bandpasses in a reverberent chamber. This would emphasize the power response of each driver while downplaying the axial response. When looking at the woofer and its crossover, and the tweeter and its crossover, in a reverb room, the primary difference would be the woofer's response. As any woofer rolls off considerably off axis, it would effectively have a lower crossover point in a reverb room. This would downplay the overlap issues and might suggest a larger capacitor value as optimum.

AR also prefered a balance that followed measurements of concert halls (room R converted to a response curve). This, along with optimizing woofers to be flattest in 2 pi (half space), is the reason why every early generation product seems to have a family curve that crowns at mid frequencies when the system is measured in a free field. The 2pi-4pi difference gives an uphill trend in the woofer range and the down-tilt in the treble matches the concert hall trend.

None of these are matters of "right philosophy, wrong philosophy", but are typical af a general industry-wide evolution in design approach over the years. We certainly saw the same thing within AR: the AR 9 is designed to a flatter free field response and has key features such as the acoustic blanket, that only impact the direct response.

Its also worthwile pointing out the contribution of test equipment over the years. Much of the Linkwitz/Reilly approach to crossover design, which is the basis of my discussion of "adding an order to get the phase more in line" would have been, as a practical matter, impossible in the 60's. The essential component of measuring phase response couldn't be done on a frequency sweep basis. The first tool for doing that that I recall was the B&K phase meter of about 1980. It still didn't allow removal of excess phase, the 1 or 2 meters of air path delay, so it wasn't a great help. (My rusty recollection.) It really took MLSSA, TEF and FFT methods to give easily made phase curves.

I don't think capacitor cost was a factor. But I'm sure that a more complex crossover, certainly a third order network, would have been rejected for reasons of cost. This is again typical of the times. JBL made a lot of money selling L100's with 2 caps and 2 L-Pads, nothing else. An inductor on the woofer was already an extravegence on AR's part, in a budget system.

Regards,

David

Thanks for the explanation, although I don't understand all the technical details, I get the gist of what you said. I can certainly see the difference on the curves.

I did do an A/B comparison with the old block cap hooked up in one and a new in the other. Sometimes there wasn't much difference in sound, but other times it was very apparent, such as acoustic guitar and some vocals. I would say the one with the new cap sounded a bit 'boxy' compared with the old cap. I'll probably keep these relatively stock, but would definitely try the mod if I run into a second set of these.

Interestingly enough, my 4x's have the tweets in phase with the woofers(green wire attached to wiper, and yellow to #2) they're not the super early cross hatch woofers, but must be shortly after. I'll probably start another thread soon to document some of the differences on mine.

[Crossover mods for the AR4x]

If you wanted to stick with a first order network, this was a little better than stock. It is 10uF and reversed phase (reversed from as sold, but actually in-phase with the woofer). The overlap between drivers was less and and the 1200 Hz bump is gone but there is still a broad depression above.

First of all, great thread Dave! I have a simple question.

Since your suggestion here is so simple here, why didn't the original designers do the same thing? You mention that cost was a big factor in the design, and this wouldn't cost any more for materials or labor. The only thing I can think of is 20uf caps were maybe more common or cheaper at the time.

I'm not saying the original design was better, I will probably try these when the designs are complete. This is just the first thing that came to my mind when I read this. Thanks!