The AR4x in measurements

[speaker dave]

(Since my long-winded redesign of the AR4x has found a home in the Mods and Tweaks section, here is a shortened version of the initial exploration of the system as designed by AR)

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. Before all that lets take a look at the stock system to see what our $58 bought us in the late 60s.

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. 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 relative 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.

David

[speaker dave]

I ran across this curve on the Stereophile website. It illustrates the problem of a first order network on the tweeter. While first order (a series capacitor) should give a 6dB per octave electrical rolloff, this only works well when the load is a resistor. When the load is a tweeter and its realworld impedance curve, then things won't be so tidy. At the tweeter's resonance its impedance will go quite high. This is a "light load" on the network and the voltage will rise also (in the graph the bump at 500Hz). At frequencies above that, the tweeter impedance will drop to near its DC resistance and voltage at the tweeter terminals will fall. Finally, at highest frequencies the inductance will bring the tweeter impedance up again and the voltage will rise again.

One solution for this would be a Ferrofluid of the heavy damping type. It would drop the impedance rise at resonance (lower Qm) and give a more classic highpass.

For these reasons text book crossovers are never very successful. A software based approach must always bring driver impedance into the calculations as the load on any proposed network.

David

[speaker dave]

Anyhow, this shows the individual curves again plotted with the combined response of the whole system. The dark curve is the system response, the combination of the green and magenta woofer and tweeter.

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

David

[speaker dave]

I thought I would try reversing the tweeter polarity to see what the sum is like. Note that the stock network has the tweeter out of phase with the woofer. Best phasing in a system depends on a lot of variables including woofer depth, crossover order, crossover frequency, etc. you can't generalize and generally must use the phasing that gives the best addition.

As I mentioned it isn't good to achieve flat response via cancelation but it was worth a look. Here it might be worth listening to as it knocks about 7dB out of the peak and leaves a less offensinve trough around 2k.

David

[speaker dave]

Here is a look at the effect of the grille. It is surprisingly minor with just a little bit of loss at 5k and 8k with the grille on. This indicates that the standard cloth is very transparent. Also that reflections off the side of the grille are minor. I suspect that the tweeter being fairly directional helps reduce the side reflections.

David

[speaker dave]

The stock network for an AR4x is 20uF on the tweeter, followed by the wirewound pots. The pot has the usual problems so I've bypassed it and plan to use fixed resistors for treble level. When I connected an external substitute for the internal network I noticed significant differences between the measured response of the internal 20uF and an external 20uF. Since the cap is 40 or so years old this isn't too surprising. Note that the capacitance seems to be down (loss at the low end of the tweeter) and there is loss up to 10k also. This would have to be due to higher ESR (effective series resistance).

I'd note that many who restore these old speakers rush to replace all the caps. In this case a new cap will make a significant change but you have to wonder if it really is an improvement? If the pot is working you can have whatever treble level you like, but the replacement cap will raise the 1200 Hz bump, arguably not an improvement. (something to think about).

David

[speaker dave]

Here I played with the external network values until I got a reasonable fit to the response of the internal part. 14uF and 2 ohms got the two curves to overlap so I have to assume that is the value, due to ageing, of the AR unit.

So there you have it. Your $58 (or was it $57?) bought you a lot of speaker back then. Nice cabinet, nice woofer, okay tweeter and enough crossover to get the woofer tweeter balance about right.

Don't forget to visit the Mods and Tweaks section to see what we can do with a little bit of crossover redesign with our AR4x as a starting point.

David

[Carlspeak]

Dave, I think you must work for the USPS part time. I hereby dub you the 'POSTMAN'.

NICE JOB!

[RoyC]

Here I played with the external network values until I got a reasonable fit to the response of the internal part. 14uF and 2 ohms got the two curves to overlap so I have to assume that is the value, due to aging, of the AR unit.

That is very interesting information, and certainly illustrates the potential audible differences (for better or worse) between old and new caps!

Thanks for all your work and insight , Dave...very enjoyable and informative.

Roy

[Steve F]

Here I played with the external network values until I got a reasonable fit to the response of the internal part. 14uF and 2 ohms got the two curves to overlap so I have to assume that is the value, due to ageing, of the AR unit.

So there you have it. Your $58 (or was it $57?) bought you a lot of speaker back then. Nice cabinet, nice woofer, okay tweeter and enough crossover to get the woofer tweeter balance about right.

Don't forget to visit the Mods and Tweaks section to see what we can do with a little bit of crossover redesign with our AR4x as a starting point.

David

$57 in oiled walnut, $51 in unfinished pine. When my Dad bought his 4x's in July 1969 (238xxx), the price had risen to $63 walnut, $57 unfinished. The Lafayette salesman gave him the walnuts for the 'unfinished' price of $57 ea. Such a deal. (A few years later of course, AR was discounted to H*ll, all over the place, especially through mail order. So much for dealer profitability and dealer loyalty. But that's another tale.)

The 4x's absolutely walked all over his 15" 3-way triaxial Goodmans in 6 cu. ft. floorstanding enclosures. I mean walked all over them, top to bottom. My Mom was thrilled that those big boxes were finally out of her house, my Dad was thrilled that he finally had his AR's, and I was so fascinated that such small speakers could sound so good that I was hooked on 'hi-fi' from that point forward.

Steve F.

[genek]

(A few years later of course, AR was discounted to H*ll, all over the place, especially through mail order. So much for dealer profitability and dealer loyalty. But that's another tale.)

AR was always discounted, as the company chose not to exercise its rights under the "fair trade" laws that allowed manufacturers to set prices until 1975; they sold to any reseller who would buy a minimum amount of each model and provide space to demo the speakers. The local dealer I bought all my equipment from in those days was a discounter, and they were extremely loyal to AR; when they moved to a larger location in the late 70's, they fitted out a small listening space just for their AR speakers (they had previously been playing their demo speakers on shelves in their front office, but needed more space when AR began selling floor models).

[Steve F]

AR was always discounted, as the company chose not to exercise its rights under the "fair trade" laws that allowed manufacturers to set prices until 1975; they sold to any reseller who would buy a minimum amount of each model and provide space to demo the speakers. The local dealer I bought all my equipment from in those days was a discounter, and they were extremely loyal to AR; when they moved to a larger location in the late 70's, they fitted out a small listening space just for their AR speakers (they had previously been playing their demo speakers on shelves in their front office, but needed more space when AR began selling floor models).

Some dealers had a personal affinity for and loyalty to AR, but that was the exception, not the rule. Most retailers took the "path of least resistance" and sold what the customer said sounded better to them on a quick A-B demo--usually a pair of Advents or EPI's, both of which had zippier highs than a 4x-6-2ax-3a in a retail showroom. 1000's of Large Advents and EPI 100's were sold at retail with quick A-B's "proving" their superiority to a dull AR.

I remember a dealer in Springfield MA around 1972 or 73 who personally loved ARs and had every single model (from 7 to LST) on display--including the rare 2x. But for every dealer like that (or where you bought yours), there were 100's who hated AR and sold away from them. Can't re-write 1970's retail history. What happened, happened--rare exceptions not withstanding.

Steve F.

[genek]

One of the big things that happened in the 70's was the end of "fair trade." Discount dealers who previously couldn't get Advents, EPI's and JBLs (or if they could had to sell them for as much as double what they could price equivalent ARs) were no longer restricted (it would take five years or so for the manufacturers to adopt today's common practice that delivers warranty service through "authorized dealers" rather than manufacturer service), and combined with Teledyne sitting on its hands and not updating products or marketing to fit changing consumer tastes doomed AR to a loss of market share it never recovered from.

The reasons for changing consumer taste (rock music, unfamiliarity with live acoustic performances, etc., etc.) have been discussed at length, but the effect it has on consumer purchasing choices is indisputable. It's one thing for customers to resign themselves to turning up speaker level controls and amplifier treble when the speakers they're using are half the price of the "zippier" ones they might otherwise have chosen (the AR instructions about level controls in the CSP library demonstrate that enough AR owners asked about it to call for a response), but when you can get them all for pretty much the same price...