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Modifying AR9 Bass Crossover section

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Sometime back I brought up the topic of modifying the AR-9 bass crossover network. Many thanks to Roy C for his comments and insight in this area. For those of you not familiar with this topic I have included the bass crossover schematic attachment

My original question was based on speculation about what were the sonic effects of cutting loose the coil/cap that form a variable attenuation for this crossover. Holl noted in his design notes that these additional components contributed two aspects to the crossover function: 1) the combination of cap/coil added resistance at all frequencies higher than resonance thereby protecting the generally less than robust amplifiers of that era ('79 to '81) and 2) the extra R (as realized by these two components tended to "linearize" the bass performance in the general area of 30 Hz to 100 Hz (mid/upper bass).

Note carefully that this cap is the infamous 2500 uF, about the size of a 16 ounce beer can, that so many people hate to replace (expensive, et cetera).

I pondered the schematic for a considerable time and I could understand the basis of Holl's thinking. Amps of that era did not deal well with low impedances - and those two woofers in parallel formed an impedance that dipped as low as 2.5 ohms (back breakers to most receiver style amps). In addition the extra R would tend to keep a "mid/upper" bass rise in response from forming.

My own analysis indicated that in THIS era of robust amps the extra impedance was NOT needed - my amps (Odyssey Audio Khartago Mono Blocs) are stable with a two ohm load - and those are not extremely expensive amplifiers. In addition the extra R contributed by these two components would tend to raise the Q of the circuit - thus making the bass above resonance more "wooly" and less precise.

But I couldn't be sure.

So I cut the darn things - right at the red mark in the drawing. Quick clip of the diagonal cutters and viola - the parts were out of circuit. I did however leave the parts inside the cabinet. (next time I go in there the entire crossover is "coming out" to be installed in an external box).

So how does the resultant speaker sound? Pretty darn good - I will have an F/R plot for review sometime next week (friend has to come over and measure) but my subjective appraisal goes along these lines;

All the bass has become "tighter" and is obviously being generated by a lower Q system - there was no "boom" before but now the bass is supple, tight and detailed. The bass in the region of 35 Hz to about 80 Hz has evidenced a significant increase in perceived volume - rather startling actually. Again the increase in perceived volume is not an increase in boom - but instead a lot more of a "tight" and agile bass response.

If I am hearing - without any loss of quantity a higher quality bass, i.e. tighter and more focused (in the 80 to 160 range) then in truth the quantitative response (volume) HAS increased - boomy, high Q bass is typically perceived as louder - my bass is now lower Q but has maintained the same volume level (in the upper bass).

The bass at or near the resonant point (28 Hz) has not particularly changed - so 25 Hz to 35 Hz is about the same - the deep notes, per those in "Dark Knight" actually seem to have slightly more volume (and those are low 20's) - with of course a more focused sound.

This is a HIGHLY recommended modification. With one MAJOR CAVEAT. Do NOT - repeat do NOT perform this modification if you are using an amplifier from the "vintage" era. Those amps/receivers - no matter their size, shape or reputation cannot handle the low impedance of the two woofers in parallel. You will end up "smoking" your prized vintage amplifer/receiver. Only attempt this mod if you are certain that your amplifier is absolutely stable with a 2 ohm load. if you are not certain - then don't burn down your amp - and/or your home.

The AR-9 was always highly praised for its great bass response (among many other aspects of its performance) - but that bass really becomes exceptional with this modification.post-111787-0-16235800-1380925375_thumb.

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Are you saying that all you did is cut the wire that you have marked in red?

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I am very interested in seeing the F/R plot before and after the modification. The LC circuit you cut out is similar to that used in Infinity Waltins woofer 2-ohm coil.

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Answers to questions;

Roy C - yes all that I did was clip the wire (actually NOT where marked but where it connects to the ground - or return point).

I should have the F/R plot next week. Depends on a friend's availability.

Robert S. Are you the forum traffic director? I rather believe that this topic should be here - where all kinds of people have posted all kinds of thoughts regarding rebuilding, modifying or otherwise doing things to these very old speakers.

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Any frequency response that can be achieved through modifications at the crossover network level can be achieved at the preamplifier signal level with active equalization, invariably with much greater control. Further control over FR and relative levels can be achieved with active crossover networks and bi-amplification. The advantages are many including not presenting a risk to the power amplifier by creating a very low impedance load that could destabilize or damage it.

IMO the current design trend for very complex crossover networks to replace better control offered at the low signal level so that amplifiers can be operated without such signal processing is an unfortunate choice. So called room correction systems are nothing more than automatic equalization with a different name. Unlike manually operated equalizers they don't offer the same degree of user control or flexibility.

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Soundminded,

Yes - you are correct - within some rather restrictive limitations. The main problem being that the active equalization will, almost invariably, be implemented using a rather large number of flat-pack IC "op-amps". Along with their feed-back loops, and a host of resistors and capacitors within those loops.

The net result, according to many ears, is that the sound quality of the signal passing between the preamp, then through the EQ device and into the amp - that small and delicate signal will muddied and otherwise distorted by these low quality signal modifiers.

Active EQ, not unlike an active crossover, promises a great many gains. However in practice it is almost always a step towards signal degredation. This of course does not even touch on the phase shift problems, the filter slopes, et cetera. A simple solution that in reality is exceedingly complex to implement.

As for amps driving low resistance loads? A real problem in the '70s and '80s and somewhat into the '90s. But modern amp designers have learned to design their amplifiers to be stable at offered loads of 2 ohms - and sometimes even lower. Even modest modern amplifiers - such as my own - are absolutely stable when presented with a 2 ohm load. Of course I am not suggesting that a vintage Marantz 2265B or similar Yamaha can withstand such use. In fact I would rather expect low impedance speaker loads, i.e. less than 4 ohms, to more or less fry those vintage units.

I tend to stay away from tone controls, active EQ, or even active crossovers. I have never heard any implementation of such that sounded particularly "clean".

best,

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Answers to questions;

Roy C - yes all that I did was clip the wire (actually NOT where marked but where it connects to the ground - or return point).

I am Pete Basel not Roy C.

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I don't know how old you are but are you aware that during the 1970s and 1980s

there was a major push for high current amplifiers that could drive 2 ohm loads?

Sure, there were some that would not, but there are many that will. Hafler and

Adcom quickly come to mind. And then there were the high dollar arc welder amps

from Krell, Levinson, etc.

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Pete B,

I apologize for the mix-up - for you see - I am old - when we went to school the earth was still cooling, dinosaurs were making their tracks and we walked up-hill BOTH ways ;-)).

In the '70s (a period that I can dimly remember afer being released from the military in 1971 ;-)) most of the amplifiers available to the "general public" (a la Pacific Stereo out in LA) would burst into flames when presented with a 2 ohm load. Perhaps in the '80s the "better" amps came on the scene - but when Terry Holl designed the Mighty Nines (apparently you have your own pair now - congratulations) generally available amplifiers could not service 2 ohm loads.

In our current era there are dozens of very good amplifiers that have no problem with 2 ohms (or less) loads. Transistor manufacturers have gotten MUCH BETTER. Designers? Well they can do no more than what the silicon makes possible - the big ticket in this era appears to be the Sanken transistors - pretty snappy sand.

BTW: I offered to send you some walnut to replace some dings on your new MIGHTY NINES - give me a PM and I will be happy to send you the wood (envelope sized) and a step-by-step pictorial on how to replace a ding in a veneered surface. You will need a sharp chisel (maybe the 3/8" size would be best) and of course patience and a well controlled hand-eye ability. Some "Titebond III" wood glue is also necessary (any Homeless Depot or Lowes will have that in the paint section).

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Mach 3,

By changing the resonant circuit in the AR9, you might end up having frequency-response issues such as excessive-bass output in the below-200 Hz region. That circuit was carefully designed and tested to give flat output with optimal damping; the effective Q on the AR9 is approximately 0.75, so changing the Q will not result in cleaner low-bass output or "tighter", as you say, bass than the original. I'm unsure that the Q would change, but a Q of 0.5 will only attenuate response; it will not improve transient response at resonance. The higher damping will cause response to roll off prematurely in the region of resonance. The term "tighter" is an inaccurate and somewhat meaningless description. If the woofer is not ringing at resonance and the response is flat as it crosses the resonant frequency, then the damping is correct. More damping makes things worse. By the way, the AR9's -3dB point is 28 Hz; the resonance frequency is 34 Hz.

You would be much better off not trying to modify the original design, but repairing the crossover by replacing any out-of-spec capacitors. The caps might be okay, and it they are Sprague capacitors, they are not likely to be out of spec, as Roy has mentioned many times in the past.

Rest assured that your amp will be seeing an impedance of below 2 ohms with the circuit disconnected, and with the current demands of the AR9, even the best amps will be drawing inordinately large amounts of current and having to dissipate a lot of heat (unless you have one of the class-H pro amps that can do this more efficiently).

--Tom Tyson

AR9_Response-Q.pdf

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Mach3,

While we are at it...

I see in another recent AR-9 thread you are still recommending "completely rewiring" the AR-9 with 14 ga wire, with the added nugget of replacement of 22 watt resistors with 10 watt resistors in the midrange circuit.

Specifically you said:

At the fear of awakening the "all wire is the same" crowd of flat-earthers I will also suggest completely rewiring the speaker. The original wire, particularly that going to the woofers is far too small. Use 14 gauge spool wire - I used double runs of that to my woofers.

AND

As for the resistors - the specification (original) is 22 watts - for my UMR (the 1.5" dome) I went to 10 watts - therefore at high volumes (high current) the resistance goes UP - and keeps the UMR in line (in terms of volume) with the rest of the drivers. I found that the UMR wants to "get out front" at high volumes - and this subtle modification serves to hold it down. Perhaps eventually the resistor will overheat and crack - and when that happens I will simply replace it.

You already know my and the other "flat earthers'" view on the rewiring issue (did you think your comment would go unnoticed?), but where did you get the resistor idea? Do you really think compromising the power handling of the crossover for yet another imaginary sonic "improvement" is a wise thing to do? What method of measurement did you use for these conclusions and recommendations?

Roy

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As Pete B. Gets into his 9's it will be interesting to follow his thread.

Agreed and looking forward to it.

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To all the Flat Eathers and my other fans ;-)).

By the numbers.

TysonTom - the pdf file with the response plot is somewhat irrelevant. It is from Holl's papers on the design of the AR-9 and I think it is referring to cabinet size changes - not removing the coil/cap combination that Holl installed to ease the drop in impedance caused by the two woofers being in parallel. I will have to go read his papers again.

As for the lowering of Q - such will attenuate the response but in truth it means that the woofers demonstrate lower sustain, i.e. they "ring" less (high Q rings more on an impulse). So the perception is that the bass is tighter, e.g. the note sounds and then ends - exactly like a real lower note. A high Q system, such as a ported box, will produce "more" bass but it will be "boomy" and tend to be one note ringing its way across your sonic landscape.

As for repairing the "original" 2500 uF cap - I did so on my first rebuild - all new caps (five 500 uF NPE caps in parallel) so I am very familiar with the sound of the speaker with that particular crossover topology in place. Believe me - in my room, to my ears the removal of that band-aid for '70s style amplifiers resulted in a tremendous increase both in the quantity of bass and the quality of bass. Such of course is the absolute benchmark as I have a stereo for my enjoyment.

In regards the impedance - I have measured a low of about 1.7 ohms. I am using Odyssey Audio "Khartago" mono-blocks - and they seem to handle the excursions into deep bass with aplomb. No problems (yet). My reference recording is a CD of "Dark Knight" - another fantastic soundtrack from Hans Zimmer. Highly recommended as there are some rather low frequencies in Track 1. Again this is subjective - but then again what isn't? The plots you offered are over 34 years old - and I beleve are out of context in this discussion.

For RoyC and TomTyson;

Look folks - maybe you do not believe that wire (specifically Interconnects and speaker wire) makes any difference. Some yahoo sits down with a length of "audiophile" wire and measures the same LRC values as that of a similar length of 18 gauge solid core wire and to you the story is over. But what the yahoo doesn't measure - such as finding the impulse response of the wire or the dielectric absorption of the wire is the real story. Gross measurements do not a refutation make.

I wish you lived near - for we could settle the thing quickly by my bringing over some "real" wire and demonstrating the difference between that and your radio shack spool stuff. Which is how I learned that my flat earth viewpoint was totally wrong.

Suffice it so say - if you don't believe that wire can make a difference - well then in the end it is your loss not mine. I believe the original wire in the Nine was deficient and was used merely because it was cheap and looms could be easily made - not because anybody investigated any sonic aspects of the wire. If anybody else is reading this - rewire the thing - and solder your connections. The original wire was cheap crap.

If wire makes no difference then tens of thousands of people, spending tens of millions of dollars for wire, are all seriously deluded. Perhaps. But maybe the delusion lies on the other side of that divide.

In re the attenuation resistors;

In MY ROOM, with MY SPEAKERS, the UMR (1.5" dome) seems to get louder in a non-linear fashion compared to the rest of the drivers in the speaker as the volume increases. This means that the suitable attentuation at lower volumes is too little at higher volumes. So given the characteristics of a resistor in a situation where the temperature of the resistor in increasing, i.e. the resistance goes UP, I am attempting to introduce a resistance that varies (increases) with volume. Higher volumes will cause the underspecified attenuation resistor to begin to heat and its resistance will increase and I will be getting "more" attenuation at that point.

Pretty darn clever if I do say so myself as it works extremely well.

To RoyC;

My perception is of course my perception. Since we are not geographically close enough to share a listening session I can only relate my subjective impressions. Of course all you can do is the same - relate YOUR SUBJECTIVE impressions. Perhaps we do not share the same perception - OK - so who cares? I don't. I am on this forum to share MY experiences with others - if you do not agree that is good - and you have shared your take on these subjects. So all readers see that there are two sides to these suggested modifications and design approaches.

While I think VERY highly of AR - and particularly of the AR-9 (to my taste one of the finest speakers ever made) - I do NOT believe that what Terry Holl did in 1978 was the last word in design - particularly in his crossover (which with the cabinet is about the extent of the entire design - the drivers were already in production). Film caps change the sound tremendouly, better wire changes the sound impressively, removing that extra "bass attenuation" increases the depth and the quality of the bass.

Try these modifications and report back to all of us on what you think of the changes. What will it cost you?

I have a friend who has the F/R measuring equipment - but he is such a pain to listen to that I hesitate to have him bring his measuring equipment over. So I may or may not be able to get some plots up. I will include a plot (below if I can figure out how to do an insert picture) taken with an Apple phone app of my F/R immediately after I rebuilt the crossover (and rewired the drivers). This plot shows the 6 uF cap in shunt on the UMR (later fixed with the proper 8uF - note the spike at 7 KHz), and without the bass crossover improvements. Enjoy

Well I couldn't determine how to get the plot in the message - I will be back later with that.

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Here is the F/R plot taken several years back with an Apple phone app (which proved to be fairly accurate). This is in room response taken from my listening position. The red lines are "about" where the crossover points are - I don't remember what the blue line was supposed to mean - I have slept since I took this measurement.

I concluded that the response increase near 7 KHz was due to the wrong capacitor being in shunt with the UMR - for it was pretty bright at the time this plot was taken.

Sooner or later I will endure my friend with the full-up measurement equipment (a nice guy but he really likes to spend his time downgrading other peoples rigs - but I guess that is a big part of the hobby for some people - we seem to spend more time arguing about the kit than we do listening to it - at least some of us) - and then we can compare the two plots.

post-111787-0-97327000-1388516712_thumb.

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TomTyson,

The "plot" you forwarded was completely BOGUS. It was a plot that can be found on the CSP (under notes on the AR9 design) - and a copy is embedded below. Basically the "curve" you pointed out as indicating that my mod to the bass section reduced bass output ACTUALLY describes a cabinet with TWICE THE VOLUME of the AR-9 (Holl was illustrating the effect of cabinet size on Q).

So - there is NO PLOT that shows that my modification in ANY way detracts from the bass output. In fact my subjective listening tells me VERY CLEARLY that the change significantly increased both the depth of the bass AND the quality of the bass. Go read Terry Holl's notes on how he came up with the crossover design - and it has everything to do with protecting amplifiers of that era and NOTHING to do with sound quality.

Jeez Louse. Do you even have a set of AR-9? If so then go try the mod for yourself. You will be amazed.

post-111787-0-95431900-1388522329_thumb.

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"The "plot" you forwarded was completely BOGUS. It was a plot that can be found on the CSP (under notes on the AR9 design) - and a copy is embedded below. Basically the "curve" you pointed out as indicating that my mod to the bass section reduced bass output ACTUALLY describes a cabinet with TWICE THE VOLUME of the AR-9 (Holl was illustrating the effect of cabinet size on Q)."

The solid curve is from the standard 12" woofer in an enclosure of AR3a(or11) size. The twice the volume refers to the same 12" woofer in an enclosure twice the size of AR3a(or11), about 3.4 ft3, hence the lower Q and faster rolloff . AR9 has an internal volume of about 4.25ft3 from my memory.

Even by-passing the capacitor, the twin woofers in AR9 still sees a 10 mh coil and a 2.9 mh coil in series and shunted by a 470uf capacitor. The series resistance of the two coils could be quite significant. I would estimate about the order of 2 ohms or more. Of course the response curve of this arrangement will be quite different from the original AR 9 crossover. However, without actual measurement we are just speculating.

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Mach 3,

Just to recap a few of your points:

-Old measurements are irrelevant...even though you have no "new" measurements to refute them, or to support your position.

-You fancy yourself smarter than Tim Holl, design engineer of the AR-9.

-You have obviously bought into much "audiophile" hype relative to speaker cable and capacitors....even though you have no way to prove it. (You are simply lowering the resistance of the circuit...resistance which was actually a measured part of the original design).

-You consider successful speaker design professionals and engineers who do not agree with you to be "flat earthers" and "yahoos".

-You accuse your friend who has actual measurement capabilities of "downgrading other people's rigs"...hmmm. (Speaking of "downgrades", I still do not buy that resistor swap idea of yours as a prudent recommendation.)

-Having an amp stable into less than 2 ohms is required for one of your major "upgrades".

The fact of the matter is I only agree with one of your statements:

<<My perception is of course my perception>>

If you had prefaced your comments with this statement I may have found your posts on the matter more entertaining, and somewhat less condescending.

Enjoy your tinkering. I have no doubt you are hearing differences as the result of some of the things you have done to your speakers. Subjective differences, however, are often not improvements to others' ears, especially in different listening spaces under different circumstances.

Roy

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Mach 3:

First of all, I think all of us here are trying to understand your passion and enthusiasm for your modifications to your AR9s; however, you should probably take your AR9 tweaks to the "Mods and Tweaks" section and keep it off the discussion board. If people are willing to get in there and modify their speakers, that's fine, but it shouldn't bog down the regular discussions on these speakers. If you choose to cut out portions of your crossover in the AR9 for your own speaker, that's great, and more power to you. But most people don't want to hear a bunch of utter nonsense about improved low-end response, better damping and so forth as a result of removing part of the AR9's vital low-frequency crossover se3ction. It is an insult to the intelligence of most people here.

It's hard to know exactly what the end result would be by removing the resonance circuit in your 9s, but I'm pretty sure it would make things much worse, not better. For one thing, the very low impedance of that part of the circuit would mean that more current would flow through the woofers for a given voltage setting on the amplifier, thus the output would be greater than with the circuit in place. Since the low-frequency output of the standard, unmodified AR9 is very flat and uniform down to the resonance of 34 Hz, removing the resonant circuit in the AR9 crossover would result in excessive bass output and coloration in the frequencies above resonance right up through the 200 Hz crossover, and above, probably.

You said:

"...The bass in the region of 35 Hz to about 80 Hz has evidenced a significant increase in perceived volume - rather startling actually. Again the increase in perceived volume is not an increase in boom - but instead a lot more of a 'tight' and agile bass response...."

The ultimate extension (i.e., system resonance itself) would not be affected by cutting out the resonant circuit, so the speaker's ability to dip lower in bass would not occur except for the per4ception of deeper bass because of the imbalance of the low frequencies with respect to the

upper bass and midrange, etc. So, your perception of greater bass output is correct, except that the output in bass is only in relation to the overall balance of the AR9 and at the expense of flatness and accuracy; in other words, you would be getting an increase in output from the bass section as though you turned up the bass control on your preamp. The "tight" and "agile" bass-response descriiptions you made are pur bulls**t, and those comments are totally meaningless.

You said:

"...As for the lowering of Q - such will attenuate the response but in truth it means that the woofers demonstrate lower sustain, i.e., they 'ring' less (high Q rings more on an impulse). So the perception is that the bass is tighter, e.g. the note sounds and then ends - exactly like a real lower note. A high Q system, such as a ported box, will produce 'more' bass but it will be 'boomy' and tend to be one note ringing its way across your sonic landscape...."

Mach 3, I would suggest that you consider going to the public library and checking out some textbooks on loudspeaker design and theory, and from that maybe you might try to understand the meaning and definition of damping, resonance and some other important topics relating to loudspeaker low-frequency performance. From your discussions and the heated rhetoric, you don't appear to understand the basics of damping and how it really works; perhaps you have read too may of the popular blogs and tweak magazines.

As for the woofer crossover in the AR9, it was designed by Chief Engineer Alex deKoster of AR. There is no "Terry Holl." The VP and Director of Engineering at AR at that time was "Tim Holl," and he was responsible for the overall design of the AR9. The circuit for the AR9's woofers operate such that a the resonance frequency of 34 Hz -- where impedance is usually highest anyway -- the big capacitor and the coil are out of the circuit (tank resonance), also effectively eliminating the series resistor and allowing both woofers to have greater (by 6 dB) output. As the frequency rises above resonance, the impedance of the resonant circuit rises, too, and the speaker's output goes down, keeping the response uniform. Therefore, the resonant circuit is very, very important to reduce the woofers' output above resonance to blend in with the Lower Midrange and Upper Midrange drivers and tweeters. The tank circuit also provides an 18 dB/octave slope for the woofers, which is very important since the 8-inch Lower Midrange driver and the two 12-inch woofers pretty much act as one unit, and the woofers must be cut off sharply to avoid overlapping the response with the 8-inch unit.

From this it should be fairly obvious that the deKoster crossover design was not designed to simply raise the impedance to "save 1970s amplifiers," as you have suggested. It is much more involved than that simplistic interpretation. Besides, AR had many high-powered amplifiers on hand to use with the AR9 -- such as Crowns, McIntosh, Krell and Thresholds -- that could easily handle any such low impedance loads.

--Tom Tyson

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Interesting thread for sure. I know little to nothing of xover design althou I do understand, to a small degree, what is doing what when I look at the schematic. Since I have a pair of AR90s and am in the hunt for AR9s I became interested in following this thread.

It has me wondering why the AR90 doesn't have an appropriated sized cap in it's circuit; say an ~1875uF. Is it simply due to the woofer sizes? As designed, I believe, both speakers do not drop below 3.2 ohms impedance. Sheer curiosity has me asking this and nothing else.

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Interesting thread for sure. I know little to nothing of xover design althou I do understand, to a small degree, what is doing what when I look at the schematic. Since I have a pair of AR90s and am in the hunt for AR9s I became interested in following this thread.

It has me wondering why the AR90 doesn't have an appropriated sized cap in it's circuit; say an ~1875uF. Is it simply due to the woofer sizes? As designed, I believe, both speakers do not drop below 3.2 ohms impedance. Sheer curiosity has me asking this and nothing else.

In AR9 we have two 4 ohm 12" drivers wired in parallel, in AR90 we have two 8 ohm 10" drivers wired in parallel.

So, two 4 ohm woofers wired in parallel would give 2 ohm impedance without additional impedance correction circuit that we have in AR9.

AR90 doesn't need this, since its two 8 ohm woofers wired in parallel give 4 ohm impedance.

This is how I understand this, maybe I'm wrong?

I was in a hunt for AR9 too, but since I got AR90 I decided not to bother.

The two speakers are identical except for the woofer section.

AR90 seems much easier to drive, I do not know if few extra Hz in AR9's bass response is worth it...

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That makes sense.

Thanks

I am happy with the 90s but I want to bi-amp. I'd keep the 90s and give them to my son so I can get rid of his horrid Polk RTA12Cs.

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DavidR, I have both the AR9's and 90's. They do sound very much the same except the 9's do have more bass. I can't say anything about bi-amping the 9's because 2 of my amps are still out for repair but when they come back I will try it and let you know. I will say this about the 9's, they are way more speaker then most people will ever need and the 340 watts per channel from my Crown XLS 1000 amp is NOT ENOUGH. LOL. But if you are like me, you wont stop until you get them and you will enjoy them. I just hope you have a very large room to listen to them in and a lot more power then I have to feed them.

Harry

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