Jump to content

Thoughts on the AR-9 Bass section crossover


Mach3

Recommended Posts

An open question to all concerning the AR-9 crossover - or more particularly the bass section of that crossover. Below is the crossover as reproduced from this forum (just the bass section).

The red line is NOT part of the original schematic - it is there to aid our discussion. Please note that the AR-9 features two 12" woofers wired in parallel. Because of the parallel approach the effective impedance as presented to the amplifier will be VERY low (~ <= 2 ohms) at the resonant frequency of the woofers (~ 18 Hz or so - I think).

According to design notes (located on this site) from Terry Holl, the AR-9 designer, the 2500 uF capacitance was installed to change the effective Q of this circuit at or near the resonant frequency - thus keeping the effective impedance presented to the amplifier from dropping too low.

The reason this was done was primarily because the amplifiers of that era ('79 - '81) were for the most part incapable of driving such a low impedance load - most of the receivers of that era struggled with a 4 ohm load as all were designed for a nominal 8 ohm load. Hence the circuitry in the crossover. Holl described this as equivalent to an "automatic" clutch that would only be in play when needed. Very clever if I say so myself.

In our current era however most amplifiers - or most serious amplifiers - can drive a 4 ohm load or even a 2 ohm load without any real difficulty. Hence I have been considering just cutting the 2500 uF cap out of the circuit. (Side Note: I have rebuilt my AR-9 crossovers with all new caps (metallized film from Solen and Jantzen) and all new wiring - I replaced the original 2500 uF "Beer Can" with a bank of NPE caps (5 x 500 uF)). The point at which I would be cutting the caps loose is indicated by the RED line in the above schematic.

So I am asking the folks on the forum if they have any opinions or experience with this matter. Would enjoy hearing your thoughts on this matter. Will I get deeper bass? Will my impedance drop too low at resonance? (too low being defined as < 1 ohm). Will it even make any difference?

Thanks in advance.

Link to comment
Share on other sites

I owned a pair of AR9s once. If I remember correctly the designer Tim Hall explains, at the box resonance frequency around 28 Hz the impedance is sufficiently high so it is safe to most amplifiers. However the regions before and after resonance(most sealed woofers have lowest impedance after the resonant frequency) the impedance may get too low that it may cause problems for amplifiers. I believe the extra circuitry in the woofer XO is meant to modify the impedance for before and after box resonant regions. If the impedance is modified than the frequency response will be affected as well. My guess is by removing 2500 uf from the circuit the frequency response before and after the resonant frequency will be boosted and you may or may not like the sound.

Link to comment
Share on other sites

The woofers in parallel have a very low DC resistance, low impedance above resonance and high impedance at resonance. An advantage of this situation and high voltage sensitivity is that the large crossover inductors can have high resistance in order to bring the system impedance back up and so they can be more economical. There will be loss in amplitude but in regions where less output is advantageous, that is below resonance and above. Think of the top two inductors combined with the 470 uF cap as a traditional 3rd order crossover network. The 10 mH is the large one with high DC resistance. The series RC has a minimum impedance at resonance which is probably set to the woofer in box resonance to help minimize the losses at that frequency and get a bit more LF output.

Link to comment
Share on other sites

What I stated is in agreement with that to a large extent. They are showing the inductor

as a resistor because at LF it is mainly the DC resistance of the inductor that matters,

the impedance of the inductor is used as part of the XO low pass function which is not

being discussed in that paper. They show a greater difference in output because I

believe that their low Q attenuated curve is the woofers directly driven without any

resistance. Add some resistance and the system will be easier to drive, and the Q will

go up providing much more bass output around resonance. Add the RC network and

you'll get a few more dB on top of that also around system resonance by shunting the

series resistance at frequencies where the system impedance is high.

We saw the system Q difference in Unibox years ago when we simulated the basic

AR3 woofer with and without the crossover inductor resistance.

Link to comment
Share on other sites

Pete, think any "gains" obtained would be minimal compared to possibly larger "negative" changes in tonal balance @ frequencies affected. I'm pretty sure that even ABOVE system resonance, bass "contouring" will change. Also, the "ultimate slope" an octave above the resulting X-O point may affect lower midrange response as well.

To the best of my knowledge, few/any speakers other than the AR9 employ a 200 hz X-O point "passively". As you noted , the L & C values are HUGE (and costly).However, If you don't like the "original" sound , then it seems worth a shot.

If I was to attempt this, I would remove the ENTIRE passive X-O and low-pass the woofer section using an electronic X-O.

Off course, I'd be twiddling forever to "blend" what I think would be "correct" at the 200 hz crossover point.. LOL.

Link to comment
Share on other sites

I know just enough electronics to be dangerous to myself.

Looking at the crossover of AR 9LS, which replaced AR 9, it seems in AR 9LS, AR used a 12-inch woofer and a 10-inch woofer in parallel to achieve same -3 db(28Hz) performance as AR9. The 12-inch woofer and the 10-inch woofer will receive different power inputs (due to the difference in impedance). If anything, my gut feeling tells me this is a less ideal situation than using two woofers of the same kind? The low pass crossover is pretty straightforward. 2 large inductors(3.8 mh and 2 mh in series with a shunt 470uf plus a 0.5 ohm resistor for a 200Hz third order low pass. In real life, the 3.8 mh and 2 mh even wound with 17 gauge wire would probably add a DCR of 2 ohms. This would have effects on Qts(raising it) and some FR leveling.

http://www.sound-thinking.org/index.php?showtopic=772&st=30

I opened up a pair of AR TSW 410s once, Sure enough there were a 8-inch woofer and 6-inch woofer sharing the same volume and connected in parallel in the bass. Maybe AR knew something we didn't:)

Link to comment
Share on other sites

AR used that design quite a bit in the mid-80's. I think the head of engineering at the time was Alex DeCoster, and he must've liked that approach. If I remember, the TSW-410 looked like a 3-way, but was actually a "2 1/2"-way. The 8" and 6" woofers both covered the bass (sharing the entire enclosure volume), bit the 8" was rolled off before it entered too high into the midrange, leaving the 6-incher to carry on alone and cross over cleanly to the tweeter.

AR's so-called 'high-end' range in that time period, the Connoisseur Series, had a Model 35, which used the same driver compliment: an 8," a 6," and a tweeter, with the same type of crossover. The 35 actually made it onto Stereophile's Recommended Component list. The 410 was very favorably reviewed by High Fidelity magazine .

Steve F.

Link to comment
Share on other sites

An open question to all concerning the AR-9 crossover - or more particularly the bass section of that crossover. Below is the crossover as reproduced from this forum (just the bass section).

The red line is NOT part of the original schematic - it is there to aid our discussion. Please note that the AR-9 features two 12" woofers wired in parallel. Because of the parallel approach the effective impedance as presented to the amplifier will be VERY low (~ <= 2 ohms) at the resonant frequency of the woofers (~ 18 Hz or so - I think).

According to design notes (located on this site) from Terry Holl, the AR-9 designer, the 2500 uF capacitance was installed to change the effective Q of this circuit at or near the resonant frequency - thus keeping the effective impedance presented to the amplifier from dropping too low.

The reason this was done was primarily because the amplifiers of that era ('79 - '81) were for the most part incapable of driving such a low impedance load - most of the receivers of that era struggled with a 4 ohm load as all were designed for a nominal 8 ohm load. Hence the circuitry in the crossover. Holl described this as equivalent to an "automatic" clutch that would only be in play when needed. Very clever if I say so myself.

In our current era however most amplifiers - or most serious amplifiers - can drive a 4 ohm load or even a 2 ohm load without any real difficulty. Hence I have been considering just cutting the 2500 uF cap out of the circuit. (Side Note: I have rebuilt my AR-9 crossovers with all new caps (metallized film from Solen and Jantzen) and all new wiring - I replaced the original 2500 uF "Beer Can" with a bank of NPE caps (5 x 500 uF)). The point at which I would be cutting the caps loose is indicated by the RED line in the above schematic.

So I am asking the folks on the forum if they have any opinions or experience with this matter. Would enjoy hearing your thoughts on this matter. Will I get deeper bass? Will my impedance drop too low at resonance? (too low being defined as < 1 ohm). Will it even make any difference?

Thanks in advance.

My advice; "Don't fix it if it ain't broke."

My AR9s refoamed, recapped except for the two metal cans in the woofer section, carefully positioned, and carefully equalized continue to pour out bass second to nothing else I've ever heard in my 4000 cubic foot acoustically moderately live room with a 60 wpc amplifier. It seems to produce strong and sometimes overwhelming bass down to the lowest sounds I can hear and then some without the slightest signs of strain or distortion. In this regard it lacks for nothing. What could be achieved by tinkering with it except to degrade it?

Link to comment
Share on other sites

Hey - if you "don't break it - it can't be fixed".

I am suprised you are getting such bass with 60 per side - I am running Odyssey Audio mono-blocks (conservatively rated at 110 wpc) and don't seem to get that much response.

But my main thought here was that the speaker was designed during an era when amplifiers (mostly receivers) did not have much in the way of "drive" - the four ohm nature of the speaker meant that almost any amp/receiver was already being asked to do more than it was designed for - and Holl did clearly state that the main reason for the 2500 uF cap was to keep the minimum impedance high enough not to fry amplifiers.

So this being a "hobby" and myself being a dedicated "tweaker" the thought occured to me that perhaps I would get even "MORE BASS" by removing the capacitance from the bass crossover. Though the responses I have received seem to indicate - not very clearly I might add - that to remove that cap from the circuit will modify the entire F/R curve from 200 down. Though NOBODY has been very exact in their reasoning - mostly just a sense of "this might not be an improvement" - which I already knew and hence was asking opinions here.

I have plotted the response of the AR-9s with pink noise and a spectrum analyzer - and there is a roll-off as you move below 100 Hz (and a peak at 200 where the 8" overlaps with the two 12" - but that - I think - was intentional to remove the dread floor bounce suckout that almost all speakers demonstrate in that region). Though the low mounted side-firing woofers were designed by Allison to also address this "floor bounce" problem.

I guess I will go in there and cut them loose and see how it sounds - pretty easy to do.

Link to comment
Share on other sites

The quantity and quality of bass, to a large degree, depend on the size of the woofer and displacement of the woofer. A better magnetic circuit will allow some headroom beyond the linear travel of the woofer and still manage to sound decent. An optimal woofer/enclosure design(in AR 9 for example) will not only maximize the bass response(within limitation) but provide some control below the enclosure resonance point to minimize the unavoidable increase in distortion. A 5% distortion at 20 Hz at 10 watts is not meaningful until you give the actual spl at 20 Hz.

Here is an example of testing a perfectly decent 12 inch subwoofer with twice the linear p-p travel of AR 12 inch woofer. However, the limitation in bass extension vs distortion and max slp is quite obvious.

http://www.hometheatershack.com/forums/subwoofer-tests-archived/978-diy-infinity-kappa-perfect-12-a.html

BTW, I have heard 40 Hz from the smallest Hi Fi Speaker-Goodmans Maxim, designed by Ted Jordan. It has a tiny woofer that could travel half-inch p-p. It has a frequency response like that of a KLH 17 from the tests done at Wireless World if I remember correctly. At low listening level it can fool you into believing it is a much bigger speaker.

http://www.eastmarinedrive.com/audio/2003/maxim.html

If you like to experiment, I would suggest using an amplifier with a parametric equalizer with a variable highpass filter. You may find extending the bass too low will overburden the woofer so much that it sounds thick and muddy. Personally I found cutting the subsonic frequency from the woofer actually makes it sound more musical. But again my hearing may be different from yours

Link to comment
Share on other sites

Hey - if you "don't break it - it can't be fixed".

I am suprised you are getting such bass with 60 per side - I am running Odyssey Audio mono-blocks (conservatively rated at 110 wpc) and don't seem to get that much response.

But my main thought here was that the speaker was designed during an era when amplifiers (mostly receivers) did not have much in the way of "drive" - the four ohm nature of the speaker meant that almost any amp/receiver was already being asked to do more than it was designed for - and Holl did clearly state that the main reason for the 2500 uF cap was to keep the minimum impedance high enough not to fry amplifiers.

So this being a "hobby" and myself being a dedicated "tweaker" the thought occured to me that perhaps I would get even "MORE BASS" by removing the capacitance from the bass crossover. Though the responses I have received seem to indicate - not very clearly I might add - that to remove that cap from the circuit will modify the entire F/R curve from 200 down. Though NOBODY has been very exact in their reasoning - mostly just a sense of "this might not be an improvement" - which I already knew and hence was asking opinions here.

I have plotted the response of the AR-9s with pink noise and a spectrum analyzer - and there is a roll-off as you move below 100 Hz (and a peak at 200 where the 8" overlaps with the two 12" - but that - I think - was intentional to remove the dread floor bounce suckout that almost all speakers demonstrate in that region). Though the low mounted side-firing woofers were designed by Allison to also address this "floor bounce" problem.

I guess I will go in there and cut them loose and see how it sounds - pretty easy to do.

I've got to use considerable equalization to get this kind of response. There's probably about 10 db or more boost at 30 hz. The LMR level switch is set at -3db. Evidently my ears confirm what you measured. Two stages of LF filtering are available to reduce infrasonic and low frequency rumble and other noise inherent in some cds. Further equalization for each recording is required and available on in my system.

If you look at the Fletcher Munsen curves or whichever you trust instead and think beyond the fact that hearing is relatively insensitive at the frequency extremes compared to where it is most sensitive around 4 khz, you'll also notice that as you get to the extremes small changes in actual loudness results in relatively large changes in perceived loudness. This points to how critical it is to carefully equalize for the room, the speaker, the recording and the playback loudness. Each of these variables will affect the perceived bass loudness and relative tonal balance. MY amplifier seems very stable and can be operated in bridged mono for 250 watts output into 8 ohms. Clearly it has a good power supply. I've never heard it clip even though it has been used at loud but not deafening levels. IMO once an amplifier can provide sufficient power over its required range without audible distortion further electrical performance improvement is of no value. While I don't completely agree with Ken that all solid state amplifiers sound exactly alike, (I don't think they do), except for those with unusually high harmonic distortion or which are not stable with their load, virtually all audible differences are attributable to relatively small differences in frequency response which can easily be corrected for with equalization. Therefore I have not found it compelling to buy a very expensive amplifier. I see no merit in it. What's more, should I require one with much greater power, I think a new generation of class D amplifiers will perform excellently at low to moderate cost, if not fully assembled then DIY.

Link to comment
Share on other sites

The quantity and quality of bass, to a large degree, depend on the size of the woofer and displacement of the woofer. A better magnetic circuit will allow some headroom beyond the linear travel of the woofer and still manage to sound decent. An optimal woofer/enclosure design(in AR 9 for example) will not only maximize the bass response(within limitation) but provide some control below the enclosure resonance point to minimize the unavoidable increase in distortion. A 5% distortion at 20 Hz at 10 watts is not meaningful until you give the actual spl at 20 Hz.

If you like to experiment, I would suggest using an amplifier with a parametric equalizer with a variable highpass filter. You may find extending the bass too low will overburden the woofer so much that it sounds thick and muddy. Personally I found cutting the subsonic frequency from the woofer actually makes it sound more musical. But again my hearing may be different from yours

Allison speakers could be "improved" by using their optional ESW (electronic sub-woofer). But there is no "free lunch' here either. The lower the frequency below system resonance, the more power needed to make an "audible" improvement. Personally, I feel this improvement is worthwhile only if playback levels is kept low to moderate. Many (most?) listeners are very tolerant of high woofer distortion as long as the woofers don't "bottom". Even if not "bottoming", the distortion generated by such large excursions do tend to muddy things up.

Excessive cone motion modulates the upper bass frequencies when driven hard. The audible effects are very similar to ported systems reproducing frequencies below their port tuning frequency; The woofers are excesssively moving, but nothing is "heard'. I think that having a subsonic filter is a "must". The Apt/Holman pre-amp incorporated such a filter. It's main benefit was to keep turntable rumble and ultrasonic garbarge induced by poor tone-arm/ phono cartridge combinations from wasting amplifier power reproducing signals the speakers can't reproduce in the first place.

I'm not an organ lover, so IMO, anything below a 30-35 hz roll-off is not necessary for the vast majority of even "hard-core" audiophiles. That's why I think the AR 9 is that "perfectly enginnered" loudspeaker that does just about everthing . All you need is a hefty amp capable of LOTS of CURRENT at low frequencies. The Snell Type A is also a contender for "good bass" but I don't have the strength to move it for optmum bass response. At least with the AR 9, I can "walk it".

I can't emphasize enough how speaker / listener placement for ANY given room speaker combio SWAMPS anything promised by a "spec sheet" !

Link to comment
Share on other sites

I sold my AR9's years ago. Subsequently I bought a pair of HSU Research TN 1220 HO's as my subwoofers. Initially I used Adcom GFA 555 ll to drive them without equalization. but I was not getting enough low bass, however. Looking at Audio Magazine August 1998 issue and it showed that while a single TN 1220 was capable of 105 db at 20 Hz( 115 db with room gain), the vented alignment was designed for -6 db down at 20 Hz . Later I bought a Datyton HPSA1000-R 1000W subwoofer amplifier with built-in parametric equalizer and subsonic filter(high pass at 18 Hz). I engaged a full 6 db gain centered around 20 Hz and now they began to produce satisfying low bass to my liking. Therefore, even with a woofer with very potent capability the proper use of an equalizer can bring more enjoyment out of it, IMO. The acoustic suspension AR-9's should have better subsonic control over a vented design and they may or may not need a subsonic filter. However, too much power goes into subsonic region may generate a lot of heat in the voice coils and this may change the sound all across the woofer's operating range.

Link to comment
Share on other sites

My suggestion, in case it was not clear is NOT to remove or open the LC

circuit. You will get less bass at and around the system resonance and it

will probably not be a very large difference. Of course, go ahead and try

it to satisfy your curiosity if you want.

Driving the woofers directly is going to completely alter the response and

you'll have to match the crossover response in the active section - but I

don't think this is what you had in mind.

Hey - if you "don't break it - it can't be fixed".

I am suprised you are getting such bass with 60 per side - I am running Odyssey Audio mono-blocks (conservatively rated at 110 wpc) and don't seem to get that much response.

But my main thought here was that the speaker was designed during an era when amplifiers (mostly receivers) did not have much in the way of "drive" - the four ohm nature of the speaker meant that almost any amp/receiver was already being asked to do more than it was designed for - and Holl did clearly state that the main reason for the 2500 uF cap was to keep the minimum impedance high enough not to fry amplifiers.

So this being a "hobby" and myself being a dedicated "tweaker" the thought occured to me that perhaps I would get even "MORE BASS" by removing the capacitance from the bass crossover. Though the responses I have received seem to indicate - not very clearly I might add - that to remove that cap from the circuit will modify the entire F/R curve from 200 down. Though NOBODY has been very exact in their reasoning - mostly just a sense of "this might not be an improvement" - which I already knew and hence was asking opinions here.

I have plotted the response of the AR-9s with pink noise and a spectrum analyzer - and there is a roll-off as you move below 100 Hz (and a peak at 200 where the 8" overlaps with the two 12" - but that - I think - was intentional to remove the dread floor bounce suckout that almost all speakers demonstrate in that region). Though the low mounted side-firing woofers were designed by Allison to also address this "floor bounce" problem.

I guess I will go in there and cut them loose and see how it sounds - pretty easy to do.

Link to comment
Share on other sites

I sold my AR9's years ago. Subsequently I bought a pair of HSU Research TN 1220 HO's as my subwoofers. Initially I used Adcom GFA 555 ll to drive them without equalization. but I was not getting enough low bass, however. Looking at Audio Magazine August 1998 issue and it showed that while a single TN 1220 was capable of 105 db at 20 Hz( 115 db with room gain), the vented alignment was designed for -6 db down at 20 Hz . Later I bought a Datyton HPSA1000-R 1000W subwoofer amplifier with built-in parametric equalizer and subsonic filter(high pass at 18 Hz). I engaged a full 6 db gain centered around 20 Hz and now they began to produce satisfying low bass to my liking. Therefore, even with a woofer with very potent capability the proper use of an equalizer can bring more enjoyment out of it, IMO. The acoustic suspension AR-9's should have better subsonic control over a vented design and they may or may not need a subsonic filter. However, too much power goes into subsonic region may generate a lot of heat in the voice coils and this may change the sound all across the woofer's operating range.

Poh Ser of Hsu Research is a dear friend of mine; daughter's "stuff" is stored in my bedroom until she resumes school @ MIT. As "

payment", gave me his "mid-level" VTF Mark 4 system. At 78 pounds shipping weight, it's gonna stay boxed despite giving me all I could ask (and then some) for "accurate" low frequency reproduction . It has ALL the versatality I could possibly need to suit my needs.

Even if I owned AR 9's (never did), I'd STILL use his subwoofer "high -pass" between 40-90 hz using the built in 24 db Linkwitz/Riley X-O. Hearing his very early protypes reproducing "clean" 16 hz material was something arwesome; made me nauseus for some reason.

What surprised me was that switching the sub on and off made NO audible difference to the MAIN speakers (forgot what they were...some decades back..possibly Rogers LS5 or something like that) at the crossover point as long as the main speakers were not over-driven. With the subwoofers engaged, the main speakers retained all their famed properties AND able to play much louder without getting "muddy".

For the above reasons, I'm not a big fan of EQ'ing anything above 100-150 hz in the main system. Rather experiment with room placement to get mid-bass/soundstageing/imaging optimized to my taste. For the AR9's, messing with 200 hz crossover network just isn't worrh the risk, especially given the availabity of high -performance/ low -cost products like Hsu Resarch .

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...