94 replies to this topic

[speaker dave]

Listening test: preamble

In the last weeks I’ve worked on an increasingly complex crossover revision for the venerable AR4x. From the series capacitor (1st order) stock network I’ve worked up to a third order tweeter network and even added an order to the woofer network. A number of you have asked about the sonic differences and so, at your requests, I’ve just finished a long session of listening to most of the variations that I had previously measured. In most cases I compared my # 2 system with the crossover internal, to the #1 system with various crossover iterations attached via external wiring. Finally I compared the fully modded #2 system to a Snell K5, a similar sized 2 way bookshelf unit that I designed 7 or 8 years ago.

Am I biased? Of course I’m biased. I know that the 3rd order network measures considerably flatter than the stock network. I can remember the approximate response curves of each variant. So it wouldn’t be too hard to predict what the sonic balances of the various iterations are. You’ll just have to trust that my descriptions are “fair and balanced”. I will do my best to give useful descriptions of what is heard, in a manner that anyone can relate to. No nebulous “speed and timing” type descriptions but more how each speaker’s balance impacts a variety of instruments or the human voice. I’ll include a real time analyzer curve taken of each paring taken after I listened to the particular pair.

To keep things simple I picked one sampler disc that had a variety of classical and jazz tracks, all from the Delos catalog and all recorded by John Eargle (much missed). Four tracks were principally used for every comparison. For a symphonic work: Prokofiev, Three dances from Romeo and Juliet. A Haydn Piano Concerto for the sound of piano, a female soprano singing “Before I Gaze at You Again” by Loewe and Joe Williams a “Mean Old World” for a Jazz piece. I listen to plenty of rock music, at least 60s and 70 folk and geezer rock, but these particular tracks covered the bases well. Rock music would have gotten me more focused on the frequency extremes and output level issues. Rather, my focus in listening was for matters of balance and midrange neutrality, as this is what you would be striving for while finalizing a crossover.

The speakers were on tall stands and well away from the walls, driven by a Scott integrated amp of about the same vintage. I sat about 2.5 to 3 meters away on a listening axis about even with the top of the woofer. At that position the bass was smooth if a little light. With the chosen pieces bass was not a big factor and of course the crossover differences wouldn’t impact bass.

Please realize that the lowly stock AR4x will suffer some criticism, but we love it just the same since it will prove to have more potential then ever we could have guessed! I’d also note that the losers in these comparisons are frequently speakers that we would be perfectly happy to listen to music through. Comparison tests magnify the differences, while long listening sessions let us get used to the “personality” of a system.

[speaker dave]

Stock network vs. 3rd order network

The stock tweeter network is a 20uf capacitor leading to the 15 ohm potentiometer, with the tweeter reverse phased from the woofer.

My notes ascribe a “shouty” megaphone quality, especially on the Joe Williams track, to the stock AR4x. On switching from the modified 4, especially on the piano piece, there was a strong nasal quality. Some times colorations can be described as contributing vowel sound qualities and the stock AR4 had an “aw” vowel quality. Massed strings playing in their upper registers sounded okay but Clarinet and woodwinds shared the nasal quality. After some minutes of listening to the stock 4 the balance was more palatable but the midrange prominence and a general treble dullness was always there. Immediately upon changeover, though, the nasalness was striking.

The modified 4 was clearly flatter in the midrange and had more upper harmonics more of an “eh” lower treble vowel sound, at least in comparison. My notes say “voice is sharp but not resonant.” “Piano has a slight split with the lower register warm and the upper register light”.

I brought my wife in to see if she would notice the differences and how she might describe them. All I said was “here are two systems, based on similar parts but both of which had been modified. Can you hear a difference?” We switched back numerous times between the two while listening to the Soprano singing Loewe. Finally she said: “I prefer the one on the left (the 3rd order network). It sounds crisper and wider.” “The one on the right sounds like a room with no drapes.” That seems like a pretty apt description of the resonant quality of the stock system.

The curves show the comparison of the two. The orange curve is stock and the green curve is the fully modified system.

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[speaker dave]

Revised first order network vs. 3rd order network

I had previously found that a first order network worked a little better with the 20uf cap reduced to 10uf and tweeter changed to in-phase with the woofer rather than reverse phase. I adjusted the #1 system to that configuration and compared it to the 3rd order modified system.

The listening notes again concentrate on the characteristics of the system with the simple network. “Piano: dull, Soprano: dull, hollow. Saxophone projects” (3rd order system: “fizz on saxophone”). Symphonic music was a better match to this simply modified 4: “Strings in upper range good. Slight mid emphasis.” “Oboe bad, “aw” sound.”

By comparison the heavily modified 4 had “more edge to voice” but I noted that, in general, the differences between the 2 systems had been "cut in half" relative to the stark differences of the first pairing.

Curves for this pairing show much less 1200Hz bump but still a quick drop above that frequency and a low general level for lower treble. Instruments that straddle this transition are clearly going to be effected. This relative difference between mid level and treble level led to the mid emphasis and dull sound.

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[speaker dave]

3rd order without woofer shelving vs. full 3rd order network

For the third test I had connected my second 3rd order tweeter network to the #1 system, but not the extra 2 components that convert the woofer to a damped 2nd order network. By this point the differences are becoming minor and a preference is less consistent. From the notes: “System 1 (all mods but woofer network) darker balance, rich”. “Piano a little dull, sax nice, voice a little “shouty”. The fully modded AR4: “sax thin, better balance between fundamental and harmonics on Soprano.”

At this point the differences were still fairly obvious in a direct comparison but either system would probably sound fine in isolation. If you like a little bit of the classic AR balance then this version (#1,no woofer mod) would probably suit you well. It was really nice on symphonic music and had a nicely rounded quality to it. I thought the fully modded system sounded a little more neutral, but might be accused of having a bit of treble edge.

The curve shows the difference the second order woofer network addition makes. Orange curve is tweeter net mods only. The green curve has the woofer net mods as well. It pulls down the region from 800 to 2kHz or so by about 2dB. I wonder if pulling this down too much has left the 3kHz region a bit exposed (“Fizz on Sax”)

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[speaker dave]

AR4x with 3rd order network vs. Snell K5

Just to throw another system into the mix I did a final comparison between the fully modded AR4x and a Snell K5 bookshelf system. This was a fairly expensive unit with Peerless 6 ½” woofer and metal dome tweeter, a damped laminated baffle and sophisticated crossover. (Also a beautiful Rosewood cabinet made in our factory in Haverhill Massachusetts.) Of the Snell products we did in that era this was fairly successful and something I would offer up as a well balanced system.

From the notes on the fully modded AR4x #2: “A richness in the lower 100s. Nice overall balance. More ‘eh’ sound to treble.” On the K5: A little more mid emphasis.” “Reminiscent of previous comparison (somewhat like the 3rd order AR4 without the woofer additons).” “Piano slightly dull. More ‘ow’ sound.”

And finally a pink noise comparison: “More upper mid/lower treble” (on full mods AR4x) and More “polite” or recessed for the K5.

Note that the K5 has more bass extension but it gave up nearly 3dB of sensitivity to get it. The AR4 is not as insensitive as we would guess. In truth AR speakers were only low sensitivity relative to the 50s and 60s systems that preceded them.

The curves for the two speakers are surprisingly similar, with no clear trends to suggest either is superior. The flip flop of levels between 800Hz and 2kHz is probably what leads to the balance judgments but we are getting to the limits of what our curves can tell us.

Conclusions?

As I stated before, I have to admit to a heavy amount of bias in this test in that I had modified an AR4x to measure flatter and was likely to prefer it no matter what! Still, I believe the descriptions of the sonic qualities and the magnitude of differences that I have described are accurate and proportionate, and that other experienced listeners would have similar observations. I might tweak the 1kHz level up a tad and the 3kHz down a smoosh, but otherwise I think it is about right.

I think the end result is a very good speaker that I could happily listen to any type of music over without the system ever getting in the way. These are not exotic drivers by today’s standards yet the end result doesn’t have anything to apologize for. It really is competitive with modern systems of similar size and bigger systems would only surpass in areas of bass extension and output capabilities. So it meets my initial goal of being based on AR parts, still looking classic, but not sounding dated.

For those that have stuck with the thread this far: Bravo! Hopefully you have picked up a bit about the design process as some of us practice it. It is a bit science and a bit art, but no real magic is involved.

Its time for some others out there to try their own modifications and see if they like the results.

Regards,
David

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[genek]

AR4x with 3rd order network vs. Snell K5

Dave, what's happening between 100 and 200 Hz? Both speakers show the same dip, and it isn't anywhere near the crossover point of the 4x. Is that a room effect? Does it play any role in your descriptions of the speakers' sound?

[speaker dave]

Dave, what's happening between 100 and 200 Hz? Both speakers show the same dip, and it isn't anywhere near the crossover point of the 4x. Is that a room effect? Does it play any role in your descriptions of the speakers' sound?

Yes, that is the floor bounce dip. The mic was about 3' 10" in front of the speakers. Tall stands had the systems about 3ft off the floor, so the dip frequency could be calculated from that. I tried the mic back at the listening position and at that resolution (1/6th octave) the dip is gone, replaced by lots of room effect wiggles that average out.

I didn't notice it in the sound (actually it didn't appear, as shown, at my position) but I did notice that the sound was a little "phasey" when I leaned back in the couch and a little clearer when I leaned forward. These were spatial effects and didn't seem to "overwrite" the frequency effects.

David

[vanquishfist]

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

[speaker dave]

Here is a schematic of the final network. Pardon the hand drawing, perhaps someone can do up a proper drawing?

In the end I have put the variable resistor back into the circuit as an aid to those with unequal tweeter sensitivities (a seemingly common problem) and also to let you "adjust to taste". Early on I found that just a series R (rather than potentiometer connection) suited the response shape I was trying to get, so the pot is connected differently than the standard AR configuration. The 10 ohm resistor around the pot reduces its total range and puts the expected 4 ohm value about 1/2 way along its travel.

From my adjusting I know that most of the component values are non critical, with the exception of the tweeter inductor. When I get a chance I'll measure the DCR of that choke and add that to the info. It was a fairly fine wire over one of the compressed dust cores, nothing special.

Note that my speaker pair have the 1.0 mH AR inductor. If you have the 0.8mH inductor you should add 0.2 in series. Also note that the tweeter polarity is reversed from stock.

Good quality low loss electrolytics will work fine. Please don't do anything silly like buying a film 50uF (just so you can put 6.8 ohms in series with it!). You can substitute 47 for the 50 and 20 for the 22, if that is what you find locally.

Let us know if you mod a pair and how it comes out!

David

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[speaker dave]

Here is a nice PDF drawing of the final modified network, courtesy of Richard Lowe (thanks Richard!).

I need to explain that there is a little controversy over the woofer inductor. I've measured mine twice and get 1mh (or 0.99mH) with a digital meter. The AR lists show a number 4 inductor as 0.88 and the #5 as 1.2mH. I believe mine is a number 4 but the value is not to spec. If you are going to do the mod and have a #4 measuring 0.88, then don't worry about the exact value or adding inductance to up the value to 1.0, at least until we can see if there is a consensus about a possible third value. My system serial numbers are FX141819 and FX157677. It would be nice if others could measure their inductors and see if any serial numbers close to mine are 1.0 or 0.88. Those with a 1.2 can do the full mods as well but optimally the 50uf, 6.8 ohm combo might need a little adjusting (the 6.8R would probably increase). Hard for me to say without having the systems to try. (Zilch, what is your woofer and inductor stockpile like??)

Anyhow, here it is and have at it.

David

Attached Files

•  AR4x_Mods.pdf   35.51K   384 downloads

[RoyC]

Here is a nice PDF drawing of the final modified network, courtesy of Richard Lowe (thanks Richard!).

I need to explain that there is a little controversy over the woofer inductor. I've measured mine twice and get 1mh (or 0.99mH) with a digital meter. The AR lists show a number 4 inductor as 0.88 and the #5 as 1.2mH. I believe mine is a number 4 but the value is not to spec. If you are going to do the mod and have a #4 measuring 0.88, then don't worry about the exact value or adding inductance to up the value to 1.0, at least until we can see if there is a consensus about a possible third value. My system serial numbers are FX141819 and FX157677. It would be nice if others could measure their inductors and see if any serial numbers close to mine are 1.0 or 0.88. Those with a 1.2 can do the full mods as well but optimally the 50uf, 6.8 ohm combo might need a little adjusting (the 6.8R would probably increase). Hard for me to say without having the systems to try. (Zilch, what is your woofer and inductor stockpile like??)

Anyhow, here it is and have at it.

David

Nice drawing...

John O'Hanlon and I compiled data for literally dozens of 4xs a few years ago. The era indicated by your serial numbers was well represented, and I have seen many 4xs since. I'm convinced AR never installed a 1mh inductor in a 4x on purpose.
Attached is the AR coil chart posted by Tom Tyson awhile back. Your 4x should have a #4 or #5 coil.

 excel_coil_chart.zip   3.72K   234 downloads

Roy

[graham]

Thanks for the schematic Dave, and more thanks to RoyC for plugging it into a computer. If anyone has sourced the parts could they share the details of suppliers and prices please?
the extra 0.10mH air coil inductor - parts-express have 14/15/18/20 gauge - which is best ?
also what is the source for an iron core inductor of 0.25mH / 250uH ? part-express don't go down that low.

[pb8000]

Thanks for the schematic Dave, and more thanks to RoyC for plugging it into a computer. If anyone has sourced the parts could they share the details of suppliers and prices please?
the extra 0.10mH air coil inductor - parts-express have 14/15/18/20 gauge - which is best ?
also what is the source for an iron core inductor of 0.25mH / 250uH ? part-express don't go down that low.

I am restoring an AR4x using a Parts Express phenolic ring tweeter. I added a 2nd order high pass filter - cap followed by a coil - and used an air core .25mh, 18 gauge inductor from PE.

[d052057]

I am in process to restore my AR-4X. I would like to hear from people who have done this mod and tell where to get all of the parts.

So far, I've resanded the cabinets and applied boiled lindseed oil. It looks great now. I took the potentiometers and I tried to clean them. They are really broken and can not be repaired. I've heard that by passed the pot is an option as well but I wonder is mod is really really worth it.

Thank you

[phivates]

I've gone back to my starting point, AR4x, as the standard for essential listenability. They were 5 bucks, and they're still good. The screech of most two ways, like the Polk 10s, is mostly absent, and I will now upgrade the x-o to make them one of my reference tools for the tweaking I'm unable to resist. THANKS DAVE.