Chorus II mods

[g3dahl]

Lynn Olson and I have begun work on our Choruses. Lynn has a pair of the originals, and mine, which arrived several days ago, are the later "II" version.

We made some improvements to Lynn's speakers, but didn't change parts values. Crossover caps were replaced by Hovlands and North Creeks. The mid and tweeter horns were treated with rope caulk. They sound quite promising, but the speakers do have a treble emphasis, and some peaks in the lower treble. More work will be done to these speakers during the coming weeks.

Yesterday, my Chorus II's were put on the operating table. At the start, I had already replaced the crossover caps with Hovlands, Auricaps and North Creeks. One of the tweeter's phenolic diaphragms went south the previous night. I re-did both tweeters, because my replacement diaphragms were the polymer type, which Klipsch specifies for the Chorus II. For some reason, they used different diaphragms types but kept the same model name for the tweeter (K-79-K).

We began by performing some MLSSA scans. The tweeter was horrible! It was about 10 dB down from the level of the mid, and very peaky. I wasn't too concerned, though, because I had already planned to use the Fostex FT17H tweeters I had on hand. I installed the Fostexes. The level was still below the mid, but very close to that of the woofer. This was promising, because we wanted to get away from a type of response that rises with increasing frequency.

The next step was to lower the output of the mid. We put a resistor in parallel with the autoformer to bring it down by the right amount. This caused a new problem though, because the crossover was now mis-tuned, resulting in a "hole" between the woofer and mid. We added capacitance in the mid crossover, but this didn't help. The horn cutoff was so close to the crossover frequency, the actual response wouldn't respond to the increase in capacitance.

The solution was to run the woofer up a bit higher. We decreased the capacitance until we got as smooth a curve as we could. I should add that the best final results were achieved with the mid connected in reverse polarity. The Fostex tweeter was connected in the same (reverse) polarity as is standard for the Chorus II. (BTW, this reversal is done on the board, not at the tweeter. You still connect the green wire to the tweeter's "+" terminal, and the black to "-").

Last night I got the mods installed securely in both speakers, and had a nice long listening session. Wow!! With the response flattened up, the sound is much better. There almost none of that annoying "bite" in the lower treble, the upper treble is much more extended and airy, and the lower midrange has finally emerged.

My only complaint right now is that the Fostex tweeter sounds like it needs a steeper slope (jeez, it's already 18 dB/oct!). I suppose I'll be on the lookout for a better tweeter anyway...

The next step is going to be way over the top for this speaker, though. I have a pair of Altec 311-60 large-format sectoral horns, and a pair of 290 drivers which will go on top of the Choruses, to replace the midhorns. I have ordered some Universal Transformer autoformers (as used by ALK) which will provide enough taps to lower the 290s' output suitably. Of course, this will take a complete re-do of the crossover, but with Lynn Olson and his MLSSA system around, it will be in good hands.

Many thanks to Lynn Olson for his crossover expertise, patience and persistence! Hopefully he will chime in on this thread, and maybe post some of the MLSSA plots.

Gary Dahl

[LynnOlson]

Here's the first image, showing the before-and-after of Gary Dahl's Chorus II's. The green trace shows the modified system, the orange trace the original. This is a 6mSec FFT that excludes room reflections, and should not be considered accurate below 300Hz (due to window length). It is, however, an unsmoothed graph, so the full depth of the HF nulls are apparent.

[LynnOlson]

Here's a different view, with 1/10th octave smoothing and the FFT window opened up to 20mSec. The notch around 200Hz is caused by comb filtering from the floor bounce (all speakers do this), and the smoother appearance of the HF is an illusion created by the smoothing algorithm. If smoothing is not used, the traces appear very rough due to multiple room reflections falling in the 6-20mSec window. Those fortunate enough to make free-field measurement in an anechoic chamber, or outdoors hanging from a crane, can use FFT windows as large as 20mSec or more, removing the need for smoothing.

The use of smoothing is controversial in speaker-design circles. Some feel it makes graphs easier to read by directing the eye to broad trends, and others feel it hides fine detail, making for prettier graphs the marketers like. I'm in the second group, but I'm used to the wrinkles that real-world drivers have.

It should also be noted that 1/10th octave smoothing is MUCH LESS than many driver mfg's like to use ... 1/3 octave smoothing (much heavier) is pretty typical when measuring rough PA drivers. By comparing these two graphs in your browser, you'll get an idea of the appearance of smoothing, so you'll know it when you see it in mfg's data sheets.

[LynnOlson]

So much for the easy stuff, guys. Now I'm going to show the raw time data from which MLSSA calculates the freq response. This is the stock Chorus II in the time domain. The twin-peak structure reveals that it has mid and tweeter horns with dissimilar path lengths to the microphone (1mSec equals 14" distance).

It is standard design practice to trim the phase response of the crossover to smooth out the crossover region, but the 1mSec discrepancy in the time domain remains and is visible on MLS, FFT, or TDS systems. There are two "fixes" if the design thinks close time-alignment is desirable: either move the HF tweeter towards the back of the cabinet, sitting just behind the mid voice coil, or use a digital crossover and slide things around in the digital domain.

Most speaker designers (but not all) believe time distortion of 1mSec or less is not audible. Delays of 3mSec, though, are more controversial, with the argument going back to the original Shearer horn for MGM, and the resulting Altec 2-way theatre systems.

[LynnOlson]

And here's the Gary Dahl-modified Chorus with the Fostex tweeter. No difference in the two-spike time response, but you can see the mid driver is attenuated by about 2dB relative to the stock Chorus, and the Fostex has a little tidier reflection performance (fewer ripples in the time domain). The reversal of phase for the mid horn is also visible compared to the stock speaker.

[LynnOlson]

So far you guys have been getting off light, now I'm going to throw you the serious data. The Cumulative Spectral Decay, 3D graph, or Waterfall curve shows time, frequency, and resonant structures all in one graph. The rear-most curve is the unsmoothed freq response - in fact, most MLS systems provide no means for smoothing this data, so if you're suspicious of the mfg's freq curves, take a close look at the Waterfall curves and look at the real, unsmoothed freq response.

D.E.L. Shorter of the BBC first wrote of "delayed resonances" in Wireless World in the mid-Fifties, and this referred not to major resonances that show in freq resp graphs, but subtler resonances that continue to ring on after a musical transient. Kind of like a quiet off-key bell that plays alongside the music - and all drivers do this, the only problem back then was measuring it. Shorter used an exotic 10-cycle-on, 10-cycle-off chopper circuit that triggered a scope display when the sine wave cut off, allowing him to visualize narrowband stored resonances as he gradually swept an oscillator over the working passband of a driver. Unfortunately transatlantic prejudices and the old Not-Invented-Here syndrome kept this technique out of the USA for more than twenty years. It wasn't until the Crown TEF system, designed CalTech's Richard Heyser in the early Seventies, that allowed quick visualization of the stored resonances.

The advent of PC-based MLS systems in the late Eighties brought powerful analytic tools to the PC owner for the first time when the Doug Rife MLSSA system hit the market. These days, you can do this kind of analysis with a sound card, although a high sample rate (at least 96kHz) and a built-in lowpass filter set to 25kHz or higher is still desirable. The measurements you see here, for example, have a sample rate of 117kHz and a Butterworth lowpass set to 25kHz (the MLSSA system has tunable lowpass filter, allowing choice of frequencies and a selection of Chebychev, Butterworth, or Bessel filter shapes).

[LynnOlson]

And here's the CSD of the modified Chorus. I've picked out the dominant resonance, which seems to be around 6kHz. Note this high-Q resonance doesn't appear at this frequency on the freq response graphs shown earlier - this is a good example how freq resp graphs are misleading when tuning notch filters. You want to chase out the high-Q resonance you see on the CSD, not necessarily the bumps on the freq response curves.

The high-Q resonances, since they are so narrow, are not so much excited by musical harmonics as they are by musical transients, which have a broad spectral excitation. At these frequencies, resonances lend a metallic coloration, due to the bell-like high Q and fairly long decay time. A long 2.5mSec decay may not seem like much, but remember, that's a decay tail that's nearly 3 feet long!

[LynnOlson]

Here's an Energy-Time-Curve the TEF guys like to use, although I don't use it very much myself. It basically shows the overall decay in energy over time, but in dB vs time. It shows nothing of which frequencies are dominating the energy storage - which is why I don't use it - but it does provide a rough figure-of-merit showing how quickly a speaker quiets down after it gets a transient. The ideal speaker would shut up instantly, but we have to live with real-world driver with physical mass and many many resonances.

By the way, most audiophiles misunderstand the term "transient response". They think it's the risetime, but that's trivially a function of HF extension - in other words, the higher the HF extension, the shorter the rise time. Morever, the ear mostly perceives the rising edge as nothing more than a click.

The real differences in speaker drivers, and audibility, is the decay time, where the ear/brain has something to go on, and is looking for echoes and resonances. It would be nice if all you heard was musical resonances and echoes from the performaning space, but real speakers interfere with their own set of reflections and resonances that have nothing to do with musical values. Chasing out these resonances is the hard part of speaker design, since they are inevitable with physical drivers, horn-mouth reflections, and cabinet diffraction.

[Deang]

Awesome stuff. Now get Gary away from those Chorus' before he hack's them up.

[g3dahl]

Hi Dean,

Funny you should mention keeping me from "hacking them up." As a matter of fact, one of my projects will involve converting them from Chorus II bass (passive radiator) to Chorus I-type bass reflex. The Chorus II is tuned lower, but the the result is poorer transient response. Before I do it, though, Lynn and I will get our I's and II's in the same room for a direct comparison. I am not concerned with bass extension (that's what subwoofers are for!) but only with quality and realism for what is there.

Also, I think the Choruses look great with the (still silent) 311-60's on top! Of course, I will set them up so they also can be run alone, with their own mids instead of the Altec stuff. I should have the 311-60's up and running by the end of next week, if those autoformers show up soon.

Fun, fun, fun!

Gary Dahl

[leok]

Gary,

Very interesting. Observing similar graphs in Stereophile reviews, I've wondered what various Klipsch designs would look like.

Do you have the Chorus-IIs in a corner? I've come to consider the corner simply part of a speaker if there is a rear-firing passive element of any kind.

Caps I used in mine were Jensen foil pio for the 6uF on the mids and Hovland foil/polyprop. on the tweeters. The Jensens definately needed several weeks of break-in .. they were just terrible at first. Hovlands seemed fine all along. Now, if I can just keep grunge out of my tube sockets, and no more of my expert solder joints go cold, the speakers are very clean sounding.

On the K-61-K (I think that's it) mid. My Chorus-IIs are '95. My Forte-IIs are (or were) '90. Both use the K-61-K mid horn. I tried the '90 mids in the Chorus-II and they were too loud for the other two drivers (as they tend to be in the '90 Forte-II also. The '95 mids were a much better match for the other two drivers. That may accound for some of excessive mid power you found.

Thanks for posting your results.

leok

[Q-Man]

Gary,

Is the sensitivity rating of the Chorus around 100dB? I find the 290 with the 311-90 horn to be a perfect match with the 104dB Klipschorns. I was surprised that no attenuation was necessary.

I'm curious how the 290 and 311-60 will sound on the Chorus.

[LynnOlson]

The Beyma CF22 and CF25 look interesting as replacements for the stock tweeter, if the Beyma curves at US Speaker are to be believed. Efficiency is higher, but with the arrival of the new autoformers, correcting for that is no problem. There is some evidence of smoothing on these curves, but not as heavy as Fostex, where the factory curves look pretty different than the MLSSA data. (Warning! The Fostex PDF is a 10MB download!)

Very likely that Gary will use the Altec 311's on his Chorus', and I will keep the stock mid and try the Beyma tweeters in small sub-enclosures sitting on top of my Chorus I's. That way I can time-align the speaker without a lot of trouble, and get better treble while I'm at it.

[LynnOlson]

Sorry - Beyma CP22 and CP25, not the numbers posted above. The two look different, and each has its appeal. The CP22 has noticeably lower distortion than the CP25, and the horn profile at first glance seems smoother considering the throat-to-horn-mouth distance.

By contrast, the CP25 has (much) wider dispersion, and more extended mid response, simplifying the crossover design. Anyone who's been using the Beyma tweeters is welcome to chime in with their experience.

[g3dahl]

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On 8/10/2003 10:29:04 AM Q-Man wrote:

Gary,

Is the sensitivity rating of the Chorus around 100dB? I find the 290 with the 311-90 horn to be a perfect match with the 104dB Klipschorns. I was surprised that no attenuation was necessary.

I'm curious how the 290 and 311-60 will sound on the Chorus.

----------------

Yes, the Chorus II is rated at 101 dB/W/1m, which appears to be about right for the mid, but the woofer looks more like 98 dB. I will expect to bring everything in at an appropriate level to match the woofer.

I don't know which version of the 290 you have, but my 290-8K's are rated at 109 dB in the 311-60's and 106.5 dB in the 311-90's--the narrower pattern of the '60's results in higher on-axis output, so I do expect to need significant attenuation from the autoformers. If your 290's are earlier alnicos, perhaps their output has dropped a bit over the years, and is just right to match your K-horns...in any case, great luck for you!

Your enthusiasm for the 290's was a significant factor in my decision to buy a pair. I had previously owned 311-60's, and already knew they were good. I nearly bought a pair of 311-90's, but I realized that they would look pretty silly on top of a pair of Choruses! On the other hand, the 311-60's are just an inch wider than the Choruses, and look pretty cool up there. My listening room is fairly small, and I think it will be just as well to have the '60's anyway.

Gary Dahl

[g3dahl]

----------------

On 8/10/2003 10:03:04 AM leok wrote:

Gary,

Very interesting. Observing similar graphs in Stereophile reviews, I've wondered what various Klipsch designs would look like.

Do you have the Chorus-IIs in a corner? I've come to consider the corner simply part of a speaker if there is a rear-firing passive element of any kind.

Caps I used in mine were Jensen foil pio for the 6uF on the mids and Hovland foil/polyprop. on the tweeters. The Jensens definately needed several weeks of break-in .. they were just terrible at first. Hovlands seemed fine all along. Now, if I can just keep grunge out of my tube sockets, and no more of my expert solder joints go cold, the speakers are very clean sounding.

On the K-61-K (I think that's it) mid. My Chorus-IIs are '95. My Forte-IIs are (or were) '90. Both use the K-61-K mid horn. I tried the '90 mids in the Chorus-II and they were too loud for the other two drivers (as they tend to be in the '90 Forte-II also. The '95 mids were a much better match for the other two drivers. That may accound for some of excessive mid power you found.

Thanks for posting your results.

leok

----------------

Hi leok,

Yes, Stereophile's graphs are produced using MLSSA, just like Lynn's. When comparing Lynn's graphs with Stereophile's be sure to check the length of the sampling window--you can make the results appear much more flattering by reducing the sample length, or changing the scale of the waterfall plot.

I don't have the Chorus II's in a corner. I do realize that having them against the wall or in the corner would affect the lower frequencies, and probably bring the mid into balance with the woofer (with the stock crossover). In my room, however, it wouldn't be a practical placement.

Thanks very much for the info about the later K-61-K drivers. I think my Chorus II's are earlier units. I wonder what makes your later ones different...the diaphragm, or perhaps the whole driver? My K-79-K tweeters changed completely when I replaced the phenolic diaphragms with the newer polymers. Kind of frustrating; I wish Klipsch would use different designations for different parts. If it's not the same, it shouldn't be called the same thing!

Gary Dahl

[Q-Man]

Gary,

Altec never used alnico magnets in the 290 drivers. I'm using 16 ohm diaphragms and that puts the out put about 3dB lower then the 8 ohm diaphragms. I already guessed why you went with the 311-60 vs. the 311-90. Even thought the 90 fits on top of the Klipschorn it is kind of intimidating looking.

Good luck to you too. Once you get them dialed in let us know what you think of the sound.

Q.

[Deang]

Ya bunch of loons.

[g3dahl]

Hi Q-man,

The K series, introduced in 1979, was the first non-alnico 290. All previous units (C, D, E, G and H) had alnico magnets. The ferrite units are easy to identify, because the magnet is larger in diameter than the rest of the driver and is visible as a 1"-wide black band protruding around the middle of the driver, just ahead of the rear cover. The alnico magnets, having different proportions, fit entirely within the casting of the driver.

My 290-8k's will arrive Monday. I am anxious to see if they have Tangerine phase plugs like my 291-16k's (which are really 299's because I installed 8-ohm Pascalite diaphragms). Supposedly the Tangerines help in the HF extension department. The higher the 290's will go, the easier it will be to integrate a tweeter. Altec says 7k, which would be great, but they also claim 20k for the 802/902's, which is pretty iffy.

Gary Dahl

[djk]

"Gary,

Altec never used alnico magnets in the 290 drivers. I'm using 16 ohm diaphragms and that puts the out put about 3dB lower then the 8 ohm diaphragms. I already guessed why you went with the 311-60 vs. the 311-90. Even thought the 90 fits on top of the Klipschorn it is kind of intimidating looking.

Good luck to you too. Once you get them dialed in let us know what you think of the sound.

Q."

http://www.lansingheritage.org/images/altec/catalogs/1975-pro/page11.jpg

Early 290 units were alnico, later they went to ferrite and the tangerine phase plug.

Earliest units could swap diaphragms with the 288, later the pin locations and bolt circle were changed.

4 ohms was standard in early units, they used the copper wire the woofers were wound with. Later both 8 and 16 ohm units became available.

The large alnico and later ferrite drivers had about 2dB more sensitivity than the early smaller alnico motors.

Sensitivity of a 290G or later K, L, would be 108.2dB/W/1M on a 311-90 horn, the 106.5dB spec is at 4', not 1M.

Any impedance 290 may be used with a Klipsch network with the T2A autoformer by simply adjusting the tap, no cap change is needed.

A 292 is a 290 without the transformer cover, it has a flat back like the 288.

OT, the JBL2404 can be mounted either way, below 12.8Khz there is very little difference in dispersion.

This horn sounds very good with a 290 and a 6dB crossover at about 700hz and works well with an Eminence Delta Pro type 12" driver.

http://www.newark.com/product-details/image/mcm/image/54-330.jpg