LynnOlson

Don't forget the path-length difference between the speakers in the corners and the center speaker, in addition to the 5 foot bass delay in the corner horns. In theory, the most accurate correction would be a HT controller that provided an adjustable delay for the center channel between 5 and 8 mSec. (You need a tape measure to set the delay correctly. At sea level, 1 millisecond is 13.8 inches long.) The bass would then be path-length-synchronized for all three channels, and everything could be wired in-phase. It's true the center mids would then be out of sync by five feet, but then, that's the case for the horns anyway, so the audibility would probably be minor.

Yes, I will. Things still plenty busy around here - the afternoon/evening session with Gary was the one open spot in the last couple of weeks. The American Standard/Trane 14i heat pump/AC is finally installed, and now I'm fine-tuning the settings on the new thermostat. If it ain't one thing, it's another. I have a renewed respect for what the stock Klipsch does - I too would like the Cornwall/Chorus/Forte, or a similar speaker, to join the Heritage line. They're BIG step up from the Heresy, and have a different presentation than the all-horn Klipsch speakers. They're also a good compromise between the mainstream low-efficiency audiophile approach and Altec/JBL school. Looking at marketing demographics, quite a few DHT and vintage-tube enthusiasts would like a Heritage version of the Cornwall/Klipsch/Forte - there are very few speakers on the market today that have good-quality horns from 700Hz on up, a prosound 15" woofer, and a well-balanced autoformer-attenuated crossover.

The crossover description wasn't the easiest thing to follow, so here's a sketch that Gary made for the Chorus II Mod II crossover. As noted before, this crossover is NOT SUITABLE for a stock Klipsch Chorus II; it only works for a time-aligned version with a Fostex tweeter. During the brief time the Klipsch tweeters were working, though, the time-aligned version looked good, and the same smoothness we saw with the Fostex was also present. Lining up the voice coils definitely simplifies the mid-high crossover design, at the expense of esthetics.

Here's the pic that should have appeared with the first post, showing the impulse response of the Mod II version.

OK, here's how the crossover works. Starting with the bass, there's 3.5mH series inductor (stock part), followed by a parallel element. This parallel element consists of a 20uF cap in series with 3 ohm resistor (not stock). The 3-ohm resistor is a fairly critical value, with 2 ohms being too low and 4 ohms too high. The woofer is connected in-phase, as per the stock crossover. The mid horn crossover is the most complex element, with a Universal 3619 replacing the stock Klipsch autoformer. Starting at the amp side of things, there's a series 6uF cap. (The stock series resistor is deleted.) This is followed by a 10 ohm parallel resistor, which parallels the entire autoformer. The amp-side of the circuit is connected to the topmost tap of the autoformer, Tap 5. The circuit ground is connected to Tap 0. The signal emerges from Tap 1 of the autoformer, and before it gets to the mid-horn there is a parallel 1.75mH inductor (stock part). The mid-horn is connected out-of-phase (this is reversed from the stock crossover). The Fostex FT17H tweeter crossover is fairly close to stock, starting with a series 2uF cap (stock value). This is followed by a 160 microhenry parallel inductor (stock). The signal then goes through a 2uF cap in parallel with a 4.7uF cap, or more simply, a single 6.8uF cap (value is not critical). Before the signal gets to the tweeter there is a 50-ohm parallel resistor across the tweeter terminals. Tweeter phase is reversed, same as the stock crossover. Note this crossover DOES NOT WORK for a stock Klipsch Chorus II, or the Mod I version with the Fostex replacing the Klipsch tweeter. It ONLY works for a Chorus II with a Fostex FT17H mounted almost all the way towards the back of the speaker, about 6 inches off the top surface, with some kind of damping material covering the forward top surface of the speaker. By now you are getting the impression that crossovers are NOT a one-size-fits-all exercise, especially if the drivers are moved from the usual locations. For a stock Klipsch, the stock crossover is close to optimum for a speaker intended to placed against a wall or in a corner. When you pull the speaker out into the room, though, the mid and tweeter (mostly the mid) can get a little too hot, about 2dB or so. This is where autoformer taps come in handy.

Here's a different-looking set of data for the Mod II; it shows a 1/6-octave-smoothed response for a 20mSec window, which counts most of the early reflections as well as the direct sound. The participation of the room is obvious in the bass response, which starts to appear with longer window length. Measuring bass response, although easy to model, is actually very difficult. The near-field technique is usually preferred, since that shows total power output into the room, but of course the room (any room) is then excluded from the measurement. In the old days, speakers were measured outdoors suspended from guy wires, but it's generally considered not accurate to measure the bass response of a speaker in a free-field, since room loading dominates the level of the bass. In other words, a speaker designed to be flat in free air is going to be very bass-heavy in any room, even an auditorium, much less a home.

Here's the frequency response of the Mod II using a FFT with a 6mSec window, ending just before the first room reflections. This shows the first-arrival frequency response, which is an important subjective factor to the sound, but not the only factor. The ear/brain also independently processes the room sound, which is a summation of the frequency response into the room (i.e., the first arrival plus all room reflections). Speaker designers usually consider the first-arrival *and* the total response into the room when designing a speaker system, along with other factors such as directivity vs frequency, IM distortion vs frequency, time response, etc. etc. The weighting of these factors (no speaker at any price can do all of these well) is why speakers sound different; each designer picks their own set of priorities. PWK, for example, was famous for putting low IM distortion (and high efficiency) first on the set of priorities. He was almost alone in the industry, but stuck to his guns, to his credit. That design philosophy is why Klipsch sounds the way it does.

Here's the step-response of the Mod I version, with obviously poorer time response. The two different arrival times of the mid-horn and tweeter are clearly visible about 0.8mSec apart. One nice thing about MLSSA, or Sample Champion, or other MLS programs is that you can store the raw *.TIM files on disk and post-process later. I have files I measured back in 1990 on the Quad ELS, CLS II, and other speakers, although my measurement technique back then wasn't quite as good as it now, with a dedicated low-diffraction mike-stand and the ACO Pacific instrumentation microphone.

Here's the step-response data for the Mod II. As you can see, it looks really different than the impulse data, yet is calculated from the same dataset. What's going on is impulse data is heavily weighted towards the high frequencies, while a step-response (similar to a long-duration square-wave) is weighted towards the bass. A "perfect" step-response would rise (or fall, depending on phase) quickly, then gradually decline back to zero (no speaker can reproduce DC, so a return-to-zero is inevitable). Impulse response data, although the native format of MLS systems, can be difficult to interpret, since it mostly shows details that are associated with HF rolloff and things like that. A step-response, as you can see here, makes it much easier to line up the time response of a speaker, if that's your goal.

Here's the Mod I impulse data. True, the crossover was a bit different, but I can assure you a passive crossover can't easily create a discrete 1mSec delay like you see here. The time misalignment is quite visible compared to the first photo ... which by the way, shows a system that is only approximately time-aligned. If we'd spent more time at it (using the step-response data shown in the next photos), we could have dialled it in more exactly, to a few millimeters. But I think the Mod II is within an inch or so of being correct, judging from the impulse photo.

OK, here are the pics promised earlier. The first one is a MLSSA impulse response, with time-aligned Fostex tweeter. The next post will have the earlier Mk I mod, with a somewhat different crossover and the Fostex mounted in the usual tweeter location. Why Fostex instead of the usual Klipsch tweeter? It's not that the Fostex is startlingly better, just that Gary and I are having trouble with the Klipsch tweeter. The data we posted last time for Chorus II with the Klipsch tweeter was no good; or rather, the tweeter was no good, since the replacement diaphragm didn't fit, and was rubbing against the pole-piece. This ruined the measurements. Well, Gary got another set of replacement diaphragms that fit better, but the first one failed in the first half-second of MLSSA testing yesterday. We made lots of checks to make sure no DC was getting through the crossover, no wiring errors were present, and the power amp was free of ultrasonics. No dice, the tweeter must of just been getting ready to fail. Anyway, to make a long story short, we didn't have any good Klipsch tweeters to measure, although for a brief interval the MLSSA data looked good, with nice extension out to 20kHz. So the Fostex tweeters from the last go-round came out of storage and we adjusted the crossover and looked at how it did in a time-aligned setup. So here's the first graph, showing impulse response. The next post will show the conventionally-mounted Fostex tweeter.

Hi all, sorry for the late posting, but things have been busy around here, with a new HVAC system installed today. The twenty-year-old Carrier heat pump that came with the house quit (gee, these things don't last forever?) and finally got the new one installed and running today. The weather here in the Puget Sound region is usually mild, but it has a way of throwing curves at us every now and then. Had another measuring and modifyin session yesterday (I'd like to name this thread Chorus II Part Deux) and liked what we've done so far. Time-aligning the tweeter really chases out those annoying hill-and-dale ripples that seem to plague horn designs. Usually, you twiddle with the phase response (altering crossover slopes, reversing driver phase, etc.) but that seems to just chase the nulls from one part of the spectrum to another. When a design is not responding to the usual crossover tweaks, it's trying to tell you something ... to stop messing with the crossover, and look a little deeper at what's going on. In this case, the mid and tweeter are misaligned by about 1mSec. This is obvious in the time response of the earlier measurements, where you see two peaks, one from the mid, and the one from the tweeter. A millisecond doesn't sound like much, but it's five wavelengths at 5kHz. Looking at it that way, it's a lot. It's a truism in speaker design you can't correct a time error with phase adjustments. Phase is relative to one frequency only, usually the nominal crossover frequency. This implies a phase adjustment that's right for the crossover will be way out an octave above and below. And that's just what we saw on MLSSA - nulls and peaks that moved around, but never going away. Since I'm used to working with direct-radiators, this was a new one for me, ripples that just refused to go away. But I thought a bit about the severe, many-wavelength misalignment of the mid and HF driver. It's not pretty in the time domain - two spikes for the price of one - and has to be having bad effects on the FFT, which is really allergic to reflections of any kind. Even a heavily damped floor reflection at 3mSec (with 20dB of attenuation from many pillows) still creates small ripples in the FFT, so what can two big peaks 1mSec apart be doing? A lot, as it turns out. Once Gary and I moved the tweeter towards the rear of the enclosure (centerline of the tweeter about 6 inches above the cabinet top with several layers of towels on the cabinet top in front of the tweeter), the crossover pretty much dropped into place. The two spikes in the time display merged into one, and the crossover started to behave normally, with the crossover region nice and smooth for a change. In fact, the entire response of the Chorus/Fostex fits into a 5dB window - that's right, plus or minus 2.5dB, with no smoothing at all. MLSSA data to follow, stay tuned.

Might be a while 'till we get to the Khorns, since I don't think Gary or I are planning to buy a new house any time soon. Belle Klipsch or La Scala, yeah, a possibility there. One thing I have to mention is that MLSSA shows all horn systems in an unflattering light. What horns do best is very low IM distortion, and this is difficult to measure with MLSSA, or any system, without an anechoic chamber (big bucks there). MLSSA favors speakers with good time domain performance, and the champs here are electrostats and a few direct-radiators. By comparison, horn speakers don't come off well when you look at MLSSA data - this measurement bias is reflected in the high-end biz in general. So why I am messing around with MLSSA and Chorus at all? Well, I've owned my own MLSSA system since 1991, so I know how to drive the thing (working at Tektronix Spectrum Analyzers and before that, Audionics, helped in this respect). Even though it doesn't conveniently measure IM distortion, I can use it to optimize the time and frequency performance of a horn speaker, keeping in mind that decreasing crossover slopes exacts a price in higher distortion (true with any drivers, not just horns). It's also a little zany and offbeat to apply BBC/KEF/Quad philosophy to a horn speaker, which is usually considered the polar opposite in the speaker-design community. But why not? No reason to keep the sonic balance of the mid-Forties Altec Duplex around forever; it was state of the art then, but we can measure things better today, and can do better with modern crossovers. I have to admit an all-horn system is a lot bigger challenge than the Cornwall/Chorus/Forte series. Horns don't like being "stretched" out of their natural bandwidths (lots of lumps, lots of distortion), and the region where the bass horn quits (300Hz or so) and where the mid horn comes looks tough. Neither horn is really comfortable around 400 to 500 Hz, and that's a critical region of the spectrum. It's the center of the musical power band (most energy), the region where voices come in (bottom of telephone bandwidth), and the few studies that indicate that phase is audible indicate that 500Hz is where it is most audible. The room is starting to get goofy at 300Hz and below, which doesn't help. In fact, unless you have a *really big* anechoic chamber, most measurement techniques require you to switch to nearfield below 300Hz since room effects start to dominate the measurements. If you're getting the idea that 200 to 500 Hz is a rough region for a crossover - much less horn crossovers, which don't "stretch" in any meaningful way - you're right. The Cornwall/Chorus/Forte make all of this a lot easier since getting a 15" or 12" woofer up to 1kHz is no big trick. You filter off the midrange bump (if any) and you're all done. Instant 700Hz crossover. But the Khorn/Belle/LaScala are another story altogether - getting the bass and mid horns on speaking terms is not a simple matter. It can be brute-forced by going with a *big* Altec theatre horn, or an even more gigantic Edgar salad-bowl horn. These are all big, room-dominating horns, looming over the top of a Khorn. Al K has a more elegant approach with an ultra-steep crossover, maybe even an elliptic filter, in order to keep the horns well-behaved on the edge of their usable range. But no free lunch there either; elliptic filters have substantial phase distortion, but then again, the several-millisecond path-length difference between bass and mid horn probably renders phase distortion moot. The waters get really deep with an all-horn system, mostly around the bass-mid transition. The Chorus lets me take little baby steps with horn systems, letting me find out what horns like to do and what they don't like to do. I think my next experiments will be with the Beyma tweeters - the mid-tweeter crossover is a lot simpler.

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.

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.