Chorus II Mods Part II

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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.

[LynnOlson]

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

[LynnOlson]

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.

[g3dahl]

I have now had the opportunity to listen to the Chorus II's with the completed modifications. The results are fabulous! Wow!!

The flatness of the frequency response is immediately obvious when listening to music. There is a beautiful transparency, a very strong "you are there" sense. I have never heard a horn system that was completely free from audible peaks and resonances, especially when listening to orchestral music. Strings are *really* hard to get right with horn systems (or even with direct radiators, for that matter), because any such peaks or resonances, or bits of distortion, destroy the delicate balances that allow strings to sound like they do in real life.

Last night when listening to the Telarc SACD of Berlioz' Symphonie Fantastique, I was treated to the most sublime orchestral reproduction I can recall. During dense passages, the tone colors of individual instruments were clearly delineated--the lack of any sort of glare was revelatory. The resulting sound was also quite refined. The delicate violin passages in the opening of the first movement brought goosebumps!

I'm not ready to say that the Fostex tweeter offers the last word; there are some very nice units out there that I'd love to try at some point. But if it doesn't get better than this, I am already very pleased.

There is also a minor hump in the midbass response that I would like to get evened out. Experimenting with placement might help, but I am also curious about how the original Chorus (vented bass reflex) alignment sounds compared with the Chorus II's passive radiator. I wouldn't hesitate to ditch the PR and cut some ports if it will improve bass smoothness.

With that being said, the bass sounds quite good even with the hump. The hump is broad enough that melodic lines played in the lower ranges don't seem to move in and out of an exaggerated response area. In fact, to my ears, the bit of extra warmth makes cellos and basses sound more realistic than they often do in systems that measure "flat". This has always bugged me; in real life, lower strings make a big, warm sound, not something you would call "lean" or "tight". So...what's really right?

Looking forward to more listening. Congratulations Lynn, you've really outdone yourself this time! I think you should come over tomorrow for a listen. You'll really like what you hear.

Gary Dahl

[sunnysal]

you guys take the cake! facinating! thanks for the graphs and the schematic for the crossover. lynn, get over to gary's and tell us what you think of the sound as well! I bet someone else here will be trying these mods soon. tony

[LynnOlson]

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.