Restoring the standard bass horn throat in a '62 pair of Klipschorns

[LarryC]

Around 1980, I bought this pair of 1962 mahogany laquer Klipschorns, KB-ML 1A836 and 1A837, from a friend, their second owner, who was vaguely dissatisfied with their sound:



As I also came to think they didn't sound very well-balanced, I soon updated the drivers and crossovers. This unexpectedly produced a very peaky, uncomfortable sound. Because the problem seemed to be in the upper range, I sent the tops to Klipsch to have them tested and reworked as necessary. Klipsch modernized the top section, flush-mounted the tweeters, and shipped them back all without charge! My 1985 letter to Audio in praise of this act is on pp. 174-175 of the Barrett and Klementovich biography of Paul Wilbur Klipsch.

The speakers sounded better, but still seemed not to have the large, powerful sound Ive always associated with K-horns. Numerous equipment changes over the years helped but never solved it in my mind.

Then, around 1994, while changing out the woofers, I was astonished to see these wooden block inserts attached immediately above and below the other side of the 13" X 3" opening to the throat in the first stage of the bass horn:



I contacted Klipsch and faxed them the following cross-section diagram of the bass horn throat. The inserts, which were pyramidal wood blocks, are marked in pink in the diagram. Also shown is a separate woofer mounting board, which was apparently discontinued in the early 1970s:



Aside from the mounting board and blocks, this portion of the K-horn, including the splitter, ½-inch plywood motorboard, 3" slot width, and 3" front-to-back width of the first horn stage, is supposed to have been standard in K-horns since at least 1961. This is a picture of the inside of one of the bass bins at the start of this project (the green items are anti-resonance strips):



Because of the 32-year interval between manufacture and my query, current Klipsch staff were not aware of the inserts. Notes found in the files indicated that this was a production change between 1961 and 1963, purpose unstated. I asked whether the blocks could be responsible for what I thought I was hearing in the middle range and bass, and whether they could be removed. Klipsch engineers were not sure about the possible sonic effects, and doubtful that they could be pried out without damage to the motorboard.

Given these uncertainties, I looked at what else might be responsible for the problematic sound. For example, Id had the speakers in a series of inadequate rooms some were too long, some had incomplete corners, and my current room is small and has a badly-located glass-panel door. Perhaps these rooms just couldnt produce good bass. New components and the AK-4 upgrade which promised terrific bass never seemed to fully deliver.

I was also aware that Klipsch had had to undertake a major change in its drivers when Electro-Voice discontinued the powerful 15WK woofer a year or two before these horns were made (the original drivers and network were long gone at this point), and wondered if the blocks were part of this change. More recently, I was able to locate and try out a pair of 15WKs. For the first time ever, the bass became very powerful and impressive, although it did not complement the overall sound. Also around this time, GaryMD heard a pair of 2003 K-horns that he felt produced far better bass in a room probably not that much better than mine.

By some sort of logic and intuition, these experiences led me to conclude that, rather than the room, the problem was almost certainly the blocks. I again raised the issue with Klipsch and asked them to consider further the likely effect of the blocks. After review by Roy Delgado, Jim Hunter, and Mark Kauffman, the view was conveyed that the blocks were probably originally intended to improve the upper response of the low frequency horn, around the 400-500 Hz range, but that they would hurt the bass.

Now that all the signs pointed in the same direction, I arranged for a local woodworking/antiques restoration firm to come to my place to remove the blocks. The prevailing view was that, unless the blocks could be pried out without damage, sections of the motorboard would have to be cut out and replaced by gusseted and glued sections of baltic birch plywood. As it turned out, the expert woodworker was able to use a hammer and pry bar to break the old glue and pry out the blocks and long staples that held them in without damaging the motorboard. Below are pics of (1) the blocks, (2) the horn mouth now without the blocks, and (3) GaryMD holding up a block to show the size and shape.







After reassembly, Gary and I were immediately astonished at the increased power and depth of the bass! Cellos and basses were now full and dramatic where before they had been pallid and sometimes hard to hear. Lower mid-range instruments had a far more natural, real-life fullness, and even upper mid-range instruments were fuller and more connected with the bass. It was not easy to believe that they were the same speakers.

So, am I all set?

NO! something was still not right! Violins were shrieky in the upper, E-string range, and response seemed thin below that. The E string, the highest on the violin, sounds at 680 Hz, and the apparent peak was centered above that, most likely too high to be directly affected by bass horn output. But, if the upper part of the bass horn output were depressed, would it not leave higher frequencies standing out as a relative peak? It didnt seem difficult to subjectively assess this: the AK-4 crossover point between bass and mid-range horns is 450 Hz, easily "heard" in the mind's ear as the nearly-identical 440-Hz "A" used in tuning an orchestra, and the frequencies below this would be produced predominately or entirely by the bass horn. It seemed that something was lowering the upper range output of the bass horn.

The only remaining major difference I knew of between my horns and todays, for which the current network and drivers are designed, was the separate woofer mounting board. Perhaps, after removing the blocks had reduced the 400-500 Hz range, the additional thickness of the 13" X 3" passage now reduced it too far. Klipsch staff thought it would increase compression at the throat, but were not sure of the effect. Below is a pic of the mounting board in place, sans woofer, showing the more extended passage combining the mounting board and motorboard throat, compared with not having the mounting board in place as seen in the pic before last:



Mark Kauffman of Klipsch advised mounting the woofer directly to the plywood motorboard, using the largest wood screws that would fit through the woofer mounting holes. Carting the woof to the local Ace Hardware, I determined that #14, 1¼" screws would just fit through the basket mounting holes and reach nearly all the way through the plywood. I then used the bolt-holes in the mounting board as a template to locate and pre-drill the eight new holes in the motorboard, using this helpful gizmo:



Finally, using the helpful lower mounting board rail to hold the woof up until the first two screws were started, I was able to tightly secure the woofer directly to the motorboard:



Astounding success, at last! The whole range of both speakers now sounds just right powerful, very clear bass, full, accurate, even mid-range and highs.

Well-deserved thanks go to the very committed and helpful staff at Klipsch! It is remarkable that this dedication and expertise continues to be available from the company that built these classics 42 years ago. I also want to express great thanks to GaryMD, whose quick, accurate judgments of sound and speaker quality were extremely helpful and supportive while I puzzled through what, if anything, was wrong, and what to do about it. I think I finally have the real K-horns that I bought over two decades ago.

Larry

Edited July 27, 2015 by LarryC

[Guy Landau]

Interesting read.

Glad that you finally enjoy them after all these years.

[j-malotky]

Larry - Wow. Thanks for sharing the pics, made your story much easier to understand.

I am very familuar with the midrange, tweeter and crossover evolution, but not the bass bins.

Would be very interesting to find out the Khorn bass bin evolved over the years.

Maybe somebody here knows and would be willing to share with us.

Just wondering how my '83's compare to your '62s, compared to the new ones etc?

Larry, thanks for sharing. Was very interesting to me.

JM

[fini]

Great example of the scientific process there, Larry! Excellently illustrated and described. Did you ask the woodworker to come over in a suit, too? (Just ribbing you. Gary was probably on lunch break, eh? He does look like he'd be comfortable sipping tea from a fine china cup, pinky extended... )

[NOSValves]

Excellant and very informative post ! Thanks for sharing

Craig

[Deang]

"Klipsch staff in 1994 had not been aware of these inserts. Notes found in the files indicated that this was a production change from 1961 to 1963, purpose unstated...Klipsch engineers were noncommittal about the possible sonic effects...I raised the issue with Klipsch and asked them to reflect further on the likely effect of the blocks. After review by Roy Delgado, Jim Hunter, and Mark Kauffman, the view was conveyed that the blocks, which were probably originally to improve the upper response of the low frequency horn, would hurt the bass and boost output in the 400-500 Hz range."

I find it remarkable that Klipsch knew so little if anything about the inserts -- clearly seen in the Transparent Klipschorn below.

At any rate, welcome to the world of the AK-4 Klipschorn!!

[dubai2000]

Larry,

thanks a lot for sharing your bass bin experience....I suppose it's time at least to have a look into my '73 model .

Glad you got at last to the point of being really happy with your speakers.

Wolfram

[LarryC]

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

On 3/17/2004 12:49:05 PM DeanG wrote:

I find it remarkable that Klipsch knew so little if anything about the inserts -- clearly seen in the Transparent Klipschorn below.

http://forums.klipsch.com/idealbb/files/plexi.jpg">

At any rate, welcome to the world of the AK-4 Klipschorn!!

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

Dean, I think what you're seeing in the transparent horn is the splitter, the triangular cross-section piece on the forward wall of the first stage that points toward the rear -- take another look at my diagram. These blocks were on the rear wall of that first stage pointing forward, and were paired above and below the 13" X 3" throat -- they would have shown up rearward of the splitter and both slightly above and slightly below it.

I think that 30 years between the speakers' manufacture and my asking about it had erased that institutional memory.

And, yes, I do agree re the AK-4.

[Chris Robinson]

Larry, wonderful post in many ways.

It really puts a lot of attention on the LF section of this speaker, which is a mystery to many when they're in my living room trying to understand why and how there is no grill on the front of the lower section. I usually try to explain it, but your dialogue really pulls disparate pieces together.

Dean, I never saw the plexiglass version of the Klipschorn. It's VERY helpful to understand what's going on in there!

PS: I second Larry's comment above. I think you misread his photos

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[Deang]

...I think you misread his photos

I sure did. I should have spent more time looking at his drawing.

[garymd]

I was more than happy to assist although I didn't do all that much but listen and give my opinion. It was a remarkable improvement to say the least. I've been listening to Larry's khorns for quite a few months now on a regular basis and while the highs and mids never cease to amaze me, there has always been that lack of the smooth, deep bass I associate with khorns.

After the first step (removal of the blocks), the bass improved but was slightly muffled, and as Larry said, there was some loss of clarity and volume in the lower midrange. Once the woofers were directly mounted to the motorboard, all that changed. The bass is deep, smooth and powerful while the lower mids regained their clarity and punch. They sound phenominal!

One thing Larry did NOT mention was the improvement in the soundstage and imaging after the mods. Before, I had a difficult time finding the sweet spot in Larry's room. Trust me, I searched. There didn't seem to be a spot where I could hear the depth of a performance or a voice coming from the center on recordings which are very familiar to me. Now there seems to be a huge sweetspot and voices seem to come from the picture of the little girl hanging on the wall between the speakers. Classical music is as lifelike as I've ever heard.

Yes Fini, I was on my lunch break as I usually am when I visit Larry and his cats.

[Deang]

"...and voices seem to come from the little girl hanging on the wall between the speakers."

Well, I've heard of some pretty extreme tweaks to enhance imaging -- but hanging a little girl between the speakers is clearly stepping over line. O.K. Larry, time to let her down.

[fini]

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

On 3/17/2004 2:13:25 PM garymd wrote:

Now there seems to be a huge sweetspot and voices seem to come from the little girl hanging on the wall between the speakers.

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

I've got to say, that sounds a little creepy (especially if you're playing a Barry White tune). Could be enhanced by somehow installing some "Clutch Cargo" lips within the painting.

I really envy you guys being so close to be able to work on this, and to get together for "music appreciation." Thanks for the report!!

Dad-gum dean beat me to it!

[garymd]

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

On 3/17/2004 2:22:50 PM DeanG wrote:

"...and voices seem to come from the little girl hanging on the wall between the speakers."

Well, I've heard of some pretty extreme tweaks to enhance imaging -- but hanging a little girl between the speakers is clearly stepping over line. O.K. Larry, time to let her down.

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

Referring to the picture of the little girl which is hanging on the wall between the speakers Dean (see 1st picture).

[NOSValves]

Larry,

When did you get this all straightened out ?

Craig

[LarryC]

Craig,

Very recently -- the blocks were removed in the first week of March, and I re-mounted the woofers within a week. The build-up to that took a lot longer because of uncertainties about whether the blocks were really the problem and whether they could be removed without irreparable damage.

My decision to move came after several things seemed to show these speakers had the potential to generate powerful low bass in my room, e.g., the temp 15WK, and Gary's report on Tom Longo's 2003 K-horns, which sealed the deal for me, since my drivers and network would be identical to his. Does that answer your question?

Guy and Wolfram -- I didn't mean to leave the impression that I was always unhappy with them. I was very unhappy initially, but became more and more satisfied as I upgraded my system. It's actually been sounding very good over the past year or two, especially after I got a tube amp and the AK-4 helped improve a very peculiar blending problem between the three horns on each side. It was just that I always knew from what I'd heard before that they should be a lot more imposing.

Larry

[3dzapper]

Larry,

A very interesting saga. A good story for a snowy New England winter day, complete with a happy ending.

Your perceverance was amazing! Good work! Now, ENJOY!!!!

Rick

[tillmbil]

What I need is to borrow Gary and Larry's ears so that I could properly set up my system.

[Tom Mobley]

Well, thought the 60's I bought in Nov sounded pretty good. Still do. After I first got them set up I got out one of the Stereophile test CD's and tried it, seemed like the response died off between 40 and 50Hz, just like my DIY LaScalas. After reading this I got curious, got out the Heathkit audio generator and SPL meter. Looks like my perception was right, these things are great to about 50, and not much below there. This is using my HF-81 amp, one channel driven.

I know that my woofers are mounted on removable motor boards, I've seen them. But I haven't had them out to see if those other pieces are installed. I have a request in to Klipsch for info on my speakers, dates and whatnot. I believe they were built in early '60. Serial numbers are 1505 and 1517. A sticker on one of the mid horns is in early Dec of '59.

I'll try it again with a known good SS amp I have before tearing into them. I have a pair of current production K-33 in my LS, might try one of those too.

Interesting deal here. Thanks go to Larry for taking the time to do it right, and doc it up so nicely.

I have a question for anybody that knows, what is the proper procedure for measuring Kilpschorns? Setting up a SPL meter 1 meter in front of the face of the bass bin seems a little strange. Any words of wisdom here? Out in the room?

[Q-Man]

Thank you, I think you just saved me from making a mistake.

I have two pairs of later model Klipschorns. I recently picked up a pair of the bass horns only from the 60's to use as a center channel speaker. This 60 pair has the removeable mounting board like yours, but not the inner deflector boards. This weekend I was going to mount a woofer to the mounting board and install it. Now I'll trash the mounting board and mount the woofer directly to the motorboard like my other pairs. Here I was guessing that Klipsch did away with this mounting board for a cost advantage.

Always learning something.