Let's redo some 1960's DIY Khorns!

[jpc2001]

I recently bought a pair of DIY khorn replicants and would like to bring them up to par. They have some bugs. Literal and figurative. This post should make you feel better about whatever you're listening to.

These have Klipsch type A crossovers, Jensen P15LF woofers and Electro-Voice T35 tweeters. One unit has a Klipsch K-55-V midrange driver, the other has an Atlas PD-4V midrange driver. So far so good! Oops: all drivers are 16 ohms which is wrong. It should be 4-ohm woofer, 16-ohm midrange, 8-ohm tweeter to get the right crossover points and to equalize the sensitivity for all three.

These were probably built before 1970 judging by the ubiquity of flat-head screws, age of parts, and absense of zip-ties or anything plastic.

The cabs are sturdy, all joints look and feel tight. The bass bin has the same dimensions as the real thing. There are some oddities: the woofer cutout is 6"x13", double the size of the modern 3" wide cutout. These are missing the triangular piece in front of the woofer that would guide the wave toward either the upper or lower horn. They are also missing the triangular piece at the top of the upper horn, and bottom of lower horn, to guide the wave back toward the corner.

These are a bit shorter than a factory Khorn. The tweeter mounts through the front baffle, in the upper section of the bass horn. Could be an intermodulation distortion risk. Only the mid horn is atop the bass bin.

I'm thinking of replacing the woofers and tweeters with correct ones, probably Bob Crites' units, selling the spares, and recapping the crossovers. Then see if the midrange drivers are good enough. If the speakers sound "the same" it might not matter that the mid drivers are mismatched. (I'm in denial that I might need to replace all six drivers.)

Questions.

What are the effects of using a K-33 equivalent with a 6x13" cutout versus 3x13"? How important are the missing triangular wave-guide pieces? What's different about an Atlas PD-4V versus a K-55-V / Atlas PD-5VH?

[Sancho Panza]

Welcome to the Klipsch Forums!!!

I'm sure someone can help & others, like me, will watch with excitement.

[The Dude]

Welcome to the forum. There are 15's that work better in the 6x13 vs the 3x13, the 6x13 was the correct opening for the older units. I had a 1953 that had a 6x13 opening. If you could find a pair of EV 15wk's I think those are the ones that sound best in the 6x13s, or even a pair of K43, which is the pro version of the k33, which would give you better mids I believe.

That's about all I could tell you. Like Sancho said there will be other along that are more knowledgeable and we will sit and watch with excitement, to see what unfolds. Good luck.

[Deang]

You will need to dump the PD4 and get another PD5.

The Jensen woofers are nice. If they're in good shape, keep them.

I know it's hard to believe - but you can still run your Type As without changing anything.

You just need a PD5 and some nice Jensen paper in oil capacitors.

[mike stehr]

What's the specified free-air roll-off of a Jensen P15LF?

[jpc2001]

The mid horn is a mystery. It's built into the cabinet and most of it can't be seen. The mouth is 20" wide and 5" high, measured at the inside edge of the mouth. The depth is 23" measured from the front of the horn to the closest part of the driver screwed on the back. Does that sound like a K400, K401, or something else?

I tried using 'hornresp' to try to understand the effects of having a 3x13" vs. 6x13" throat on the bass horn. I copied many of the horn physical parameters from a speakerlab K example on diyaudio and used the T/S parameters for Bob Crites' stamped frame woofer. The result is, it doesn't make a difference. The frequency response is similar to a first order. The wider throat may even have a smoother roll-off at the high end. Assuming I didn't drop a decimal point somewhere, and assuming the speakerlab K numbers are close enough, the simulation just doesn't steer you one way or the other. It could have said that a 3x13" cutout is clearly better in some obvious objective way and it didn't say that.

[jpc2001]

More observations.

One of the caps in the type A crossover should measure 13uF. Instead it measures 19uF on one side and 23.5uF on the other. Ouch!

With the aid of a flashlight or camera flash it's possible to see the internals of the mid-horn through the grill cloth. As the horn moves forward away from the driver, it reaches full height before even starting to widen out. The rear end of the throat looks like wood. I'm not sure if somebody built a horn into the cabinet or built the cabinet into a horn. See pics. Sorry about the quality, they were shot through the grill cloth.

File this under things you really could ABX: the PD-4V sounds different from the K55. The PD-4V is lower-bandwidth like a polite table radio. The K55 sounds airier, there's more HF content. Together these drivers hardly develop any stereo image. That could be due to phase differences, which could be partly due to the 19uF vs 23.5uF issue on the crossovers.

Is a new PD-5VH a close match for an Apollo-era K55? Are those 50 years basically inaudible?

[drboar]

Bruce Edgar studied the "rubber throat" of the Klipsch and concluded that the throat constriction was changing path lengths for the driver-horn start and that this benefical for the output in the upper range of the horn. If that still holds true and how driver dependent that is I have no idea.

[WMcD]

Here is the article and many comments.

https://community.klipsch.com/index.php?/topic/56237-article-solving-the-klipschorn-throat-riddle-edgar/?hl=%2Bthroat+%2Briddle

WMcD

[jpc2001]

WMcD, thank you. If an expert writing in a technical magazine isn't sure how the Khorn throat works, I won't speculate about how any given geometry change would affect the sound.

So, let's copy the Khorn geometry exactly. I built and added the missing Huygens reflectors over the weekend. They're screwed, glued, and caulked with silicone to be airtight.

I have new 16" squares of plywood, with a 3x13" slot cut in each one, ready to go over the top of the old 6x13" slots. This won't be an exact copy: the woofer will sit 3/4" further back due to the added plywood square, and there will be 1/2" more space in front of the slot. So that's a modest increase in the throat chamber volume, and a small-percentage reduction in the volume of the sealed back chamber. I hope that's close enough to not matter.

The other option was to reduce the size of the existing slot by adding plywood strips and affixing them with mending plates or something. I suspect it would be difficult to make this strong enough to not flex without adding any bracing that might interfere with woofer cone movement. So I'll overlay new woofer plates instead.

That should do it for the cabinets. Except, perhaps, for the question of whether the back chamber is truly sealed. I don't know if I should test for that or preemptively seal them with silicone.

Edited June 24, 2014 by jpc2001

[jpc2001]

I have one finished speaker. A sweet Khorn mono rig!

This should have the same bass horn geometry as the real thing, to a real close approximation, with the added Huygens reflectors and 3x13" slot. It's got Crites' CT125 tweeters and CW1526 stamped frame woofers, and Atlas PD-5VH squawker drivers. All new. All have specs matching the real thing, notably impedance. It was expensive...

The bass is there, it's clean, it doesn't burp out or resonate. It sounds clean down to 34 hz on some youtube test video, then drops off very suddenly. There's nothing below 30hz. The midrange can sound "honky" at higher volumes. There are probably some resonances in the custom mid horn. It is possible that some DIY carpenter's idea of a horn in 1966 wasn't a perfect modern tractrix.

To quiet down the response peaks above 6kHz on the Atlas, I added a small low-pass inductor to the Type A crossover. The added 2uF cap is an attempt to keep the relative response of tweeter and midrange in equal proportion. The modified parts are screwed into a terminal strip for the sake of experimentation. Adding a 6db-per-octave low-pass only rejects the Atlas' 9.5kHz peak by about 5db compared to the stock Type A, according to spice sim. So it's not much better but it's better. Maybe enough to take the edge off any upper-midrange glare, real or imagined.

Someday I'll try an unmodified Type A network. It would be nice to measure frequency response, I don't have an SPL meter or a mic for that.

Edited June 25, 2014 by jpc2001

[jpc2001]

This crossover works better than the previous one. The previous one was all upper midrange, very little treble. This one is a better balance with the Crites CT125 tweeter, which is a little less sensitive than a T35/K77 would be.

[jpc2001]

Here are pics from putting speaker #2 back together.

1. The new Huygens reflectors in front of the woofer. In the pic it looks a bit diagonal, but the camera exaggerates this, it's just a couple of degrees out of true. The same structure in the other speaker was perfect. Honest.

It's not easy to shoot the reflectors at the top and bottom corners of the bass horn. They came out great. They fit into a perfectly rectangular corners so they weren't tricky. The center reflectors must fit up against the angled sides of the bass horn and that was a challenge for a non-woodworker. I got help from a friend with a table saw to make the longer, 45 degree beveled cuts so those are all perfect. It was only the short cuts on the ends of these pieces that I did myself, free hand, with hand tools, and those are close enough.

2. New woofer plate with 3x13" slot. These came out perfect! This is a very flat and smooth piece of poplar plywood, 22/32". My woodworker friend had lent out his jigsaw, so we did the center cutout with a rotozip spiral saw. It took a little longer but the result is perfect.

I drilled 8 mounting holes through the new woofer plate, and bolted through the woofer, plate, and down to the recessed nuts in the original motorboard. This meant buying longer bolts.

Originally I was planning to use wood glue under the woofer plate to ensure a perfect seal. Now, some of the recessed nuts in the original motor board were a little loose and one actually fell out. Well, I didn't want to get it 90% built including glue and then have to tear back into it to retrieve a dropped nut. So I skipped the wood glue. I don't think it matters. The motorboard and the new plate are smooth and flat, with a little sanding they sat perfectly flush. Eight bolts later, there's no way air will get through that interface.

3 & 4. Super fake k-horns! I haven't done anything to the external appearance, that's what they looked like to start with.

[jpc2001]

I went back to the factory Type A crossover network. It has a nice, lively sound.

I also tried the Type A with a 350uH inductor added across the tweeter terminals. That gives a steeper crossover slope for the tweeter, and brings the tweeter in at a somewhat lower frequency, near full strength at 8kHz. This sounded nasally, tinny, just too hot at HF. This topology might work with different C and L values to move the crossover point higher, or with a resistor to just cut a dB or two. I guess the CT125 is sensitive enough to match the PD-5VH; it'll more than match it with the right network.