New Diaphragm for Old K-55V?

[BEC]

This might be a good place to post a finding on Jhawk's K-55s.  I recently had them on my bench for testing.  The idea was to see if there was something to be gained by replacing the diaphragms in these K-55Vs, even though they were still working and working well after 36 years.  I put them on a k-400 horn and drove the K-55Vs with a sweep of 20 to 10Khz at about 1 watt and looked at the output with a calibrated mic connected to a spectrum analyzer.

The intent was:

1.  Test each of the old K-55Vs.

2.  Test a new Atlas PD-5VH as a reference.

3.  Replace the diaphragms in the old K-55Vs.

4.  Retest the old K-55Vs with new diaphragms installed.

Findings:

1.  Man, with one watt a K-400 is LOUD.

2.  Steps 1 and 2 from above produced 3 traces that were so close to identical in every respect that the test was stopped.  It would be impossible for steps 3 and 4 to produce a result that could show improvement.

I will go ahead and say that these findings are not what I expected.  I thought I would find that over the 36 years the effect of some age related changes to the diaphragms would be measurable.

My thanks to Jhawk for allowing this test to be done.  He was willing to "foot the bill" for the diaphragm replacement to see how this would work out.  Turns out that we got a pretty good proof on this subject without him having to spend the money.

Bob Crites

[Al Klappenberger]

Bob,

Were the K55s the old push-pin version with the single-piece phase plug or the newer version? Did you see any change in the "glitch" at 9 Khz when the diaphragm was changed?

Al K.

[BEC]

Al,

I said above that I did not go ahead with the diaphragm change since in every important respect, the spectrum I ran on the old ones were the same as a new PD-5 VH that I used as a reference.

These were the push pin type and the new PD-5VH has solder lugs. There is a small peak at around 9 Khz on all three tested but I consider it rather insignificant in that it is about 15db down.

Bob

[jhawk92]

As I posted over in 2-ch, I appreciate Bob's info and candor on these mods. He is a great forum asset. I actually sent him my K-77s for re-diaphragm-ing as well as my stock Type A crossovers to check the caps. We both came to the conclusion that these speakers must have been babied by their original owner. For '68s to look as good as they do, as well has have components that weren't too far out of spec, they must have led a charmed life.

Bob did replace the K-77 diaphragms and the caps in the x-over were only about 5-10% out of spec, but he did replace those. Looking foward to getting everything back home and reassembled. I will also have a K-401 horn mounted in one of the K-horns to do a bit of A/B comparison. The K-400 horn sure rings once it's out on its own.

[Al Klappenberger]

Bob,

OOps.. Ok.. I misunderstood. I though you replace the diaphragm good or bad. I was just hoping to identify what was causing that 9 Khz glitch. Is it the phase plug as I assume or the diaphragm.

I think the K55 diaphragm will last for ever so long as it is not driven to the point the voice coil burns open.

Al K.

[BEC]

Al,

Can you describe the glitch you are talking about. I can see a small peak at 9 Khz, but it does not look very significant to me.

Bob

[Al Klappenberger]

Bob,

Here's plots of the new and old K55V drivers. The 9 KHz glitch is pretty bad on the old one.

Al K

[BEC]

Al.

You are seeing a lot higher peak at 9 Khz (compared to the output at 6 Khz) than I saw. I wonder if that difference could have something to do with the different horns we used. You used a K-500 where I used a K-400.

Bob

[Al Klappenberger]

Bob,

I doubt if the horns are the reason. I got a phone call from a friend who has had some experinece with this "glitch" (The "father" of the "P trap"). He said that it is inconsistant. Not all the early drivers even have the glitch. The two in the plot were from y 1975 (ish) Belles.

Al K.