Replacement 1n3996 in AA Xovers?

[Al Klappenberger]

Bob,

The 2% number PWK mentioned is probably based on the power distribution of normal music. That number rises drastically when an amplifier is drive into clipping. So, as long as your amp doesn't clip and your ears can stand it your tweeter will be ok. But if your amp is driven into clipping by the lows then the energy in the tweeter spectrum will be a lot more than 2%.

Al K.

[BEC]

Al,

I was aware of the forward drop of the zeners. Just did not add that in for simplicity. Usually I consider the forward drop for a silicon device to be .7 volts. So peak to peak (positive peak to negative peak) would be around 11.6 volts.

Now, lets get to the "shorts the amp" question again. I thought this one was so obvious that I could just treat it with logic, but, once again, you have "provoked me" to testing.

You stated " you can easily find the zeners, but where is the series resistor that limits

the current? There answer is that it is IN THE AMP!"

I believe you are completely overlooking the fact that the zeners are at the end of a network. They are not in any direct way connected to the amp. Even if they could short, that would not be a short to the amp. For the premise of this discussion though, lets consider what would happen if they could become a direct short.

Test conditions:

Test #1

Klipsch Type AA crossover with all outputs terminated into resistive loads of 8 ohms-tweeter, 16 ohms-squawker, 4 ohms-woofer. Impedance meter connected to crossover input terminals.

Z at 120hz=4.795 ohms

Z at 1khz=16.40 ohms

Z at 10khz=11.64 ohms

Test #2 Same test as above except with at dead short (jumper) across the zeners.

Z at 120hz=4.794 ohms

Z at 1khz=16.42 ohms

Z at 10khz=21.54 ohms (not a misprint, tested it over and over again, impedance really goes up)

Al, please either agree with me that the zeners do not and cannot "short the amp" or give me another way to test to prove that they do.

Bob

[Al Klappenberger]

Bob,

INTERESTING!

I did some testing too. Attached is the complex impedance plots (rectangular

R +-jX) of an AA network terminated in 5 Ohms, 13 Ohms and 8 Ohms (top plot

NORM.PLT). The other plot (bottom SHORT.PLT) is with the tweeter shorted.

Both plots are up to 40 KHz.

It looks like the Zo does go down but at a lot higher in frequency than I

thought. The marker is at 15 Khz on both plots. At that frequency the Zo is

9.5 -j3.9 Ohms normally and goes to 6 -j1.2 Ohms when shorted. If you figure

out the equivalent circuit, it comes to 9.5 ohms in series with 2.7 uFd

normally and goes to 6 ohms in series with 8.8 uFd when shorted. Even though

the complex impedance starts looking more like a capacitor than a resistor, I

wouldn't think either impedance would damage an amp unless it doesn't like a

capacitive load. What's interesting is the big jump in Zo around 8000 Hz.

That shouldn't be dangerous either though. BTW: Computer analysis of the AA

confirms all this.

It could be that the story of zener diode protectors causing amplifier damage

was just a rumor that I bought into because it sounded logical. If this is

the cast though, why did Klipsch abandon the zeners in favor of the poly

switch? Maybe it was because of the distortion it causes when it clips lows

that get through the tweeter filter. Maybe the polyswitch just costs less!

Al K.

[BEC]

Al,

I expect that you hit it when you suggested that the polyswitch may be cheaper. The high power zeners PWK used are very expensive. They run almost $10.00 each and there are 4 per pair of crossovers.

Bob Crites

[Deang]

Considering how many in the mainstream run their equipment, I still think if there was any truth to the diodes causing amplifier damage -- there woud have been plenty of ruined amplifiers -- which certainly would have prompted and investigaton by Klipsch, and a subsequent redesign to the protection scheme.

The Zeners may sound like crap when they kick in, but if they are operating, then the output is approaching garbage anyways.

[Al Klappenberger]

Guys,

I did a little Google search and found that those zeners do cost OVER $10 each in small quantities!

If the blown amp reputation is true it may have been in the days of early SS amps that were not protected form weird loads. Also, I think some vacuum tube amps doint like high impedance load. Running one with no load at all could cause arcing. Maybe it's just old problem that's no longer a consideration with modern amps.

Al K.

[Deang]

Partsexpress has polyswitches, uh, a lot of them. What would be a good one to use for the AA? These work in series with the tweeter though, and might impact the sound even when they're not doing their thing -- unlike the Zeners.

[nk2j]

Wow lol that was informative! never dreamed i'd start such a ruckus lol

now for another question.. i got these Cornwall E's 1972's i think the label on the back says they were manufactured for some audio company in frankfort germany... .. now there are no grills for these speakers no attachment holes etc and the baffle is a beautiful finished wood grain...

am i correct in thinking klipsch made them with no grills?? and does anybody know if they are considered good and/or how good.. they do sound amazing to me lol i guess thats all that really matters but i am curious..

i ordered crossover caps from Bob as well as diaphragms for the tweeters one was blown...

[Al Klappenberger]

John,

I am not aware of any special Cornwalls for Europe, but the grill frame on my old Cornwall (about 1985) was made of stiff cardboard and was held on my velcro. It could be yours was a "decorator" model and simply had no grill cloth / frame.

Al K.

[BEC]

Bump for current discussion.

Bob Crites

[WMcD]

Boy, I love this. I reference to this bump from the one about AAs.

Bob's experiment with shorting the Zeners and driver does make sense. My view was overly simplistic. But so is Bob's. This is all said in good spirit. No offense, buddy. I think we're getting closer to the full story.

By way of background, please recall my recounts of Bob G.'s (?) warning of operating speakers with no woofer load on the crossover. If there is a second order crossover, the LC series creates a low impedance (short) to ground, at some freq., and it creates a higher impedance at others. We can graph that.

Let's look at the AA schematic he posted. There is a third order high pass in the tweeter with the Zobel and the polyswitch. In another design there is the Zener clamp.

Bob is replacing the Zener clamp with a short. That is to say, basically, we have a whole new circuit which loads the amp.

It is the input cap which feeds a parallel LC, or tank, to ground. We can put that up on a simulator and see what comes out in terms of input impedance. Here I'm quite sure that Bob and Al's experience will be shown correct.

The LEAP simulator can do that and I'll crank it very soon.

A bigger problem is to recreate the effect of the Zener clamp. LEAP can't do that but a SPICE simulation can. That will take a bit of doing. Gulp, I don't know if I can model the input impedance of the K-77.

For the sake of accuracy, does anyone have a schematic of the Zener circuit. I wonder if it has the Zobel.

One thing I'd like to investigate is the actual impedance seen as the Zeners turn on in the midst of a complicated L - C circuit.

This may take more time than I have until the holidays. Still, I do believe that my previous statements were overly simplistic and there is a lot to investigate.

Best,

Gil

[djk]

" Without the resistor the diode would blow up
with a bang the instant it was subjected to a stiff voltage source greater
than its breakdown voltage. If you look at the schematic of the AA network
you can easily find the zeners, but where is the series resistor that limits
the current? "

A zener requires a ballast to work properly. For DC<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

this is usually a resistor. For AC it may be a

resistor or a reactive component. A reactive component

is better because it dissipates negligible power. The

crossover capacitor serves as a reactive ballast.

A square wave is a sine wave with all its odd

harmonics added out to infinity. In practice it looks

pretty square after the 29th harmonic.

The 3rd harmonic of 6Khz (the crossover point) is

18Khz, most people can't hear that.

The reason the zeners sound bad is the discharge of

the non-linear capacitance from the series

back-to-back connection when the waveform crosses zero

due to the high frequencies involved.

A series connected fast diode for each zener, with

each diode/zener combo then connected in reverse

parallel, with a 0.001µF cap across the zeners will

make them sound much better. 

Klipsch switched to a higher voltage zener later on, 

after the flat lead K77 came out, it allows about 3dB

more output before clamping.