AA Xover Modification

[Al Klappenberger]

Shawn,

Sorry I missed the light bulb and resistor in the tweeter circuit. This is supposed to be a tweeter protection circuit. I do not like it! A light bulb gets higher resistance when it heats up, but a tweeter will blow long before a light bulb will heat up! What's needed here is the Polyswitch used in the later Klipsch networks. The best thing is to forget the protection circuit and just keep a lid on the volume control!

You are correct in that cheap amps have a higher output impedance. These amps are even more dependant on a constant impedance load. If the output impedance is high, then a varying load will casue the frequency response to vary. A dead battery is the analogy. It's internal resistance is high compared to a good one. This is also a reason why an autoformer is a better attenuator than an "L" pad. An L pad has a higher source impedance so it's attenuation changes with load. An autoformer has it's attenuation locked tight to the turns ratio no matter what load it sees!

Al K.

[lynnm]

OW ! OW ! OW!

I think my head will soon explode and screw up the decor in my computer room !

AKAside mis amigos I really appreciate the comments and work that you all have put into this.

I should note that I have never been unhappy with the sound of my Khorns.

I will leave the Xovers set up as A's for the time being as I have made several changes of late and need to live with them for a while before any further tweaking ( I am after all a married man and wish to stay that way).

An aside: When the horns first arrived mein frau looked a tad dubious about their size but obviously decided that after 40 years with this old fart she would live with these additional monsters. I hooked them up and she listened to them for perhaps a minute and told me that she couldn't believe how sweet they were. A few months later she walked in on me as I was caulking the midrange horns. The upper cabinets were in pieces and she asked - not very sweetly:

What the hell are you doing to our speakers ??

At that point I knew that my Khorns were permanent members of the Marshall Menagerie ! I did the Xover mods when she wasn't around to avoid getting the family jewels caught in the wringer if the results weren't salutary but she seems pleased with the result.

[Al Klappenberger]

Lynnm,

I should point out here that, even though I point out a lot of things in the Klipsch heritage speakers that I don't think are what they could be, I still believe Khorns are one of the best speakers ever built!

Al K.

[lynnm]

Amen !

[John Warren]

x

Edited December 17, 2013 by John Warren

[John Warren]

Does it matter what I think?

Edited December 17, 2013 by John Warren

[John Warren]

Here is the impedance plot with an 8 ohm non-inductive load at the tweeter output.

Edited December 17, 2013 by John Warren

[lynnm]

Does it matter what I think?

You betcha !

[3dzapper]

"Does it matter what I think?

You betcha !"

Ditto.

Rick

[John Warren]

Edited December 17, 2013 by John Warren

[djk]

"! A light bulb gets higher resistance when it heats up, but a tweeter will blow long before a light bulb will heat up! What's needed here is the Polyswitch used in the later Klipsch networks. "

Well spoken by someone who has not bothered to measure either device!

The 561/211-2 is MUCH faster to respond than a PolySwitch.

http://ps.circuitprotection.com/docs/RXE050_rev_G.pdf

Uhh, it is rated for 4 seconds to trip at 500% of the hold current.

The lightbulb only takes a few 10s of milliseconds to heat up.

For a two-way speaker like the Kg4 the lightbulb by itself may be TOO FAST.

For the tweeter in a three-way it is just right.

For the two-ways I recommend wiring a 2.2 ohm 2W resistor in series with a 561 or 211-2 bulb, and parallel this with a RayChem RXE050 PolySwitch. This goes in the hot lead to the tweeter.

The resistor forces most of the signal to go through the PolySwitch. After the PolySwitch trips most of the signal goes through the lightbulb. If you overdrive it hard, the lightbulb will glow very bright.

On old Kg models with clear cones, they will light up when driven too hard.

On newer models with opaque cones a LED may be mounted where it may be seen behind the grill. No electrical connection is required for the LED, just get the lightbulb somewhere near it (IOW, the LED is just a colored lense).

For the audio purist that objects to having what is in effect a semiconductor in series with his tweeter, and views the 1.75dB loss associated with the 1.17 ohm post trip resistance (this is what you actually get in the LaScala type AL networks), you have several choices.

Run naked and risk it.

Run the lightbulb only.

Build the EV STR tweeter protector designed for this tweeter. This substitutes the relay for the PolySwitch (no loss when not tripped), with the lightbulb in parallel with the relay. Effectively out-of-circuit until needed.

B&W also uses this circuit in some models, their trade name for it was APOC.

[Al Klappenberger]

djk,

Man, I must intimidate the tar out of you! You just live to pick apart anything I say.. You never have anything positive to add to what I post! Maybe it's because you are afraid I can do anything you can do withOUT the BSEE degree. You and I could have learned a lot from each other but instead you prefer to try to make me look bad at every opportunity. I am sick of it!

"Well spoken by someone who has not bothered to measure either device!"

Why don't you just call me an idiot like you have in the past?

The fact is that I once built a quelch circuit out of 2 lamps like that and two resistors in a bridge circuit around a speaker for a communications receiver back in about 1972. It worked very slowly! Light bulbs change their resistance in proportion to their brightness. What's faster, a tweeter diaphragm hitting a phase plug or a light bulb heating up? Next you are going to tell me to protect a tweeter with an instrument fuse.

I stand by what I said.. A light bulb is TOO SLOW!

GO AWAY or try acting like you have not been elected by Klipsch to be the personal Al K watcher.

Al K.

[djk]

"Man, I must intimidate the tar out of you!"

No, you just hand out bogus advice.

"You just live to pick apart anything I say.."

I do tend to hold you to a higher standard because you tend to hold yourself out as an expert.

"Maybe it's because you are afraid I can do anything you can do withOUT the BSEE degree."

You can't, plus I flunked out of high school the second half of 10th grade.

"You and I could have learned a lot from each other but instead you prefer to try to make me look bad at every opportunity. I am sick of it!"

Sounds like a personal problem to me.

"Why don't you just call me an idiot like you have in the past?"

I just show how foolish some of the things you say really are.

A lightbulb is really a very high tech protection device. The only real problem with small ones is they may be too fast.

"The fact is that I once built a quelch circuit out of 2 lamps like that and two resistors in a bridge circuit around a speaker for a communications receiver back in about 1972. It worked very slowly!"

Different application, not a valid comparison.

"Light bulbs change their resistance in proportion to their brightness."

No kidding, that's whay they work as a tweeter protector.

"What's faster, a tweeter diaphragm hitting a phase plug or a light bulb heating up?"

You didn't even bother reading the spec sheet on the PolySwitch, did you? At 200% of the hold value they can take up to ONE HOUR to trip, at 500% of the hold value they still take 4 SECONDS.

The diaphragm hitting the phase plug has to do with the crossover slope and frequency, and nothing to do with the thermal protection we are discussing.

"Next you are going to tell me to protect a tweeter with an instrument fuse."

They work faster than a PolySwitch, and I must say that an AGC-1A blows faster than an AGC-3/4A does, so in this case the bigger fuse offers more protection.

"I stand by what I said.. A light bulb is TOO SLOW!"

A 1KW PAR64 is slow, a 10W 561/211-2 may be TOO FAST for a two-way.

I, of course, have measured the resistance ratio changes using a current probe and an oscilloscope.

"GO AWAY or try acting like you have not been elected by Klipsch to be the personal Al K watcher."

As soon as you quit handing out bad advice.

[Al Klappenberger]

djk,

You have all the tact and social skills of a Mack truck! There is a term for you, it's "Know-it-all". You should listen to my bogus advice, YOU might learn something, that is if didn't have such as closed mind. Besides, who appointed you to be the Klipsch bogus adive police dog trained to ATTACK everybody who is even slightly wrong, especially me! Pure jealousy! With all the bogus advice on this board, how come it is ME that you have decided to attack at very opportunity! You can't stand it can you! Go get a life!

I repeat.. I light bulb is too SLOW. If a polyswitch is also too slow it is useless also. There is nothing more dangerous than a safty device that doesn't work! You are better off without it. Rely on it and when it fails you are dead! The only way that stupid light bulb protects a tweeter from a transient is by its static attenuation. You could accomplish the same thing with an "L pad".

Al K.

P.S.

It's very sad that a nice discussion about mods to the AA network and filters could degenerate so quickly.

[Al Klappenberger]

Guys,

My on-going irritation with djk has gotten me thinking about the tweeter

protection problem. While I still believe that having no protection at all is

the best protection because it keeps the informed listener honest with his

volume control, the problem of the exuberant teenage son still exists! The

best protection for the tweeter is the back-to-back zener diodes used in the

AA network. It simply had the nasty habit of blowing up your amp to save your

tweeter! This is because it forms a virtual short circuit the instant it's

breakdown voltage is exceeded. It might be possible to combine the slow

average responding protection of a light bulb with the instantaneous response

of the zener without shorting out the amp in the process. If the resistor

that is connected across the tweeter in the first order network post earlier

was replace by the zener diode pair of the AA network, the light bulb would

provide the series element to absorb the extra voltage rather then the

internal source impedance of the amp when the zeners start to draw current.

It's true that you might hear distortion when the zeners kick in, but what's

worse, a blown tweeter or a nasty distortion reminding you to cool it!

Al K.

[JohnA]

Y'all need to quit the pissing contest.

The E-V literature of the time claims a 5 watt continuous/50 watt program and a spec I've never quoted before of 100 watt instataneous for the T-35A (and by inference, the K-77-M).

So, in light of that and the fact that there is not much energy in music above 5k, the audio enthusiast community doesn't need much protection. My experiments, assuming I did them correctly, showed the bulb limiting the DC power to 2 watts. This should be analogous to average power. If the average is kept under control, spikes to 100 watts will not hurt the tweeter.

That looks like protection enough to me. I would also prefer an additional resistor in parallel to the tweeter so that the high-pass filter crossover point is not shifted too far down in frequency, introducing even more energy to the bulb and tweeter.

[Al Klappenberger]

John,

I agree 100% about the P__ing contest. I had simply had all I could take from him! I get tired to taking it on the chin from people like that. It's not that I can't take criticism, I just believe there is a right and wrong way to tell somebody he's wrong. It can be done without insulting them. After all, you learn more from your mistakes than your sucesses. One of my best friends started out our relationship by pointing out a mistake I made (Mr "P" who designed the "P trap"). In fact, that mistake, once corrected, led to my design for the Cornwall network I used to build.

I think the 100W peak specification on the T-35 isn't the entire story. From that it sounds like a T-35 should take a lot of abuse. I have heard of too may of them blowing out, even ones protected by the polyswitch. I think the big offender, back when the AA was desinged, was monkey-chatter from fast forward or rewind of reel to reel tape machines. That's not as big a problem these days though. Something definitly still blows them. To come up with a really bullit-proof protection circuit would take some serious research. I'm sure the light bulb helps some, but I put it in the catagory of false security. I wouldn't trust it for a minute!

Al K.

[Al Klappenberger]

Guys,

I did a bit of investigating into the zener diode tweeter protection scheme

used in the AA network. I started by looking up the breakdown voltage of the

zeners PWK used. They are two 1N3996 diodes. These break down at 5.1V. The

two are in series which means the forward barrier potential of the other

diode must be added to this. That's usually about 0.6V making the actual

breakdown about 5.7V. If you figure that the RMS voltage is .707 times the

peak voltage you can compute the maximum allowed power that can be produced

by the tweeter before the diodes start to conduct. It comes to 4.03V RMS.

The next thing I did was to connect one of my collection of old T-35s to my

new toy, a Spectral Dynamics SD375 FFT analyzer, in a bridge circuit to

measure the vector impedance of it. The results are attached. The scale is

V/V of 0 - 2. This corresponds to 0 - 20 Ohms with 4 Ohms / division. That

makes the second division 8 Ohms. The top plot is the impedance phase. As you

can see, it is virtually resistive with the "R" component going from 7 Ohms

to 8 Ohms. This agrees nicely with the plot John Warren post earlier.

If you figure an nominal impedance of 7.5 Ohms, power is voltage squared over

Zo. That comes to 2.17W at the point just before the zeners start adding

nasty distortion. Since saturating the diodes to the point of turning a sine

wave into a square wave, where peak voltage is equal to RMS voltage, the

maximum becomes 4.33W. That is the maximum that the T-35 could be subjected

to using the diode scheme of the AA network even on an instantaneous peak.

If the specks John Albright just posted is correct, and I have no reason to

doubt it, the diode breakdown voltage was picked to be ultra conservative.

This was, no doubt, to save Klipsch from giving way so may replacement

tweeters! I also suspect it may be the reason so many people clip out the

diodes. I think maybe part of the answer to tweeter protection is to replace

the diodes with ones with a slightly higher breakdown voltage. Of course some

series resistance would also be needed to save the amp from the short circuit

they represent when they start to conduct. The light bulb might be a good

choice.

Could somebody give me some information on the resistance of the proposed

#561 lamp at various current levels so I can do some figuring?

Al K.

[JohnA]

The T-35A/K-77 (AlNiCo) carries a 2w cont./20 watts program. These were the ones used when the Type AA was designed.

[Al Klappenberger]

John,

Good point! That says that the protection circuit really needs to be tweeked for each type of tweeter.

Do you have any info on the #561 lamp or do I have to get off my bucks and buy one to experiment with? All I have been able to come up with is 12.8V at .97A which is 13.2 Ohms. That is maximum resistance when fully lit and hot. I would like to know what it is cold.

Al K.