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Zener Tweeter protection


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51 minutes ago, jason str said:

The 561 bulb you found here confirms, this was for K 77.

I found this:

 

 If I built a set using K-77 tweeters, I'd build DHA3s.  The smaller tweeter cap reduces a bump in the response of the K-77 around 6k to 8k.  Since the K-77 is really a 105 dB/w/m driver, the bulb and resistor form an L-pad in addition to tweeter protection.  If you use Bob Crites' tweeters, or similar, you can use the DHA2 and maybe we'll need to adjust the 65 ohm resistor. 

DHA2.jpg

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42 minutes ago, Crankysoldermeister said:

Dumb question, this must be the 561 or 211-2 bulb. correct?

 

I have a bunch of Sylvania 1895's in my garage.  Made in the USA so I probably bought these in the 1980s.  --- OK I answered my question, the 1895's measure about 6 ohms.  DJK said the 561 is about 1/2 ohm. 

 

I have  7605 bulb on teh shelf that is much larger than the 1895 and it measures about 0.6 ohms.  So the big 561 at 1/2 ohm makes sense. 

 

Hard wire it in?  A holder might be hard to come by. 

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50 minutes ago, garyrc said:

@Curious George Since I have a Klipschorn AK4, I evidently have a polyswitch.  In what ways are they better?  Do you have any articles to recommend?

Polyswitch’s don’t clamp like Zeners, they slowly increase their resistance until the trip point. That is why they are usually bypassed with a resistor for audio use. They also need a cool down period to re-establish their thermal cycle. 

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#561 bulb is a 12.4 watt bulb at 12.8 volts, gives us a current of 970mA

 

Let's convert the bulb to an operational resistance of 13 ohms.

 

2 watts into the tweeter for protection is 500mA

 

Not sure what the cold resistance is of the bulb, someone seems to have written .6 ohms, sounds about right. Just a guess but at the 500mA of current we know the bulb won't be 13 ohms because it doesn't meet the 970mA spec needed for 12.8 volts across it giving us the 13 ohms so we are probably around 6-7 ohms at 500mA? The best way to tell would be to adjust a current source to 500mA into the bulb and see what voltage we end up at across the bulb, that would give us our answer to how much protection it actually adds in terms of impedance and voltage drop.

 

Should work quite well for tweeter protection so long as the cold state is fairly low like the .6 ohms stated.

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16 hours ago, captainbeefheart said:

There are many forum members swearing by taking the two 1N3996 Zener diodes out of the circuit, just unbolt one of them from the bracket and let float to remove from circuit.

 

I was intrigued so I felt like taking some distortion measurements to see what is going on. In theory these diodes should not be doing anything unless you reach a peak voltage of 5.737v to where they would start to conduct and clamp the signal there, nothing higher can pass. In terms of RMS power it works out to a maximum of 2 watts going into the tweeters. Testing confirmed that the diodes indeed do clamp the signal exactly at 5.737v. BUT there was an unexpected outcome to taking distortion readings below this value.

 

Voltage readings are taken at the tweeter output across the diodes. They are peak voltages unless otherwise stated.

 

What I found was surprising. I didn't see any difference at 2.8v, both with and without diodes had identical distortion profiles. It's when you start to reach about 4v where things start go awry for the circuit with the diodes. As soon as you near 4v distortion starts to quickly build up with increase in amplitude. The pictures below are the difference between the network with and without the diodes at 5v. The zeners are clearly starting to do something at 4v, I am guessing it's the very beginning of their 'knee' where very little current is starting to pass and the diodes are somewhere in their dynamic impedance range, datasheet shows 300uA they are 100 ohms and linearly decreases impedance vs increase in current, at 600mA the diode is .2 ohms. Regardless of the mechanism they just don't clamp at 5.737v, clearly current is passing at lower peak voltages, I am seeing 4v is where the difference is happening at.

 

Now you might say to yourself that's still a lot of power. 4v peak is about 2.8v RMS, or 1 watt power. But you must remember dynamic range in music. If you are hovering around say 1 watt at the input of the crossover network, 20db dynamic headroom, a heavy crash cymbal may push a peak transient over the 4v for a very short period of time. 20db is 10x, so for 4v that's only around 400mV. I may be completely wrong but I can completely see even though we know the average power into the tweeter will be low, there must some signals passing over the 4v. Members have made it quite clear that taking these diodes out have impacted the sound positively, so they must be misbehaving during even modest play levels. My engineering mind was doubtful, the diodes are suppose to clamp at 2 watts and up, certainly must not be heard at in home listening levels, well it turns out that distortion is increasing rapidly at anything over just 1 watt or 4v peak into the tweeter. You may not be averaging 1 watt into the tweeter but you may hit it with dynamic music. If for whatever reason you are blasting away and you happen to reach the clamping voltage peak, distortion grossly increases, at the point of clipping you are already at 5% THD and it just gets worse from there.

 

Clearly in home listening is reaching the point where these diodes are creating distortion and effecting the sound quality. You can see the higher harmonics jump right up at 15kHz and 20kHz.

5vpeaknozener.png

5vpeakZener.png

Ok, I have a question. Looking at the two traces, the harmonics between the fundamental of 5 KHz and the second harmonic of 10 KHz look to be less with the zeners acting. Also the odd-fractional harmonics between 10 and 15 KHz (third harmonic) look lesser. Clearly there is more distortion at 2nd, 3rd and 4th with zeners  conducting. Also note that the "zener conducting" trace seems to have more low-level noise than the "non conducting trace." My question: Given the power distribution of most music, just how audible can the 3rd and 4th be where the zeners are conducting? I

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9 hours ago, tigerwoodKhorns said:

Dumb question, this must be the 561 or 211-2 bulb. correct?

 

I have a bunch of Sylvania 1895's in my garage.  Made in the USA so I probably bought these in the 1980s.  --- OK I answered my question, the 1895's measure about 6 ohms.  DJK said the 561 is about 1/2 ohm. 

 

I have  7605 bulb on teh shelf that is much larger than the 1895 and it measures about 0.6 ohms.  So the big 561 at 1/2 ohm makes sense. 

 

Hard wire it in?  A holder might be hard to come by. 

 The headliner is a thin fiberboard and this design was used for some time, you should be able to easily remove the whole light assembly and cost minimal if removed at auto recycling center. Aftermarket supplies may be available as well.

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Back when I had my EV "D's" (bought 1978) one thing I thought was cool about them was.....they  had this light bulb.  I really had no clue how it worked.  I rarely saw them lit.  In fact, I can only think of once when I saw one light up (was in college at the time.....yeah, they were stuffed into the dorm room along with the LaScalas)

 

Here's a schematic for them.  Parts number 8, 35, 28 (illustrated at the bottom of the diagram)  They had the bulb on the inside of the speaker but then a clear, plexiglass type ""rod" that extended through the cabinet, flush with the exterior.  So looking at the outside you only saw this dot and this dot is what would glow when they were in protection mode, letting you know.  The "light" is at the top of the lower half of the cabinet, near the right side corner.

 

Just throwing this out here since I think this is interesting stuff.  I doubt it will add any value to the conversation.

 

 

interfaced.jpg

SentryVI-crossover.png

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4 minutes ago, Coytee said:

Back when I had my EV "D's" (bought 1978) one thing I thought was cool about them was.....they  had this light bulb.  I really had no clue how it worked.  I rarely saw them lit.  In fact, I can only think of once when I saw one light up (was in college at the time.....yeah, they were stuffed into the dorm room along with the LaScalas)

 

Here's a schematic for them.  Parts number 8, 35, 28 (illustrated at the bottom of the diagram)  They had the bulb on the inside of the speaker but then a clear, plexiglass type ""rod" that extended through the cabinet, flush with the exterior.  So looking at the outside you only saw this dot and this dot is what would glow when they were in protection mode, letting you know.  The "light" is at the top of the lower half of the cabinet, near the right side corner.

 

Just throwing this out here since I think this is interesting stuff.  I doubt it will add any value to the conversation.

 

 

interfaced.jpg

 

Cool stuff indeed.

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13 minutes ago, Coytee said:

Back when I had my EV "D's" (bought 1978) one thing I thought was cool about them was.....they  had this light bulb.  I really had no clue how it worked.  I rarely saw them lit.  In fact, I can only think of once when I saw one light up (was in college at the time.....yeah, they were stuffed into the dorm room along with the LaScalas)

 

Here's a schematic for them.  Parts number 8, 35, 28 (illustrated at the bottom of the diagram)  They had the bulb on the inside of the speaker but then a clear, plexiglass type ""rod" that extended through the cabinet, flush with the exterior.  So looking at the outside you only saw this dot and this dot is what would glow when they were in protection mode, letting you know.  The "light" is at the top of the lower half of the cabinet, near the right side corner.

 

Just throwing this out here since I think this is interesting stuff.  I doubt it will add any value to the conversation.

 

 

interfaced.jpg

SentryVI-crossover.png

Cool in the fact that the EV 'D' and their vented midrange were the coolest thing back in '78!

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4 minutes ago, Curious_George said:

Cool stuff indeed.

 

IIRC, Dennis had mentioned that Community Light & Sound had also used a bulb in some of their PA speakers, which would let an alert sound guy know he was cranking the levels to high, as they could see the light at the mix position.

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4 hours ago, Curious_George said:

 

Here is a socket for the 211-2, which is a festoon type bulb. Looks like just what we need. 

 

https://www.ebay.com/itm/3x-42MM-Festoon-Bulb-Socket-Base-Holder-Metal-Contact-for-LED-Halogen-Lamps/183980436091

 

I wonder if the 561 bulb with metal wire on the ends is a better choice if you just hard wire it in.  What are the chances of a bulb burning out compared to adding solder joints and a press fit? 

 

I's like to add these to Chorus, Fortes and some JBL Centurys, each has an eight ohm tweeter.  I don't want to have to replace any of these tweeters.

 

These limit to a few watts correct?  I ask because I have K Horns and La Scalas in storage but have JBL and Beyma tweeters for them with much higher power ratings than the K77s.  The JBls are rated at 40 watts, that should never happen. 

 

 

 

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12 minutes ago, jason str said:

Ah yes, Alligator clips.

I guess you could burn those tweeters out if you really tried but don't think it's needed here.

 

I live in Nevada, they are roach clips around here. 

 

OK, I do not listen at any loud levels.  I already finished the X Overs and did not plan for these so if I were to add them, it would be a separate board or cram them into my design.

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