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Are the diodes in AA crossovers necessary?


jhoak

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I'm toying with the idea of converting my DHA2 crossovers to Deang's "Super-AA"s. The Super-AAs have the tweeter protection diodes in them. I'l be driving a pair of CT125s so I'm not all that worried about toasting the tweeters.

The question is do the diodes bring anything "positive" to the party? Is there a compelling reason to not leave them out of the circuit?

THANX!
Jeff

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Guest David H

I love my DHA2 crossovers. The diodes were in the ALs for tweeter protection, and I can't imagine you would need them using Bob's CT125 tweeters.

I agree.....

Dave

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Guys,

The "super AA" is actually my design and was intended to be a turn in your AA and have it modified but Dean ran with the design and built them as a product. It was later renamed the "ALK Jr". It's actually a simplified version of my Universal. Anyhow, the Zener diodes in the AA do absolutely nothing until the AC voltage to the tweeter exceed a certain level, then they clip the tar out of the signal and generate distortion. They were added by PWK to keep from having to replace tweeters under warranty. Unless you have the old round-magnet K77 tweeter, you just don't need them. If you do have the old K77, I would leave them in there. They actually do no harm so long as you keep the volume level to a reasonable level. I personally think they were really an attempt to save the tweeter from reel-to-reel recorder monkey-chatter created on fast forward or rewind. That makes a lot of high frequency sounds at a high level that could be what was blowing tweeters back then.

Al K.

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Guest David H

For in-home use, the zeners are really not needed at all. Like Al said the zeners kept the tweeter from blowing during the warranty period. The fact of the matter is the A,B, and E crossovers did not use tweeter protection, and all used the round or square magnet K-77's. The AA crossovers were often used in Lascala's for pro-audio applications, I imagine the constant abuse caused tweeter failure regularly, thus the need for the protection circuit.

Dave

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The K-77 was good for about 2 watts. When EV started using copper-beryllium flat leads on the diaphragms, that power rating became about 5 watts. The zener diodes start to conduct to protect the tweeter at about 2 watts. PWK in one of the DFH papers said it was not high power that blows them, but instead usually transients took them out. The zeners act very quickly to protect the tweeter. Down side is that they are pretty expensive.

The CT125s are good for 45 watts, so the diodes (rated at 20 watts) would blow before the tweeter.

Bob Crites

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Thank you all for the input. The decision has been made. No diodes.

The parts are ordered. I'll take some shots of the results prior to installing.

Here's one of the "donors"...

So that there was at least a bit of Klipsch "provenance" they were built on plywood salvedged from my La Scala rebuild.

post-34422-1381959912115_thumb.jpg

post-34422-1381961742162_thumb.jpg

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NOW YOU DID IT! You got me interested in Zener diode tweeter protectors.

I connected my sillyscope, true RMS voltmeter and HP analyzer to the tweeter output of an old AA network.

There is output voltage and power, and the harmonic distortion for various levels:

Watts Vrms % distortion

2.5 4.5 6.7%

2.0 4.0 1.68

1.5 3.5 0.34

1.1 3.0 .08

0.8 2.5 .08

0.50 2.0 .08

0.28 1.5 .08

Next I ran the output back to 4.5Vrms and removed the zener diodes. The output moved up to 4.9V and the distortion dropped to 0.4%. The input level was 8.8Vrms. BTW, That represents 5.8 dB loss through the tweeter filter. That is typical for the poorly designed filter it has and its age. I'm sure Bob Crites is going to ask, so I measured the output 2 uF cap. It was 2.02 uFd with ESR=.66 Ohms. This particular network also has the steel screw holding the tweeter filter inductor to the board. That should be removed.

Conclusion: If you expect to exceed about 1.5 W to the tweeter the zeners start to distort. Below that they do virtually nothing.

Al K.

post-2934-13819599205428_thumb.jpg

post-2934-13819617524284_thumb.jpg

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Al, if you are using AA networks, is that an actual 2watts aplied into the network that will distort the tweeter, or is the tweeter seeing less than the full wattage applied?In other words how many watts can your amp put into the network before the tweeter sees 2?

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Mark,

That depends on the loss through the network. The one I tested had 5.8 dB. An AA will have at least 3 dB with brand new caps. That means 4 - 7.6 Watts in to get 2W out. That is at 6 kHz. up higher there will be less loss so you will approach 2W in for 2W out. So the actual answer is that you really can't be specific.

Al K.

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That is specific enough. 10-20 watts would be WAY too much. So much for the guys running 100 and 500 amps into their Heritage speakers. You can't even use 10 watts without risking a tweeter. I never knew that. In 30 years of using purely Heritage speakers I never broke a driver either. I guess I was lucky. Several of my friends routinely carried their tweeters down to Superior Sound in Syracuse for replacement.



Edit: Wait a second. Let's do the numbers again Al. If you lose 3 db in the network doesn't that cut the power down by some large amount (maybe x10)?

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Guest David H

Mark, I was going to take a stab at explaining the power consumtion, but ALK is going to do a much better job.

Basically the lower frequencies ie. woofer eats up most of you power followed by the mid then the tweeter. Attenuation can and does play a role, but the tweeter is not attenuated in many of the networks.

Dave

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Mark,

That depends on the loss through the network. The one I tested had 5.8 dB. An AA will have at least 3 dB with brand new caps. That means 4 - 7.6 Watts in to get 2W out. That is at 6 kHz. up higher there will be less loss so you will approach 2W in for 2W out. So the actual answer is that you really can't be specific.

Al K.

Al, how are you measuring power as an input to the tweeter filter? Just voltage won't do that because you are working with the impedance of the filter below the crossover frequency plus the tweeter impedance plus ESR of the caps. When you are across the crossover point, above 6khz, you are then working with only the impedance of the tweeter plus ESR of the caps. You have said that you approach 2 wattts out for 2 watts out there. I think you will find that power out is very close to power in regardless of the frequency once you take impedance into account. I don't believe a 3 db loss (power) inside a new AA tweeter filter. I am, however, ready to be convinced otherwise.

Bob Crites

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Bob,

The power measurements were taken with a true RMS AC meter connected across an 8 Ohm power resistor connected to the tweeter terminals. Power is E x E / 8. Part of the problem is that the tweeter filter in the AA is NOT and 8 ohm filter. I forget what it actually is. It's also a constant-K filter designed to work between equal impedance, not a voltage source and 8 Ohms. In short, all bets are off with it! I did a lot of playing with it years ago trying to figure why it has a 3 dB loss even with brand new parts. The computer confirms that 3 dB too. I forget what the loss is at 20 KHz. I could put this particular AA on the analyzer looking through the tweeter filter, but I don't think it would mean a lot considering the old caps that are in it.

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