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Blown (2) K-77 diaphragms while Tri-amped


Cantilope

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What I am running:

 

1978 Klipsch Belles w/ all original horns and drivers, Tri-amped

Xillica 4080 DSP – (HP Xover set at 4900 HZ LW 24 dB) for the Tweeters

(2) First Watt M2s – A left and right feeding the MF and HF.

McIntosh C32 Pre

McIntosh MC2205 – Feeding the bass bin

Gain Delta set via volt/ohm meter, attenuation on the McIntosh knobs.

 

I have been on the exploratory path of digital xovers.  I have in the last few months added the M2s and the Digital xover into the system and went the tri-amp route on the Belles.  I am very happy with the sound and feel I have it at about 80% there although I am fine tuning.  But I have run into a problem that I can’t seem to solve.

 

This past weekend I discovered that my Right Tweeter was no longer working and I discovered it to be an open circuit.  The diaphragms I run are replacements that are less than a year old from Simply Speaker.  So I ordered up a replacement and on Tuesday replaced the diaphragm.  This diaphragm lasted about 15 seconds under mild use and stopped working while I was testing.  I shut it down and proceeded to check the ohms and saw a funny thing, it was floating around, not a direct short nor was it an open circuit.  I checked all the wiring to the xover and the freq slopes and this took about 10 minutes.  Upon one last check with the OHM meter the diaphragm measured OK so I plugged it and it worked for a about 10 seconds then I heard a crackle and now it measures completely open.  I suspect I have overpowered or overheated it.  But have no idea how or why.  After it blew it the second time I tested the outputs of the M2 and was at less than .5Vrms which is under 1 Watt per the M2 sensitivity specs.  My theory is that I overheated it to the point it seized the first time and after it cooled it was OK until I tried it again this time going all the way with it.

 

I contacted Nelson Pass, (what a great guy):

The M2 does not amplify DC, so that is unlikely, but you should check for DC at its output with a voltmeter (I test none).  I assume that you have a capacitor in series with the tweeter in any case.  The M2 also does not amplify very high frequencies, so that’s kind of unlikely also, but there is a possibility that ultrasonic oscillation exists in the system.  I think the most likely source of the problem is that the crossover frequencies or slopes may be too low, sending more energy to the drivers than you think.  (See setting above, this setting is widely used)

 

One of the things I wanted to try and do was blow the other one… I flipped the system left for right and cannot get the left one to blow using the exact same setup.  Maybe I just had a bad diaphragm?  Or I have one that just won’t blow…  I don’t know.

 

I do not have a capacitor in series, do I need one?  Is that to protect against the DC?  Would this fix my problem?  Tomorrow I have 2 more diaphragms that will arrive, and I am really thinking of pulling that active and the tri-amping and going back to one amp and a passive.  It has been recommended that I go with a zeener diode, but if that is the only logical solution, I will go back to the passives.

Although I feel though that the improvement the tri-amping this has made, is not worth giving up on just yet…  Please share your thoughts even if they include "go back to passives".

 

I have attached an unrelated photo of my system as I try and fix this issue and run some dedicated outlets.

 

Thanks for your time.

Joe

post-8016-0-05900000-1452879176_thumb.jp

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I just noticed this 4900 hz. Change it to 6000 hz for sure before installing new diaphragms.

 

No, that's a much steeper crossover filter than the one that's in the passives that come with the Belles.  That's not likely the issue.  However, the crossover itself has an "output limiter" - see page 17 of your manual.  Set that to a lower level than default to avoid threshold/attack/limiter levels that could eat the tweeter.

 

I don't know why I didn't think about that earlier.  Sorry about that.

 

Chris

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Install a fast blow 1 amp fuse in series with the positive input to the crossover and it should protect the tweeter and still maintain plenty of volume. The fuse will blow before the tweeter. I did this many years ago when I had teenagers at home blowing my tweeters and never lost a tweeter again. A lot cheaper replacing a fuse than a tweeter.

 

I just noticed this 4900 hz. Change it to 6000 hz for sure before installing new diaphragms.

I happen to have (2) 250V 1 AMP Fast Acting.  Is that voltage OK?  I will add these for sure, I remember blowing my dads advents, to find a blown fuse.  What a relief that was, I should call him and tell him that story, hell love it.

 

Made the xover change, per your recommendation, that seems like it could be the problem as it echos NPs thoughts.

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Install a fast blow 1 amp fuse in series with the positive input to the crossover and it should protect the tweeter and still maintain plenty of volume. The fuse will blow before the tweeter. I did this many years ago when I had teenagers at home blowing my tweeters and never lost a tweeter again. A lot cheaper replacing a fuse than a tweeter.

 

I just noticed this 4900 hz. Change it to 6000 hz for sure before installing new diaphragms.

I happen to have (2) 250V 1 AMP Fast Acting.  Is that voltage OK?  I will add these for sure, I remember blowing my dads advents, to find a blown fuse.  What a relief that was, I should call him and tell him that story, hell love it.

 

Made the xover change, per your recommendation, that seems like it could be the problem as it echos NPs thoughts.

 

My first thought is that if it is frying the driver within 15 seconds, then double check and triple check that no DC is getting through. Are you confident that you are using the Volts DC and not Volts AC setting on your VOM? (I mean no disrespect, but others have made this error). Even if the crossover point is at 4kHz and not 6 kHz, it should not be causing damage so quickly. 

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My first thought is that if it is frying the driver within 15 seconds, then double check and triple check that no DC is getting through. Are you confident that you are using the Volts DC and not Volts AC setting on your VOM? (I mean no disrespect, but others have made this error). Even if the crossover point is at 4kHz and not 6 kHz, it should not be causing damage so quickly. 

 

No disrespect taken... no worries.  When I set it to ACV I get a voltage reading that raises and lowers with signal around 1V at 75% volume.  When I set it to DCV, I get a reading of -0.032V that does not change.

 

However, the crossover itself has an "output limiter" - see page 17 of your manual.  Set that to a lower level than default to avoid threshold/attack/limiter levels that could eat the tweeter.

 

Chris

 

You'll have to help me with this one.  I see where to enter it, but would have no idea what a safe number would be.

.

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You'll have to help me with this one.  I see where to enter it, but would have no idea what a safe number would be.

 

What kind of amplifier is driving the tweeter-- brand and model number?

 

Chris

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You'll have to help me with this one.  I see where to enter it, but would have no idea what a safe number would be.

 

What kind of amplifier is driving the tweeter-- brand and model number?

 

Chris

 

Chris, I understand your thinking ... it's just that the damage seems to happen so quickly. I think this is a big screw up and not just big amps clipping or something. 

 

OP: if you have access to a scope, I would suggest substituting a power resistor for the tweeter. Run the set up, with the scope attached to the "tweeter" (i.e., the resistor) and see if you see anything odd (big surge, low freq, or  DC or whatever). At the very least you can put a cap in series with a cutoff a couple octaves below the tweeter crossover point. That should buy some protection along with a smaller fuse (not slow blow). Just a note: in general fuses are not the best way to protect a tweeter.

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Tom: the crossover is a lot faster than almost any other way to protect the tweeter because you're limiting the output power of the amplifier to a max voltage value for the tweeter channel(s) only.

 

Joe: How about 4 dBU threshold, 5 ms attack time, 16x release time.  If it sounds bad, then let us know.

 

Chris

Edited by Chris A
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I suspect that what "Pappa" Nelson Pass suggested is correct. You might have some d.c. offset at the output of the amp feeding the tweeter. You'd expect zero d.c. volts at the output terminals of the amp when there is no music (a.c.). But sometimes things get out of adjustment and there is a (hopefully small) d.c. offset from the zero volts.

(You may heard a crackle in the headphones when you plug them into a computer or Walkman. That is d.c. offset.)

You can set your multimeter to d.c. volts and see what you get at the amp output. I'd try with no speaker connected and, say, the two volt setting on the multimeter, if there is a setting. Some multimeters just autorange.

Nelson is suggesting a capacitor because a cap will not pass d.c.

This sort of thing is not a problem with typical boxes which have crossovers because the mid and tweeter are coupled to the input to the box though capacitors.

OTOH, the woofer is not coupled through a capacitor and this is why you can measure what is essentially the woofer voice coil resistance (plus the dcr of an inductor) by attaching a multimeter to the input terminals of the box.

WMcD

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I contacted Nelson Pass, (what a great guy):

The M2 does not amplify DC, so that is unlikely, but you should check for DC at its output with a voltmeter (I test none).  I assume that you have a capacitor in series with the tweeter in any case.  The M2 also does not amplify very high frequencies, so that’s kind of unlikely also, but there is a possibility that ultrasonic oscillation exists in the system.  I think the most likely source of the problem is that the crossover frequencies or slopes may be too low, sending more energy to the drivers than you think.  (See setting above, this setting is widely used)

 

 

I do not have a capacitor in series, do I need one?  Is that to protect against the DC?  Would this fix my problem? 

 

Joe

 

 

Tom: the crossover is a lot faster than almost any other way to protect the tweeter because you're limiting the output power of the amplifier to a max voltage value for the tweeter channel(s) only.

 

Joe: How about 4 dBU threshold, 5 ms attack time, 16x release time.  If it sounds bad, then let us know.

 

Chris

 

 

I suspect that what "Pappa" Nelson Pass suggested is correct. You might have some d.c. offset at the output of the amp feeding the tweeter. You'd expect zero d.c. volts at the output terminals of the amp when there is no music (a.c.). But sometimes things get out of adjustment and there is a (hopefully small) d.c. offset from the zero volts.

WMcD

 

 

I went ahead and entered the threshold and attack limits. I replaced the driver and so far so good. I listened for 2 hours at a decent level. I'll give it another go today.

DCV of all outputs from the M2s.

-.040 -0.032 and -0.032 and -0.027

 

 

 

For anyone experimenting by moving from passive crossovers (placed between the amplifier and the speaker driver) to active crossovers (placed before the amplifier where the amplifier is now directly connected to the speaker driver) this thread contains very good information that more people should take the time to understand. 

 

I'm not sure of which passive crossover you had before the switch to active; however, the Klipsch Type AA crossover used zener diodes because of the great frequency in blown tweeters when using the Type A crossover network. 

 

The zener diodes in the AA crossover are designed in the passive crossover circuit to limit the amount of power to the tweeter in the 100 watt continuous rated Klipschorn, Belle and La Scala to 2 watts.  Essentially, I believe that the zener has the effect of limiting the power to the tweeter to ~ 2% of the actual power the amp is delivering to the crossover.  What I believe that ChrisA was getting at with the changes to the settings of the limiter in the active crossover is adjusting the crossover to lower the amplifier output to the tweeters similar to the purpose of the zener diodes used in the Type AA crossover network.

 

Where Nelson is stating, "I assume that you have a capacitor in series with the tweeter in any case. Essentially, by removing the passive crossover network from between the amplifier and speaker drivers, you have removed all of the capacitors that would have also served as DC blocking in the event of the power amplifier passing any unwanted DC voltage for any reason.  The amplifier is now directly connected to the speaker driver with nothing in the path to block unwanted DC voltage. Too much unwanted DC voltage can easily destroy a voice coil or diaphragm in a speaker driver. This DC voltage and DC offset voltage is part of what WMcD (Gil) refers to above.

 

I believe that the most common "protection" (alluded to by Nelson in your quote) when using an active crossover before the amplifier, is inserting a capacitor in series with the tweeter driver to protect from unwanted DC voltage being passed by the amplifier directly to the tweeter driver.  The selected value of the capacitor would need to be of a size to not change, or have an impact on the active crossover point or slope where the capacitor would only act as DC voltage blocking. 

Edited by Fjd
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The EV T-35 tweeter (i.e., generically the Klipsch K-77 tweeter before it underwent incoming QA at Klipsch) has a rating of 5 watts music power with a 6 dB crest factor (that's a DR Database rating of "6" which is actually pretty low).  That's not much power handling capability.  The Beyma CP25 ("baby cheek" tweeter) for instance can handle 50 watts music power.  That's a bit of a problem in terms of resistance to even nominally high SPL levels of music.

 

The T-35 actually has a very clean output in terms of its frequency response, phase, and harmonic distortion curves, but I've found it to be less listenable than the CP25, presenting a significantly harsher upper register that sounds a lot like "frying bacon", or in other words, sounding like much higher levels of higher order modes (HOMs), than the CP25 presents.  However, the CP25 requires frequency response correction, which is easy to do with an active crossover.  I highly recommend the CP25 for those reasons instead of the K-77.

 

YMMV.

 

Chris

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  • 2 weeks later...

It has been a few weeks, things are still working well. I cannot hear any affect this setting has created sound wise.  I think I will give Chris the solved button.  Thank You.  I would take some suggestions on which filter cap to use to block the DC, I don't see how this extra bit protection could be a negative.

 

I do want to say now that it is back together, and I have my freshly re-freshened McIntosh C32 back in as a pre, I cannot get over how amazing this sounds.  I have been seeking audio bliss for many, many years and feel like I am right there, with the stock Belle being my limitation.  I have had my friends and my father over and they all are amazed with what it has done to the quality of the sound.  From Deadmau5 to Gloria Estefan, tri-amping, and QUALITY digital xovers are the way go.

 

I hope this thread has not discouraged anyone from trying this.  Do it! Do it! I will never look back, I just need 3 more of these xillicas...

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