AR9 - both tweeters out simultaneously

[dingus]

i dont have my hands on a voltmeter at the moment, but it is forthcoming. i highly suspect the root of the problem lies elsewhere and the tweeters took the hit. i didnt notice anything untoward in the sound when it happened, they just dropped out, mid's, mid-bass and woofers all sound fine. all connections and wiring checked out ok as well. any ideas on what to look for, or what could have caused this? i was going to pirate the tweets from my AR91's but dont want to risk blowing them too.

[dingus]

while confirming that both tweeters are dead with the volt meter, i noticed that the leads to the negative terminal on both tweets has been severed. is this indicative of a one-time anomaly, or is it an ongoing problem with amplification or a failure with the crossover? if anyone would want to rebuild the crossovers for me please give me a PM.

[soundminded]

while confirming that both tweeters are dead with the volt meter, i noticed that the leads to the negative terminal on both tweets has been severed. is this indicative of a one-time anomaly, or is it an ongoing problem with amplification or a failure with the crossover? if anyone would want to rebuild the crossovers for me please give me a PM.

I didn't want to be the bearer of bad news so I let you confirm what I suspected. I can think of at least two reasons this could have happened.

If the crossover networks have not been rebuilt, it is possible and even likely that the capacitors have changed value. As has been pointed out, as non polarized electrolytic capacitors of the type used in the original manufacture age, their capacitance increases. For the capacitors in series with the tweeter, (and midrange) this lowers their crossover point making them more susceptable to being overdriven. Frequencies below their intended operating range would cause greater excursion since most musical signals have more energy in the range below the 7 khz crossover frequency than above it. The tweeters may have therfore been overdriven due to capacitor aging. Failure at the same point on both tweeters suggests mechanical overexcursion as the cause of failure rather than melting the voice coils. If there is no gap between the two ends of the wire at the break this may be further indication of overexcursion.

The other possibility is that the amplifier was the cause, possibly intermittent where a parasitic high frequency component is induced. This signal at high power levels can damage the tweeter even if it is not audible. It could only be detected with an oscilloscope and may have any one of many causes. These could range from power supply defect, coupling of a local rf source at any point in the signal chain but especially at the preamplifier input cables, a defective source such as a problem with a tuner or cd player, or the use of exotic speaker wires that may have inadvertently turned the output circuit into an oscillator often resulting in amplifier failure. At the risk of being sued, I am not going to mention the brand name of one company whose product for a while was famous for this propensity.

Unless the source of the failure is identified and corrected, replacing the tweeters will only likley result in another failure.

AR9 tweeters although fairly robust are really not designed to withstand high power levels especially sustained high power levels. For this reason alone, extreme caution should be usd especially when playing loud rock music. Unfortunately, the industry has established internally wired crossover networks as a standard method of manufacture. This makes troubleshoting, repair, and installation and replacement of individual fuses for each driver more difficult and time consuming. I recenlty replaced all but the two largest capacitors in my AR9s at the time I had the woofers and LMRs refoamed last fall. I had noticed that at least one of the tweeters occasionally distorted and I was concerned that the caps had changed value. It was not a project I enjoyed. Working through the woofer openings with large hands was a little awkward. I think it is best to remove both woofers and lay the speaker on its back when doing this project. I had mine off the floor stradling two chairs but a table would have been even better. I checked the results audibly before I reinstalled the woofers to be certain I'd gotten it right and that both speakers were identical.

[dingus]

thanks for the input. i have pulled the amp in question, but i've no way to verify if it or the crossover is at fault here. if the only problem with the tweeters is the broken lead, is there any reason the connection cannot be restored with a bit of solder?

[soundminded]

thanks for the input. i have pulled the amp in question, but i've no way to verify if it or the crossover is at fault here. if the only problem with the tweeters is the broken lead, is there any reason the connection cannot be restored with a bit of solder?

At this point you have nothing to lose by trying. Be very careful though. Use a low wattage pencil iron, no more than 15 or 30 watts max. Do not apply more heat than is absolutely necessary, these leads melt very easily. Use a low temperature solder if you can get some. There may be some alternatives using a thin strand of wire and some conductive glue. There have been other threads about that, I think someone has come up with some. Personally I'd replace the series caps to the tweeter just to be on the safe side. That would be the 4 and 6 mfd caps. You can see that as a 3rd order filter, AR wanted a very steep slope, about 18 db per octave. This sharp cutoff substantially reduced energy below the crossover frequency from reaching the tweeter. I'm sure they had a good reason for it. I used ordinary NPTs to recap mine and my digital multimeter has a capacitance checker so I verifed that the replacements were the correct values. There is someone on e-bay who claims to repair AR tweeters at $40 each. I've had no experience with him though.

[dingus]

rather than do piecemeal work on the crossovers, i would rather have them completely rebuilt. problem is i lack the ability to do a competent job so i would need to have the work farmed out.

[dingus]

in my unscientific examination i did a visual inspection of the crossovers. all original as far as i can tell, nothing out of the ordinary, no discoloration, leaking or bulging detected. installed the tweeters from my AR91's, disconnected the amp driving the high-end (Yamaha B-2x) and ran them with only the amp formerly driving the lows (Yamaha M-2). at the very lowest volume setting i could easily hear a hash from the mids..., tweets and mid-bass sounded right. disconnected the M-2 and reconnected the B-2x alone. trying again at the lowest volume setting everything sounds right, no hash or distortion. giving it a little volume and everything sounds right. i miss the separation and a not as prominent low end, but its better than the alternative.

because both tweets went out at the same time, it makes sense to me that the amp would be the culprit. what doesnt make sense that it would be the M-2 since it was driving the woofers only. is it possible that a spike from the low end to take out the tweeters in a bi-amp configuration? i'm confused.

[dingus]

ok, it would appear that the M-2 was the issue, though i cant figure how it cause the tweeters to blow when it was driving the woofers. i pulled out my Luxman R-117 receiver and have it running the woofers and the B-2x on the highs. sound is more relaxed and smoother than with the M-2. i'm guessing that the M-2 has been steadily deteriorating so that i didnt notice that something was wrong with the sound. now i need to source some more tweeters, also i'd still like to have the crossovers rebuilt if anyone is willing.

[soundminded]

ok, it would appear that the M-2 was the issue, though i cant figure how it cause the tweeters to blow when it was driving the woofers. i pulled out my Luxman R-117 receiver and have it running the woofers and the B-2x on the highs. sound is more relaxed and smoother than with the M-2. i'm guessing that the M-2 has been steadily deteriorating so that i didnt notice that something was wrong with the sound. now i need to source some more tweeters, also i'd still like to have the crossovers rebuilt if anyone is willing.

"ok, it would appear that the M-2 was the issue, though i cant figure how it cause the tweeters to blow when it was driving the woofers."

I have noticed that isolation between the woofer section and the rest of the crossover network when the link between them is open for bi-amplification is less than perfect. I attribute this to mutual transconductance where the coils of one section and the other are linked as in an air core transformer. Because there are no issues with core losses as in a conventional transformer, coupling can be equally efficient at high frequencies as at low frequencies. The problem is created by the close physical proximity of the coils to each other. Just one theory. Another is that the two amplifier inputs in parallel connected to the preamp as you had it somehow caused some sort of very high frequency feedback, possibly above the audible range. Were the two power amplifiers close to each other? You may not know such a signal is present if it is above the audible range. As I said previously, it might only be detectable by seeing it on an oscilloscope. These things are rare but they do happen. Exotic speaker cables with high capacitance or inductance to alter the sound can sometimes inadvertently turn the output circuit into an oscillator.

[dingus]

the amps are on adjacent shelves in the same rack, one above the other. they have been in this configuration with the same cables and ic's for the past 18 months or so with no issues until now. with the R-117 in place there is no mistaking the improvement in the sound from top to bottom.