I have a pair of infinity Qa's that I love and adore. They does have a couple issues though. The left speakers ribbon tweeter has started going in and out and I believe it is the crossover capacitor going out that needs to be replaced anyways. Turning it up to 2-20 watts usually kicks it back in, and I'm not sure if that gives any clues to if it is the cap or the driver that is having problems. I have a mcintosh mc2205 with power guard and rarely turn it loud, so I don't think I would've blown the driver. Also the potentiometers scratch when they are turned for the treble adapter.
Otherwise the only thing that bugs me is as a 2 way, they quickly run into intermod distortion. I am adding some DIY subs to the low end so I am going to cross the infinitys at ~100hz will go by ear, but that should keep distortion very low on the woofer. But with a first order crossover, the tweeter distorts quickly as well. My plan is to replace the cap, add an inductor in parallel to make it second order, and add an inductor in series with the woofer to make it ~ second order in conjunction with its own "first order" roll off which is utilized by running it full range in the original crossover.
My question is, what should I target the component sizes too? I'm thinking targeting a -3db at 2.5kHz for the inductor on the woofer, and a -6db at 2.5 kHz on the tweeter, which I believe corresponds to a .255mH inductor in series with the woofer, and a 7.96uF cap in series and .509 mH inductor in parallel with the woofer, assuming ideal 4ohm drivers. I'm not sure what to do with the potentiometer for the tweeter. Should I leave it? Seems like all it would do is mess with the crossover frequency.
Anyways any advice would be highly appreciated. Thanks all!
Otherwise the only thing that bugs me is as a 2 way, they quickly run into intermod distortion. I am adding some DIY subs to the low end so I am going to cross the infinitys at ~100hz will go by ear, but that should keep distortion very low on the woofer. But with a first order crossover, the tweeter distorts quickly as well. My plan is to replace the cap, add an inductor in parallel to make it second order, and add an inductor in series with the woofer to make it ~ second order in conjunction with its own "first order" roll off which is utilized by running it full range in the original crossover.
My question is, what should I target the component sizes too? I'm thinking targeting a -3db at 2.5kHz for the inductor on the woofer, and a -6db at 2.5 kHz on the tweeter, which I believe corresponds to a .255mH inductor in series with the woofer, and a 7.96uF cap in series and .509 mH inductor in parallel with the woofer, assuming ideal 4ohm drivers. I'm not sure what to do with the potentiometer for the tweeter. Should I leave it? Seems like all it would do is mess with the crossover frequency.
Anyways any advice would be highly appreciated. Thanks all!
The Quantum series are notorious for suffering oxidation in the treble adjustment pots causing exactly what you describe. While the caps definitely need replacing by now, look there first for an immediate fix. The Qa used a smaller 2Ω 5W potentiometer vs. the big 5Ω 25W pots of the larger Quantums, so you won't be able to simple disassemble those for cleaning like you would the others. You'll have to figure out some way to work a little contact cleaner in there and work the wiper back and forth to clean things up without getting it everywhere. Once you do, you should have cleared up the intermittent tweeter and scratching sound. Should.
The older EMITS do sometimes suffer another corrosion/resistance related problem up where the contact points are for the diaphragm. It's been a lot of years since I worked on one of these, but if I'm remembering correctly the little dollop of solder there at the base of the leads sometimes needs a little touch up with a low wattage soldering iron and you'll be good to go. (But be very careful here as it's easy to get the diaphragm too hot melting a hole and then the EMIT is ruined.) Do the potentiometers first and you'll know if you need to take that risk.
As for crossover modifications, I'll leave that to more knowledgeable people to make suggestions there. However, it's worth noting that later series' models using repackaged versions of that same EMIT tweeter moved the crossover point up to 4.5kHz. The last and best models did that with fourth order slopes and diode protection at that. Though the Qa woofer can't go that high, it's something to keep in mind.
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