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Post by stoopalini on May 17, 2014 9:56:33 GMT -5
Thanks Keith, I appreciate the info. I also thought something was going wrong with the amp, but the more I research this issue with these Adcom amps, I'm finding the same info the repair shop told me; that it's normal operation due to the design. Again, I am not qualified to look at the schematic and make my own determination, but considering these Adcom GFA-545 amps can be had for around $150, I don't see it worth spending $200 for a repair shop to look at it. Last night I built the filter using 250v .22uf capacitors, and it works like a charm. No more popping I have a professional RTA app for my iPhone, as well as calibration microphones, I use for large venue calibration. So I plan to measure the frequency response of the front heights at 90hz with, and without, the filter to see if the .22uf value is causing any dB loss at the UMC's crossover point. If it is, I'll replace the .22uf caps with 2.2uf ones. Thanks again guys, the wife is VERY happy the pops are gone!
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Deleted
Deleted Member
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Post by Deleted on May 17, 2014 12:43:09 GMT -5
Pop has been removed.
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Post by srrndhound on May 17, 2014 17:22:07 GMT -5
Ok, does that mean the phase shift only occurs within the vicinity of the cutoff value? I actually ordered .22uf capacitors when I thought I would be using a 300k resistor, so I don't have any 2.2uf ones. I could order 2.2 ones, but with the 22kOhm resistance value and a .22uf capacitor, the cutoff value is 32.88Hz; and since these channels are crossed over at 90Hz by the UMC anyway, I imagine it won't matter. At this website you can see the response and phase shift plots for your filter. sim.okawa-denshi.jp/en/CRtool.phpPlug in 22k and .22u, and you'll see the phase shift at 100 Hz is about 20 degrees. Not sure that it will be audible, but it's an easy and cheap fix. Get a pair of 10 uF electrolytic caps and put them in parallel with the ones in the box. Radio Shack has non-polar caps for $1, but it's ok to use regular polarized caps in this application, since the DC across the cap will be extremely small. And if you connect the + side of the cap to the preamp input side, whatever little DC that exists will be in the "correct" direction since the DC offset at the amp input is negative.
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Post by stoopalini on May 17, 2014 19:58:30 GMT -5
I agree it's probably not worth messing with if it's 20 deg at 100Hz. Considering one cycle (360 deg) of a 100Hz wave is 0.01 seconds, which is 10ms, correct? So 20 deg would equate to a 0.55ms shift of a 100Hz tone, unless my math and comprehension of this is wrong...
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Post by stoopalini on Oct 8, 2014 15:27:04 GMT -5
Just as an update to this: I finally got around to taking some SPL measurements with the filter box in line, and with the filter box removed. the greatest dip is a 2dB drop around 55Hz, and no other real impact. The speakers are NHT Super Ones, and I have them crossed over at 80Hz. The mic was placed in the listening position, aimed at the speaker during the test. I know the graph isn't pretty, but unfortunately the UMC-200 doesn't offer PEQ for the front heights/rear surround channels All my other speaker measurement graphs look great after making the appropriate PEQ adjustments.
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