trioid Posted August 9, 2006 Share Posted August 9, 2006 Hi all: Anyone know about the impedance curves for the new Cornwall III? I assume the crossovers are different?? Question comes from my experimenting with the 4 and 8 ohm taps on my PP tube amp (zero feedback, class A, ~ 6 W). I assume they still dip fairly low due to K33, so DeanG suggestion for a parallel 15 Ohm resistor would be a good idea?[8-)] thanks, jim Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 9, 2006 Share Posted August 9, 2006 Sounds like your best bet is to move to a constant-impedance crossover...I don't think a resistor in parallel will achieve that, but it should smooth things out. Quote Link to comment Share on other sites More sharing options...
trioid Posted August 9, 2006 Author Share Posted August 9, 2006 Hi Mike Yup, that was my idea. If they still drop to something like 6 ohms min, that size resistor in // should flatten things out.... i think jim Quote Link to comment Share on other sites More sharing options...
Guest " " Posted August 9, 2006 Share Posted August 9, 2006 I'm a little puzzled as to why we would use a resistor in parallel with an entire speaker system to attempt to match or adjust impedance. Thinking out loud, a. if we use the 4 ohm tap, the resulting connection to a speaker using a k-33 would result in an impedance mismatch of about +50% more than what the amp expected to see. Adding a 15 ohm resistor would close the gap to about +33% more of a load than what the amp expected to see, but the cost penalty is the juice the resistor would use. can't afford this on this amp. b. If we were to use the 8 ohm tap, the resulting connection to a speaker using a k-33 would result in an impedance mismatch of about -25% less of a load than what the amp expected to see, adding a resistor, would boraden the gap for the worse. c. if, rather than adding a resistor, you added an autoformer in between the amp and the speakers, the following adjustments can be made. Using an atlas AF140 autoformer (52 bucks each), and connecting using the 1.56 impedance step up multiplication factor, you would wind up with a load of 9 ohms, using the 8 ohm tap on the amp, this would only be 12% higher than what the amp expected to see. This would be my prefered solution. d. if, rather than adding a resistor, you added an autoformer in between the amp and the speakers, the following adjustments can be made. Using an atlas AF140 autoformer (52 bucks each), and connecting using the .64 impedance step down multiplication factor, you would wind up with a load of 3.84 ohms, using the 4 ohm tap on the amp, this would only be -0.16%% lower than what the amp expected to see. While this may sound better than option © above, I think the resulting sound of option © would be better than the resulting sound of option (d). Quote Link to comment Share on other sites More sharing options...
trioid Posted August 9, 2006 Author Share Posted August 9, 2006 Well, here is my quick, and perhaps not well thought out answer. If I just look at pure resistive load (ok, that is a bad assumption), and I have a speaker impedance that bottoms at 6 Ohms, and add 15 Ohm in parallel, I get a low end impedance of a little over 4 Ohms, so I use the 4 ohm tap on my output transformer and my tubes are fat and happy; everything is near and above 4 Ohms (and the whole impedance curve is 'flattened' due to the parallel resistor - the 40 Ohm peak is reduced). Would not want the 8 ohm tap since that will load the tubes at the low impedance end and we are right back where we started. I will loose a couple db, but with a mighty 6 Watts, I am willing to do that. That's why I like Cornwalls.... OK. so if I am ignorant and missed the point, GOOD. I am more than willing to get educated : ) jim Quote Link to comment Share on other sites More sharing options...
Guest " " Posted August 9, 2006 Share Posted August 9, 2006 The use of resitors was very popular about 35 years ago, back then, they refered to them as dampening resistors. For your amp, a 15 watt autoformer would cost 11 bucks per speaker. 22 bucks total plus shipping. http://www.edcorusa.com/transformers/em/em3015.htm It would eliminate the resitors, and let you recover the power being drained off as heat (abot 33%). Only giving you options, as well as some rationale behind the recomendation. Quote Link to comment Share on other sites More sharing options...
trioid Posted August 9, 2006 Author Share Posted August 9, 2006 speakerfritz -- your point of view is more elegant, plenty cheap, and the right way to go. I thank you for the input and the link! jim Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 9, 2006 Share Posted August 9, 2006 The 'problem' here is that a speaker does not have a flat impedance curve for every frequency. It is not an issue of the nominal rating. At some frequencies there will be dips as low as 4 ohms (actually a bit lower) and peaks as high as 30? ohms. It's been a while since I've looked at the plots and I never saved any of them. A 15 ohm resistor in parallel with the 4 ohm dips results in a 3.2 ohm impedance - basically no difference. A 15 ohm resistor in parallel with 30 ohms results in 3.5 ohms - a very big difference. The benefit being that the amp without feedback now sees a nearly flat impedance response (again, that's per frequency). Throw the amp on the 4 ohm taps and you're home free. The downside to such an approach is that this will change the frequency response of the speaker and there are losses in the resistor. Not quite a free lunch, but at least it keeps the amp without feedback from crapping up. Quote Link to comment Share on other sites More sharing options...
trioid Posted August 9, 2006 Author Share Posted August 9, 2006 Hi Mike: www.belgaudio.com/kcmeasurements2.htm BUT: back to my original question: does anyone have imp vs. freq. data on the new Cornwall? I am (perhaps ignorantly) assuming that : 1. The crossovers are different 2. The crossovers affect the impedance curve Cheers, jim Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 9, 2006 Share Posted August 9, 2006 1 and 2 are both good assumptions. There is even a different tweeter in the C3 which is going to further add to the differences. I'm pretty sure there aren't any plots on the internet considering it's a relatively new speaker. You might try your luck with Klipsch: 1-800-Klipsch They have a good service department and might be able to help you out. Quote Link to comment Share on other sites More sharing options...
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