JohnA Posted December 21, 2003 Share Posted December 21, 2003 How do I size the power ratings of L-pad resistors? I need to knock down the output of a pair of Altec 902-8Bs from 106 to 104 to try a 2-way La Scala. An L-pad seems like the best way to deal with just 2 dB, but I don't want them to burn up. Quote Link to comment Share on other sites More sharing options...
sfogg Posted December 21, 2003 Share Posted December 21, 2003 John, Considering the efficency of the 902s 10w resistors should be plenty since under normal useage you won't be putting nearly that much power into the 902s and they can only handle 15w max anyway. Where are you crossing them and at what slope? Good luck with the 2 way project, I converted mine a few days ago with Al's crossovers and am very happy with them. Shawn Quote Link to comment Share on other sites More sharing options...
JohnA Posted December 21, 2003 Author Share Posted December 21, 2003 Shawn, I always thought the 902s were rated at 35 watts, but 15 is correct. Tom Brennan says the 902/802/806 drivers on 511Bs will handle a 1st order crossover at 500 Hz nicely. So, I'm going to use a 1st order at 650, an L-pad to drop the 902 from 106 to 104 dB and a parallel notch filter at 200 Hz to suck out the mid-bass hump I hear and you showed. I should get a nice looking impedance curve and a really flat FR. Quote Link to comment Share on other sites More sharing options...
sfogg Posted December 21, 2003 Share Posted December 21, 2003 John, " will handle a 1st order crossover at 500 Hz nicely." When I was running the 902s with the ALK type A it was crossing them at 400hz first order and even there they weren't complaining. Though the very deepest stuff was being additionally rolled off fourth order at 60hz which probably helped a bit. Shawn Quote Link to comment Share on other sites More sharing options...
JohnA Posted December 21, 2003 Author Share Posted December 21, 2003 How have you padded down the 902? I have considered calling UTC and asking about an autoformer with 2 dB steps. I don't want to use the T2A/3916 because it would have the 902 running at 103 dB. I have found 1 dB is audible as sort of a "flavor". It may not be clearly identifiable, but there nonetheless. My brother's constant impedance Type A is a good example. The tweeter circuit runs about 1 dB hotter than my Type AAs (less losses) and his La Scalas are brighter sounding. I might not notice until I hear music I know. I want to keep the forward, "in the room" sound La Scalas have. Quote Link to comment Share on other sites More sharing options...
WMcD Posted December 22, 2003 Share Posted December 22, 2003 Dear John, I think one of my ham radio handbooks might have some info, if you are very concerned. I could look it up. My overall hazy engineering view is to consider an L pad with 3 dB attenuation. There, half the power would go into the resistors and half would go into the load (driver). That gives us some idea of the maximum power disapated by any one resistor. In view of that approximation, 5 watt or 10 watt resistors would be enough given rational drive levels. Gil Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted December 22, 2003 Share Posted December 22, 2003 John, If you give me the resistor values you plan to use in the attenuator I can put them into the computer and give you EXACT power losses in each resistor with any input level. I can also synthesize a constant impedance highpass filter at 400 Hz with a notch at 200 Hz. Al K. Quote Link to comment Share on other sites More sharing options...
Q-Man Posted December 22, 2003 Share Posted December 22, 2003 John, Do you know what your getting into? These little improvements in sound quality that you may hear are going to lead you down a long and winding road. It serves you right though, your one of the ones that made me start tweaking a few years ago. I finally settled on a combination that I can live with. Good luck. Quote Link to comment Share on other sites More sharing options...
sfogg Posted December 22, 2003 Share Posted December 22, 2003 John, "How have you padded down the 902? " Using the autoformer, it is on taps 2-0. I'm not sure exactly how much attenuation that is through the autoformer though since the driver is 8 ohm instead of the 16 ohms Al's chart is for. "I have found 1 dB is audible as sort of a "flavor". It may not be clearly identifiable, but there nonetheless." Human hearing can detect changes down to about 1/3 of a dB. Like you said it won't necessarily be heard as a change in volume but it will influence the reported sound quality and can be reliably detected. Shawn Quote Link to comment Share on other sites More sharing options...
JohnA Posted December 22, 2003 Author Share Posted December 22, 2003 Oh Yes! It serves me right! I've been putting this off, but it's Christmas Vacation time and I need something FUN to keep me busy (besides honey-dos). The 511Bs will stay on top of my unmolested La Scalas, but I will have another crossover in the back that will allow it to work. One of the nicest changes I expect is losing the timing error between the tweeter and squawker. I did not consider it a real problem (and don't think it too bad now) until i got my upgraded ACT-3. It now allows the distance to all speakers to be set, even the subs. The result is a focus or tightening that I didn't have before and didn't have after until I finished the setup and measured all of the distances. I still don't know how I'll damp that 511B. I could have a box made and stained to match and screw it into it. That would hide what ever I did. Quote Link to comment Share on other sites More sharing options...
sfogg Posted December 22, 2003 Share Posted December 22, 2003 John, "I still don't know how I'll damp that 511B. " One of my 511B is a later one that Altec cut the welds on the fins and put a rubbery substance in the resulting gaps. It also doesn't have the small vertical stiffener near the throat of the horn. Just tapping that one compared against the fully welded 511Bs is a big difference. It still rings but only very briefly. As you know the welded 511s ring for a long time. I'm going to be doing that to the rest of my 511bs. Shawn Quote Link to comment Share on other sites More sharing options...
WMcD Posted December 22, 2003 Share Posted December 22, 2003 John, Are you assuming an 8 ohm or 16 ohm driver, or something else? Gil Quote Link to comment Share on other sites More sharing options...
JohnA Posted December 22, 2003 Author Share Posted December 22, 2003 Hi Gil, My 902s are 8 ohms (902-8B). Is that what you are asking? Quote Link to comment Share on other sites More sharing options...
WMcD Posted December 23, 2003 Share Posted December 23, 2003 Try: www.webervst.com/lpad.htm Quote Link to comment Share on other sites More sharing options...
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