cry Posted September 10, 2009 Share Posted September 10, 2009 Hi guys, 200KG of MDF later I've built my first set of speakers. Exciting and frustrating.... Given that it's my first attempt, I've taken the lower risk path and am using an active crossover (high chance I'd muck up a passive crossover But I have a couple of extra dB that I need to pad from the tweeter - and the pad needs to be capable of handling the 125W from the amp. I've done the calculations but I'm having problems identifying suitable resistors. Does anybody have good sources for high power resistors? Cheers, David Link to comment Share on other sites More sharing options...
ehtcom Posted September 10, 2009 Share Posted September 10, 2009 Hi David. Welcome to SNA. If your going down the active path, you should/may be able to adjust the output level for each output on your crossover. Or use amplifiers with input gain adjust. What crossover and amplifiers are you planning to use? We will also need pictures of your project Cheers, Earle. Link to comment Share on other sites More sharing options...
cry Posted September 10, 2009 Author Share Posted September 10, 2009 Cheers - I've actually been a lurker for a while buying bits and pieces from other folks. I figured that I'd better step up to the plate and try something harder Given that (a) I'm inexperienced (:o I'm cheap I've elected to reuse a Emotiva UPA-7 for the power amp and I've grabbed a CX3400 for the crossover. The cabinets are bonded sheets of 9 and 18mm MDF and the drivers are peerless HDS. According to WinISD this should be flat down to around 40 - if I can get the tweeter under control (810921). I've got the crossover at the full -6dB for the high end but it is still too overbearing (for my ears). I've got some time constraints so short term I'm looking just to add an extra -2dB and tweaking the x-over to taste. But the perfectionist in me would like to balance the drivers out so that no fiddling is required on the input. And I have a chief documenter taking pictures which should be uploaded at the end (provided she doesn't kill me for all of the "how does this sound to you?"s) Hi David. Welcome to SNA. If your going down the active path, you should/may be able to adjust the output level for each output on your crossover. Or use amplifiers with input gain adjust. What crossover and amplifiers are you planning to use? We will also need pictures of your project Cheers, Earle. Link to comment Share on other sites More sharing options...
ehtcom Posted September 10, 2009 Share Posted September 10, 2009 Hi David. What level is the low end set at? Link to comment Share on other sites More sharing options...
cry Posted September 10, 2009 Author Share Posted September 10, 2009 Hi David.What level is the low end set at? The low is set at +6dB - so in theory 4 x stronger than the high end. That actually has me scratching my head, as whilst the level adjust on the x-over does have an effect, it is no where near as pronounced I would expect.... The other thing in the back of head is that the woofers are Darcher's old MW-1077s. Old but the cones are in great nick - no sag...etc. Perhaps I should replace .... Link to comment Share on other sites More sharing options...
little blue penguin Posted September 10, 2009 Share Posted September 10, 2009 Rather than use a high power resistor to the speaker driver, wouldn't it be far easier to rig up a passive volume control between the Active Crossover and the Power Amp? This would allow you to adjust the level as required. Later you could replace the passive volume control with a fixed attenuator made to suit. Link to comment Share on other sites More sharing options...
cry Posted September 13, 2009 Author Share Posted September 13, 2009 Rather than use a high power resistor to the speaker driver, wouldn't it be far easier to rig up a passive volume control between the Active Crossover and the Power Amp? This would allow you to adjust the level as required. Later you could replace the passive volume control with a fixed attenuator made to suit. This would allow me to tweak the control rather than the x-over for the short term solution.....cheaper up front. Good idea. Running some more tests though - I've found some softness at the mid/high x-over point....might forgo the perfection and get an equaliser.... Link to comment Share on other sites More sharing options...
andyr Posted September 13, 2009 Share Posted September 13, 2009 But I have a couple of extra dB that I need to pad from the tweeter - and the pad needs to be capable of handling the 125W from the amp. The pad resistor does not need to handly 125w. That might be what your tweeter amp is capable of producing but I suggest the tweeter itself will draw a fraction of this. 125 watts into 8 ohms is 4 amps, continuous ... I would suggest the tweeter would melt with 4 amps going through it! Given that it's my first attempt, I've taken the lower risk path and am using an active crossover (high chance I'd muck up a passive crossover ) IME, active XOs normally have an in-built method of adjusting output for each band ... are at least the output of all other bands relative to one. Are you sure yours doesn't have something like a Bourne pot on the PCB? The gain of all the amps you are using the same should be the same. If your tweeter amp has higher gain than the other(s), it will feed a louder signal to the tweeter. Regards, Andy Link to comment Share on other sites More sharing options...
Hugh Dean Posted September 13, 2009 Share Posted September 13, 2009 Cry, as rule of thumb, if the amp is 125W, assume no more than 20%, that is 25W, will ever pass to the tweeter. Even that is optimistic, and would only occur with large orchestras, massed violins, upper octaves, and percussion to the max. So, 25W into a 6R voice coil would be 35Vpp, or 17.5Vp. That would be just shy of 3A peak, which the tweeter can certainly handle for short periods. So, your L pad will need to cope with a peak current of 3A peak. Let us assume you choose 2.2 ohms, a typical value. From Iexp2R, we have a dissipation at 3A of 9 * 2.2 = 19.8W. However, this peak will only occur for very brief periods, and resistors have a lot of thermal inertia. In truth, you'd probably see these sorts of peaks about 10% of the time, and even then they would be intermittent, not continuous. So you could derate it around 10 times, to 2W, and for good measure use a 3W resistor and never have a heat problem in 50 years use...... Incidentally, use a wire wound resistor if you can as they are more robust. It will have a small amount of inductance, but nothing like the inductance of the tweeter voice coil, so it will be insignificant. In any event, this would help roll it off over 100KHz, which is no bad thing. Hugh Cheers, Hugh Link to comment Share on other sites More sharing options...
Nigel Posted September 13, 2009 Share Posted September 13, 2009 Good thread. Thanks Andy and Hugh. I was thinking of replacing a wirewound with a metal film resistor on the primary side of my ESL step-up transformer. Do you think there is any point ? I suspect that I may not hear any improvement. Link to comment Share on other sites More sharing options...
Hugh Dean Posted September 13, 2009 Share Posted September 13, 2009 No, not on an ESL primary.... any amp driving a large electrostatic display sees a very large capacitance, upwards of 2uF on the ESL63, and a tiny amount of inductance from a WW resistor, considering also the inductance of the trafo, won't have any impact at all. In fact, the voltage rating of the WW will be higher, so best to leave it be. Hugh Link to comment Share on other sites More sharing options...
Nigel Posted September 14, 2009 Share Posted September 14, 2009 Thanks Hugh. Link to comment Share on other sites More sharing options...
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