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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Jon Risch, Hi Efficiency Xover design considerations posted by Edp on March 8, 1999 at 15:54:47:
Edp,Crossover input impedance is not a big issue, it does need to be largely non-reactive, and reasonably high, so that multiple cabinets can be paralleled. That is more of an issue than maintaining a high overall impedance profile for high sensiitivity.
The crossover must avoid large phase angles, anything beyond about 40-45 degrees for very long can tax cheap PA amps, causing them to thermal, or outright blow up. Many bands overload the amp, trying to get max power by using a lower impedance total load, such as three 8 ohm cabinets on a 4 ohm rated amp channel!
Minimum impedance numbers for the motional impedance generally need to be around 70 to 75% of the nominal rated impedance, or it is pushing things too far. There is an impedance standard that pro's are supposed to use, but no one pays any attention, and rates any way they want to.
Many of the speaker companies that mine competes against use very loose interpretations of what constitutes a valid spec. For instance, one manufacturer rates their powerd speaker as having 300 watts of power for the woofer. Sounds like more than our 200 watts, right? Well, the 300 watt rating is into 4 ohms, and is Class G (non-continuous), while the speaker impedance is a nominal 8 ohms. So they deliver maybe 150 watts, or a little more for real. Another competitor is notorious for calling what should be marginally rated as a 6 ohm system, an 8 ohm system. Parallel two of these, and the amp better be another brand than theirs!
Driver sensitivity is the big one, and I work with 100 dB sensitivity woofers (1W, 1M), 110 dB plus compression drivers, and 98 dB cone midranges and 106 dB horn loaded mids.
With that in mind, the most important factor in crossover design is the use of low DCR inductors, for the woofer, so as to avoid loosing any more of the woofer output than absolutely necessary.
Typical DCR for my inductors is from 0.1 to 0.4 for the really large ones.
A proprietary air gapped laminated iron core design is used to achieve this. I recently redesigned them for 6-10 dB less distortion, depending on the size. Used to be below -40 dB, now even less. Minimum spec is 15 amps before saturation, and most of them will take over 20 amps before they even begin to look bad. This is so transients do not saturate the core, or cause inductive kick-back when it does hit hard saturation, which will destroy most amps. There is a lot of energy stored in the field of a 2.5 mH inductor when it collapses at over 20 amps!The issue is high power handling for the whole system, including making a nominal 40 watt continuous rated compression driver hang in there with over 400 watts rated continuous system input! Peaks are another matter, most of the systems/components can take about 10 dB more on peaks (that ten times the power level for those who left there log tables at home), and this is from a base of continuous operation at near rated power.
With the advent of 2 and 3 thousand watt amps, being run into a single speaker, or a pair of speakers, the entire system has to be designed to survive from the beginning. We had to finally go to a special protection circuit I helped design, in order to help the compression dirvers survive.
Amazingly enough, even though I have done systems that can handle 400W continuous, thousands of watts peak, they still manage to blow them up with sheer abuse.Ever wonder how loud a 100 dB sensitivity system gets at full rated power input? With enough clean amp power (over 3000W), around 128 dB at one meter on peaks. That allows for instantaneous power compression, cable losses, etc.
Hope this rambling was along the lines of what you were after.
Jon Risch
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Follow Ups
- Re: Jon Risch, Hi Efficiency Xover design considerations - Jon Risch 03/8/9920:47:38 03/8/99 (4)
- Let me restate my question for further focus - Edp 10:28:16 03/9/99 (3)
- Re: Let me restate my question for further focus - Jon Risch 18:29:12 03/9/99 (2)
- Oops! - Jon Risch 18:33:01 03/9/99 (1)
- Much thanks, filed away, whens the book anyway? - Edp 21:13:45 03/9/99 (0)
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