BEGINNER: Stupid questions about wattage, crossovers, etc.

jdegelder

Member
2020-01-06 10:46 pm
Hi guys,

New here and wondering if someone can help me understand some very basic questions. I have done a bunch of reading on DIY speakers, but I think my foundational understanding of a couple of things is a little shakey, so apologies for the really stupid questions

1. How should I think about system wattage, generally speaking? Are there rules of thumb around how amp, crossover, and driver wattage relate to eachother? (Should one always be less than, equal to, or more than any other?)

1.5: What how does adding more drivers affect required input an outputs? Is a loudspeaker wattage calculated as a sum of all drivers, or by the most powerful driver? Ex. Lets say I have a loudspeaker with a 100amp 2 way crossover, and a 50 watt mid and a 20 watt tweeter. What happens if I want to add a 2nd identical mid? Am I now at 50+50+20= 120 watts? or am I still effectively at 70 watts but with higher resistance?

I understand the function of crossovers, but how should I think about designing systems with more than one driver at each range? I understand how one would wire up a 3 way system with one driver for each range, but what if I wanted to add a 2nd driver for each range, or I wanted, say, 1 woofer, 3 mids, and 2 tweeters. Do I need to add a 2nd crossover, or if I use the same one, how do I wire up the additional drivers and how will that change the usual factors (impedance, wattage, etc).

Again, I apologize for the proufoundly stupid questions. My understanding at this point is enough to recognize that these are very stupid, but not quite enough to know exactly why. That's the knowledge hump I'm trying to get over.

Thanks!
 

Galu

Member
2018-04-17 6:50 pm
  1. In theory you should match up the rms power ratings, but in practice a great deal of flexibility is tolerable.
  2. Adding drivers does not draw more power from an amplifier, it is the setting of the volume control which dictates how much power goes to the drivers. The greatest percentage of the supplied power will go to to the bass driver, less to the mid driver and even less to the treble driver. So the power rating of the bass speaker is the main indicator of the power handling of the speaker system.
  3. Doubling up mid drivers will alter the impedance in the midrange (e.g. two 8 ohm mids in series result in an impedance of 16 ohm). In that case, the values of the midrange crossover components have to be changed to match the new impedance (same for tweeters).
 

jdegelder

Member
2020-01-06 10:46 pm
  1. In theory you should match up the rms power ratings, but in practice a great deal of flexibility is tolerable.
  2. Adding drivers does not draw more power from an amplifier, it is the setting of the volume control which dictates how much power goes to the drivers. The greatest percentage of the supplied power will go to to the bass driver, less to the mid driver and even less to the treble driver. So the power rating of the bass speaker is the main indicator of the power handling of the speaker system.
  3. Doubling up mid drivers will alter the impedance in the midrange (e.g. two 8 ohm mids in series result in an impedance of 16 ohm). In that case, the values of the midrange crossover components have to be changed to match the new impedance (same for tweeters).


Thanks for your response! Super helpful. Follow up question on number 3: If a 3-way crossover is rated for 8ohm, does that mean 8 ohm on each "way" or range (8ohm bass, 8ohm mid, 8ohm high) or does it mean 8 ohm as the sum of all drivers connected to it?
 
Thanks for your response! Super helpful. Follow up question on number 3: If a 3-way crossover is rated for 8ohm, does that mean 8 ohm on each "way" or range (8ohm bass, 8ohm mid, 8ohm high) or does it mean 8 ohm as the sum of all drivers connected to it?

If you are referencing a prebuilt commercial crossover rated at eight ohms, please be aware that such crossovers most often are built around the assumption that each of the downstream driver's loads (woofer, mid, tweeter) are actually 8 ohms at and everywhere around the crossover points. And also around the assumption that, were it not for the insertion of the crossover, the speaker's performance beyond the crossover's rolloff would be constant (when in actuality, it won't). If you pull up speaker driver specifications and look at the impedance curves, you will see that with rare exceptions, nearly all drivers have anything but a flat impedance curve, and that the "nominal" 8 ohms only holds over certain frequencies- sometimes very narrowly. Drivers frequently swing to much higher impedances at their low and high frequency operations. As soon as you are into that "non-nominal" impedance, then the behavior of the pre-baked commercial crossover is going out the window, fast. It's probably the rare exception that a combination of a pre-baked crossover and a particular combination of drivers is going to result in a neat, tidy crossover point that matches the pre-baked crossover's nominal crossover point. If you are lucky, the crossover point may just shift upwards or downwards, but not too radically, and you might still have a relatively flat overall combined frequency response. If you are unlucky, you could get really lousy results, or even damaged tweeters.

Designing and testing really good crossovers is an art unto itself, based in science, but requiring a lot of insight and intuition and experience, and probably most often some trial and error. I don't consider myself a skilled practitioner of that art, but from studying others' work, I'm gradually gaining insight into how the different considerations interact.

I'm not aiming to be a snob in this response, just to highlight something that is far from self evident to those first venturing into this, and that can be a stumbling block. No reason at all to not get your feet wet, try things, make mistakes, learn, adapt- just that you cant place a lot of reliance in off the shelf crossovers
 
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