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Originally posted by RONM Why do some speaker makers use a 4 ohm design and others use a 8 ohm design?If 8 ohm is more efficient and easier to amplify what are the benefits of 4 ohm speakers ? 
Originally posted by Grandarf Why would 8 ohm speakers be easier to drive than 4 ohms?! Ohms is a unit for resistance, the greater the number, the more resistance. An amp able to feed 100 watts at 8 own would be able to feed something around 140 watts at 4 ohms. So an amp can feed more power to 4 ohms speakers than 8 ohms.. So its the other way around, 8 ohms are less efficient than 4 ohms. No? 
Originally posted by Az Barber Lower output impedance acts as a shunting resistor between the speaker terminals and increases control over the speaker cone. Simplified, this generally means a higher damping factor for a lower impedance amp. (Damping factor is a ratio between load impedance and the amp's output impedance at a specific frequency) Of course, the inductor in passive crossovers introduces additional resistance and makes the damping factor mostly pointless. 
Originally posted by Grandarf Why would 8 ohm speakers be easier to drive than 4 ohms?! Ohms is a unit for resistance, the greater the number, the more resistance. An amp able to feed 100 watts at 8 own would be able to feed something around 140 watts at 4 ohms. So an amp can feed more power to 4 ohms speakers than 8 ohms.. So its the other way around, 8 ohms are less efficient than 4 ohms. No? 
Originally posted by Rop 4 Ohm is not "easier" or "more difficult" to drive than 8 Ohm (and viceversa). It's all relative to what the amplifier was designed to drive. For an amp to deliver, say, 100 Watt into 8 Ohm a rail voltage of about 3035 Volt is needed. To deliver 100 Watt into 4 Ohm would take only around 2025 Volt is needed. That means when that first amp drives a 4 Ohm load at a 100 Watt it will have to burn that extra 10 Volt into heat, at 5 Ampere effective that means the amplifier has an extra 50 Watt per channel in heat to deal with. 
Originally posted by Rop A 4 Ohm amplifier does not have 4 Ohm as its output impedance. Neither does an amplifier for 8 Ohm speakers have 8 Ohm output impedance. For audio there is no impedance matching (where load, source, and transmission line would have the same impedance, common at high frequencies). All amplifiers, pretty much regardless of type or power rating, have a very low output impedance, generally a small fraction of an Ohm. Rob 
Originally posted by Morbius Rob, I'm afraid the above is somewhat misleading. <<SNIP>> Unquestionably, a 4 ohm load IS more difficult to drive than an 8 ohm load  it just plain takes MORE CURRENT to do so relative to an 8 ohm load. 
Originally posted by Rop A 4 Ohm amplifier does not have 4 Ohm as its output impedance. Neither does an amplifier for 8 Ohm speakers have 8 Ohm output impedance. For audio there is no impedance matching (where load, source, and transmission line would have the same impedance, common at high frequencies). All amplifiers, pretty much regardless of type or power rating, have a very low output impedance, generally a small fraction of an Ohm. Rob 
Originally posted by Rop Morbius, In the paragraph after the one you quoted I stated it takes more current for the same power at 4 Ohm vs. 8 Ohm. Not to keep whipping a dead horse, but there is not just one limitation (constant power, constant current) when it comes to amplifiers. Yes, their power supply has a finite capacity when it comes to delivering power. So with voltage fixed that means there's a limit to the current it can deliver. However, all amplifiers I'm aware of will deliver quite a bit more current than what they are rated for at an 8 Ohm load. They generally can drive a 4 Ohm load, but total dissipated power starts to be a problem. With, these days, 7 channels in a receiver of fixed size (and a need not to have noisy fans) power dissipation is a big problem. I stand by my statement that it's all about what the amplifier was designed to drive. Driving one vs. another load is not inherently more difficult. It is more difficult for any amplifier to drive a load (higher or lower) it wasn't designed for. As mentioned, I know of one design that was meant for driving a ribbon (less than 1 Ohm impedance). It could do that comfortably. Driving an 8 Ohm load would be a big problem for that amplifier though, it doesn't have the voltage to generate much power in that load. Rob 
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