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Old 8th October 2012, 01:54 PM   #11
Pano is offline Pano  United States
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Amplifier source "resistance" or source "impedance"?
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Old 8th October 2012, 01:57 PM   #12
tvrgeek is offline tvrgeek  United States
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And as has been shown many times to be not that important. As you determine Qes', you will find any DF over about 20 is just not important. The tube camp seems to think 5 is about right as do the ultra low global feedback camp.

Passive crossovers do not degrade transient response any more than active do. Crossovers are FILTERS. Each driver does what it is told based on the filter transfer function and it's basic capabilities. Of course you are preventing the midrange from moving fast. That is what it does. "Speed" and "transient response" are totally bogus magazine reviewer audiophile BS. If your speaker lacks "impact" then it was designed poorly.

Now, active filters have the advantage of letting the amp deal with the ugly impedance problems of the drivers so they do not effect the crossover transfer function. They also have the advantage of allowing very complex equalization like a little bump, which is not easy in a passive. Don't forget to take into account any phase shift from the CD blocking caps that are still mandatory with a active crossover.
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Old 8th October 2012, 02:01 PM   #13
tvrgeek is offline tvrgeek  United States
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Originally Posted by Pano View Post
Amplifier source "resistance" or source "impedance"?
For the overly simplistic equation presented here, treat the impedance as resistance. The complex function is way the heck out of scope here. Over in the lounge are a few folks you know who are prepared to talk at that level. Of course they will tell you it does not matter anyway.
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Old 8th October 2012, 02:21 PM   #14
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I read all the time that active filter are better because the transfer function doesn't change with changes in the driver's Z due to a variety of dynamic effects. Again, that is not an accurate statement. The thing that is overlooked is that this is limited to the voltage transfer function of the filter. But in reality, what determines the acoustic output of a driver is the current flow through the VC. Even in that case where the amplifiers output Z is zero applying a fixed voltage transfer function across the driver terminals does not mean the acoustic output remains constant under dynamic conditions. The driver VC Z changes with time (heating) and excursion and these factors will affect the current and therefor the radiated SPL. Active can make a lot of things easier, and I and a fan of actives. But many proponents of active make arguments for them which are overly simplistic and ignore many of the real issues resulting form the dynamically changing VC Z.
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Old 9th October 2012, 12:53 AM   #15
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Actually, Q of the driver never changes. What changes is Q of the system composed of the driver, any series resistance, and Rg of the amp.
That's understood. However if I wanted to convey that perspective, then I'd have to come up with a new symbol like Qe-sys and I'd rather stick to the original T/S terminology as much as possible.
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Old 9th October 2012, 12:58 AM   #16
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That's understood. However if I wanted to convey that perspective, then I'd have to come up with a new symbol like Qe-sys and I'd rather stick to the original T/S terminology as much as possible.
I am glad you understand that. Most people don't. The lack of considering the entire system, whether active or passive, is typically what leads to a lot of the claims about the superiority of active crossovers, many of which go by the wayside when the complete system is considered.
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