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Old 21st October 2004, 08:10 PM   #1
AndrewT is offline AndrewT  Scotland
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Default where to place active filters?

Hi all listeners(readers?),
what are the pros & cons for locating the active filters for a 4 way stereo speaker system.
a. one big box with 8 filters, psu, interference suppression etc, located next to the preamp.
b. 2 boxes each contained half stereo set as above but located near the monoblock power amps behind the speakers.
c. 4 little boxes with one filter in each plus one psu box, all five located at the back of their respective power amps. & repeat for other channel.
d. psu & filter inside each power amp with two inputs, flat and filtered.
I'm inclined to opt for d. but most manufacturers sell a.
are there any better options I have missed?
regards Andrew T.
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Old 22nd October 2004, 01:14 AM   #2
sek is offline sek  Germany
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Hi,

Regarding sound quality, I'd recommend keeping the number of interconnects/leads between (to and from) filter stages as low as possible, as they are prone to picking up noise and hum. This applies to audio signal lines as well as power supply wiring, so a single unit would be preferable.

Regarding cost, the number of enclosures and transformers is what greatly dominates the total project costs, so a single unit would also be the most economic.

This would opt for solution (a).

Solution (b) with only one case and one PSU more is 'nearly' as inexpensive and integrated as (a), but looks more beautiful.

Solution (c) is the one with the highest number of enclosures, and I would consider the single PSU to be more difficult to integrate into the system (thick wiring is difficult to get reliable, inexpensive and noise free at the same time).

Solution (d) is superior to (c) but a little difficult to get started, I think. You would have to fit a PSU and the filter into each amp enclosure, so that neither of them interferes with the other. Other than that, the maximum number of PSUs also doesn't let it appear attractive to me...

That said, of course most manufacturers sell (a).
Why should they offer 8 boxes and a PSU when customers only want a single functionality (have you ever seen someone selling a 'desintegrated' preamp with plenty of gear to modify, just because it connects to more than one source)? And of course using a single and readily prepared external unit is the only way for them to keep safety and warranty issues under control.

As a general rule in active filters, it is wise to keep noise away from low pass filter circuits (as induced noise is never filtered any more past the LPF.) and hum away from high pass filter circuits (as imposed hum is never filtered anymore past the HPF stage). Depending on your filter topology and circuit it could be overly complicated to separate LPF and HPF from each other, which could increase complexity (and thus cost and size) and bad influences on the signal path due to the 'too' many opamps (or whatever).

Both solutions (a) and (b) should be good - assuming proper design and construction - with (a) being the most economic and (b) the more beautiful if you don't want to have another unit under or next to your preamp.

Indeed, you could design the xovers matching your monoblock amps...

Just my couple of cents...

Which filter circuit do actually you want to use?

Sebastian.
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