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Old 25th June 2015, 11:42 PM   #1
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Default Blocking cap for subs

Should I install a blocking cap in series with my subwoofers to protect them? How many volts are appropriate? The speakers (Peavey LO Max) are rated at 1200 watts continuous. (ill be using a pair)The amp is rated (at four ohms bridged) @ 2200 watts RMS. I can figure my values myself if someone will give the necessary formulas.
m
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Old 26th June 2015, 12:12 AM   #2
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I can't answer your first question.
Ohm's law answers the second: V=I/R.
Last is: capacitance (farads) equals the reciprocal of 6.28 times the cutoff frequency times the speaker impedance, or C=1/2piFR.
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Old 26th June 2015, 12:25 AM   #3
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Thanks I had forgotten the math......
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Old 26th June 2015, 12:29 AM   #4
Sonce is online now Sonce  Macedonia
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Do not use a blocking capacitor, you need an active (electronic) high-pass filter with at least second-order (12dB/octave) slope, at about 30 Hz.
Do not bridge the amplifier, connect one loudspeaker to one amplifier channel, and the other loudspeaker to the other amplifier channel.
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Old 26th June 2015, 12:32 AM   #5
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why would I not bridge the amp? it is rated at stated output at four ohms...I am using two 8 ohm cabs.
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Old 26th June 2015, 01:09 AM   #6
Sonce is online now Sonce  Macedonia
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Better stability, less heating of the amplifier... unless you have to squeeze the last watt from the amplifier. Nothing wrong with bridging the amplifier with the 4-ohm load if the amplifier is a good quality, though. Amplifier with lesser quality struggle with 4-ohm load when bridged.
Which make and model is the amplifier?
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Old 26th June 2015, 01:17 AM   #7
freddi is offline freddi  United States
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a blocking cap could be useful to protect in advent of an amp dumping its rail - but it can alter the impedance of some systems and effectively give a useful "boost" (sized correctly) - or unwanted bump and excessive cone excursion

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Old 26th June 2015, 01:19 AM   #8
infinia is online now infinia  United States
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contact Peavy factory technical support for bridging recommendations. Some amplifiers are set up for bridging but not really ready for prime time, at max power, stuff can break.
good bi-polar blocking caps at that power level are not easy to find, again contact the factory concerns for protection advice. IDK they should know better than most all opinions. back to back electros can do in a pinch but I wouldn't.
Polarized power electros want to have bias voltages to fully form the dielectric.
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Last edited by infinia; 26th June 2015 at 01:21 AM.
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Old 26th June 2015, 01:30 AM   #9
djk is offline djk
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Will be almost impossible to get a high-current capacitor for PA use.
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Old 26th June 2015, 02:02 AM   #10
Sonce is online now Sonce  Macedonia
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Quote:
Originally Posted by freddi;4370548
a blocking cap could be useful to protect in advent of
an amp dumping its rail * but it can alter the impedance of some systems and
effectively give a useful "boost" (sized correctly) * or unwanted bump and
excessive cone excursion
A blocking capacitor is useful only with sealed designs with Qtc=1.1 or so. Low
Qtc designs with less than 0.7 will show SPL loss of 2 or 3 dB (see the simulation
above) in the important bass range from 70 to 150 Hz (gain below 70 Hz is OK).
The majority of powerful PA 18" subs are vented designs, so blocking capacitor is not a good solution here. More appropriate here would be a line-level capacitor filter (first order).
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