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Why are capacitors at times connected

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Burningmace

New Member
They're usually for DC filtering on the speaker input. If you put a small capacitor in series with the speaker, it eliminates any DC in the input whilst preserving the audio signal.
 

crutschow

Well-Known Member
Most Helpful Member
They're usually for DC filtering on the speaker input. If you put a small capacitor in series with the speaker, it eliminates any DC in the input whilst preserving the audio signal.
A cap to block DC is usually only at the amp output. I've never seen one on the speaker for that purpose. And the capacitor has to be large enough to pass the lowest frequency of interest without significant attenuation into the low impedance of the speaker. This requires a large capacitor for the lower frequencies.

Any capacitor connected directly to a speaker is for crossover frequency division between speakers. In inexpensive speakers this may just consist of a capacitor in series with the tweeter to roll-off the lower frequencies.
 

Burningmace

New Member
Sorry, I misunderstood. I was thinking of speaker units that have an internal amplifier and that accept an external signal from a PC or stereo system.
 

Bob Scott

New Member
Why are capacitors at times connected to the terminals of speakers and how do they vary in their different sizes
If the capacitor is in series with the speaker, it may be there as a DC blocker and/or a high pass filter. The corner frequency where the lower frequencies will start to roll off can be calculated. You did not provide the speaker impedance. eg: 8 Ohms, 4 Ohms or 3.2 Ohms, etc. You did not specify the capacitor value in microFarads (uF). If it is several hundreds of microFarads or higher, it's just a DC blocker. If it is only a few (less than 10 uF), it's a tweeter high pass filter.

Use the equation to calculate the -3dB corner frequency of the high pass filter:

f = 1 / (2 * Pi * C * Xc) where:

f is the corner frequency where the signal is down 3dB (voltage down to 70.7% & power down 50%)
Pi = 3.1416
C is the capacitor's capacitance in Farads ( 1uF = 0.000,001 Farad)
Xc is the loudspeaker impedance in Ohms.

I have an old Sony STR110 receiver upstairs. Internally, its amplifier has 470uF capacitors in series with the output terminals for blocking DC. I can tell by using the above equation that, if I connect it to 4 Ohm speakers, the bass rolloff starts about 85Hz. Hardly acceptable for Hi-Fi with no bass below 85Hz!

Bob
 
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