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Old 1st February 2010, 05:17 AM   #1
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Default DC Blocking

All,

everyone puts a very large emphasis on protecting speakers via some DC blocking mechanism (be it a DC detection circuit and a relay or a DC blocking cap).

I am trying to avoid either for simplicity and wonder this:

1. If I have 100W RMS speakers (around 150W peak), my power supplies can deliver at most 24V and my speakers are 8 Ohm nominal (i.e. DC impedance), than I do not see why I cannot simply say that the maximum current is 24/8 = 3Amps. Power = (3^2)*8 = 72W < 100W.

So my question is, can I not oversize my speakers a bit to avoid having to introduce any DC protection mechanism? I realize some of that 100W is going into the high end drivers, however I consider that to be a minimal amount of power compared to the low end drivers (the ones who will see the DC and the ones who contain most of the power rating).

Regards,

James
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Old 1st February 2010, 05:18 AM   #2
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O yes, and I am thinking passive crossover stereo speakers. Therefore the high and mid range drivers will not see the DC.
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Old 1st February 2010, 05:26 AM   #3
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It doesn't take much DC to burn off even very robust speakers, and DC on the output also upsets the equilibrium of the speakers so that they will always give non-linear (and very annoying) distortion if present in more than miniscule amounts.
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Old 1st February 2010, 06:38 AM   #4
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It seems your perspective is based entirely upon a DC world. Impedance relates to AC (I believe "DC Impedance" is an oxymoron), and the impedance of a speaker driver varies dependent on the frequency of the AC imposed upon it. Around the driver's resonant frequency the impedance is highest. A nominally 8-ohm woofer, let's say, probably has a DC resistance of under 2 ohms. It may have as high an impedance as 15 or 20 ohms at its resonant frequency (say, 45 Hz), and as low as 3-ohms at, say, 20 Hz. Like @Saturnus, I think you should avoid DC at all costs, as even if the voice coils aren't melting you're pushing the driver into non-linearity.
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Old 1st February 2010, 12:48 PM   #5
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The DC resistance of nearly all 8 ohm woofers is between 5 and 7 ohms.

Some DC can actually improve or degrade linearity depending on driver design.

DC protection is intended to prevent speaker damage when the amplifier fails resulting in the output sticking to one rail and you are not there to turn it off quick enough. A basic DC protection would just cut power to the amplifier in case a few volts of DC are detected. Actually, the criteria used to design it is usually the opposite, to allow full output down to 20Hz without activation (and to require a few volts, like >3V, because less DC is not that dangerous).
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Old 1st February 2010, 09:36 PM   #6
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Quote:
Originally Posted by Eva View Post
The DC resistance of nearly all 8 ohm woofers is between 5 and 7 ohms.

Some DC can actually improve or degrade linearity depending on driver design.

DC protection is intended to prevent speaker damage when the amplifier fails resulting in the output sticking to one rail and you are not there to turn it off quick enough. A basic DC protection would just cut power to the amplifier in case a few volts of DC are detected. Actually, the criteria used to design it is usually the opposite, to allow full output down to 20Hz without activation (and to require a few volts, like >3V, because less DC is not that dangerous).
Alrighty, it seems to risky to let it slide. I'm just going to design in a DC detection relay toggle.
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