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Old 12th March 2008, 04:51 PM   #1
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Default PFC in car amplifier

Nowdays, SMPS (offline) usually equipped with PFC, even a small notebook SMPS.

Is it makes sense to put PFC for car amplifiers SMPS?
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Old 13th March 2008, 11:35 AM   #2
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PFC is ment to force the current waveform to follow and look like the voltage waveform(usualy sinewave) but a battery is constent DC, current can be drawn anytime with out a problem.

I hope this makes sense, basicly PFC is for AC only not DC.
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Old 13th March 2008, 12:37 PM   #3
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There's no need of PFC for DC sourced SMPS?
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Old 13th March 2008, 03:13 PM   #4
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Not in the least.

PFC is required to produce a better harmonic spectrum back to the AC power system. It makes a computer power supply look more like a resistive load than the typical capacitor input power supply. Key element is that a PFC is an AC/DC converter.

PFC is not applicable to a DC system. The 14.4V from your battery goes right to the bulk input storage on the SMPS. If you were using an AC source, the PFC circuit would produce this DC voltage at the bulk input. Since you already have DC from the car, there is no need to use a PFC to convert AC into DC.

The SMPS proper then converts the DC into a specified output, say +/- 40V for the amplifier.
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Old 13th March 2008, 03:36 PM   #5
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Thanks for the info
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Old 14th March 2008, 01:36 AM   #6
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In car amplifiers LC input filters are still required in order to prevent too much high frequency currents from flowing through the power wiring, which is usually quite inductive and prone to produce strong stray fields that contaminate line and speaker signals.
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Old 14th March 2008, 04:50 AM   #7
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Hi, EVA,

Do you know how Rockford amps can get away without such 12V LC filter?
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Old 14th March 2008, 01:00 PM   #8
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I have a power supply out of an amp that also has no LC filtering. I've never noticed that it causes any noise issues.

http://i32.tinypic.com/2ik2i5w.jpg
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Old 14th March 2008, 01:29 PM   #9
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You may be talking apples and oranges here.

An LC filter, applied to "prevent too much high frequency currents from flowing through the power wiring, which is usually quite inductive and prone to produce strong stray fields that contaminate line and speaker signals" would have to be installed close to the battery post, in order to keep said currents out of the long power lead going from battery to the rear of the vehicle. In case you are wondering why you never see any of these LC networks installed at the battery, it's because they are unnecessary. Noise does not couple in a car that way, assuming shielded interconnects. Should you get coupling into speaker wire because you unwisely ran your power wire parallel to it, installing an LC filter anywhere in the system will do nothing to remedy the situation.

Now there are many amps which install LC filters inside the amp, right at the input. The Rockford Punch 45 is an example. The reasons for this are very different, though. The idea is to produce a low impedance source for the internal SMPS. The long power wire, especially at high frequencies, is inductive and results in poor performance of the SMPS. The LC filter isolates the car's system from the SMPS, providing for a low impedance local source suitable to supply the high frequency currents demanded by the SMPS. Most of this is done by the capacitor. The inductor both stores energy to keep the capacitor charged, as well as isolating the car's system from the SMPS in higher frequencies.

Maybe not the best description, but it works.
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Old 14th March 2008, 02:01 PM   #10
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So how much more effective is LC vs. just C?

I have a rather large inductor of unknown value with two 2.5mm wires in parallel. This came from a high current SMPS battery charger. Would there be any benefit to adding this to the input?

http://i31.tinypic.com/1111hxz.jpg
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