I've been meaning to post about the capacitor controversy but have been designing a valveless engine in CAD. Anyways, I took a break and maybe i can clear some confusion from the thread.
As you may know, capacitors used for coupling have a unique frequency response depending on the impedances of the source output and amp input. Attached is two files, a picture of the test circuit in Multisim 10 which was designed to mimic typical values for output and input impedances, and a PDF of the frequency responses of four typical valued capacitors. Input to test circuit was 10mV peak-peak with 15mV DC offset (values found in iPod nano 3G DAC).
To keep a frequency response absolutely flat in any given iPod/amplifier stage values ranging from 1uF to 33uF can be used with frequency responses of not worse than -0.01db for a 1uF. Values smaller than 1uF will begin to create a high pass filter around 200 Hz for the test circuit. Phase difference is irrelevant since both channels being coupled will be out of phase equally.
The use of
polarized electrolytics isn't recommended. It's simply because of their construction, when an AC voltage is presented to a polarized capacitor, the thin aluminum oxide that forms the dielectric on the positive plate is punctured by the reverse voltage which creates a short circuit between the two plates. This short allows the capacitor to pass any DC bias to the next stage which will be amplified with the AC signal coming from the source. Because the puncture of the dielectric (which is 0.025" thick) starts out small at first, it won't be noticeable until the voice coils overheat and fail. Considering the hundreds if not thousands of dollars some of you spend on headphones and speakers, amplifying DC bias isn't tolerable at all,
So don't use polarized capacitors for coupling.
Materials used in construction determine the qualities and capacitance of a capacitor. Obviously higher purity/quality materials will yield a higher quality capacitor. General choices in materials suitable for coupling capacitors are paper, plastic, and non-polarized electrolytic. Each have their strengths and weaknesses.
Electrolytics are cheap, small physical size for their capacitance, but have low temperature stability so their capacity is substantially reduced at lower temperatures.
Paper based capacitors are more temperature stable than electrolytics, but have less capacity vs physical size and are more expensive than electrolytics.
Plastic capacitors - any dielectric having "poly" in the name. These capacitors have very good temperature stability, are generally smaller than paper capacitors, but are more expensive than electrolytic and paper capacitors.
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