And sometimes they are - Jon Risch - Propeller Head Plaza

Propeller Head Plaza

Technical and scientific discussion of amps, cables and other topics.

And sometimes they are

In Reply to: "Often these things are not always fully engineered (calculated)." ... posted by andyr on January 16, 2012 at 23:56:39:

The value of a coupling cap does seem to be a straight forward kind of calculation, and on first glance, seems simple enough: get the LF corner down to the bottom of the audio band (or just below), and all is well.

However, upon further thought, it is NOT that simple.

1st, the phase shift at the -3 dB point is now at 45 degrees off of the original signal, and this tends to smear the time domain somewhat.

Most equipment has more than one coupling cap. If it has ten coupling caps with a - 3 dB point at a nice low 5 Hz, it will be approx. -30 dB down at 5 Hz, -10 dB down at 10 Hz, and -3.3 dB at 20 Hz.

Most recording equipment has WAY too many coupling caps in them, and not all of them are set that low. Some might be multi-pole filters that RING.
So the playback equipment must minimize any further roll-oof and associated phase shift.

So giving yourself some extra lee-way is just a smart thing to do.

Finally, most of the ill things attributed to coupling capacitors in terms of capacitor dielectric aberrations and distortions happen when the capacitor is being almost fully discharged and charged up, that is, operation below the cut-off frequency. If you make the cut-off frequency extremely low, you minimize this aspect of the capacitor distortion.

Realize that for vinyl LP playback, the fundamental record warp frequency is 0.55 Hz, and any coupling cap LF roll-off's must be well below this frequency in order to minimize the above aspect I just mentioned.

Live sound has near DC transients in the form of mic thumps, stage footfalls, etc.

Not so simple any more........
Jon Risch

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Follow Ups Full Thread

Topic - Pinging EEs - cap rolloff equation?? - andyr 19:34:44 01/14/12 (10)

RE: Pinging EEs - cap rolloff equation?? - Tre' 11:35:41 01/18/12 (2)

Thanks, Tre ... - andyr 13:52:44 01/18/12 (1)

RE: Thanks, Tre ... - Tre' 09:20:41 01/19/12 (0)

A cigar is due! - clifff 15:47:21 01/16/12 (6)

"Often these things are not always fully engineered (calculated)." ... - andyr 23:56:39 01/16/12 (5)

And sometimes they are - Jon Risch 20:45:44 01/17/12 (2)

RE: And sometimes they are - Tre' 09:29:55 01/19/12 (0)

Thank you, Jon - a very interesting take ... - andyr 23:24:43 01/17/12 (0)

RE: "Often these things are not always fully engineered (calculated)." ... - clifff 09:56:06 01/17/12 (1)

Hah! ;-)) ... - andyr 11:22:35 01/17/12 (0)

Follow Ups

And sometimes they are - Jon Risch 20:45:44 01/17/12 (2)

RE: And sometimes they are - Tre' 09:29:55 01/19/12 (0)

Thank you, Jon - a very interesting take ... - andyr 23:24:43 01/17/12 (0)

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