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In Reply to: RE: coupling cap -3db point posted by Tweaker456 on May 16, 2016 at 15:24:04
Nor can any system in any audiophile home reproduce 2Hz, which is a good thing, because it would make you sick. The issue is phase. The low pass filter effected by the coupling capacitor affects phase to some degree at frequencies 10X the -3db frequency. Thus, if your filter has a -3db point at 2Hz, the signal will achieve 0 phase shift at 20 Hz, a frequency at the low end of the audio spectrum. I think this is why most designers shoot for -3db at 2Hz. In a phono circuit, some would say that you want your -3db point to be higher in frequency so as to filter out turntable rumble and other extreme low frequency energy such as from LP warps. Furthermore, whether one can really hear phase shift is controversial.
Follow Ups:
What I wanted to know was if people can really hear it. One argument againt going so low is the excursion from a greater amount of low frequencies that cannot be heard. Some pretty smart folks like the 2hz number.
I thought I answered that question; you cannot hear it, partly because it is not there in any feasible audio system, and partly because you could not hear it if it WERE there, in which case you would FEEL it but not really hear it. OK?
If anyone told you that he or she can hear 2 Hz, either he was pulling your leg or you misunderstood the comment. Possibly the person could have meant that he or she can hear the effect (at 20Hz, for example) of having the -3db point at 2 Hz vs, for example, 4 Hz.
You are misunderstanding the question. It's the consequences of phase shift I'm talking about, phase shift that can be at 10 times the -3db point. Of course you can't hear 2hz.
I did offer that as an alternative interpretation of your question. And my additional comment is that if someone says they can hear something (anything), there is really no way to argue the point so long as we are talking about frequencies within the range of human hearing. So, if you're worried about phase shift at 20Hz, then make your -3db point as low as possible. That's the best anyone can do. However, the possible downside of that has to do with phono, mostly. Very low frequencies are spuriously generated in creating LPs (from the cutter head, or so I am told) and in playing them back (warps, turntable rumble, etc). If these spurious signals are sent to the line stage and downstream, they can sap amplifier power uselessly or muddy the speaker sound, or both.
I recently installed an output coupling capacitor on my phono section that probably creates a 2 Hz, or even lower, -3db point. I used that particular capacitor because it sounds so good at mid and upper frequencies, and I don't have a similar type in lower value so as to effect a higher bass cut-off. The result is that I think I am hearing some extreme low frequency junk with phono that is polluting the sound a bit. I am still trying to decide how to deal with that without spending more bucks on smaller value but equally good sounding output coupling capacitors.
Lew, Your comment about having to do mostly with phono make a lot of sense. The dac came with a unnecessarily high output cap that and I was wondering why even go to 2 hz if it was risky in terms of woofer excursion. Good explanation. Thanks, Tweaker
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