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Old 4th July 2009, 01:50 AM   #191
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Hi everyone, having fun? Newbees, it is not easy to understand what is going on here, I recommend easier threads. There are plenty to chose from. Let the rest of us try our best to be productive in this difficult environment.
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Old 4th July 2009, 02:38 AM   #192
iko is offline iko  Canada
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Are you serious? I hope this is a joke, because such terrible attitude doesn't look good on anyone, let alone someone who demands some sort of respect.
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Old 4th July 2009, 03:40 AM   #193
anatech is offline anatech  Canada
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Hi Pavel,
Yes, I understood that. So far, I have had no reason to use feedthroughs in an audio design. Once I can worry about super low noise levels, I would choose carefully. For use in a preamplifier, I imagine that the higher impedances might demand a careful choice in dielectric.

I am somewhat confused about some of the descriptions for the feedthrough. What I'm used to is a wire with distributed capacitance, usually in the 10s of pF. Possibly up to 1 nF maybe (power supply feeds?). For higher impedance audio connections I would be thinking of an L-C section maybe. Something designed to kill 100 KHz (as a starting guess) and up. Since I have not used feedthrough caps for any audio signals, it's all guesswork on my part.

When values in the neighborhood of a few F are being talked about, I start thinking of coupling capacitor that mounts through a bulkhead. Why? Isn't that an unnecessarily expensive component compared to a standard feedthrough and a standard coupling capacitor? Not only that, but the current trend is to eliminate all capacitors in the audio path, and a coupling cap would seem to buck the trend.

Hi John,
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PMA, I don't mind sharing, but I wish that I COULD get in a word or two on this thread.
I don't think that's a problem for you! Just start banging away on your keyboard.

-Chris
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Old 4th July 2009, 04:36 AM   #194
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Fellow audio loving adults, I do hope that we can go forward with some new ideas on this thread as it has continued, almost without break for 3 years or more, and we have discussed a few new ideas over this time. I do not wish it to be too difficult for those who are calculus challenged, yet not too easy, as if it were a high school class. There must be a happy medium, if it is to be productive. (Is this better?)
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Old 4th July 2009, 06:09 AM   #195
PMA is offline PMA  Europe
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Chris, I speak about feedthroughs on DC PSU wires, wires that interconnect 2 separate shielded spaces in a milled box. No one uses them in audio, AFAIK. I do, now.
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Old 4th July 2009, 06:21 AM   #196
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PMA there is nothing intrinsically wrong with feedthrough caps, and they have been around in laboratories since the very beginning of radio, I am quite sure. Jack Bybee, once showed me some 10 uF Sprague feedthrough caps taken from a defunct radio station more than a decade ago. The big problem with feedthrough caps is the material used to make the capacitance. It is usually ceramic, and I don't think this is good.
Now, do feedthrough caps do what they are supposed to do? Yes. Do we really need them in order to have a relatively quiet interior environment in a typical low RF environment, such as a home? I am not sure. Is the tradeoff between adding ceramic to the power supply buffering worth the relative purity of the power supply without the ceramic added, worth it? That is up to each one of us. I don't think that I would be comfortable living in such an RF environment, but when I was young, about 45 years ago, there was a radar pulse that would overwhelm virtually every audio system about every 15 seconds from a mountain above us. It might help in that case.
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Old 4th July 2009, 04:56 PM   #197
Blues is offline Blues  United States
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On my living room couch's sweet spot there are 2 AT&T cellphone RF signals at 850MHz and 1.9GHz (GSM & UMTS). Levels are at -47dBm and -62dBm respectively.

I'm probably well done by now...
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Old 4th July 2009, 05:08 PM   #198
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That's interesting, I wish that I could measure that high. This might imply that PMA is RIGHT in being concerned. I need some way to evaluate my own environment.
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Old 4th July 2009, 06:01 PM   #199
KSTR is offline KSTR  Germany
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Homes aren't low-RF zones anymore, I fear. Just the contrary.

I gave up listening to foreign radio stations (on MW) for just this reason, it is all drowning in noise from the cell-phone relay stations, SMPS's etc. At least in cities.

I now use my hand-held receivers as noise detectors (hint to everyone... you'll be surprised how much dirt for example a PC spits out through its openings and from any cables. The receiver properly positioned and tuned can tell you a lot).

I take Pavels recommendations very seriously, he knows this stuff *and* can measure it... on occasion I also recommended feedthtrough caps for speaker outputs here, which I consider as a must, today.

- Klaus
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Old 4th July 2009, 06:26 PM   #200
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How does RF impact speakers?
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