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Audio discussions - friend over at your place?
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<blockquote data-quote="Ampdog" data-source="post: 21800" data-attributes="member: 144"><p>Mostly, Croak, but we must be careful here. With the abundance of very high frequencies all over the show these days, a shorted cable (or to whatever degree a loudspeaker load is an r.f. short) can also act as an antenna, depending on its length in terms of multiples of quarter-wavelengths of any r.f. I did encounter systems where this caused problems because of NFB (oops! - I must quickly add: The poor application of NFB.) Especially where there is a phase-compensating capacitor over the series NFB resistor, this can infrequently channel said r.f. to the power amp input stages, etc. Now this is somewhat theoretical (designers should by now be aware of this possibility), but as said, it does happen.</p><p></p><p>And yes; the vagaries of pre-amp designs ....</p><p></p><p>In my similar comment to Byrd's about combining channels for a sub-woofer (the quote seems to have disappeared somehow), combining resistors <em>can</em> be used without a summing amplifier <em>if the preceding pre-amp output impedance is sufficiently low</em>. In that case the fed-back signal from the unwanted channel will see such a low impedance at the end of (say) 100K, that signal rejection should be sufficient. But as we both know, anything is possible these days ..... (Now what about those "passive" pre-amps??)</p></blockquote><p></p>
[QUOTE="Ampdog, post: 21800, member: 144"] Mostly, Croak, but we must be careful here. With the abundance of very high frequencies all over the show these days, a shorted cable (or to whatever degree a loudspeaker load is an r.f. short) can also act as an antenna, depending on its length in terms of multiples of quarter-wavelengths of any r.f. I did encounter systems where this caused problems because of NFB (oops! - I must quickly add: The poor application of NFB.) Especially where there is a phase-compensating capacitor over the series NFB resistor, this can infrequently channel said r.f. to the power amp input stages, etc. Now this is somewhat theoretical (designers should by now be aware of this possibility), but as said, it does happen. And yes; the vagaries of pre-amp designs .... In my similar comment to Byrd's about combining channels for a sub-woofer (the quote seems to have disappeared somehow), combining resistors [I]can[/I] be used without a summing amplifier [I]if the preceding pre-amp output impedance is sufficiently low[/I]. In that case the fed-back signal from the unwanted channel will see such a low impedance at the end of (say) 100K, that signal rejection should be sufficient. But as we both know, anything is possible these days ..... (Now what about those "passive" pre-amps??) [/QUOTE]
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Audio and Video Talk
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Audio discussions - friend over at your place?
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