92b67022   
   From: ruffrecords@yahoo.com   
      
   Patrick Turner wrote:   
   > On Oct 22, 9:03 am, Engineer  wrote:   
   >> On Oct 21, 4:47 am, Ian Bell  wrote:   
   >>   
   >>   
   >>   
   >>   
   >>   
   >>> George Papadi wrote:   
   >>>> 1)I'm rebuilding my Theta mono tube amps&    need to buy quality,great   
   >>>> sounding, moderately priced film&    foil polypropylene 0.47uf 600/630   
   >>>> volt coupling&    by-pass capacitors. Does anyone prefer the sonics of   
   >>>> Audio Cap Theta series($12), Multi Cap PPFXS($13), Auricap ($22),   
   >>>> Mundorf ZN series($11) or Sonicap Generation I ($5). Cap length can't   
   >>>> exceed 1.3". For several reasons, leads must be solid, not   
   >>>> multistranded. On a price performance basis, I'm thinking Honda&    Lexus   
   >>>> rather than Mazeratti or Ferrari quality. Only the Auricaps seem too   
   >>>> costly. I realize that sonics are subjective but would appreciate your   
   >>>> recommendations?   
   >>   
   >>> When used a coupling capacitors there is no sonic difference because they   
   all   
   >>> have zero signal across them.  Use orange drops.   
   >>   
   >>>> 2)Need to replace resistors as well. Carbon film, carbon composition,&   
   >>>> metal films are in original circuits. Any recommendations?   
   >>   
   >>> Metal film every time.   
   >>   
   >>> Cheers   
   >>   
   >>> Ian   
   >>   
   >> Ian is quite right.  Incidentally, coupling caps work by NOT charging   
   >> (on the AC signal that is.)  I use orange drop caps and  metal film   
   >> every time, if I have them.  Else, whatever plastic caps of the right   
   >> specs I have to hand.  Don't use ceramic caps in audio as they are   
   >> alleged to be nonlinear against voltage... or, since they don't   
   >> charge, is that current? I use them for RF only.  Anyway, there's no   
   >> need to use them in audio as we have plenty of plastic cap choices.   
   >> Cheers,   
   >> Roger- Hide quoted text -   
   >>   
   >> - Show quoted text -   
   >   
   > Coupling caps do actually have an AC voltage across them. Their   
   > reactance becomes high as frequency becomes lower, and so there is   
   > some ac voltage always there although it is a negligible amount. If   
   > the cap is a plastic type the amount of THD or IMD generated by the   
   > cap being in the circuit is utterly negligible even at LF of say 20Hz   
   > when the signal voltage across the cap becomes considerable relative   
   > to the following grid biasing resistance.   
   >   
      
   Properly sized coupling capacitors will have practically zero AC across then at   
   the frequencies of interest by definition.   
      
   > AFAIK, plastic film or foil type caps have never been been the cause   
   > of any significant or audible distortion that the most golden eared   
   > audiophile can identify in a careful AB test. But some audiophiles   
   > still say caps affect the sound and that different brands of the same   
   > type of cap sound different, or better or worse, or whatever.   
   >   
      
   For the latest on this check out Doug Self's new book which demonstrates that   
   some film/foil caps are measurably better than others though the difference is   
   tiny OHMO. The place you really need to take cars is in frequency sensitive   
   circuits like RIAA where there will be considerable AC voltages across caps.   
      
   Cheers   
      
   Ian   
      
   > A change of input tubes in a preamp may cause a sound change even   
   > though THD<  0.01%, and artifacts of the tube should be entirely   
   > inaudible if below -80dB below signal.  Perhaps it is due to subtle   
   > microphonic effects.   
   > Just how plastic caps change the sound is not within my capability to   
   > explain.   
   > I just use generic 630V rated Wima polypropylene caps and nobody   
   > complains (very much).   
   >   
   > When a guitar amp is seriously overdriven the coupling caps do begin   
   > to get a Vdc charge that is greater than the Vdc because of the grid   
   > current flow. The increase is Vdc charge effectively increases the   
   > output tube grid bias so much on big bashes of signal that the OP   
   > tubes work in class C to make a near square wave to the speaker.   
   > Usually low C values are used, say 0.01uF with say a following Rg =   
   > 220k, giving a quick recovery from overload. AFAIK, the presence of a   
   > steady Vdc charge across coupling caps in amps well short of clipping   
   > or overload causes negligible THD/IMD.   
   > Electro coupling caps used on base circuits of bjts are regarded as   
   > sonic poison though.....   
   >   
   > Sure, use ceramics for RF bypassing. Their C value changes with   
   > temperature. I've not seen significant THD caused by them though, but   
   > I never use them in audio circuits unless I want to bypass something   
   > at RF, and the presence of a ceramic cap has such high reactance at AF   
   > that its loading/distortion effect on the AF is negligible.   
   >   
   > Patrick Turner.   
      
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