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"Often these things are not always fully engineered (calculated)." ...

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andyr

In Reply to: RE: A cigar is due! posted by clifff on January 16, 2012 at 15:47:21

Thanks for confirming the equation was right, clifff. I'm think of using a value that will deliver 1uF with whatever Rsource I have. :-))

So WTF is the "English Riviera"? I'm presuming the South coast near Brighton? :-)) (I was at Sussex Uni from 1970-73.)

Regards,

Andy

Follow Ups:

And sometimes they are - Jon Risch 20:45:44 01/17/12 (2)
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
- RE: And sometimes they are - Tre' 09:29:55 01/19/12 (0)
  Jon, I think you have made very important points but, I believe, you should have prefaced them with my point above.
  The bypass cap is not bypassing the Source resistor value alone but the Source resistor value in parallel with the Source impedance.
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  
  Edits: 01/19/12
- Thank you, Jon - a very interesting take ... - andyr 23:24:43 01/17/12 (0)
  It's actually not a 'standard' in-series coupling cap ... it's a bypass cap across the Source resistor of a JFET gain stage, in a phono stage. The original circuit had this (for reasons that I can't imagine) set to 51K - with a cap value of 100uF.
  
  In my investigations into the circuit (with the aim of making the phono stage sound better), I determined that about 10K gave a better current through the JFET and I was interested to know whether the cap value could be decreased.
  
  So now I will not lower it as much as I was thinking of doing! :-))
  
  Regards,
  
  Andy
RE: "Often these things are not always fully engineered (calculated)." ... - clifff 09:56:06 01/17/12 (1)
Bournemouth (Wimborne actually), mate. I can see a palm tree from my window.

Brighton is well east and for London refugees. Don't go there!

;-)

Cliff
- Hah! ;-)) ... - andyr 11:22:35 01/17/12 (0)
  Never went to Bornemuff.
  
  Regards,
  
  Andy

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