Why do we need -70v cv for Fairchild?

[strangeandbouncy]

Hi Guys,

 
  Can someone enlighten me? I have ascertained that a Fairchild has a gain of 16dB with no compression. I have read several times on this forum that we need to be able to pull -70vdc for the control voltage. This I can understand if there is far more gain on tap to make it up, but I am not sure that -70v is the necessary to achieve only 16dB of GR . . . Have I missed the point? is it the CAPABILITY of running to -70v that is required to achieve the speed required? is this not a function of the slew-rate/rise-time of the control amplifier(in conjunction with the impedance of the load being driven)? I frequently push compressors WAY past 20dB of GR, but it is fairly pointless with the Fairchild, unless you use another line amp afterwards. I always found that the 660's I regularly used huffed and puffed like crazy if you hit 'em too hard, unlike many other compressors I know. (They might not have been in the peek of performance, especially considering the standard of maintenance generally at that particular studio!)

    I am not trying to be a smartarse, but I just don't get WHY we need such a powerful amp in the sidechain just to acheive a meagre 16dB of compression. Many times I have read that you need it, but I still don't get why!


    KIndest regards,



      ANdyP

 

[tardishead]

You need that kind of power because you are driving 8 grids per mono channel instead of 2 grids in a single 6386 design. The right power also ensures the correct attack time. I cloned a fairchild with a SS sidechain slightly underrated and  it can change the attack time considerably. PRR has gone deep on this on a few occasions. The attack time of the Fairchild is fast enough to kill more or less any spikes and fast transients. If you don't have the power and you try to emulate the same attack time you will end up with distortionor an inherently slower attack time. Attack time is affected by the impedance of the driving source - it has to be low. You need 10 watts or more to drive those grids fast enough for heavy limiting.

 

[strangeandbouncy]

Thank you for that,


  but what I don't understand is I am asking about voltage, not power! 6386 cuts off at @-30V according to my estimation from curve. We are not asking it to do anything like that, just 16dB of GR. If it is power we need to drive parrallel screen grids, surely we need more current, not more voltage? Especially a voltage that seems to me to be WAY greater than is needed.

  I am still stumped . . .


  AndyP

 

[tardishead]

There's a couple of amazing PRR posts that say it all

 

[strangeandbouncy]

Ok,


  thanks, I haven't found them yet! I'll keep looking.


    Kindest regards,



      ANdyP

 

[lolo-m]

6386 doesn't cut off at 30v but at more than 70v... Don't make any mistake in curve reading. Data sheets are DC and we're dealing with audio (AC). 20db audio gain reduction is around -50 v CV in a Fairchild. The power of the SC AMP determine how fast you can charge the cap of the RC Network and so your attack time.
EDITED

 

[strangeandbouncy]

Hi Lolo,


  thanks for reply. -70Vdc cut-off and @-50v for 20dB GR-That makes more sense.



      - Except I thought we WERE dealing with a dc control voltage? (hence rectification, and I guess, smoothing from the capacitor in the timing network . . .) I see that this voltage will not be static(obviously!), but is it ac? is a changing dc voltage ac? the current is not alternating(Aagh! I wish i could fathom all this out . . . )

  I can find a few PRR posts vv Fairchild, but not the one's I need . . . .



    Kindest regards,



        AndyP

 

[lolo-m]

You're right, I didn't mean this clearly:
- Audio is AC.
- CV is for sure DC.

You need that kind of power because you are driving 8 grids per mono channel instead of 2 grids in a single 6386 design. The right power also ensures the correct attack time. I cloned a fairchild with a SS sidechain slightly underrated and  it can change the attack time considerably. PRR has gone deep on this on a few occasions. The attack time of the Fairchild is fast enough to kill more or less any spikes and fast transients. If you don't have the power and you try to emulate the same attack time you will end up with distortionor an inherently slower attack time. Attack time is affected by the impedance of the driving source - it has to be low. You need 10 watts or more to drive those grids fast enough for heavy limiting.

Just a little correction on this post:
Number of grids does not eat power. Number of gris determine the maximum value of the resitor you must have for the grids. But this resistor will be used to discharge the cap ( release ) and so determine the cap yo will use to get the slower release. Then you know the cap you've got to charge to have the attack and so the power of the SCamp to charge it...
Read the PRR post here to understand this:
http://www.groupdiy.com/index.php?topic=31088.0

 

[strangeandbouncy]

Hi Lolo,



  thank you very much! Some clarity at last! AND the PRR post that was eluding me!


      Merci, Lolo!


        A bientot,


          ANdyP ;D ;D ;D

 

[strangeandbouncy]

Hi,


  If anyone is wanting a Link to another of PRR's most excellent explanations, here it is


    http://www.groupdiy.com/index.php?topic=11910.0


  Kindest regards,


      ANdyP