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30th January 2013, 11:09 AM  #1 
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Join Date: Dec 2009
Location: Copenhagen

GK2 cutoff frequency for SS section
Hi Hugh,
When I created the SS output on my GK2, I used a 1 uF teflon cap. When we discussed this prior to my implementation, I remember you said it should just be between 1 and 10 uF. Just out of curiosity  what would the cutoff frequency ( 3 dB point) for the SS output be with the 1 uF cap? It is looking into the Ground Sound filter's 20 k input impedance. I do not feel there is a problem, but since I have already implemented a very steep (48 dB/octave) sub filter at 10 Hz in the Ground Sounds, there is no need for additional sub filtering, and if that turned out to be the case I would simply change the SS output cap value to ensure I do not "get too much of a good thing" Cheers, Jens
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30th January 2013, 01:08 PM  #2 
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Simple: the 3dB roll off frequency is when the reactance of the capacitor (1/(2 * pi *frequency * capacitance)) equals load resistance. In this frequency, phase angle between voltages becomes 40 degrees and voltage across loads becomes .7071 of the input voltage.
If you get a calculator, then F 0 1/(2 pi capacitance [microfarads] * resistance[megaohms]) will give frequency in hertz. Fortunately, this calculae is independent of capacitor brands and personal tastes. Is as is. Good luck.
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Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987. 
30th January 2013, 01:59 PM  #3  
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Quote:
Unfortunately, I'm not much of a mathematician and have never used a scientific calculator, so for me this is not simple. So what is "F 0 1"? And what "resistance" are we talking about? Cheers, Jens
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30th January 2013, 02:54 PM  #4  
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Quote:
lets consider a 1 microfarad ( µF, or 10E6) farad and a 20Kohms load. So the "corner" frequency, say, the frequency at which the signal after the cap is 3dB (.7071 times) respect to the signal before the cap will be: f = 1 / ( 2* pi * 20000 ohm * 10E6F) = 15.91Hz. Then, at about 16Hz, it is considered the lowest frequency your circuit will pass without "severe" attenuation and or distortion. It is independent if the load resistor is a single 20000ohm or two 40000 in parallel, or or two 10K in series, or any other combination of resistances that gives you 20000ohms as result. Aclareta: 1E6 means .000001 in scientific notation, or 1/1 million or 1 millionth part of any measure.
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Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987. 

30th January 2013, 04:09 PM  #5  
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Quote:
But I probably do not understand the notation ... Anyway, since we do not know the resistance, I guess we can just as well wait for Hugh Cheers, Jens
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30th January 2013, 04:44 PM  #6  
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Quote:
Then, in the numeric right keyboard type (or in the calculator drawing press with the mouse the number) 2, then the asterisc that operates as a multiplier sign, with the mouse click the "pi" button, the * again, then type the value of the resistance load in ohms, the *, then the capacitance in farads (Again, 1µF is .000001F). Then, press equal button. Once did it, press with the mouse, the 1/x button, and you'll get the value I post in the previous msg, (15.91xxxx) with lots of decimal numbers. In practice, only the first or two first digits are of interest. But, if you don't know the load impedance form the output of the cap to ground, all this thread is meaningless. Cordially, Osvaldo .
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Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987. Last edited by Osvaldo de Banfield; 30th January 2013 at 04:56 PM. 

30th January 2013, 06:26 PM  #7  
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Quote:
Not all in vain  once Hugh tells us what the resistance is, I'll use your calculation to adjust my cap value, if necessary Thanks, Jens
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30th January 2013, 06:38 PM  #8 
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Congratulations, guy!!!
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Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001. Argentine Ham Radio LW1DSE since 1987. 
30th January 2013, 07:53 PM  #9 
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Join Date: Nov 2003
Location: Melbourne (Oz, not Florida!)

Hi Jens,
There's an easier equation to remember/use  although it's a slight approximation. This is: 160,000 / R * C (where C is in microFarads). But, yes, this gives the 3dB rolloff of the RC pair but you need to remember that the effects of the rolloff (in terms of phase) will extend up to 10x the 3dB frequency ... so in Oswaldo's example, this would be 159Hz. Which is much too high. The output cap in the GK1/GK2 is 1uF, so when combined with the 43K input impedance of my Lifeforce/Maya amps, produces approximately 4Hz  much better! So, in your case  looking into the 20K input impedance of the Ground Sound filter  I would use 2.2uF as a minimum. Regards, Andy PS: Don't forget that if you are using the "Sub out" facility to feed your bass drivers or subs, you also need to put an output coupling here (whose value depends on the Zin of the amp driving them). Last edited by andyr; 30th January 2013 at 07:57 PM. 
30th January 2013, 08:46 PM  #10  
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Quote:
This whole thread is in fact about the "Sub Out" (= SS output) The capacitors I am talking about are on the SS output, as recommended by Hugh, but they may need to be a somewhat higher value anyway. Let's see what Hugh says! Cheers, Jens
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