Cathode Follower output cap
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 27th August 2010, 03:58 AM #1 alexg   diyAudio Member     Join Date: Jan 2005 Location: Philippines Cathode Follower output cap In most cathode follower schematic on the net, the output capacitor is usually around 2.2uf. Can these be lowered? Is the computation of output capacitor for cathode follower different the other topology? Thanks.
 27th August 2010, 04:19 AM #2 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland The size of the output cap is determined by the load and by the required LF rolloff by the usual formula (f3 = 1/(2piRloadC). For a load resistance (e.g., power amp input impedance in the case of a CF output of a preamp) of 100k, a 2.2u cap gives a LF -3dB point of 1/(2pi*2.2E-06*100,000) = 0.72 Hz. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 27th August 2010, 04:21 AM #3 TheGimp   diyAudio Member     Join Date: Aug 2009 Location: Johnson City, TN The output capacitance is dependent on both the driving resistance (impedance) and load resistance. If you lower it, you effect the low end transfer characteristics (that is, you raise the low end cut off, -3db point, etc). You need to specify the load before you can calculate the output capacitance. (SY beat me to it with a more detailed answer.) Last edited by TheGimp; 27th August 2010 at 04:31 AM. Reason: SY beat me to it.
 27th August 2010, 05:22 AM #4 DualTriode   diyAudio Member   Join Date: Aug 2009 Location: Sacramento Hello, Remember there is more going on at that -3db roll off point than lower output. There is also phase angle shift happening. Minus 3db is a JND just noticeable difference. The phase angle is 45 degrees off as well. Take a look at Douglas Self’s’ books, he documents an increase in distortion near roll off. Capacitor caused distortion. Good reasons to select capacitors for roll off well below audible range frequencies. DT All just for fun!
 27th August 2010, 08:41 AM #5 alexg   diyAudio Member     Join Date: Jan 2005 Location: Philippines @Sy, thanks. I thought CF follows a different formula. I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.
 27th August 2010, 11:13 AM #6 SemperFi   diyAudio Member   Join Date: Jan 2002 Location: nowhere That's what's called a 'rumble filter' and was common in the old days when vinyl was the norm. I have the same 'problem' in my system, but not worried about it. (I don't use a rumble filter).
Brit01
diyAudio Member

Join Date: Jul 2008
Quote:
 I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.
I had this issue with my SS power amp.

I bought 2 devices which fit on the RCA inputs of the amp which filter out frequencies below a specified level. I chose I think 20 or 30Hz. can't remember the company right now.

Nice pieces, didn't effect the sound at all and solved the woofer wobbling instantly. You won't hear bass frequencies below this level anyway or your speaker won't be able to produce them.

SY
On Hiatus

Join Date: Oct 2002
Location: Chicagoland
Quote:
 Originally Posted by alexg I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.
Yes, you can, but you don't want to overdo it. I wouldn't set the f3 much higher than 2 Hz for the reasons DT mentioned. You may want to dig a bit deeper to see what the LF noise is and try to eliminate it at the source. Possible causes include power supply (poor regulation), heater supply (ditto), heater-to-cathode leakage, and the tube itself.
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"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."

 27th August 2010, 12:38 PM #9 DF96   diyAudio Member   Join Date: May 2007 Valve amps with feedback often have an LF response peak around a few Hz. This is caused by the accumulation of LF phase shifts bringing the feedback loop near to instability. This is in the same region as record warps, arm resonance (with a poor cartridge-arm match) and mains voltage variations. Two options: stop the subsonic stuff from getting in by using a smaller capacitor, and accept the resulting phase shift; reduce the feedback by increasing the feedback resistor, which will increase the amp sensitivity but reduce the LF peak. First thing to do is confirm the source of the subsonic: is it coming from the cartridge or the phono preamp? Do you get cone flap when you are all set up for LP, but the stylus is not actually touching the record?
kevinkr
diyAudio Moderator

Join Date: Sep 2004
Location: Boston, Massachusetts
Quote:
 Originally Posted by DualTriode Hello, Take a look at Douglas Self’s’ books, he documents an increase in distortion near roll off. Capacitor caused distortion. Good reasons to select capacitors for roll off well below audible range frequencies. DT All just for fun!
Practically speaking this is mainly applicable in electrolytic capacitors which have a variety of distortion generating mechanisms (voltage coefficient, etc) that good film caps do not.
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Last edited by kevinkr; 27th August 2010 at 06:00 PM.

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