Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th August 2010, 02:58 AM   #1
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Default 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.
  Reply With Quote
Old 27th August 2010, 03:19 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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.
__________________
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it." Upton Sinclair
  Reply With Quote
Old 27th August 2010, 03:21 AM   #3
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
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 03:31 AM. Reason: SY beat me to it.
  Reply With Quote
Old 27th August 2010, 04:22 AM   #4
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!
  Reply With Quote
Old 27th August 2010, 07:41 AM   #5
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
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.
  Reply With Quote
Old 27th August 2010, 10:13 AM   #6
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).
  Reply With Quote
Old 27th August 2010, 10:18 AM   #7
Brit01 is offline Brit01  United Kingdom
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.
  Reply With Quote
Old 27th August 2010, 10:26 AM   #8
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by alexg View Post

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.
__________________
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it." Upton Sinclair
  Reply With Quote
Old 27th August 2010, 11:38 AM   #9
DF96 is offline DF96  England
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?
  Reply With Quote
Old 27th August 2010, 04:52 PM   #10
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally Posted by DualTriode View Post
Hello,
<snip>
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.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan

Last edited by kevinkr; 27th August 2010 at 05:00 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pentode cathode follower output impedance Bitrex Tubes / Valves 0 7th January 2010 02:19 AM
U-Follower: Output from Cathode or Plate? sgerus Tubes / Valves 5 23rd May 2007 08:30 PM
cathode follower vlljpior Tubes / Valves 6 22nd March 2005 07:50 PM
Question about direct coupling a anode follower into a cathode follower. G Tubes / Valves 45 29th July 2004 06:47 PM
Cathode follower Class A output stage? Circlotron Tubes / Valves 14 15th May 2003 11:11 PM


New To Site? Need Help?

All times are GMT. The time now is 05:38 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2