Small value caps (newbie alert) - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

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
Thread Tools Search this Thread
Old 18th September 2005, 08:33 PM   #1
diyAudio Member
Join Date: Sep 2005
Question Small value caps (newbie alert)

So, having done a repair job on my old Cyrus I am interested in getting a bit more understanding of how it works. Looking at the schematic here I can understand mostly what's happening. One thing I struggle with though is the function of the various small value capacitors.

The large value caps all make sense, but the small caps like C33, C35, C41, C47/C49 and most of all C1 don't seem to do anything useful at audio frequencies. Can someone enlighten me as to what purpose they serve?
  Reply With Quote
Old 18th September 2005, 08:51 PM   #2
diyAudio Member
darkfenriz's Avatar
Join Date: Jun 2004
Location: Warsaw
C33 is for radio frequency input filtering. Very usefull unless you want to pick up AM radio station. It also prevents the amp from being driven to its slew limit by too high frequency components for example from dirty DAC.
C35/47/49 make a frequency compensation. They prevent the amp from oscilation caused by total 180 degrees phase shift at 100s of kHz/ several MHz. 180 deg. of phase shift is the same as invertion for a sine, so negative feedback becomes positive. Frequency compensation is a filter within feedback loop, which makes the frequency of 180deg. phase shift to have less than unity gain.
If you want to learn more the keywords are: frequency compensation, bode plots, nyquist stability criterion, dominant pole.

C41 is unusual, most designers would leave this 'cascode' without it. This is for preventing the cascode connection from local ocsillations/instabilities at many, many MHz. This sometimes occurs, for reasons I mentioned about a month ago in a thread about casodes.

However you are right that these caps don't do anything useful at AUDIO frequencies themselves.

  Reply With Quote
Old 18th September 2005, 09:26 PM   #3
diyAudio Member
Join Date: Sep 2005
Thanks. Very helpful.

It's been more than 10 years since I last did any control systems so all the stuff about bode plots is a dim memory! Still I can see that the -ve feeback would turn in to +ve feedback at high enough frequencies.

What about C1 on the output?
  Reply With Quote
Old 18th September 2005, 10:12 PM   #4
diyAudio Member
darkfenriz's Avatar
Join Date: Jun 2004
Location: Warsaw
So-called Zobel filter, also for stability, at some topologies at least.
  Reply With Quote


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
Incoming....Newbie alert! TiiDM Introductions 2 23rd November 2007 08:54 PM
Advanced crossover setup (Newbie Alert) bezeek Parts 7 2nd January 2007 04:30 PM
Paralleling small value caps with large value PS caps xyrium Power Supplies 16 14th October 2005 04:06 PM
Modifying amp to accept different tubes (Newbie alert!) Pickledcreeky Tubes / Valves 10 14th March 2005 05:13 PM
JLH Class-A help needed! [newbie alert] w00t Solid State 13 30th September 2004 03:01 PM

New To Site? Need Help?

All times are GMT. The time now is 12:03 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2