Unconditional stability, phase shift and feedback - diyAudio
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 11th November 2007, 07:50 AM   #1
diyAudio Member
 
Join Date: Nov 2007
Default Unconditional stability, phase shift and feedback

I was reading a bit of Morgan Jones and his comments on stability and amplifier designs. He compares the Quad amplifier with the Williamson for stability and explains that the Quad with just two stages was easy to stabilize whereas the Williamson with its preamp, concertina splitter and drivers was a considerable challenge.

Let's just say I have decided to build an amplifier with a triode voltage amplifier, a longtailed pair phase splitter and follow that with cathode follower drivers-or even regular plate loaded ones-and that I propose to put feedback across the whole mess. First, is there any reason this is particularly bad and secondly, is there an iterative procedure i should follow to determine how much feedback I may use and how to figure out all the poles and zeroes to stabilize it? I do want to accomplish a couple of objectives, I want a capacitor between the front end and the first input tube grid for DC blocking, and I want of course a flat bandpass between 20 and 20 kHz but I would particularly like to roll off the lows below 20 Hz.
  Reply With Quote
Old 11th November 2007, 09:08 AM   #2
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
You can improve the chances of stability considerably by using direct coupling between the first stage and the LTP and again between the cathode followers and the OP tube grids.
  Reply With Quote
Old 11th November 2007, 09:19 AM   #3
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
You can also simply remove the first stage by using penthode LTP

http://www.dissident-audio.com/PP_6L6/Page.html

Drastic !

Yves.
  Reply With Quote
Old 11th November 2007, 10:12 AM   #4
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
As Ray and Yves have both said, reducing the number of LF time constants by DC coupling and minimizing the number of stages is a Good Thing. You will have to adjust/optimize HF stability on test because it will largely be determined by your particular output transformer. Don't forget that HT supplies can form unwanted LF feedback paths - sometimes a regulator is a good thing. Since you don't need much current for the LTP (because it has cathode followers to buffer it from the output stage), regulation to that stage could be as simple as a couple of 150V neons.

Yves: Those are impressive 20kHz square waves on your 6L6 amplifier.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 11th November 2007, 03:02 PM   #5
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Don,

Quote:
I want a capacitor between the front end and the first input tube grid for DC blocking, and I want of course a flat bandpass between 20 and 20 kHz but I would particularly like to roll off the lows below 20 Hz.
Blocking DC is important. Even more important is protecting the O/P trafo, which is inside a NFB loop, from core saturation. A RC high pass filter tuned to approx. 19 Hz., at the I/P grid, gets both jobs done.

Quote:
Let's just say I have decided to build an amplifier with a triode voltage amplifier, a longtailed pair phase splitter and follow that with cathode follower drivers-or even regular plate loaded ones-and that I propose to put feedback across the whole mess.
What sort of O/P tubes do you have in mind? How much gain do you need? FWIW, my hunch is that the Mullard circuit using a 12AT7 in the LTP position will do the job for you. High gm is indicated for a LTP, especially in a circuit that employs loop NFB. High gm is protection against slew limiting.

If you decide to use voltage followers that are DC coupled to the "finals", I suggest you consider IRFBC20 MOSFETs for the job. The FET sounds good, requires no heater power, and (obviously) there is no heater to cathode potential issue. MOSFET Follies illustrates the technique.
__________________
Eli D.
  Reply With Quote
Old 12th November 2007, 02:21 AM   #6
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by Eli Duttman
Don,



Blocking DC is important. Even more important is protecting the O/P trafo, which is inside a NFB loop, from core saturation. A RC high pass filter tuned to approx. 19 Hz., at the I/P grid, gets both jobs done.



What sort of O/P tubes do you have in mind? How much gain do you need? FWIW, my hunch is that the Mullard circuit using a 12AT7 in the LTP position will do the job for you. High gm is indicated for a LTP, especially in a circuit that employs loop NFB. High gm is protection against slew limiting.

If you decide to use voltage followers that are DC coupled to the "finals", I suggest you consider IRFBC20 MOSFETs for the job. The FET sounds good, requires no heater power, and (obviously) there is no heater to cathode potential issue. MOSFET Follies illustrates the technique.
I was wanting to build an all triode amp just as a design exercise and not because I have anything against other devices. Would those FETs work in the form of a "Fetron" to replace the cathode follower triodes in the McIntosh 75, where there are harsh requirements?
  Reply With Quote
Old 12th November 2007, 02:22 AM   #7
diyAudio Member
 
Join Date: Nov 2007
Quote:
Originally posted by Yvesm
You can also simply remove the first stage by using pentode LTP

http://www.dissident-audio.com/PP_6L6/Page.html


Well, yes.
  Reply With Quote
Old 12th November 2007, 05:34 AM   #8
diyAudio Member
 
Join Date: Jan 2003
Location: Sweden
Quote:
is there an iterative procedure i should follow to determine how much feedback I may use and how to figure out all the poles and zeroes to stabilize it?
It seems to me that no one has answered this part of your question, as the answer is complicated I will not try to give you a complete answer but rather push you in the right direction. Yes, there are methods how to determine how much feedback to use and how to place poles and zeros in order to make an amp stable. In general an amp is unstable if the loop gain at some point has 180 degree phase shift because at that point the negative feedback will be positive and the amp will oscillate. The solution is either to use less feedback or to change the phase response by either moving poles or by adding zeros.

The easiest method on how to see if there is a risk of oscillation and what effect you get when changing poles and zeros is by a Baude diagram where you graphically plot amplitude and phase for each pole.

You can find how to do this in many basic text books on feedback systems. Another more modern method is to use a simulation tool like spice which can give you very quick results but it doesn't give you the same feel for what's going on.

As an example the original Wlliamson amplifier has quite low stability margin at low frequencies but it is also easy to rectify by introducing a phase stabilisation network, see here http://www.tubetvr.com/Williamson.pdf and here http://www.tubetvr.com/Williamson_comp2.pdf

Regards Hans
  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
phase shift of second order filter bondo Multi-Way 9 8th April 2009 02:24 AM
Phase shift smps topology darkfenriz Power Supplies 12 10th August 2008 11:30 AM
Zero phase-shift gareth Solid State 11 9th May 2008 04:45 PM
Signal phase shift through a tube amp d1camero Tubes / Valves 8 13th November 2006 03:18 PM
Zen Phase Shift, Feedback in BOZ Jay Pass Labs 3 17th February 2003 04:36 AM


New To Site? Need Help?

All times are GMT. The time now is 10:41 PM.


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