Negative feedback to differential pair driver - 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 12th March 2013, 12:27 AM   #1
diyAudio Member
 
funk1980's Avatar
 
Join Date: Sep 2011
Location: Groningen
Default Negative feedback to differential pair driver

Just wondering if anyone has used negative feedback to the differential pair used as CC drivers for the power tubes. I don't mean the phase inverter configuration, but the long tailed pair with two in- and two outputs.
  Reply With Quote
Old 12th March 2013, 05:37 AM   #2
Jehan is offline Jehan  Indonesia
diyAudio Member
 
Jehan's Avatar
 
Join Date: Apr 2012
Location: Rancaekek, Bandung
Could you upload the schematic which shows us what you've said? Nfb could make the amp design complicated with phase relationship of each input signal, if wrong phase feeded to the output, positive feedback is what you'll get.
  Reply With Quote
Old 12th March 2013, 10:28 AM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Doing this at the driver stage could cause problems with common-mode distortion, unless you arranged negative feedback to both sides. CM distortion is less of a problem when NFB is applied at the input stage, as signal levels are much lower. Applying NFB to both sides carries the risk of extra phase shift, as you might no longer have a minimum phase system.
  Reply With Quote
Old 12th March 2013, 08:41 PM   #4
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by funk1980 View Post
Just wondering if anyone has used negative feedback to the differential pair used as CC drivers for the power tubes. I don't mean the phase inverter configuration, but the long tailed pair with two in- and two outputs.
It's done frequently in the Circlotron OTL topology. No different, really, than how it's done with SS op-amps. The main thing to watch out for is that you buffer inputs that could be connected to different source impedances, as these add to the Rs resistor(s) and can unbalance/change the level of NFB you've designed for. It's not a prob with SS op-amps since these already use buffered output stages that have very low impedances.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 12th March 2013, 10:22 PM   #5
diyAudio Member
 
funk1980's Avatar
 
Join Date: Sep 2011
Location: Groningen
Thanks for all the info! I'll look into the Circlotron topology. I understand the necessity for balanced (buffered) sources, but for some reason, the workings of global NFB to the LTP elude me...
To make sure we're all talking about the same thing, I sketched up a schematic of what's in my head, as Jehan suggested. Is it still viable??
NFBtoLTPconcept.jpg
  Reply With Quote
Old 12th March 2013, 10:45 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
No. Something like that is sometimes used in guitar amps, but there the aim is not fidelity but controlled distortion. The problem is that the feedback is applied in common-mode to a differential-mode stage. You should instead put the feedback on the second LTP grid. Then to avoid common-mode distortion use a CCS tail.
  Reply With Quote
Old 13th March 2013, 01:11 AM   #7
Jehan is offline Jehan  Indonesia
diyAudio Member
 
Jehan's Avatar
 
Join Date: Apr 2012
Location: Rancaekek, Bandung
I suggest to separate the driver cathodes by removing its tail, then put a remote sensing negative feedback from output transformer's secondary legs to driver cathodes. I think it would sounds fabolous.
  Reply With Quote
Old 13th March 2013, 02:42 PM   #8
diyAudio Member
 
Join Date: Jun 2011
try r5 and r10 being doubles as each triode have its own r5&r10.

then tie a 100k pot between where the nf was one on pin1 and one on pin3 of pot. tie pin 2 of pot (wiper) to the negative FB network.

adjust for gain linearity.
  Reply With Quote
Old 13th March 2013, 03:02 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Sorry, you lost me - could you explain your proposal in more detail?
  Reply With Quote
Old 14th March 2013, 06:36 AM   #10
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
I did that in Pyramid amps, from anodes of output Gu-50 tubes, to anodes of LTP driver (R23, R28).
Attached Images
File Type: jpg Ок-Ус-схема.jpg (356.3 KB, 109 views)
__________________
The Devil is not so terrible as his math model is!
  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
How the negative feedback really works and an alternative feedback question Amit_112dB Solid State 37 21st April 2014 03:16 PM
negative feedback dscottguitars Instruments and Amps 7 13th September 2011 04:43 PM
Get away from negative feedback ? N-Well Pass Labs 3 1st March 2006 10:17 PM
Negative feedback? Solid Snake Solid State 33 14th March 2003 01:29 PM


New To Site? Need Help?

All times are GMT. The time now is 09:30 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