Revision 2. Balanced - Page 2 - 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 1st April 2012, 03:12 AM   #11
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
I switched the driver to a differential stage. I am trying to apply the negative feedback. Is it possible to somehow inject feedback along with the audio input source into the same grid?

With an op-amp you can, but it is ineffective trying to set up the tube differential stage like an op-amp using global feedback.
  Reply With Quote
Old 1st April 2012, 03:47 AM   #12
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Quote:
Originally Posted by kingneb View Post
I switched the driver to a differential stage. I am trying to apply the negative feedback. Is it possible to somehow inject feedback along with the audio input source into the same grid?
Sure. Same way as you would with an op-amp.

Quote:
With an op-amp you can, but it is ineffective trying to set up the tube differential stage like an op-amp using global feedback.
What do you mean by "ineffective"? Converting the SRPP input stage you have to a diff pair would reduce the tube count by two tubes. Though, I wonder if you could build a combination. Sorta like a diff pair with active load. Way overkill for the application, probably, but it could be fun to try.

For an example of a differential amp with global negative feedback, have a look at Morgan Jones' "Crystal Palace" amp. It's featured in his book, "Valve Amplifiers" 3rd ed. I'm sure it'll be in the 4th edition as well, when that comes out that is.

~Tom
__________________
Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 1st April 2012, 03:55 AM   #13
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
This is what I mean. I set it up the same way as you would a differential amplifier with an op-amp. With this circuit the FB is coming off of the 8 ohm transformer winding.

The gain is not reduced no matter what resistor combination I use. That is what I mean by ineffective.
Attached Images
File Type: png Untitled.png (15.9 KB, 77 views)
  Reply With Quote
Old 1st April 2012, 01:31 PM   #14
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
I am seeing a few people using LM type chips in the front end of a differential pair, which is in my opinion is just not adequate for the intended job. I think they are adequate for under output tubes - but simply do not have a linear or high enough impedance for the critical input stage. The signal will fall out of balance above about 10khz producing all sorts of nasty harmonic distortions.

Its so simple to build a two transistor CCS for this application that it's just criminal to use a LM chip.

Shoog
  Reply With Quote
Old 1st April 2012, 03:15 PM   #15
HpW is offline HpW  Switzerland
diyAudio Member
 
Join Date: Mar 2002
Location: Switzerland (Bern)
Quote:
Originally Posted by Shoog View Post
Its so simple to build a two transistor CCS for this application that it's just criminal to use a LM chip.
Shoog
My general question on CCS, while currently I do not have any practicable & else experiences on this. What is the benefit (THD or else) to have a CCS compared to resistor based. May the result is may also tube depended?

Hp
__________________
www.hpw-works.com
  Reply With Quote
Old 1st April 2012, 03:37 PM   #16
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
The stiffer the tail the better the phase balance. Effectively you are forcing the LTP to send all the signal between the cathodes, which acts as an input just like g1, The stiffer the tail the more perfect the signal transference between input valves. So the tail has to be many time higher impedance than the the valve impedance - and it is very easy to achieve with a two transistor current source. Using a resistor which is difficult to make many times higher than the valve impedance means that a substantial part of the signal can bleed away to earth via the tail.

If, as in this case, the two inputs are been driven - having a stiff tail forces the input stage to integrate any difference between the inputs.

Its a simple matter of if you are going to use a LTP - you might as well do it properly and use as stiff a tail load as possible. LM chips have a reducing impedance above about 10khz which not only makes them a poor tail load above these frequences - but also makes in a variable load with frequency.

Shoog

Last edited by Shoog; 1st April 2012 at 03:43 PM.
  Reply With Quote
Old 1st April 2012, 03:41 PM   #17
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
I can correct the tail problems later and I have thought about that.

What I am asking in the interm is how do you apply the feedback?
  Reply With Quote
Old 1st April 2012, 03:48 PM   #18
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Originally Posted by kingneb View Post
I can correct the tail problems later and I have thought about that.

What I am asking in the interm is how do you apply the feedback?
Use exactly the same cathode resistors you had originally but simply reference them to your CCS common node rather than ground. This may require another -B to give the CCS room to function.

Shoog
  Reply With Quote
Old 1st April 2012, 03:50 PM   #19
BRSHiFi is offline BRSHiFi  United States
Previously known as kingden
 
Join Date: Aug 2008
Location: Evanston, IL
Then I will look into a cascode of 3904 Q's to replace the 334? Sound good?
  Reply With Quote
Old 1st April 2012, 03:55 PM   #20
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Sounds excellent.

The suggestion about combining your two bias arrangements should work - but it will probably alter the feedback and I wouldn't like to hazard a guess at how to calculate it. It maybe a matter of experimentation with a pot to see what actually sounds best.

Shoog
  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
EAS Controller revision A pra3718 Chip Amps 9 8th March 2011 10:31 AM
Northcreek Okara Revision Question mevaniuck Multi-Way 3 2nd July 2006 08:15 AM
Cheap 24/96 DAC, Revision B. jwb Digital Source 235 28th June 2006 09:46 AM


New To Site? Need Help?

All times are GMT. The time now is 03:54 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