Improving the Futterman - 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 18th August 2003, 01:46 AM   #1
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Default Improving the Futterman

Disclaimer: I've made some assumptions here, since I don't know everything about this... I've tried to think it through though

The main problem with the Futterman is that the NFB is imbalanced. The top tube is a cathode follower, yet the bottom is a standard plate follower with no more NFB than the triode used (and with a load 1/4 or less of the plate resistance, there isn't much internal NFB left!). As a result, there will be much 2nd harmonic in the output. This is usually fixed by using a cathodyne phase splitter grounded to the output, and inverting the drive phase to the output tubes. That helps a little but is only masking the deficiencies of the topology.

I propose using shunt NFB applied to the bottom tube, as shown in the attached drawing (note: drawn for theory only). Say you drive it with 20V, and the tube produces a voltage gain of 5 in the circuit conditions. Then the plate signal output is 100V. Yet the same conditions with a CF produces an output of 16.6V, or 15.5dB of NFB. To balance the circuit requires the same amount of NFB applied to the bottom. But how?
Driving the grid through a high resistance, and applying some of the plate signal back to the grid, we form a voltage divider from input to output, with the grid inbetween. To solve this, we assume the input and output - 20 and -16.6V (it's inverse polarity remember) respectively, and then since the tube still has a gain of 5, grid voltage must be 3.3V. So there are 16.7V across the input resistance, and 20V between grid and output. The scale of the resistance should depend on HF constants (capacitance, etc.) and the possible driver Zo (for instance if driving with a pentode, the Rp will be a significant portion of the 'input resistance').
The advantage is that for two extra resistors (maybe just one if driven by a pentode) you balance the output; however since a CF's NFB is applied in series, whereas this is shunted, the characteristics vis-a-vis changing Rl will unbalance it at anything other than a certain impedance. I think!

Erm.. hope that makes sense Comments?

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 18th August 2003, 01:48 AM   #2
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
Attachment:

P.S. I was going to mess around with this a little on the breadboard... we'll see how that goes.

P.P.S. Dang, I posted this in the etc. forum? Whoops...
Attached Images
File Type: gif shunt nfb otl.gif (1.1 KB, 314 views)
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 18th August 2003, 01:55 AM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Moved.

You might look at some of the old reviews of Futterman amps, look at the measurements and waveforms, and see if they bear out your hypothesis.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 18th August 2003, 07:46 PM   #4
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Erm.. hope that makes sense Comments?
It does...
Only you seem to forget that your local NFB loop will also put a DC voltage on the output.

Cheers.
__________________
Frank
  Reply With Quote
Old 18th August 2003, 09:06 PM   #5
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
I noted it's a theoretical diagram, for the AC signal path. I'd use a coupling cap there.

Tim
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 18th August 2003, 09:34 PM   #6
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Seems we solves one problem just create another here but what the heck...

Just for the fun of it:

Cheers,
Attached Images
File Type: gif shunt nfb otl_r.gif (2.0 KB, 207 views)
__________________
Frank
  Reply With Quote
Old 18th August 2003, 11:35 PM   #7
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
http://www.tubecad.com/april_may2001/page14.html
  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
Futterman style 6AS7 OTL with other tubes Brit01 Tubes / Valves 39 22nd July 2009 06:04 PM
Futterman H-3 desperateaudio Tubes / Valves 35 23rd April 2009 05:37 AM
Technics variation of Futterman SEPP. Circlotron Tubes / Valves 38 12th December 2008 04:54 PM
Help me improving my pre-amp! thanh Solid State 0 26th January 2007 11:08 AM
Futterman OTL ferrite bead specification jwan Tubes / Valves 4 15th March 2004 08:17 PM


New To Site? Need Help?

All times are GMT. The time now is 03:20 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