Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 22nd August 2012, 09:58 PM   #1031
lhquam is offline lhquam  United States
diyAudio Member
 
Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
Originally Posted by Zen Mod View Post
solution for motor-boating is always easy - brute force - use zillion of uF

it wasn't easy in time when uF was expensive
Perhaps this isn't classical motor-boating after all. The phenomenon occurs when the only caps are the ones in the path from the secondaries to the gates. It looks to me like a LC resonance of the transformer inductance and coupling caps.
  Reply With Quote
Old 22nd August 2012, 10:01 PM   #1032
lhquam is offline lhquam  United States
diyAudio Member
 
Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
I found the output offset to be unstable if the upper bias circuit is referenced to the output. When grounded it is stable. Unfortunately, the THD appears to be increased.

Quote:
Originally Posted by Antoinel View Post
triode_al: I like [and hopefully others will appreciate] your schematic in post #1023 because:
  • The bias to the upper MOSFET is referenced to the output and not to ground. Thus; this MOSFET will always operate Class A.
  • The bias to the upper MOSFET is [hopefully] stable due to the CCS you used to establish it. I would have used [on paper] an independent power supply of positive voltage higher than the rail voltage shown. Bias in this newer instance will be granite solid or better.
  • The bias to the bottom MOSFET can be anything else that is stable; which you do show.
  • The blocking capacitors in series with the gates of the MOSFETs eliminate the dreaded direct current through the secondaries.
  Reply With Quote
Old 22nd August 2012, 10:09 PM   #1033
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
Ilquam, whynot tryputting two zeners in series taken to negative rail and pull bias for both fets off this one reference point.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 22nd August 2012, 10:44 PM   #1034
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
That might be OK for the bottom FET but, the top FET Souce is at the output and the Gate needs to be 1.2-4.5V higher, depending on FET type
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  Reply With Quote
Old 22nd August 2012, 11:04 PM   #1035
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
Yea, You need a servo to establish reference point. I was thinking of link I posted and forgot about servo.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 22nd August 2012, 11:41 PM   #1036
diyAudio Member
 
Join Date: Jan 2012
Location: Pennsylvania
Blog Entries: 1
Quote:
Originally Posted by lhquam View Post
I found the output offset to be unstable if the upper bias circuit is referenced to the output. When grounded it is stable. Unfortunately, the THD appears to be increased.

lhquam. Your bias scheme for the upper JFEt is referenced to ground. This led me to suggest a hypothesis that it may cut off. Does it cut off in your opinion? Interestingly, your prototype works whether this JFET cuts off or not. You may already have a new mode of operation for Conceptual F6!
  Reply With Quote
Old 22nd August 2012, 11:54 PM   #1037
lhquam is offline lhquam  United States
diyAudio Member
 
Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
Originally Posted by Antoinel View Post
lhquam. Your bias scheme for the upper JFEt is referenced to ground. This led me to suggest a hypothesis that it may cut off. Does it cut off in your opinion? Interestingly, your prototype works whether this JFET cuts off or not. You may already have a new mode of operation for Conceptual F6!
No, none of the JFETs cut off unless the load is lower than 8 ohms and the output power forces the amplifier into class C.

Look at the time constants involved with the gate capacitors and the bias resistance. The time constant is about 1/3 sec. A simple modification to the biasing scheme allows for DC bias to reference to ground while coupling AC from the output, getting the best of both worlds. I was experimenting with that circuit yesterday when I discovered a dangerous low frequency (< 1Hz) oscillation that must be dealt with before going further.
  Reply With Quote
Old 23rd August 2012, 12:08 AM   #1038
diyAudio Member
 
Join Date: Jan 2012
Location: Pennsylvania
Blog Entries: 1
Quote:
Originally Posted by lhquam View Post
No, none of the JFETs cut off unless the load is lower than 8 ohms and the output power forces the amplifier into class C.

Look at the time constants involved with the gate capacitors and the bias resistance. The time constant is about 1/3 sec. A simple modification to the biasing scheme allows for DC bias to reference to ground while coupling AC from the output, getting the best of both worlds. I was experimenting with that circuit yesterday when I discovered a dangerous low frequency (< 1Hz) oscillation that must be dealt with before going further.
lhquam: Thank you for your response. The bias approach in your prototype of Conceptual F6 is an interesting and unique development.
  Reply With Quote
Old 23rd August 2012, 12:22 AM   #1039
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
In trying to determine the ideal load resistor for the transformer, does the circuit need to be complete including output stage or can it be determined with just the transformer connected to the input stage?
__________________
...Shape the sound , Man!
  Reply With Quote
Old 23rd August 2012, 02:06 AM   #1040
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
Everything the Xfrmr "see's" C, R, L and FET Gate's? are all part of it's load. It can be optimized but getting close is ussually easy by the mfg's suggestion or copying a similar configuration. The FET gate will ussually dominate the C parameter you'l find difficult to satisfy in this configuration.
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  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
New F6 diy coming soon ? Nounours18200 Pass Labs 37 31st August 2012 04:35 PM
Hi Vi F6 box ioro Multi-Way 4 15th April 2012 08:27 PM
Whats happening with F6? jugoslavija Pass Labs 1 1st November 2011 07:39 AM
Hi-Vi F6 kkosiba Multi-Way 14 9th November 2007 08:38 AM
F6 traw Pass Labs 7 2nd November 2007 07:09 PM


New To Site? Need Help?

All times are GMT. The time now is 11:36 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