F6 Amplifier - Page 100 - diyAudio
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, 02:20 AM   #991
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
Quote:
Originally Posted by kasey197 View Post
flg, I, like you, took a while to be convinced, but did some thinking and agreed that it did in fact provide current/voltage gain in common source mode. Simple experiments as posted on #561 in this thread showed that was the case as well.
I just know that in the end, it will be some sematic resolution that really isn't worth arguing over, I will be the loser, etc, etc.
So, I went off and powered up the PSpice. How pany posts in this thread are about the (Corrected) Simplified F6 Schemo? And how many about a working pile of semi cunducting mess? I've heard everything, at least that I wanna read. This wasn't a post about mess's
Now I'll go find 561?
So, the 1st issue is, Avol contribution of the top FET. The Consencus seems to be, it has equal AC voltage gain contribution as the bottom FET
In the Simplified Sim Model I put together, pictured below, I think I capture the important funtions??? Fed with .1V 1kHz Most of the circuit is totally obvious, including "simplified" area's. Out of the top Transformer winding comes 5.8VAC + or -. But thats not the transformer output, it's a 1:1, that's the bias plus the transformer Sec output, plus the voltage variation at the drain of the bottom FET. So in order to see the actual Gate to source we need a diff probe not refrenced to gnd but to the Vout node (FET Source). Walla!, now we see .1V on the gate of the MOSFET refrenced to the Source of the MOSFET
And, what you've all been waiting for... ... ... Yea, the output node of the top FET is the source, and on the source is the Drain variation of the bottom FET plus .1V (+ or -). No Gain fromk the top FET.
The Bottom FET appears to have a voltage gain of about 58. Strange these numbers seem familiar?
Attached Images
File Type: jpg F6 V1.jpg (81.8 KB, 339 views)
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  Reply With Quote
Old 22nd August 2012, 02:35 AM   #992
The Dastardly Dad of Three
diyAudio Member
 
Join Date: Sep 2009
Interesting. To check your results, why dont you just measure the voltage across the load R7 and alternately remove C4 and C5 (ie leave them open circuit in turn). Based on your post, you should see 5.8V with C5 open and C4 in place. And 0.1V with C5 in place and C4 open.... is that the case?
  Reply With Quote
Old 22nd August 2012, 02:45 AM   #993
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
Quote:
Originally Posted by kasey197 View Post
hi all,
Had time...
Results:
0.2V pk-pk in, 9.8V pk-pk out. Therefore gain = 49x. Given the tx gives us 4x, that means the fets are giving us approx 12.2X gain.

Theory:

Load R = 6.8R, therefore combined bottom n top half transconductance = 12.2/6.8 = 1.8 Siemens. Each fet therefore gives us 1.8/2 = 0.9 S transconductance. This seems to tie in with what i see from the hitachi datasheets.

Explanation:
Given the above, both top and bottom fets are acting in common source mode (despite the temptation to think about the top half as a source follower providing no voltage gain). If this hypthesis is true then both top and bottom halves provide equal amounts of gain. We can test this easily given the transformer coupling by just bypassing the top and bottom tx secondaries (ie in turn, each fet would see only the bias voltage, but no signal) individually and seeing the effect.

Results:

Both secondaries connected, trace 1. Output is 9.8V.
Top secondary bypassed, trace 2,. Output is 4.875V
Bottom secondary bypassed, trace 3. Output is 4.812V

Summary: So, the sum of the two halves individually = 9.7V vs 9.8V when theyre both connected. Seems close enough to suggest that the initial hypothesis that both halves act to provide equal amounts of gain (in common source) is correct.
Wow? Lets see what the flg prototype in Pspice does? I had to think more to take out the top Xfrmr so rather than do that, I took the easy way out, I put a big resistor in series with the xfrmr...
As pictured in the previous post, I actually get 5.78Vout with .1V in. These are all Peak AC Volts. ie: .0707Vrms input.
With a 30Mohm resistor in series with the top transmormer instead of the Cap, essentially creating a huge voltage divider to the ac signal, the Vout is 5.822V. That appears to be less than 1% gain change w vs w/o the top Secondary winding
Some other results/thoughts:???
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  Reply With Quote
Old 22nd August 2012, 03:15 AM   #994
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
So, I went a little further and found that using 30M in my sim didn't make the same results as 30000000 when descibing a resistor value. I also found some strange results? For instance, now when I insert a 30Mohm resistor in the top or the bottom xfrmr I get about 1.7Vout? Gain of 17?
I need to work out a few more things to get this strait myself now
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  Reply With Quote
Old 22nd August 2012, 03:16 AM   #995
The Dastardly Dad of Three
diyAudio Member
 
Join Date: Sep 2009
Nope, not a spice dude myself so no other thoughts apart from #992
  Reply With Quote
Old 22nd August 2012, 03:59 AM   #996
lhquam is offline lhquam  United States
diyAudio Member
 
Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
I am working on the biasing issue at this moment. I first connected the bottom of the biasing network to the output, but found that the output offset voltage would not stabilize, since there is no DC negative feedback in this amplifier. I connected it to ground and achieved solid output offset stability at the cost of increased THD.

Today I made circuit changes that provide both DC stability and AC coupling of the output to the upper bias network. I am running tests now and will post results.

Quote:
Originally Posted by Antoinel View Post
kasey197: Your schematic in post #161 shows that the bias for the upper JFET is referenced to the output node. It rides up and down with the output signal Vo. The upper JFET is always in Class A.
ihquam: Your schematic in post #941 shows that the bias for the upper JFET is referenced to ground instead. The gate-source junction of the upper JFET will reverse bias as Vo goes positive relative to ground. The upper JFET can cut-off and switch to Class B

The two biasing schemes above are different with different operational outcomes: I am not sure whether my read is correct. But; which of the two biasing schemes do you think is in the actual design of Mr. Pass? I have a preference to that of kasey197.
  Reply With Quote
Old 22nd August 2012, 05:10 AM   #997
The Dastardly Dad of Three
diyAudio Member
 
Join Date: Sep 2009
Quote:
Originally Posted by Bigun View Post
sorry in advance for a dumb question !

I remember reading Nelson's article on negative feedback, it left a lasting impression on me. There are many benefits to avoiding negative feedback. And when a transformer is in the loop it adds it's own challenges. So if this circuit has -ve feedback I am curious as to why ?
i also found that an excellent article. The need for nfb is also covered in the same piece but here, for such an intrinsically linear amp, i would think that the primary need for nfb is to reduce the output impedance so that we can get closer to voltage source amp i/o a transconductance or current source amp.
  Reply With Quote
Old 22nd August 2012, 08:26 AM   #998
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
it's easy to determine gain contribution of upper Jfet - either Spice or measure on real circ:

measure output voltage as is , and with upper Jfet without modulation (disconnected secondary of xformer)

I'm too lazy to make graphical files for analogy of OS with plain resistors
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 22nd August 2012, 09:17 AM   #999
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
Quote:
Originally Posted by kasey197 View Post
i also found that an excellent article. The need for nfb is also covered in the same piece but here, for such an intrinsically linear amp, i would think that the primary need for nfb is to reduce the output impedance so that we can get closer to voltage source amp i/o a transconductance or current source amp.
The nice thing with this circuit is that the feedback has no influence on the input circuit itself.

So I plan to go for closed of loop 30 dB, not 15 dB, and see for myself. R1=10Ω, R2=300 or 330 Ω.
__________________
DAC TDA1541 S1; Kondo post-I/V filter; MC30-Super --> two stage RIAA; Pre: SP-6 clone ; F5 ; 300B PP ; ESL 57
  Reply With Quote
Old 22nd August 2012, 11:41 AM   #1000
diyAudio Member
 
Join Date: Jan 2012
Location: Pennsylvania
Blog Entries: 1
I have the privilige [or curse] at this milestone kilo post# to ask a Classic question; Are we there yet?
  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 04:12 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