Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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 16th March 2012, 11:44 AM   #51
Salas is online now Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Quote:
Originally Posted by wrenchone View Post
Salas, the whole point of this exercise was to be able to push up the impedance of the feedback network and yet have some drive available for the mosfet gate. The way you show it, you really need that electrolytic (or a great honkin' film cap), plus a buffer to drive the buffer.
Was just an idea to check where the hiss comes from on those horns. Valid points the drive difficulty and the large input capacitor indeed. So I went on investigating the theoretical THD differences. I run two FFT on LT SPICE. First is for the ~5x gain high impedance feedback network, the second is for the low impedance ~3x gain. Both on 20mA depletion mode Mosfet CCS setting. Simulated SNR -91dB HiImp, -104dB LoImp.
Attached Images
File Type: gif Hi_IMP_5xG_Sch.gif (9.8 KB, 871 views)
File Type: gif Hi_IMP_5xG_FFT.gif (22.0 KB, 863 views)
File Type: gif Lo_IMP_3xG_Sch.gif (10.2 KB, 846 views)
File Type: gif Lo_IMP_3xG_FFT.gif (22.5 KB, 835 views)
  Reply With Quote
Old 17th March 2012, 03:54 AM   #52
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
That's very nice, but if you screw up the signal source trying to drive the low impedance, you're not exactly ahead of the game. You might try increasing the bias current in the mosfet. That definitely worked for me in terms of reducing the distortion. I'm biasing the input jfet at ~5 ma, which should work nice for the 2SK117.
  Reply With Quote
Old 17th March 2012, 09:31 AM   #53
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Here's the latest effort - a bit neater that my first. There's two channels and an onboard shunt regulator. The output mosfet and its current source drain load are both cascoded. One channel exhibited high amplitude oscillations at ~350 kHZ (so low?), while the other looked stable. About 3" of "gimmick" capacitor was applied to the feedback resistor of both channels. It stopped the oscillation on the one channel, and did not obviously destabilize the other. Next up are tests to determine what is going on. I'll post schematics when I'm reasonably satisfied.
Attached Images
File Type: jpg Scade_AMP1.jpg (253.2 KB, 801 views)
  Reply With Quote
Old 17th March 2012, 04:57 PM   #54
vgeorge is offline vgeorge  Greece
diyAudio Member
 
vgeorge's Avatar
 
Join Date: Mar 2004
Location: poros island
Nice board wrenchone.
Waiting your final schematic.
I did try salas suggestion with lower resistors. Noise was down but distortion was higher.
With less sensitive speakers maybe there is no problem.
__________________
"Without deviations from the norm, progress is not possible" - Frank Zappa
  Reply With Quote
Old 18th March 2012, 04:11 PM   #55
juma is offline juma  Serbia
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Zemun
Hi Wrench,
here is my take on "turning a (MOS)FET into a SIT/triode". Fair Zin, low Zout, BW(-3dB)=1MHz, rail-to-rail swing (almost), high PSRR, Id about 15mA, sounds wonderfull - what more could a man want ?

Built, tested, measured (except for THD) - here is the schematics:
Attached Images
File Type: gif BS170_Schade_Gyrator_SCH.gif (4.7 KB, 679 views)
  Reply With Quote
Old 18th March 2012, 04:29 PM   #56
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
Juma,
R2-4, D1, and C1 are biasing network for top fet? Also, are ther two feedback loops R5/R7(schade component) and R6/R7? Very interesting thread from Wrenchone. Love following it.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 18th March 2012, 05:49 PM   #57
juma is offline juma  Serbia
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Zemun
Quote:
Originally Posted by buzzforb View Post
R2-4, D1, and C1 are biasing network for top fet?
Yes, it's called a gyrator (fake choke - the most appropriate load for the fake triode ). Voltage divider R2/R4 sets the gate of Q1 at about 2.5V above Vcc/2

Quote:
Originally Posted by buzzforb View Post
R5/R7(schade component)
Right. It sets the gain and Zin


Quote:
Originally Posted by buzzforb View Post
and R6/R7?
No. Voltage divider R5/R6 sets the Id. R5 is already involved in gain setting so the R6 is the one to be manipulated to set the Id (I find the 15mA to be an optimum value)
  Reply With Quote
Old 19th March 2012, 10:45 AM   #58
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
As always, Juma, a clear and easy to understand explanation. I don't know why less thinking R5/R6 was feedback, didn't click that these were enhancement mode devices and needed a positive voltage at the gate. I think I have seen the gyrator, minus the diode and with cap going from divider to ground, used to bias the top Jfet in an SRPP type circuit. I have been watching this thread as I am considering trying to use LU Jfet in FE for BA output stage and wrenchone always interesting circuits if for no other reason than just learning. Thanks to both of you.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 20th March 2012, 06:08 AM   #59
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Don't look now, but I've got another version of this thing up my sleeve. PSpice and the other simulators are wonderful toys (to geeks, at least).
  Reply With Quote
Old 21st March 2012, 08:11 AM   #60
Salas is online now Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Quote:
Originally Posted by vgeorge View Post
Nice board wrenchone.
Waiting your final schematic.
I did try salas suggestion with lower resistors. Noise was down but distortion was higher.
With less sensitive speakers maybe there is no problem.
Hi Vgeorge. Having a suspicion also about the CCS amplifying its degeneration resistor's noise enough for your high sens horn speakers, I looked in to just a resistor current source. So I brought back your input impedance higher for your source and pot to better handle, hoping that if there was hiss contribution by the CCS maybe we can trade what we last time cut by lowering Rf/Ri. If you still got it handy its an easy mod to test. Sims >30mA for M1 and a bit more THD 2ndH in the mix. R7 should be a 2W part.
Attached Images
File Type: gif R_Fake_It.gif (48.0 KB, 591 views)
  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
L'Amp: A simple SIT Amp ra7 Pass Labs 2037 Yesterday 03:15 PM
SIT Amp Article Out Now Variac Site Announcements 0 2nd December 2011 07:55 AM
SIT-Preamp VladimirK Solid State 36 27th August 2011 08:36 PM
SIT Power Amplifier??? mad_z Solid State 8 26th January 2009 08:31 PM
New SIT - old problem StevenOH Parts 2 13th June 2008 10:46 AM


New To Site? Need Help?

All times are GMT. The time now is 05:44 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