Amp stability unclear - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 3rd March 2009, 12:10 PM   #1
Khron is offline Khron  Romania
diyAudio Member
 
Join Date: Jul 2005
Location: Finland
Send a message via AIM to Khron Send a message via MSN to Khron Send a message via Yahoo to Khron
Default Amp stability unclear

Greetings everyone...

I'm in the process of putting together 4 of Dr Self's "Load-invariant" amps, to be used in some bi-amped speakers. They're going to handle under 2KHz signals only.

I made some component value alterations due to my somewhat restricted part inventory.

The thing i'm slightly nervous about is the fact that in an LTspice4 AC analysis, the amp still has about 6dB gain when it reaches -180deg, at the output (but before the output R//L). Is this just some simulation (in)accuracy that's fueling this "paranoia", or should i be safe in real-life?
Attached Images
File Type: gif ltspice.gif (17.6 KB, 572 views)
  Reply With Quote
Old 3rd March 2009, 01:16 PM   #2
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Albany , NY (smallbany)
Quote:
The thing i'm slightly nervous about is the fact that in an LTspice4 AC analysis, the amp still has about 6dB gain when it reaches -180deg, at the output (but before the output R//L). Is this just some simulation (in)accuracy that's fueling this "paranoia", or should i be safe in real-life?
You really won't know until you build it , the ones that
didn't sim correctly for me DID oscillate when I built them..

try a very fast rise time (10e-9+) square wave at 10k
check for ringing... Lpt, OPs

BTW , 10pf cdom is a little low. Post .asc file

Attached is pix of 'right' gain plot and a "ringing" shot
with amp undercompensated..
OS
Attached Images
File Type: gif right.gif (30.3 KB, 448 views)
  Reply With Quote
Old 3rd March 2009, 01:24 PM   #3
Khron is offline Khron  Romania
diyAudio Member
 
Join Date: Jul 2005
Location: Finland
Send a message via AIM to Khron Send a message via MSN to Khron Send a message via Yahoo to Khron
I had to "flip" the polarities of the whole front end, to be able to use some MPSA18's i have on hand, so the Cdom is the 330pF on top. That 10pF cap is something else; can't remember off the top of my head, but it's some "tweak" from Dr Self's book.
Attached Files
File Type: zip amp.zip (2.1 KB, 26 views)
  Reply With Quote
Old 3rd March 2009, 02:02 PM   #4
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Albany , NY (smallbany)
It rings.. everywhere.(attached)

How did you check open loop gain ?? I see no sweep source.
I looked at the original in the self book and you changed
a lot of things..

It might work, but it also triagulated a 10KHZ SW ,so
It might not sound good, also it showed a HUGE gain
"spike" at 1MHZ so I'm 99% sure it would oscillate in real
life.

Is this you first amp ?? Have you thought of doing a simpler
one like the standard self amp.. it is much more tolerant
about device changes.

OS
Attached Images
File Type: gif ring2.gif (9.1 KB, 431 views)
  Reply With Quote
Old 3rd March 2009, 05:23 PM   #5
Khron is offline Khron  Romania
diyAudio Member
 
Join Date: Jul 2005
Location: Finland
Send a message via AIM to Khron Send a message via MSN to Khron Send a message via Yahoo to Khron
I didn't check the open loop gain, but as far as i know, the "high end" of that should look the same, open loop or closed loop.

I did a sim with a 10KHz square wave (2Vpp), and i'm attaching the result.
V(n007) = VAS input
V(n015) = before output inductor
V(n016) = after output inductor

Indeed, there is a bit of ringing (around 2MHz), but it gets damped fairly quickly.

I know i changed a lot of things. I *DID* mention i turned the whole front end "upside down".
Attached Images
File Type: gif ltspice.gif (6.6 KB, 392 views)
  Reply With Quote
Old 3rd March 2009, 05:26 PM   #6
Khron is offline Khron  Romania
diyAudio Member
 
Join Date: Jul 2005
Location: Finland
Send a message via AIM to Khron Send a message via MSN to Khron Send a message via Yahoo to Khron
Here's a zoom in on the VAS input trace.
Attached Images
File Type: gif ltspice2.gif (5.7 KB, 394 views)
  Reply With Quote
Old 3rd March 2009, 06:24 PM   #7
fjr is offline fjr
diyAudio Member
 
Join Date: Feb 2009
Default Try decreasing Cmiller

C13 >> 10...100pF
  Reply With Quote
Old 3rd March 2009, 10:13 PM   #8
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Albany , NY (smallbany)
Quote:
Indeed, there is a bit of ringing (around 2MHz), but it gets damped fairly quickly.
But when you build it and drive a real reactive load , that
is where things get interesting.

The "change" that made the most difference is subbing
the BD's for the MJE340/350's. For a current source or
a vbias generator this is not as critical , but for the
VAS itself , a world of differance (Cob/Hfe).

"Flipping it around" makes no difference at all as far as
stability and/ or compensation.
OS
  Reply With Quote
Old 3rd March 2009, 10:40 PM   #9
Khron is offline Khron  Romania
diyAudio Member
 
Join Date: Jul 2005
Location: Finland
Send a message via AIM to Khron Send a message via MSN to Khron Send a message via Yahoo to Khron
Ok then, duly noted. It's 1:30am here now, but i'll try to find some decent MJE340/350 tomorrow and take another shot at some sims.

The assistance and guidance is appreciated, OS
  Reply With Quote
Old 4th March 2009, 04:38 AM   #10
diyAudio Member
 
Join Date: Nov 2008
Location: Brazil
Hello

You must remove input subsonic filter to make measurements, the subsonic filter change the curve of the graph but does not cause instability (not in the feedback link)

I have simulated the Low Tim seems strangely unstable >180 in 0dB (without filters)

The circuit.asc LTC can be downloaded site :

http://users.ece.gatech.edu/~mleach/lowtim/
  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
Unclear schematics folkeb Pass Labs 1 18th July 2006 10:29 PM
LF357 stability richwalters Chip Amps 3 15th March 2005 05:38 AM
stability djdamix Solid State 3 7th January 2005 07:30 AM
DoZ power supply schematic wrong or unclear? Cradle22 Solid State 7 3rd February 2003 12:19 PM


New To Site? Need Help?

All times are GMT. The time now is 09:28 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