Help with dual PS noise shunt? - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

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 20th March 2008, 08:46 PM   #11
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
First of all, the Wenzel idea is indeed working (I've tried) and it works better the more linear the correcting amplifier is (a single transistor isn't too good). A disadvantage is the increased DC/LF output impedance, one advantage is that it doesn't fiddle with the main regulators feedback loop, which brings us to Tom's ciruit.

While Tom's circuit is for sure worth a closer look and try, I feel somewhat unhappy with the additional loop gain from the opamp, it might get unstable under certain conditions (had that, too, with a related design where I sensed current high-side with a diff-amp and feed back the drop to the GND pin of a 7812).

Personally I prefer the last stage of a psu to be without feedback, a simple cap multiplier that is (and a duplicate in front of the reg, too), these tend to have way more bandwith that feedback'd approaches. And I use the LM3x7 in the lowest gain configuration, that is without the resistor divider, driving the adjust pin with a voltage (from a divider from the output) buffered with an emitter follower. This is essentially a stacking approach.. There is a post by the late Fred Dieckmann in one of the LM317 threads on this. IMO the main trick for a good regulation with LM317/78xx style regs is to load them and to have enough drop accross them -- same for the cap multipliers. The LM317 works really well with 50mA load and a 5V drop.

- Klaus
  Reply With Quote
Old 22nd March 2008, 01:04 AM   #12
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Well fellows, here is my first pass:

I tried to pay some attention to ground routing, but I'm a noob. Similarly, there may be egregious errors in this, so please beware! Anyone want to proof it before I toast one?
Attached Images
File Type: jpg psuschem.jpg (77.0 KB, 233 views)
  Reply With Quote
Old 22nd March 2008, 01:05 AM   #13
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
the layout. Notice the opamp and smd parts on the bottom, so far only a one layer board:
Attached Images
File Type: jpg psubrd.jpg (65.8 KB, 207 views)
  Reply With Quote
Old 22nd March 2008, 01:07 AM   #14
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
and here is the layout again, showing only tracks. There are actually very few ground connections...

I'm happy with it so far. It measures about 2" by 2.5". Some of the caps sizes may need to change, so layout might too. The board is designed to handle currents under 500mA, which should be plenty for preamps.
Attached Images
File Type: jpg psubrd2.jpg (40.4 KB, 197 views)
  Reply With Quote
Old 22nd March 2008, 05:36 PM   #15
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Cuibono,

In the sheet I see no connection for the opamp supplies, while in the board it seems to be connected to the reg's outputs, right?

One safety thing I would add: clamp diodes at the reg's REF (aka "GND") pins (say two antiparallel strings of a few 1N4148 each) to GND, to limit the range the output could ever swing.

Also, I would provide layout option to power the opamp from the inputs (divided down and filtered). And don't forget to load the regs with some 20mA at least (which could be partly had with opamps powered from the outputs and resistivily biased into class A).


@Tom: With which 12V reg model did you actually sim the circuit?
I assume a typical one, with a 1.2V internal reference and an integrated resistor divider, which would be 9:1 in this case. You amplify the noise with a factor of 14.7, so the internal Ref node would see a "overunity" feedback of the noise, a blip of -1mV at the output would give +1.5mV at the internal ref and a following 1.5mV increase of output, only to be damped / divided down by the (unreliable) dynamic output impedance of the reg and that of the output cap ESR.... this is what scares me a bit, looks like negative AC output impedance could happen -- did you do a output impedance sim? And, to start with, is it stable without output cap?

I think I need to sim it (also with various LM3x7 models I found)...

- Klaus
  Reply With Quote
Old 22nd March 2008, 08:00 PM   #16
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
Hi Klaus,

I used the LT1086-12 model that is included with LTspice.

I just now tried simming the output impedance, using a current source from the output with 1A 'DC Value' ('bias' current, since supply is positive-only) plus 0.5A 'AC Amplitude', with an AC analysis run, plotting v(out0/i(out). It is about 412 mOhms at low frequencies and drops from that to about 254 mOhms while going from 10 Hz to 10 kHz. Without the output cap, it's only a little different, and only drops to about 342 mOhms, and does that between 10 Hz and 1 kHz.

Transient response still looks 'OK', without the output cap. But I haven't done any rigorous stability analysis.

I am sure that the circuit could be improved, in terms of both performance and stability. (But I have started a new job and don't have much time for anything else, right now.)
__________________
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
  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
Help with Shunt Regulation and Noise dsavitsk Tubes / Valves 30 24th December 2007 06:11 PM
Squeaky Noise in ARC Dual 75 neddyboy Tubes / Valves 11 6th June 2007 04:58 PM
Low noise dual JFET pair rtarbell Parts 4 17th March 2006 10:11 PM
Low voltage noise dual OP Holger Barske Parts 7 24th May 2004 06:57 AM


New To Site? Need Help?

All times are GMT. The time now is 01:27 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