
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 
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 

Thread Tools  Search this Thread 
13th February 2020, 08:27 PM  #1 
diyAudio Member
Join Date: Oct 2007

Strange graph in PSUD2 (oscillation?)
Hi Everyone,
I am away from my soldering iron for a while, but I have my computer with me, so I have been modeling different power supply options in PSUD2 for a couple projects waiting back home. There is a recurring theme to these models: strange "V" shaped lines or sometimes slopes for the voltage graphs. These show up if I chose a delay for the sample period. The images below were sampled after a delay of 120 seconds. Is this a sign of oscillation in the power supply? see the two attached images for a phono preamp. One is the power supply as suggested on the schematic, and the second is a model of a PS based on parts in my scrap box. PSUD2PhonoDude_perSCHM.PNG PSUD2PhonoDude_42.PNG 
13th February 2020, 08:31 PM  #2 
diyAudio Member
Join Date: Apr 2008
Location: Carlisle, England

The difference in voltage between top and bottom of V is so small it is irrelevant.
__________________
2020 versions of PCBCAD51, PCBCAD360 and PCBCAD720 out now >>> https://www.murtonpikesystems.co.uk 
13th February 2020, 08:50 PM  #3 
diyAudio Member
Join Date: Oct 2007

Thanks Nigelwright7557,
I appreciate the response. This is what I felt, too, but some configurations brought up straight lines or bars really showing just the ripple. The modulation (bending of the bar) of the voltage plot shown above seemed a little odd, but I couldn't quite decide if it was just an anomaly of PSUD2, or something I should try to iron out. (or as you suggest, not to worry because we are only looking a a few micro volts difference). The graphs plotted with no delay on the sample period look nice and smooth. No ringing. 
13th February 2020, 09:03 PM  #4 
diyAudio Member

You're like down in nanovolt territory, well below self noise for most tubes.

14th February 2020, 10:09 AM  #5 
diyAudio Member
Join Date: Dec 2009

At a guess in the second pic. if you extended the time window to 100140 seconds you'll see an oscillation and slight increase in voltage as the PSU caps continue to charge, I don't think it's completely reached a steady state yet with that humongous iron and capacitance downstream of the load. Mind you the ripple is pretty much nonexistent.
Might I ask if all this is stuff in the parts bin, because otherwise with the cost of the iron chosen, caps with 2ohm ESR are an anomaly since this can easily be improved for small bucks? 
14th February 2020, 02:16 PM  #6 
diyAudio Member
Join Date: Oct 2007

Hi Jazid,
Thanks for your response. I will extend the time and see what it looks like. Didn't know I could make the sample period that long, actually. The second one with the thin red "V" represents parts in the spares box. Leftovers from various projects. But for small bucks, I don't mind placing a small order. What did you have in mind? Quick expalnation: (the 2ohm caps in the first chart are just the PSUD2 default. The ESR in the second chart are rounded off from spec sheets) (The schematics represent the whole chain of filtering including the interstage decoupling. The basic DC supply ends after the second choke. I was just seeing if I could get away with using the 3uf caps later in the chain, or if the schematic's suggestion of 47uf was necessary to achieve low ripple.) 
14th February 2020, 02:33 PM  #7 
diyAudio Member
Join Date: Oct 2007

Here are screen shots 140170 seconds out, 170200 seconds, and 200230
PSUD2PhonoDude_4_140170sec.PNG PSUD2PhonoDude_4_170200sec.PNG PSUD2PhonoDude_4_200230sec.PNG 
14th February 2020, 02:53 PM  #8 
diyAudio Member
Join Date: Jan 2020
Location: Colorado, USA

My estimation of what you are observing is a commonality to many, many numerical modeling systems. Especially when you are considering a transient, nonlinear solver. Given the nonlinear mathematics introduced by magnetics, a transient solution to that nonlinear system of equations is very likely to employ an iterative relaxation method, such as PointJacobi or GaussSeidel, to determine the "result" at each time step. The tolerance to which that iterative solver converges is finite and there is a tradeoff between accuracy and the number of iterations required. In this case what you may be observing is this inaccuracy between successive solution steps.
I don't have any visibility into PSUDII but lTspice gives little messages at the bottom of the screen about corrective alterations to the solver's parameters. My quickest speculation is summarized as: numerical solution techniques retain small amounts of error, which can be visible in adjacent time steps. 
15th February 2020, 01:59 AM  #9 
diyAudio Member
Join Date: Jun 2003
Location: Maine USA

> Strange graph in PSUD2 (oscillation?)
The physical system CAN'T oscillate (assuming everything ideal WiFi and other crap will throw oscillations into any real build). You are looking MUCH too close. Nothing in here individually moves slower than a second. Yes, if you zoomin to NANOvolts, you can see it is "still climbing" at 30 seconds the rise extends to infinity. This comes to the old joke about the mathematician and the engineer. #2 The program is approximating a continuous system with numbers of limited precision. There WILL be roundoff errors. Each one small, but after a zillion iterations they can start to show at the NanoVolt level. Long repeated computations CAN "oscillate" if they get between a roundup and a rounddown point. Your latest shows that this complicated algorithm can "jump" if rounding crosses some critical point. My uncle could explain this all to you (you could not afford his rates). This is "just an amplifier". You don't need all that precision. 270V or 290V makes no difference to us. SubmV ripple is usually good. Currentsources are *always* problematic in this type of simulation. (They should be current limiters but PSUD's currentthings seem to source power, which "fights" the power coming from the line.) Use a 71k resistor instead. And 3.8mA is mighty darn small to justify a choke, especially in this world of cheap electrolytics. Another stage of RC filter will get you to the same point, in less space and much less cost. Last edited by PRR; 15th February 2020 at 02:02 AM. 
15th February 2020, 03:59 PM  #10 
diyAudio Member
Join Date: Oct 2007

Thanks everybody for you responses!
Ti83: Yes. this makes sense. This is similar to my first intuition about the graphs. PRR: I enjoyed the jokes. Thanks. Point taken. 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
PSUD2 help  mr ed  Power Supplies  4  19th May 2019 02:38 PM 
ESP P101 using rod pcb very strange oscillation issue  brunogiovs  Solid State  12  9th March 2016 07:13 AM 
PSUD2 on Mac  Doug in Indy  Software Tools  5  29th August 2015 12:58 PM 
psud2  merlin el mago  Power Supplies  0  12th December 2012 07:13 PM 
when using PSUD2, do you...  jarthel  Tubes / Valves  13  15th August 2006 03:08 AM 
New To Site?  Need Help? 