Simple, no-math transformer snubber using Quasimodo test-jig
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Mark Johnson
diyAudio Donor
diyAudio Member

Join Date: May 2011
Location: Silicon Valley
Simple, no-math transformer snubber using Quasimodo test-jig

A power transformer snubber is a wonderful thing for reducing or eliminating RFI from rectifier-induced LCR ringing. Unfortunately it's a huge pain to design and optimize a snubber. First you have to measure the transformer's leakage inductance and secondary capacitance, at about 100 kHz, which is not especially easy. Then you have to estimate the capacitance of your rectifier(s), which does not always appear on datasheets. Finally you plug these numbers into a formula that spits out snubber values -- and then you hope it's all correct.

Shown here is a little test jig called "Quasimodo the bell-ringer" which makes this process a great deal simpler. Quasimodo connects an actual snubber across the transformer, smacks the transformer to make it ring, and you observe the ringing on an oscilloscope. Then you adjust a (25 turn) potentiometer on the jig, watching the scope to find the setting which completely damps out all ringing. And you're done! Just use the same snubber values in the end product, as you used on the test-jig, and success is yours. (The 25 turn trimpot is socketed for ease of measuring the final resistance that gave perfect damping).

Attached are some scope waveforms showing a Quasimodo jig driving an Avel Lindberg toroidal transformer (from the Akitika GT-101 power amp). I set the trimpot to 5 different resistance values, and got the 5 traces shown. When the trimpot was removed from its socket (R = infinity), I got the black trace. A setting of Rtrimpot = 109 ohms (red trace) gave "critical damping" with no ringing: see yellow arrowhead.

Quasimodo gave an optimum snubber having critical damping (Greek letter zeta = 1.0), without requiring any calculations and without measuring the transformer's inductance or capacitance. It is a quick procedure, too: set Rtrim to max, observe waveshape on scope while reducing Rtrim, stop when all ringing is completely damped out, remove Rtrim from socket and measure it with an ohmmeter. About 3 minutes from start to finish.

This Avel Lindberg transformer has dual primaries for 115V/230V operation. So I configured the primaries the other way and ran Quasimodo again: figure attached. Not surprisingly, the optimum snubber for 230V operation is (slightly) different than the optimum for 115V operation: 120 ohms versus 109 ohms. Plus or minus the error tolerance of my ohmmeter, of course!

I've also attached a .pdf note that contains more details, schematics, construction tips, user guide, more "Quasimodo in action" scope photos, plus a bit of theory and a list of references.

EDIT 1: I had a small number of extra PCBoards + kits of all parts, which I sold at my cost, in October - December, 2013. They are all gone now, and I have no plans to sell any more. I encourage any energetic and enthusiastic diyAudio member to organize a Group Buy, using the PCB Gerber files and Bills Of Materials I provide in this thread. "CheapoModo" (a low cost version of Quasimodo) kits and PCBs are available in the Vendor's Bazaar, here.

EDIT 2: I have attached the PCB Gerber files and the Bill Of Materials, for both boards (V3_SMD, and V4_thru_hole) right here, to post #1. So now they are very easy to find!

1. Build guide for V3 (SMD) is found in post #27
2. Build guide for V4 (thru hole) is found in post #103
3. 2 hour build time, quick-and-dirty Quasimodo on solderless protoboard (no PCB!) is found in post #18 and in (the CheapoModo thread)
4. Substitute parts recommendations are found in the Bill Of Materials
5. How to choose a MOSFET besides the ones in the BOM, is found in post #175
6. How to check your own BOM before purchasing components, is found in post #203
7. diyAudio members who have ordered their own sets of PCBoards from a PCB fab, using the Gerbers provided here, include: gazzagazza, luvdunhill, Borges, stormsonic, cwtim01, normundss, dsolodov, EUVL, kissmurphy, stephengrenfell, Piersma, SyncTronX, yoaudio, andrensairr. You can PM them to find out how easy or difficult it was.
. .
Attached Images
 Avel_USA.png (78.5 KB, 21698 views) Avel_Europe.png (70.1 KB, 21239 views) Quasimodo_V3_SMD_sch.png (48.6 KB, 21498 views) Quasimodo_V4_TH_sch.png (47.7 KB, 21414 views)
Attached Files
 Quasimodo_jig_revA.pdf (817.4 KB, 6524 views) QuasimodoSMD_Gerbers_and_everything_else.zip (104.7 KB, 1256 views) Quasimodo_TH_Gerbers_and_everything_else.zip (347.1 KB, 1972 views)

Last edited by Mark Johnson; 26th January 2019 at 02:22 PM. Reason: link to CheapoModo

 26th September 2013, 03:02 AM #2 Johno   diyAudio Donor diyAudio Member     Join Date: Oct 2010 Location: Tanunda SA Mark, Nice test jig and excellent application note. It surprises me that many people who you would expect to know better do not understand that it is the resistor that does the snubbing and not the capacitor. The capacitance across the secondary (or across the rectifiers) only changes the ringing frequency and the capacitor in series with the snubber resistor is only there to limit its dissipation. Somewhat of an evangelist for snubbers, I had been using the parametric measurement and calculation process for years and only recently turned to an in-circuit empirical approach similar to yours. I now use Cx = 220n and Cs = 100n on all my snubbers and simply adjust a 2k single turn potentiometer to find the Rs value for optimum damping. However I do this in the actual, live circuit at close to the rated load current (I have some very large 10R resistors). Final point is that the snubber components need to be located as close to the transformer as possible for the best results.
 26th September 2013, 03:38 AM #3 gootee   diyAudio Member   Join Date: Nov 2006 Location: Indiana Finally! Two people in a row who understand how snubbers work! (Make that three.) Good work!! __________________ The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
 26th September 2013, 03:47 AM #4 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Good stuff! __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 26th September 2013, 03:58 AM #5 TonyTecson   diyAudio Moderator Emeritus     Join Date: May 2003 Location: Maybunga, Pasig City __________________ https://www.facebook.com/tony.tecson
 26th September 2013, 12:38 PM #6 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Thank you. __________________ regards Andrew T.
 26th September 2013, 02:07 PM #7 Bixbacon   diyAudio Member     Join Date: Apr 2012 Location: Australia Great stuff What a treat, thanks for the insight __________________ Its not worth saying, if it's not heard wright.
 26th September 2013, 02:22 PM #8 ticktock   diyAudio Member     Join Date: Dec 2006 Great work !! It's true that there's still lots of half-whisdom out there, even when you look at commercial gear. __________________ thanks Blaue Leuchtdioden machen blöd. (hb) sent from NSA monitored device
 26th September 2013, 02:57 PM #9 Max Headroom   Banned   Join Date: Jan 2013 Location: West Australia, near to the beach, natural ambient sounds mostly. Hello Mark J (?), nice work, and thanks for your your thorough documentation. Recently I got hold of a variable frequency/variable voltage regenerative AC power supply http://www.kikusui.co.jp/kiku_manuals/P/PCR_L_E7.PDF to try/prove a few experiments. First on the list was to explore power supply frequency dependence of typical consumer level audio gear. Subjective testing reveled that 50Hz supply sounded best with 50Hz recordings, 60Hz supply sounded best with 60Hz recordings...lowered non musical intermod products is the resultant, but 55Hz (which is musically related) sounded best regardless of supply or recording supply frequency. With optimal snubbing this ought to be a moot point, but of course with typical audio gear none of this optimisation is implemented, and PSU ringing will crash straight through any PSRR, sub optimal earthing arangements further exacerbating the products/problems. To extend the discussion, IME optimal snubbing applied directly across speaker drivers, and then further across the input terminals of loudspeakers to achieve flat impedance characteristic pays huge sonic dividends...most amplifiers measure well into resistance loads, but many can go sour when driving reactive loads. Snubbing is a far too frequently ignored subject, and IME/IMO of paramount importance. Regards, Dan. Last edited by Max Headroom; 26th September 2013 at 03:14 PM.
Mark Johnson
diyAudio Donor
diyAudio Member

Join Date: May 2011
Location: Silicon Valley
Here's a photo of Quasimodo V.3 in operation. The 25 turn trimmer RV1 is clearly visible (dark blue box, upper right), and so is the injection capacitor C2 (light blue box, center), plugged into its socket with many pins. You also can see the three 8pin SOIC packages. It's a two layer board and if you look around you can find some vias. Frontside fill is VCC, backside fill is GND.
Attached Images
 twochan.jpg (477.9 KB, 20525 views)

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