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Toroid transformer noise
Toroid transformer noise
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Old 25th January 2018, 10:56 PM   #11
liquidair is offline liquidair  United States
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Quote:
Originally Posted by trobbins View Post
You can record a sample, or set up a live microphone test, and then use free spectrum analyser software to see if it is mainly 60Hz or 120Hz. There are also free spectrum analyser apps for cell/mobile android phones.
First, thank you so much for your excellent reply!

I just did this, there's no 60Hz component. It's all 120Hz+, looks to be 120Hz intervals throughout the entire audio bandwidth of the amp at pretty consistent amplitudes.

The rest of your post was a goldmine of information!

So I tried to install snubbers and ran into some very interesting things. First, I observed the LV supply at the fuse and saw the same nasty ringing I saw yesterday (pic 1). Then I observed the HT supply which looks pretty clean (pic 2). I then removed the HT fuse and observed the LV supply and most of that nasty ringing is gone, but you can see some ringing now on the front side of the peak whereas before the ringing was on the back side (pic 3). Finally, I added a 100nF + 100ohm snubber with a Cx of 10nF and this made the ringing worse (pic 4).

This is all very confusing...Can someone explain what is going on?

I'm now attempting to figure out the snubber values based on the Quasimodo method mentioned in that excellent article that was attached.
Attached Images
File Type: jpg LowVoltageSupply.jpg (570.9 KB, 163 views)
File Type: jpg HTSupply.jpg (601.9 KB, 161 views)
File Type: jpg LV-NoHTFuse.jpg (705.1 KB, 157 views)
File Type: jpg LV-Snubber.jpg (732.9 KB, 155 views)
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Old 25th January 2018, 10:58 PM   #12
liquidair is offline liquidair  United States
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Originally Posted by Kay Pirinha View Post
Are you sure you didn't construct a short circuit turn around your toroid?
Best regards!
Lol, why does everyone love the shorted turn? No, no shorted turn, the center bolt is well away from the chassis top, but in this case I didn't even have the bolt installed.
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Old 25th January 2018, 11:34 PM   #13
trobbins is offline trobbins  Australia
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Nice to see a real world, albeit complicated, situation as you have multiple rectified current pulses passing through different windings, and having different conduction durations and leakage inductances

For clarity, where were the probe ground and tip connected for the scope photos? And were pic 1 and pic 3 at the same probing locations, but just with CH1 gain and offset different?

More insight may also be gained by putting a trace of winding conduction current (eg. a sense resistor between bridge neg and 0V) on the screen along with the related winding arm voltage.

The low resistances related to the toroid windings and ss diodes and large filter caps appear to be showing up diode 'turn-on' ringing effects as well as the more typically observed turn-off effects. The issue is effectively the same - a current wants to step change. There is also a chance that diode turn-on recovery is also coming in to play.

Anyway, it would be easier to discern behaviour details if you could obtain dual trace setups. That would also be good to set up, as a way to show any change from introducing a tuned snubber as compared to perhaps the base case of no local snubber cap, or your default snubber cap.

Ciao, Tim
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Old 26th January 2018, 12:16 AM   #14
liquidair is offline liquidair  United States
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Originally Posted by trobbins View Post
For clarity, where were the probe ground and tip connected for the scope photos? And were pic 1 and pic 3 at the same probing locations, but just with CH1 gain and offset different?
Ground was attached to PCB screw attached to the chassis via a PEM standoff, and the measurement was made with the tip was on the fuse itself, on the rectifier side, in both cases. And ya, the gain was different, I was trying to really capture the ringing but never took a pic for a side by side comparison.

Quote:
Originally Posted by trobbins View Post
More insight may also be gained by putting a trace of winding conduction current (eg. a sense resistor between bridge neg and 0V) on the screen along with the related winding arm voltage.

The low resistances related to the toroid windings and ss diodes and large filter caps appear to be showing up diode 'turn-on' ringing effects as well as the more typically observed turn-off effects. The issue is effectively the same - a current wants to step change. There is also a chance that diode turn-on recovery is also coming in to play.

Anyway, it would be easier to discern behaviour details if you could obtain dual trace setups. That would also be good to set up, as a way to show any change from introducing a tuned snubber as compared to perhaps the base case of no local snubber cap, or your default snubber cap.

Ciao, Tim
I could likely do both experiments tomorrow. I'm a bit confused I have to admit. For the most part I get the basic concepts at play here (charging currents, different conduction durations and leakage inductances), but I am struggling to put it all together into how to capture more evidence required for a solution. It sounds like this may be a much bigger problem than just slapping a snubber on each winding and calling it a day, no?

I'll play around and see if I can come up with anything that appears useful. Should I even bother with my plan to inject a square wave with a function gen with all the other windings shorted and attempt to tune a snubber for each winding?

Thank you so much!
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Old 26th January 2018, 12:41 AM   #15
trobbins is offline trobbins  Australia
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I can appreciate it is a long haul when its you doing all the work. I can profer that it should be a good learning curve, and should allow a better understanding of the influences at play.

The tuned snubber is just about minimising the disturbance that can become noticeable at diode turn-off. By circulating that disturbance energy within the winding in the easiest damped path, it can reduce transfer of some of that disturbance energy to other secondary windings, as well as escaping in to circuitry. It also has the advantage of cleaning up scope waveforms, so as not to distract from the bigger picture.

The magnitude and shape (harmonic structure) of charging pulses won't be changed by the snubber - those pulses will always be there. The concern then rises if they are complicit in your 120Hz noise transfer hassle. It looks like that is the main concern to focus on here. How to improve that situation comes I suggest from a better understanding - starting with the waveforms of individual windings, and whether some windings cause more noise egress than others (as identified by removing the B+ winding fuse).

For probing, you may get a cleaner (less prone to other parasitic signals) waveform by connecting the ground clip to the bridge negative 0V. That is elevated for the heater winding (should still be ok to clip there - or temporarily ground that bridge neg end for testing).

Last edited by trobbins; 26th January 2018 at 12:43 AM.
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Old 26th January 2018, 08:02 AM   #16
SemperFi is online now SemperFi  Wake Island
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How far away do u go till tha hum is tolerable? The angle of your pickup should also alter the hum intensity. Something is radiating 120Hz really well which implicates a big loop in there.
Itd be interesting to see your layout in there.
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Old 26th January 2018, 03:44 PM   #17
liquidair is offline liquidair  United States
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Originally Posted by trobbins View Post
I can appreciate it is a long haul when its you doing all the work. I can profer that it should be a good learning curve, and should allow a better understanding of the influences at play.
I completely agree and I have no problem doing the leg work; spent all day researching and experimenting. My problem has always been when I don't fully understand something, I brute force it with will and determination, which is exhausting, lol.

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Originally Posted by trobbins View Post
The magnitude and shape (harmonic structure) of charging pulses won't be changed by the snubber - those pulses will always be there. The concern then rises if they are complicit in your 120Hz noise transfer hassle. It looks like that is the main concern to focus on here. How to improve that situation comes I suggest from a better understanding - starting with the waveforms of individual windings, and whether some windings cause more noise egress than others (as identified by removing the B+ winding fuse).
So, I started thinking about the current pulse, and my first thought was to simply remove a cap from each supply, cutting the reservoir in half. But I realized that would increase the charge pulse's current because the load stays the same (but the pulse width would be wider). The only way to reduce the charge pulses and to increase their width is to add resistance to the XFMR/BR/Reservoir loop. This problem seems to be a case of the dark side of trying to be too perfect: toroid transformer, schottky diodes, high value, low ESR caps in parallel (which halves the ESR). I likely designed this problem in!

Quote:
Originally Posted by trobbins View Post
For probing, you may get a cleaner (less prone to other parasitic signals) waveform by connecting the ground clip to the bridge negative 0V. That is elevated for the heater winding (should still be ok to clip there - or temporarily ground that bridge neg end for testing).
I plan to do this in a bit. I actually removed the heater elevation wondering if the elevation could have been causing some DC to flow in the coil but it appears to not. At least this will be safe!
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Old 26th January 2018, 04:47 PM   #18
liquidair is offline liquidair  United States
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Originally Posted by SemperFi View Post
How far away do u go till tha hum is tolerable? The angle of your pickup should also alter the hum intensity. Something is radiating 120Hz really well which implicates a big loop in there.
Itd be interesting to see your layout in there.
It's interesting, at about 1-2 feet away the noise drops off considerably but most of that seems to be higher harmonics. From that point I can get about 6-8 ft away (limited by headphones) and it will not decrease further. I can angle a bit and it appears the best position is 45 deg facing the unit on the left side.

I just hooked up another preamp (professionally made with EI core XFMRs) and it's buzzing is about 30% less, but the higher frequency harmonic content is drastically less than my unit. It seems like the problem actually is more above 120Hz! Oddly, with the guitar volume down or unplugged, my unit has far less noise and no buzz (the professional unit does still buzz in this case).

As far as loop areas, I tried really really hard to minimize every single loop area. Loops are either routed right next to or directly on top of one another for the most part, even the transformer leads are super short (3.75" for the primary, 1.75" secondaries).

Attached are the supply layouts. First is HV, second is the heater supply (left) and Aux supply (right).
Attached Images
File Type: png HVSupplyLayout.png (90.1 KB, 128 views)
File Type: png LVSuppliesLayout.png (162.7 KB, 34 views)

Last edited by liquidair; 26th January 2018 at 04:48 PM. Reason: Grammar
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Old 26th January 2018, 05:36 PM   #19
epicyclic is offline epicyclic  United Kingdom
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Thinking about the decrease in buzzing with distance element of your problem . Have you checked the preamp is not oscillating .
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Old 26th January 2018, 06:17 PM   #20
liquidair is offline liquidair  United States
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Originally Posted by epicyclic View Post
Thinking about the decrease in buzzing with distance element of your problem . Have you checked the preamp is not oscillating .
I just checked and no, there's no evidence of oscillation audibly or on a scope. Sound is very clean and smooth. It's not to say it's not, but it would likely have to be 50 MHz +. I was pretty diligent about using grid stoppers right on the valve bases with resistor vias on the pads of the tube. The power supply nodes are all super-clean too, the issue appears to be related solely to the transformer and immediate connections.
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