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Toroid transformer noise
Toroid transformer noise
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Old 26th January 2018, 06:31 PM   #21
liquidair is offline liquidair  United States
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Definitely just got a smoking gun. Here's a couple dual trace shots of the HT and Aux supply. The brighter is the HV winding, the LV is faint. I've never seen anything like this before...it appears the energy from the HV winding is propping up the Aux winding voltage, not allowing it to change. What is this even?
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Old 26th January 2018, 09:23 PM   #22
trobbins is online now trobbins  Australia
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Given that your scope probe gnd is at the neg end of the bridge for each trace, and you have shifted the trace down to allow the full B+ range on the screen, then the flat top and bottom sections are when the bridge diodes are conducting and the winding end is clamped either one diode drop away from gnd or B+.

When the bridge diodes are not clamping the winding ends, then there are only stray capacitances charging up and down to control the voltages that you are probing.

Lets follow the LV winding trace first - it appears to start getting clamped before the HV, and un-clamps about the same time as the HV, and the edges appear a little rounder (which is likely just the channel gain not overlaying the waveforms, but could also be related to the diode voltage drop as a % of the waveform p-p voltage).

I'm thinking the LV trace then doesn't change much for a while because of the large variation in schottky diode capacitance with voltage. The diode that was just conducting exhibits a lot of capacitance when the junction has close to 0V across it, whereas the other diode in that side of the bridge arm is significantly reverse biased and exhibits a much lower capacitance. The winding current that is charging/discharging those diode junction capacitances is not large as it is related to dV/dt, and dV/dt is relatively low for the LV winding as compared to the HV winding. Hence it takes a while for the voltage on that winding end to be charged up enough so that the previously conducting diode capacitance falls enough to not constrain the winding end voltage (and both winding ends get 'clamped' by their previously conducting diode capacitances in the same manner).

In post #11, pic 1, that region of fuzz/noise is during the time when just diode capacitances are determining your trace voltage waveform. That fuzz could be related to diode turn-off energy in one/all the windings trying to find winding current paths to take - but also could be related to you probe gnd position. The advantage of the tuned snubber on each secondary winding is that it can rapidly attenuate the winding currents that appear to be causing that fuzz.

But this is probably just an aside to your main hassle - but none the less worth appreciating.

Last edited by trobbins; 26th January 2018 at 09:31 PM.
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Old 26th January 2018, 10:09 PM   #23
SemperFi is offline SemperFi  Wake Island
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Just to verify the toroid is culprit can u unscrew the cnt bolt and rotate and vary its angle. Have the guitar laying so the noise is obvious. If it is the toroid the noise will change if not it is not the toroid. I think...
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Old 26th January 2018, 10:14 PM   #24
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by liquidair View Post
Thank you for your reply!

valve rectifier is not possible.
)
Did you use Schottky diodes ? You can get bad switching noise with normal diodes.
I made that mistake in a valve mixer, the hum was terrible.
I looked on power supply and there were big switching spikes from the diodes.
Try using HER158G's.
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Old 26th January 2018, 10:21 PM   #25
trobbins is online now trobbins  Australia
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Diode models are identified in the schematic link.
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Old 26th January 2018, 10:25 PM   #26
pieter t is offline pieter t  Netherlands
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Quote:
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The whole reason I went with a toroid was because they were supposed to NOT spew out as much EMI as EI types, and many design choices that I made were to reduce and avoid this in the first place.


IMO that is a misunderstanding.
In toroidal transformers coupling from primary to secondary(ies) is better than in EI transformers; ideally we wish coupling only of the 50/60 Hz line frequency, but toroids maintain coupling up to much higher frequencies and that's not what we want.
I did not hear you mention that there are screens between primary and secondaries; these help to shunt unwanted stray capacitance.
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Old 26th January 2018, 11:21 PM   #27
liquidair is offline liquidair  United States
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Originally Posted by trobbins View Post
But this is probably just an aside to your main hassle - but none the less worth appreciating.
First thank you for the excellent explanation, sincerely. But I really was hoping you wouldn't say that.

After a little more experimentation, I found that if either LV supply fuse was removed, the waveform looked fine. It's only when both are active. Would it be a bad idea to swap the XFMR leads on the Aux supply since the 2 LV supplies are relatively the same? I'm thinking we could maybe cancel some noise that way since the currents will be out of phase.
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Old 26th January 2018, 11:23 PM   #28
liquidair is offline liquidair  United States
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Originally Posted by SemperFi View Post
Just to verify the toroid is culprit can u unscrew the cnt bolt and rotate and vary its angle. Have the guitar laying so the noise is obvious. If it is the toroid the noise will change if not it is not the toroid. I think...
I tried this and no, it doesn't seem to matter. Which is making me wonder because it surely has to be magnetic in nature since the guitar is pickup source.
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Old 26th January 2018, 11:32 PM   #29
liquidair is offline liquidair  United States
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Quote:
Originally Posted by nigelwright7557 View Post
Did you use Schottky diodes ? You can get bad switching noise with normal diodes.
I made that mistake in a valve mixer, the hum was terrible.
I looked on power supply and there were big switching spikes from the diodes.
Try using HER158G's.
Sweet, a valve mixer!? That's awesome.

Ya, all Schottkys. Cree SiC C3D0206F on the HV (which look very comparable to the HERs) and STPS2150s on the LV supply.
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Old 26th January 2018, 11:44 PM   #30
liquidair is offline liquidair  United States
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Originally Posted by pieter t View Post
IMO that is a misunderstanding.
In toroidal transformers coupling from primary to secondary(ies) is better than in EI transformers; ideally we wish coupling only of the 50/60 Hz line frequency, but toroids maintain coupling up to much higher frequencies and that's not what we want.
I did not hear you mention that there are screens between primary and secondaries; these help to shunt unwanted stray capacitance.
No, I chose to not go with a screen and I'm regretting that now I think, lol. I think I thought my line filter/GOSS band would be enough. This could also explain the noise level discrepancy between this build and the professional one with the EI core. I just repeated the spectrum analyzer test and found that mine is actually 1dB lower in noise over the entire spectrum, but like 10 db higher in the 500 Hz to 5kHz range (which is what I hear).

Is there any harm in grounding the Goss band to (hopefully) give some of that noise an easier path back to it's source than the guitar? I read somewhere (Rod Elliot's site I think) that this does help a bit rather than leaving it floating.
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