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Toroid transformer noise
Toroid transformer noise
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Old 23rd January 2018, 11:23 PM   #1
liquidair is offline liquidair  United States
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Default Toroid transformer noise

Hi All,

I built a really nice tube guitar preamplifier with a toroid power transformer. The unit is extremely quiet except for the transformer spews out EMI (in the form of 120Hz buzz) that gets picked up but the guitar pickups (I'm using a shielded guitar with humbuckers too!). All tube amps do this yes, but this seems excessive. Turning the volume down on the guitar completely silences it.

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.

Some details:
- I overspec'd the trafo to the manufacturer on each winding by about 30%. The manufacturer mentioned they do the same.
- The trafo has a GOSS band for magnetic shielding, which oddly may be making things worse.
- Chassis is thick steel.
- Leads are kept short and twisted tightly.
- Each winding feeds a discrete bridge rectifier with Schottky diodes and a cap across each winding to reduce HF hash from the diodes.
- Using the guitar as a 'sniffer', I notice that the noise is most intense with the pickup right on the trafo, and this dies off rapidly at about 1-2 ft away, but no matter how far I go from there the noise stays the same amplitude.
- Rotating the trafo does nothing.
- Rotating the guitar changes the harmonic spectrum of the noise, but not fundamental amplitude. There seems to only be one spot with a very narrow null in the magnetic field but it's still pretty intense.

How can we reduce this noise? Thank you in advance!

Last edited by liquidair; 23rd January 2018 at 11:47 PM. Reason: spelling
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Old 24th January 2018, 06:34 AM   #2
FauxFrench is online now FauxFrench  France
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I have no experience with vacuum tubes or guitars as input source.
From my ancient experience with discrete transistor amplifiers: How is the grounding of the chassis made?
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Old 24th January 2018, 06:41 AM   #3
trobbins is offline trobbins  Australia
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I can suggest:
- Use a tuned C-RC snubber instead of a cap.
- Use smaller value for first filter cap, and then a buffer (R or L) to larger filtering.
- Use a valve rectifier instead of ss diodes.
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Old 24th January 2018, 12:28 PM   #4
kodabmx is online now kodabmx  Canada
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If it's coming from the transformer it should be 60Hz, not 120Hz which is the hum of the rectified power. Short the input of the amp. Does the hum go away? If so it's being picked up by the guitar/cable, if not it's from the circuit in which case you might have a bad or undersized filter cap. Do you have the schematic?
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Old 24th January 2018, 02:53 PM   #5
liquidair is offline liquidair  United States
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Quote:
Originally Posted by FauxFrench View Post
I have no experience with vacuum tubes or guitars as input source.
From my ancient experience with discrete transistor amplifiers: How is the grounding of the chassis made?
Thank you for your reply! The B+ supply grounds from a point after the last filter cap and ties to a switchable ground lift circuit. Connection is made with a ring washer to #8 PEM stud with a tooth washer to cut into the metal. The powdercoat was masked around the stud for bare contact.

The tube heater supply is elevated via a resistive tap from the B+ and this tap is filtered with a 22uF cap. The ground connection of the tap/capacitor is made to the B+ ground plane.

The Aux supply ground is taken from the last filter cap of the aux supply and it bypasses the ground lift circuit.

A schematic I'm working on will show this in more detail.
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Old 24th January 2018, 03:01 PM   #6
liquidair is offline liquidair  United States
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Quote:
Originally Posted by trobbins View Post
I can suggest:
- Use a tuned C-RC snubber instead of a cap.
- Use smaller value for first filter cap, and then a buffer (R or L) to larger filtering.
- Use a valve rectifier instead of ss diodes.
Thank you for your reply!

I will try the first 2 suggestions, valve rectifier is not possible.

I did try removing the caps altogether and it does not seem to make a difference. As for the smaller first cap, what is the mechanism at play there? The larger filter draws a bigger charge current from the winding, and thus more noise? For each first filter cap I use 2 rather large caps in parallel (3900uF x 2 on the low voltage, series connected 100uF x 2 for the HV).
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Old 24th January 2018, 03:16 PM   #7
liquidair is offline liquidair  United States
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Quote:
Originally Posted by kodabmx View Post
If it's coming from the transformer it should be 60Hz, not 120Hz which is the hum of the rectified power. Short the input of the amp. Does the hum go away? If so it's being picked up by the guitar/cable, if not it's from the circuit in which case you might have a bad or undersized filter cap. Do you have the schematic?
Well it's more of a buzz than a hum, just like this video, although the video has more of a hum component than I'm getting: YouTube

So yes, it does go away when the input of the guitar is shorted, such as turning the guitar volume down all the way, removing the cable from the amp input (shorting jack) or even turning the amp gain down. It's definitely not the circuit but caused the transformer/wiring.

I'm hesitant that it's a filter problem because ripple on the main caps is where it's supposed to be. What is interesting is the waveforms scoped at the secondary fuses. The high voltage winding looks like your decent clipped-top sine wave that you expect, but the two low voltage windings look like more of a square wave with reverse ringing that is like 50% of the cycle.

Schematic is attached.
Attached Files
File Type: pdf PowerSupplyForum.pdf (135.5 KB, 111 views)
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Old 25th January 2018, 06:31 AM   #8
trobbins is offline trobbins  Australia
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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.

If it is mainly 60Hz, then extra shielding around the transformer is a reasnable path to take, if the solid chassis and distance between guitar and amp can't alleviate your concerns.

If 120Hz and higher harmonics are dominant, then reducing the peak of the current waveform (and as a result widening the width of that pulse) is pretty much the path to take (other than shielding).

Having such high peak current levels can exacerbate the diode turn-off related noise issue - which is why a tuned winding snubber may assist, or go back to the future and use valve diodes.

You may be able to simply test the amp using a battery for valve heater supply and control power - that would remove charging pulse currents from the heater windings (given you use DC heater powering, and seem to have a substantial control power requirement - and may help localise where your main noise source is coming from. The heater windings could well be outermost on the toroid, and so not as shielded as the HT secondary winding.

Some background on the rectifier related noise issue is in:
https://www.dalmura.com.au/static/Po...ube%20amps.pdf

Ciao, Tim

Last edited by trobbins; 25th January 2018 at 06:41 AM.
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Old 25th January 2018, 10:01 AM   #9
Kay Pirinha is offline Kay Pirinha  Germany
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Are you sure you didn't construct a short circuit turn around your toroid?
Best regards!
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"Bless you, Sister. May all your sons be bishops." (Brendan Behan)
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Old 25th January 2018, 12:36 PM   #10
Ghianni is offline Ghianni  Greece
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Quote:
Originally Posted by Kay Pirinha View Post
Are you sure you didn't construct a short circuit turn around your toroid?
Best regards!
Yes, may also be some DC from the mains which always is dirty...
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