I am looking for a pair of toroidal transformers for a preamplifier power supply, and I recall reading @EUVL in the F5X transformers thread suggesting that a open-core transformer would be better-suited to this purpose than an encapsulated one. https://www.diyaudio.com/forums/group-buys/204203-f5x-transformers.html
I've seen this mentioned elsewhere as an unattributed comment, I think because it should have lower capacitance and transmit less mains noise across the windings?
As Patrick is known to be thorough in his testing, has anyone tested this proposition, or have direct experience of the difference between open and encapsulated cores in a power supply?
I've seen this mentioned elsewhere as an unattributed comment, I think because it should have lower capacitance and transmit less mains noise across the windings?
As Patrick is known to be thorough in his testing, has anyone tested this proposition, or have direct experience of the difference between open and encapsulated cores in a power supply?
It is not possible to increase or decrease the leakage of capacitance by using epoxy or for that matter any insulating potting compound.
Potted small toroids look neater than open core toroids, in my view.
Toroidal Transformers | RS Components
RS have a good range of them both open and potted.
Potted small toroids look neater than open core toroids, in my view.
Toroidal Transformers | RS Components
RS have a good range of them both open and potted.
I'm hoping not to see too much more of them after the amp is built, so whilst the potted units look tidier, appearance isn't as much of a consideration as performance.
My impression is that you can increase inter-winding capacitance with a potting compound because air should have about the lowest permittivity while epoxies, polyurethanes etc. have a higher permittivity than air. This is not really an issue for 50/60Hz transformers with ordinary voltages but can be in important issue for transformers with many turns like high-voltage transformers. In particular for high voltage supplies operated at higher frequencies this is important. Potting lowers the resonance frequency of the transformer.
Potting offers some important qualities for particular needs. Potted transformers are much more mechanically stable if the transformer may be exposed to high G-forces. The insulating properties of a potted transformer are better and far more reliable if there is a risk of humidity or other solvents penetrating the transformer. Potted transformers rarely makes acoustic noise.
On the negative side, potted transformers do not cool as well as not potted transformers, unless they use special materials, and potted transformers may have a reduced temperature operating range.
For not critical use, I take what is available and cheapest which is normally not potted.
Potting offers some important qualities for particular needs. Potted transformers are much more mechanically stable if the transformer may be exposed to high G-forces. The insulating properties of a potted transformer are better and far more reliable if there is a risk of humidity or other solvents penetrating the transformer. Potted transformers rarely makes acoustic noise.
On the negative side, potted transformers do not cool as well as not potted transformers, unless they use special materials, and potted transformers may have a reduced temperature operating range.
For not critical use, I take what is available and cheapest which is normally not potted.
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Toroids do not need t be potted, since they tend to be mechanically quiet. For power transformers, potting will not change performance enough to matter since they operate at a low frequency. Potting will change interwinding capacitance in an audio transformer, and that can affect its performance. It needs to be taken into account when designing the transformer.
Thanks for all the replies.
Toroidal transformers generate mechanical noise at 100Hz and harmonics due to movement of the core, or windings in the magnetic leakage flux, according to an article in Electrical Engineering: Transformers Are Never Silent
I don't know whether this has a material effect on electrical noise, but vibration of audio circuits has a deleterious effect on sound quality in my experience.
As FauxFrench says, encapsulating in epoxy and sand mix should dampen any vibrations, reduce their amplitude and lower their frequency. The effects either way may be trivial, but I'm not aware of any evidence.
I note that Pass Labs and other amplifiers use potted transformers, presumably to mitigate mechanical and/or electrical noise.
So it would seem plausible to specify an encapsulated transformer in order to mitigate any potential issues whilst the disadvantage of poorer cooling isn't relevant in a low-power amp.
Unless anyone has evidence to the contrary?
Toroidal transformers generate mechanical noise at 100Hz and harmonics due to movement of the core, or windings in the magnetic leakage flux, according to an article in Electrical Engineering: Transformers Are Never Silent
I don't know whether this has a material effect on electrical noise, but vibration of audio circuits has a deleterious effect on sound quality in my experience.
As FauxFrench says, encapsulating in epoxy and sand mix should dampen any vibrations, reduce their amplitude and lower their frequency. The effects either way may be trivial, but I'm not aware of any evidence.
I note that Pass Labs and other amplifiers use potted transformers, presumably to mitigate mechanical and/or electrical noise.
So it would seem plausible to specify an encapsulated transformer in order to mitigate any potential issues whilst the disadvantage of poorer cooling isn't relevant in a low-power amp.
Unless anyone has evidence to the contrary?
Yes, you can't just leave the windings loose. Magnetostriction is real, and the unstabilized windings will rub on each other, eventually wearing through the enamel. Then the windings short, and that's like crossing the beams in Ghostbusters. It's a definite reliability issue.
Also, 'Merican transformers generate mechanical noise at 120 Hz, not 100 Hz. Lol.
Leave the windings unpotted, and you can hear the transformer sing along with the audio signal, depending on how much rail cap you have, on top of their 120Hz (100?) drone.
Also, 'Merican transformers generate mechanical noise at 120 Hz, not 100 Hz. Lol.
Leave the windings unpotted, and you can hear the transformer sing along with the audio signal, depending on how much rail cap you have, on top of their 120Hz (100?) drone.
You've got some nerve comparing your wimpy preamp transformer needs against Patrick's F5X's!
A PAIR INDEED....
A PAIR INDEED....
Hahaha A good preamp needs a good power supply, and that's definitely the weakness of this unit.
Patrick's F5X project is fascinating. He follows a rigorous approach, however I'm not sure whether that was applied to the specification of an open-core transformer as I seem to recollect it was based on a recommendation.
There's also the possibility that a well-designed power supply circuit will reduce any mains or low-level transformer electrical noise below the level where it has a material effect on the signal.
Patrick's F5X project is fascinating. He follows a rigorous approach, however I'm not sure whether that was applied to the specification of an open-core transformer as I seem to recollect it was based on a recommendation.
There's also the possibility that a well-designed power supply circuit will reduce any mains or low-level transformer electrical noise below the level where it has a material effect on the signal.