Best toroidal power transformers for audio?
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mjk said:GOSS = Grain Oriented Silicon Steel. Apparently most transformer steel contains the element Si... something to do with increasing the electrical resistance while not affecting the magnetic properties.
I obtained some by unwinding the core of an old toroidal power transformer with open windings. Actually it was a new Hammond right out of the box with an open primary
Magnetic shielding floobymaterial is also available at http://lessemf.com in small quantities
I spec power transformers for 2X-3X the load VA if using capacitor input rectifier filters. For 15-25VA load I would use a 50VA transformer.
Still awaiting the results of my own toroid shielding experiments. Another possibility is to reduce Bmax and potentially reduce radiated EMF through using a buck winding on the primary; drop the voltage down from 120V to ~110V.
Cheers,
Michael
Very informative! Thank you Michael. Does anyone know where you could buy a strip off GOSS to wrap around a PT?
mjk
Well-known member
schmidlin said:
Not yet, but in a few days I'll have some results for a few different materials and configurations using my ultra-sensitive near-stray field detector (aka "microphone input stage"). I purchased samples of 3 different materials plus I have some GOSS strip from unwinding the Hammond core.
The lessemf.com site is full of EMF fear-mongering but a convenient source for internet ordering. There is another web source I used in the past but it apparently has a lower googlerank these days...
Stagefright13
Well-known member
earthsled I am just an amateur experimenter. That aside I use double the VA requirement for toroid transformers. At least. Avel Lindburg has them so cheap you might as well. Thru my research the flux and noise is possibly due to the load. If it works hard it gets noisey. But like I said I am an amateur. But has been working for me. And I use weird star ground schemes. That are silent.
On my 1108 pres that I am building now I spent an extra 4 bucks for the biggest toroid that fits a 1U rack. And I always go for a bit more AC than spec. More headroom? Dunno or care. Shoot the regulator with a infared gun if interested. But ALWAYS way under temp spec.
John
On my 1108 pres that I am building now I spent an extra 4 bucks for the biggest toroid that fits a 1U rack. And I always go for a bit more AC than spec. More headroom? Dunno or care. Shoot the regulator with a infared gun if interested. But ALWAYS way under temp spec.
John
I've sourced some heavy steel cases from Antek:
=3]http://www.antekinc.com/lview.php?d[]=3
Nice mounting system, too: good for external apps.
But they seem to be missing the 50VA size: I'm calling them Monday.
These cases sure seem more robust than the bands; I would *think* they sheild better for about the same cost.
=3]http://www.antekinc.com/lview.php?d[]=3
Nice mounting system, too: good for external apps.
But they seem to be missing the 50VA size: I'm calling them Monday.
These cases sure seem more robust than the bands; I would *think* they sheild better for about the same cost.
link is "unprintable" in this forum software, you need to look around a bit at that site.
pdf datasheet: http://www.antekinc.com/pdf/CA-xxx.pdf
Looks very good indeed!
Jakob E.
pdf datasheet: http://www.antekinc.com/pdf/CA-xxx.pdf
Looks very good indeed!
Jakob E.
mjk
Well-known member
I have a 50VA toroid from Antek and the 100VA cover. It's 1/8" thick welded steel 
The 50VA toroid fits over the center post of the cover and will leave enough space between the toroid and the cover for an inner shell of EMF shielding material, which will face my audio circuit. I have high hopes for this arrangement, sort of an inner and outer clam shell. If it works it saves me fabricating a transformer cover and provides a little extra shielding. In my application the transformer is mounted on the back of a 3U enclosure.
The 50VA toroid fits over the center post of the cover and will leave enough space between the toroid and the cover for an inner shell of EMF shielding material, which will face my audio circuit. I have high hopes for this arrangement, sort of an inner and outer clam shell. If it works it saves me fabricating a transformer cover and provides a little extra shielding. In my application the transformer is mounted on the back of a 3U enclosure.
Actually, this is the first commercially available magnetic shield for a toroid that I have seen. Been looking for a long time!
Thanks!
Jakob E.
Thanks!
Jakob E.
ricothetroll
Well-known member
Doing some archeology here...
I'm currently building some mic pres based around the THAT1512 chip (if any use, schematic and layout here :http://groupdiy.com/index.php?topic=56611.msg722145#msg722145). I'm experiencing some transformer magnetic noise issues. I've tried every possible position and orientation, I still have some hum, yet buried in the preamp's noise when the trafo's position is at the "sweet spot" (input pin 2 and 3 shorted) but still I'd like it to be quiter. I still have a 50Hz component, that BTW seems not to be dependant of the trafo's position, and some harmonics remaining.
Abbey Road d'Enfer said in this post (reply #2) that lowering the magnetic flux in the trafo could reduce the radiated noise. There's this french company, Audiophonics, that makes custom toroidal trafos. I guess I could ask them to make such a trafo ! I've already ordered one from them and was pretty satisfied.
What are the parameters involved ? I guess that means a bigger core but I think ther's a lot more involved and I'd like to be precise when I order it.
I guess this could be useful for other people...
Thanx in advance !
Best regards.
Eric
PS : Audiophonics' website : http://www.audiophonics.fr/transformateur-torique-sur-mesure-toutes-puissances-p-3675.html
I'm currently building some mic pres based around the THAT1512 chip (if any use, schematic and layout here :http://groupdiy.com/index.php?topic=56611.msg722145#msg722145). I'm experiencing some transformer magnetic noise issues. I've tried every possible position and orientation, I still have some hum, yet buried in the preamp's noise when the trafo's position is at the "sweet spot" (input pin 2 and 3 shorted) but still I'd like it to be quiter. I still have a 50Hz component, that BTW seems not to be dependant of the trafo's position, and some harmonics remaining.
Abbey Road d'Enfer said in this post (reply #2) that lowering the magnetic flux in the trafo could reduce the radiated noise. There's this french company, Audiophonics, that makes custom toroidal trafos. I guess I could ask them to make such a trafo ! I've already ordered one from them and was pretty satisfied.
What are the parameters involved ? I guess that means a bigger core but I think ther's a lot more involved and I'd like to be precise when I order it.
I guess this could be useful for other people...
Thanx in advance !
Best regards.
Eric
PS : Audiophonics' website : http://www.audiophonics.fr/transformateur-torique-sur-mesure-toutes-puissances-p-3675.html
Basically, you specify the transformer to work with lower magnetic flux levels, so you get away from magnetic overload. The simple way is to specify a higher primary voltage, giving more primary turns (and then taking into account the needed secondary voltage)
Example: You need a 230V:2x15V - you order a 300V:2x20V that , when run at 230V, operates at less magnetic flux. (Actually, you'd just specify a lower core flux to the manufacturer)
Other tricks are:
- Addition of a strip of core material around the outer circumference of the transformer.
- Winding back-and forth instead of continuous, so your windings has less average voltage potential between them (dunno why/how this works for reducing emission, but my transformer guy suggested this, and it further attenuates dirt)
- Keeping the output leads in a bundle instead of distributing it around the transformer - this makes it easier to position the residual dirt-flow.
But I'm no expert in this at all - it's a pretty complex area - I'd suggest that you find the oldest transformer design guy available, and pick his brain.
Jakob E.
ricothetroll
Well-known member
Hi Jakob,
Thanx a lot for your answer !
I asked the guy of Audiophonics an he answered me that it was technically possible but that he doubted that it could have any impact on my hum issue, and adviced me to use "R-type" trafos instead.
Doing some researches about flux and toroidals, I found another reference to the technique you named in Glen Ballou's Handbook for Sound Engeneers, 4th edition :
"Toroidal power transformers can have a weaker radiated magnetic field than other types. Using them can be an advantage if audio transformers must be located nearby. However, a toroidal transformer must be otherwise well designed to produce a low external field. For example, every winding must completely cover the full periphery of the core. The attachment points of the transformer lead wires are frequently a problem in this regard. To gain size and cost advantages, most commercial power transformers of any kind are designed to operate on the verge of magnetic saturation of the core. When saturation occurs in any transformer, magnetic field radiation dramatically increases. Power transformers designed to operate at low flux density will prevent this. A standard commercial power transformer, when operated at reduced primary voltage, will have a very low external field—comparable to that of a standard toroidal design."
I guess the Audiophonics guy doesn't know that trick !
About the other tricks you gave, when you talk about a strip of core material, do you mean laminations of EI style trafos ?
Best regards.
Eric
Thanx a lot for your answer !
I asked the guy of Audiophonics an he answered me that it was technically possible but that he doubted that it could have any impact on my hum issue, and adviced me to use "R-type" trafos instead.
Doing some researches about flux and toroidals, I found another reference to the technique you named in Glen Ballou's Handbook for Sound Engeneers, 4th edition :
"Toroidal power transformers can have a weaker radiated magnetic field than other types. Using them can be an advantage if audio transformers must be located nearby. However, a toroidal transformer must be otherwise well designed to produce a low external field. For example, every winding must completely cover the full periphery of the core. The attachment points of the transformer lead wires are frequently a problem in this regard. To gain size and cost advantages, most commercial power transformers of any kind are designed to operate on the verge of magnetic saturation of the core. When saturation occurs in any transformer, magnetic field radiation dramatically increases. Power transformers designed to operate at low flux density will prevent this. A standard commercial power transformer, when operated at reduced primary voltage, will have a very low external field—comparable to that of a standard toroidal design."
I guess the Audiophonics guy doesn't know that trick !
About the other tricks you gave, when you talk about a strip of core material, do you mean laminations of EI style trafos ?
Best regards.
Eric
naa - just a strip of the same material that is used for winding the transformer core, going all way around the outer circumference of the toroid - "H" in this pic:
ricothetroll
Well-known member
Ok now I see ! Do you know where those can be bought in Europe? I suppose it's better to connect it to the case, right ?
Best regards.
Eric
Best regards.
Eric
That strip is laid-in from factory if ordered. It's a magnetic shunt, needing no grounding or such.
ricothetroll
Well-known member
Hi,
I had one of those evil max-fluxed toroid I could loose for some experimentation. I removed the windings - a time-consuming task BTW - and wrapped some of the laminations around the rafo of my 8 channel green pre, that has a few channels with a slight hum (no orientation of the trafo can null the hum on all channels at the same time...). With the laminations I reduced the hum of about 5dB.
The size of the core of the frafo I sacrificed is about half the height of my 8 channel green preamp. I then wrapped it up and down to cover the whole surface. Would it be more efficient to use a bigger lamination that covers the whole trafo ?
Best regards.
Eric
I had one of those evil max-fluxed toroid I could loose for some experimentation. I removed the windings - a time-consuming task BTW - and wrapped some of the laminations around the rafo of my 8 channel green pre, that has a few channels with a slight hum (no orientation of the trafo can null the hum on all channels at the same time...). With the laminations I reduced the hum of about 5dB.
The size of the core of the frafo I sacrificed is about half the height of my 8 channel green preamp. I then wrapped it up and down to cover the whole surface. Would it be more efficient to use a bigger lamination that covers the whole trafo ?
Best regards.
Eric
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