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Old 5th June 2004, 05:21 PM   #21
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Bricolo, an isolation transformer isolates primary and secondary windings by placing isolation shields between them, reducing interwinding capacitance. That capacitance, in a non-shielded transformer, is sufficiently high to transfer high frequency noise between the windings. The best isolation transformers currently available reduce capacitively coupled noise, through shielding, 150dB. MGE UPS manufactures a variety of such transformers under the brand name Topaz, typically available on the used market at good prices. Elgar once manufactured similar transformers, but recently pulled the (single-phase) line.
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Old 5th June 2004, 05:22 PM   #22
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Default Re: toroid noise rejection

Quote:
Originally posted by serengetiplains


Maybe the best way to reject noise is to cancel it via balancing the AC on a high-bandwidth balanced transformer? The higher the bandwidth, the more high frequency noise generated before or after the transformer is cancelled? See white paper discussions on the Equi=Tech website.

I've tried (still own) lossy EI balanced isolation transformers and opposite-of-lossy high bandwidth toroid balanced isolation transformers and the latter win hands down. I mean, no contest whatever in what I heard.

I'm not sure I got it.
What's the advantage in balancing the AC? I don't see how it would cancel noise.
And AC is already some kind of balanced signal: it's not (directly) referenced to ground, and a transformer is a 2 wire thing. So it balanced or unbalanced, it doesn't see the difference
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Old 5th June 2004, 05:23 PM   #23
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serengetiplains - good post! good point! Balanced power, now you're talking. Something that just dawned on me that may be a great source for headbutting drag down arguments is that some if not all countries in Europe have 240v (???) which would be just the 2 legs of 120v (???) (i.e. balanced ac??) In the states we just have 120v single sided non-balanced power. I'd think the two would probably have different cures for maybe the same ailment sometimes? I can definitely see apples being compared to oranges here.
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Old 5th June 2004, 05:24 PM   #24
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Quote:
Originally posted by serengetiplains
Bricolo, an isolation transformer isolates primary and secondary windings by placing isolation shields between them, reducing interwinding capacitance. That capacitance, in a non-shielded transformer, is sufficiently high to transfer high frequency noise between the windings. The best isolation transformers currently available reduce capacitively coupled noise, through shielding, 150dB. MGE UPS manufactures a variety of such transformers under the brand name Topaz, typically available on the used market at good prices. Elgar once manufactured similar transformers, but recently pulled the (single-phase) line.

I'll go to the shop next week, to see how the isolation transformer is done.

What VA rating do I need? Is the 500VA ok, and 40 a good price?
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Old 5th June 2004, 05:37 PM   #25
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serengetiplains - oops, i just reread my previous post about interwinding capacitance and isoxfmrs. I messed up, so sorry. I guess I got ahead of myself and was describing the split-bobbin type. Yes, the isos are wound with an electrostatic shield that reduce the capacitance. My boner,..can't believe I did that. I need more coffee...will be back. Bricolo, also it's good to ground the static shield if it's not built in. At least all were done that way I've seen.
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Old 5th June 2004, 06:03 PM   #26
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Default Re: Re: toroid noise rejection

Quote:
Originally posted by Bricolo
What's the advantage in balancing the AC? I don't see how it would cancel noise.
And AC is already some kind of balanced signal: it's not (directly) referenced to ground, and a transformer is a 2 wire thing. So it balanced or unbalanced, it doesn't see the difference
A balanced transformer has a center-tapped split secondary. The center tap becomes the ground reference. The two legs measure + and - 60VAC from this reference, ie, are opposite phase from each other, therefore balanced. The Equi=Tech website explains this well.

As to the question of noise reduction via balanced AC, that topic seems very complex. I'll explain a little of my understanding. In a typical power supply circuit, AC current flows in current peaks, charging the input capacitors only where the input voltage exceeds capacitor voltage. These current peaks represent a form of power factor loss, and themselves create voltages across any resistance (any piece of wire will do). The current peaks, for their part, create reverse-going voltages through inductance. Imagine a wire through which current flows. That current will create a magnetic field around the current carrying wire that will collapse to the degree the current reduces. The collapsing magnetic field in turn induces current/voltage opposing the current/voltage that created the magnetic current, shooting this noise back into the AC line. This noise sometimes goes by the name current harmonics, reactive current, etc. That's one form of noise reduction.


Quote:
Originally posted by RetroAudio
Something that just dawned on me that may be a great source for headbutting drag down arguments is that some if not all countries in Europe have 240v (???) which would be just the 2 legs of 120v (???) (i.e. balanced ac??) In the states we just have 120v single sided non-balanced power. I'd think the two would probably have different cures for maybe the same ailment sometimes? I can definitely see apples being compared to oranges here.
AC drawn from 2 x 120VAC lines (= 240VAC) stepped down should be cleaner than AC drawn from 1 x 120VAC. In North America, the neutral line carries current from both 120VAC legs, so has double current harmonics, among other things. Ridding your AC supply of these harmonics is a step in the right direction. The step-down option also, as you said, offers noise reduction in the manner of balanced AC, but only to the degree the secondary windings are precisely balanced. Any winding or coupling difference in the two halves of the secondary winding will reduce noise rejection. A precisely balanced 240VAC-120VAC step down isolation transformer seems to me the best supply transformer a person can obtain.


Quote:
Originally posted by Bricolo
What VA rating do I need? Is the 500VA ok, and 40 a good price?
VA requirements depend on overall current requirements of your system. As a good rule of thumb, transformers should operate cool. Heat in a transformer indicates the transformer is working closer to capacity. Transformer distortion increases the closer a transformer is operated to capacity. Which is to say a larger than smaller transformer is generally preferred.

Mind you, a balanced transformer that reduces noise probably has much higher overall operating headroom than a transformer that does not: the former will of necessity operate with a higher power factor and thus with greater apparent power than the latter.

Search on eBay for "Topaz isolation transformer" (Audiogon also). You should find something.
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Old 5th June 2004, 06:06 PM   #27
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Quote:
Originally posted by RetroAudio
serengetiplains - oops, i just reread my previous post about interwinding capacitance and isoxfmrs. I messed up, so sorry. I guess I got ahead of myself and was describing the split-bobbin type. Yes, the isos are wound with an electrostatic shield that reduce the capacitance. My boner,..can't believe I did that. I need more coffee...will be back. Bricolo, also it's good to ground the static shield if it's not built in. At least all were done that way I've seen.
Retro, you were right. Split-bobbin windings are not as tightly coupled as bi/tri/quad-filar wound, and therefore reduce incoming noise at the point they begin rolling off high frequencies. R-core transformers are lower noise transformers for this reason.
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Old 5th June 2004, 06:35 PM   #28
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Bricolo - it sounds as though you already have your mind made up about trying an iso xfmr. While it's so easy to get caught up with all the theory spouted off around here and get taken up with what people say as gospel, you might want to back up here and think about why the original xfrmr is doing what it's doing. If it is faulty and humming because of it, an isolation xfrmr may not do what you want. Think about it,...it's hum frequency is probably that of the line, in which case it's humming just because you applied power to it. An iso xfrmr would apply power just the same. If it is faulty, is there any way to swap your own gainclone xfrmr for another known to be good? From a friend or someone? I'm of the opinion that maybe the trouble needs to be taken care of directly, not covered up. - Just a thought from someone that still needs more coffee..
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Old 5th June 2004, 07:33 PM   #29
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I thought about this too.


I can't get another transformer for my amp, I can borrow an isolation transformer from a friend.
when I'll test with the isolation transformer, there are 2 possibilities:
-the amp's transformer becomes quiet -> DC on the line (that causes the core to saturate)
-the buzz is still here, and since it's not due to DC, my trafo is faulty
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Old 5th June 2004, 11:15 PM   #30
moamps is offline moamps  Croatia
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Quote:
Originally posted by markp
WHAT!!!???? Please tell me why and how.
Hi,
DC offset can be produced if transformer is (over)loaded with unsymetrical load (for example, heaters with diode power regulation).

Regards
Milan
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