If you use multiple screens you will be further reducing interwinding capacitance and that will allow lower value x and/or y cap values.
Although PE isn’t exactly clean, neither is 10’s of volts of common mode noise that will definitely be a worse problem to deal with.
On some SMPSs, they place a small cap (5-10 nF x-cap) from the supply neutral to the secondary 0 V to effectively create a short at HF. This very effectively deals with HF common mode noise. On the transformer, special winding techniques are used to reduce HF noise. I have a paper on this latter point somewhere at home I will try to dig out.
For the kind of stuff we do here, using linear power transformers aka toroids or EI at mains frequencies, I’d recommend a screen - but if you have forgotten to spec it, it will not ruin your project.
Although PE isn’t exactly clean, neither is 10’s of volts of common mode noise that will definitely be a worse problem to deal with.
On some SMPSs, they place a small cap (5-10 nF x-cap) from the supply neutral to the secondary 0 V to effectively create a short at HF. This very effectively deals with HF common mode noise. On the transformer, special winding techniques are used to reduce HF noise. I have a paper on this latter point somewhere at home I will try to dig out.
For the kind of stuff we do here, using linear power transformers aka toroids or EI at mains frequencies, I’d recommend a screen - but if you have forgotten to spec it, it will not ruin your project.
How can you tell one from the other, by ear?
By the way, how do toroids "sound"?
Please describe different flavours.
In Australia the use of an internal screen between primary and secondary windings for protective earth connection was required by AS3108:1994 (for isolating and safety isolating transformers) to meet a class 1 spec, such that the screen winding is separated from primary and secondary by suitably rated basic insulation, and extends the full width of the windings, and has a cross-section sufficient to cause the upstream cb to open upon a short. Now the relevant standard is AS/NZS 61558.2.9.
IIRC Kapton transformer tape is rated 1000V/layer. We used to use it for industrial isolation amplifiers back when I was still doing that stuff. It’s important to take the tape well past the winding to preserve creepage distances. All the transformers we did were tested to 2500 V for 20 seconds.
I'm assuming that persons on this thread are mostly 'do it your-self-ers (DIY)', and are working in the audio / video realm. I really don't understand why there is so much concern and discussion about a screen. I have designed more than 5,000 toroids in 25 years, for companies all over the world, including VERY HIGH END audio and video companies, and a screen was never part of their specifications. For example, If you purchase a $100,000.00 audio or video system, I would strongly suspect they have specified a toroidal transformer for sure, and they would never have requested a screen. That's why I'm a bit confused about why this is of concern to this thread. Screens were usually required in medical applications, especially those where a patient was connected through probes. Also I recall a screen was sometimes requested between some of the secondary windings, where they wanted to separate power being supplied to analogue and digital circuitry. These issues have never previously arisen at the engineering level from my experience in the last few decades, so I don't have a definitive answer, but leads me to think its a non issue. This may be one of those issues where the theory is valid, but in the real world has little or no effect on product performance.
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Maybe there is a changing perception over time. For example, hum and noise and distortion and digital resolution are all 'performance' parameters that have taken many decades to 'improve on'. Sometimes that improvement has been driven by a slow evolution of aspects like speaker sensitivity, circuit design, digital bit depth and codecs. Sometimes that improvement has been driven by a slow evolution of measurement tools like from AP and using modern soundcards and software and the likes of oscillators from Viktor - sort of like a carrot and a horse, if you can measure some aspect then it can therefore be improved on. I can now measure the difference between using 1N4007 and UF4007 in a power supply from a soundcard acquired noise signature in the tens of kHz - I'll never be able to 'hear' that, but maybe people don't want it carrying through on recordings etc, or just want to improve on something.
That may not be a commercial 'real world', where bean-counters have always pushed for lower audio equipment prices from the 1940-50's, and where I wouldn't flag wave 'VERY HIGH END' audio as the doyen of performance specs (unless it could be massaged into audiophoolery advertising). In this day of enhanced measurement, and where measurement is likely driven by the DIY community (especially of aging retired professionals), then perhaps there is some reality to it.
That may not be a commercial 'real world', where bean-counters have always pushed for lower audio equipment prices from the 1940-50's, and where I wouldn't flag wave 'VERY HIGH END' audio as the doyen of performance specs (unless it could be massaged into audiophoolery advertising). In this day of enhanced measurement, and where measurement is likely driven by the DIY community (especially of aging retired professionals), then perhaps there is some reality to it.
The very best measuring amplifiers are not made by high end audio companies who for the most part are happy to sell amplifiers with 1% distortion and not much better than -80 dBr mains noise floors for $ thousands. Stereophile is littered with examples. The best amplifiers in the world are being designed and constructed within the DIY audio community. When you take an amplifiers that can achieve close -130 dBr noise floors and 1-2 ppm distortion levels at or near full power, every little thing counts. I’d be less inclined to dismiss the inclusion of a screen and more interested to understand why it was being specified. YMMV
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Stereophile’s measured results (from their tech guru John Atkinson) and some of the measured real designs from people on this forum. I’d say the only standout commercial stuff wrt to measurement is Bruno Putzeys class D stuff - but that uses SMPS.
I put together a summary of recent THD measurements from Stereophile. I did not include tube amplifiers which easily hit 1% THD.
Ed
Ed
Not to involve Bonsai’s own designs, we could use this one:
https://www.diyaudio.com/community/threads/diy-class-a-b-amp-the-wolverine-build-thread.385920/
Top dogs form the Stereophile list, compiled by EdGr, are embarrassingly far behind.
There are other examples.