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Old 6th June 2004, 01:12 PM   #1
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Default Mains Isolation

Im not quite sure where to post this but thought SS would fit ok.

Right first of all, what do you all think of isolation transformers, ie a 240-240 xformer. Putting this from the mains outlet and feeding the hifi after it, would it have any benafits a normal transformer in the powersupply wouldnt filter out??

Plus I am looking to build a good filter unit but dont really know where to start, have any of you any ideas or links to web sites with relavent information? or even schematics for the isotek devices, which I would purchase but I'd need the 2k varient and dont fancy the huge pricetag.

Cheers Matt
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Old 6th June 2004, 01:19 PM   #2
hacknet is offline hacknet  Singapore
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i have seen people change the mains voltage into DC. using a 50hz osscilator with voltage feedback to regulate the output and an amp thats capable of 230v AC out, you should get super clean 230v 50hz AC.
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Old 6th June 2004, 01:34 PM   #3
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Have a look at Jon Risch's website. It has some good info on line filters and a surge suppressor.

Regards,
Dan
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Old 6th June 2004, 01:46 PM   #4
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Quote:
you should get super clean 230v 50hz AC.
You should, but in practice you don't. The DC feeding the oscillator/amp is derived from the mains and suffers a fraction of the mains noise. I was really annoyed after noticing the mains cord of the regenerator is still clearly audible.


Most forms of mains filtering (iso transformers as well) are detrimental to the sound, at least to my ears. If it raises the mains impedance it can't possibly sound good. John Risch's filter seems better than most due to the very low resistance of the inductors and the minimal part count.
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Old 6th June 2004, 02:11 PM   #5
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Cheers for the link Ive had a look and that seems to be what I was after, something simple. I had no idea what kind of part values to use. Next time I put an order thru to maplin or farnell ill see what they have on offer to make one of em and see how if its worth it.
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Old 6th June 2004, 05:05 PM   #6
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I've made up one of Jon Risch's line filters using Wima's FKP-1 capacitors and Hammonds 1538M13 chokes . I'm planning on installing it in my CDP in the near future.

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Dan
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Old 15th June 2004, 09:26 AM   #7
skippy is offline skippy  United Kingdom
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It's a real shame that we have to invest effort with mains filters, but there are just so many hi-fi manufacturers who avoid this issue. The whole point of a PSU is to provide a clean, stable supply, no matter what the input is doing (within reason). A whole host of European test (part of the CE directive) were devised to help with this, but they can be circumvented so easily in audio.

We all know a decent PSU helps make audio sound good.

Many shun switched-mode PSUs, becasue they generate noise; however iof the designer can work with this and filter it, any mains-borne noise is not an issue.

Some companies like Linn are already using SMPSUs in their top-end equipment.

Class D amps are effectively SMPSUs and they are now receiving very good reviews.

When will the rest of the industry clean up its act? I'm fed up with reading report of a piece of hifi sounding different from one location to another, just becasue the mains power is different.
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Old 15th June 2004, 09:33 AM   #8
skippy is offline skippy  United Kingdom
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Quote:
Most forms of mains filtering (iso transformers as well) are detrimental to the sound, at least to my ears. If it raises the mains impedance it can't possibly sound good. John Risch's filter seems better than most due to the very low resistance of the inductors and the minimal part count.
The resistance of the inductors is generally trivial compared with the wiring back to the sub station! And I'm not sure why fewer parts would be better, when more stages could attenuate noise more.

Jon's filters seem to be quite good. For best effect, mains filters should really be mounted inside the hifi unit with wiring kept ideally under an inch long where possible. (Obviously this requires an appreciation of mains product safety wiring and is not recommended to anyone.)

My experience from measurements favours a ferrite-based filter followed by a wound-type common-mode filter.
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Old 15th June 2004, 09:56 AM   #9
Jennice is offline Jennice  Denmark
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Quote:
Originally posted by skippy
It's a real shame that we have to invest effort with mains filters, but there are just so many hi-fi manufacturers who avoid this issue. The whole point of a PSU is to provide a clean, stable supply, no matter what the input is doing (within reason). A whole host of European test (part of the CE directive) were devised to help with this, but they can be circumvented so easily in audio.
Well, the directive only specifies limits (in Amperes) of the harmonic currents. For everything other than a power-amp, these levels are typically highter than the mains current consumption. Hence, these criterias are usually met without special attention to the subject.

Quote:
We all know a decent PSU helps make audio sound good.
Many shun switched-mode PSUs, becasue they generate noise; however iof the designer can work with this and filter it, any mains-borne noise is not an issue.
If the SMPS is well designed, there may be noise left, but above audible frequencies. That's the clue... the frequency spectrum. You will never be without noise.. not even from batteries or capacitors - it's a choice of which noise is the least audible or annoying.


Quote:
Originally posted by skippy

The resistance of the inductors is generally trivial compared with the wiring back to the sub station!
IMHO there's something left out of this comparison/calculation. You need to look at more than the route from your outlet to the nearest sub-station (transformer). When "seen" from your wall outlet, it's a parallel load of the substation's impedance and all the houses/appliances it supplies. (assuming that you don't live in the middle of nowhere, and have 100's of kilometers to the nearest transfomer).


Just my 2 cents...
Jennice
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Old 15th June 2004, 10:26 AM   #10
skippy is offline skippy  United Kingdom
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Quote:
IMHO there's something left out of this comparison/calculation. You need to look at more than the route from your outlet to the nearest sub-station (transformer). When "seen" from your wall outlet, it's a parallel load of the substation's impedance and all the houses/appliances it supplies. (assuming that you don't live in the middle of nowhere, and have 100's of kilometers to the nearest transfomer).
Agree entirely. I was only thinking about 50/60Hz as being the main factor affecting the supply voltage to the hifi unit's PSU.


Quote:
Well, the directive only specifies limits (in Amperes) of the harmonic currents.
I'm thinking of susceptibility to main-borne interference. The directive has a very useful burst-noise test which can help enormously with designing a resilient PSU. My Naim CD player, for example, was designed so it didn't lock-up under this condition, which they considered to be a CE pass. I would rather it was designed so the noise didn't adversely affect the audio output, because the PSU would then be better in a normal domestic environment.
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