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Old 26th July 2001, 04:34 PM   #21
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Quote:
Originally posted by ALW
I've had good results from transformers (toroidal) that incorporate an electrostatic shield - they can provide some significant attenuation of noise, but you don't often find them on off-the-self parts.
Check out the toroidal medical isolation transformers from Plitron ('http://www.plitron.com/'). The 'type 1' transformer has an electrostatic shield, a magnetic shield, and an overtemp switch. They are available in HUGE capacities (up to 12.5KVA). They have dual 100-120 inputs and dual 115 outputs, so they can be used to convert 220V to 115V and vice versa. All things considered, they are fairly inexpensive.

Just one of these would be able to clean up the power for a complete stereo system .

Good luck.
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Old 26th July 2001, 07:38 PM   #22
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Mains noise must be a real problem as manufactures of domestic electronics that contain a micro fit as standard double pi sections and mov's after the mains input.
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Old 26th July 2001, 10:58 PM   #23
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Default Chicken or the egg?

I think we are looking at two issues here:

1. Diode (in this case) switching noise getting into the rest of the power supply

2. Diode (in this case) switching noise getting back out on the net and into another device.

I believe I have experienced both. You can test number 2 by putting a mains filter on a detachable power cord. Putting such a filter into my CD player (and covert it to detachable cord at the same time) is one of the bigger upbrades I have done (with the system I had at that time). My power amp sounded much better, but my Stax OTL headphone sounded no different. Whether this was diode induced or not is another matter.

Regardless of whether I was experiencing digititis or diode swiching noise, I am convinced that anything you do to remove HF ringing at it's source should be done.

Schottky diodes are not very expensive these days but get hot at high voltages.

Hexfred's or equivalent are not really that expensive either ....

At least one can put some caps onto the devices. I would suspect ceramics are the best (shame on me!) as noted by other(s) previously in this thread.

Petter
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Old 26th July 2001, 11:50 PM   #24
ALW is offline ALW  United Kingdom
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Default Filters - yuck!

Quote:
Putting such a filter into my CD player (and covert it to detachable cord at the same time) is one of the bigger upbrades I have done (with the system I had at that time)

and

as manufactures of domestic electronics that contain a micro fit as standard double pi sections and mov's after the mains input
These filters (particularly the filtered IEC inlets) are commonly used, but some systems don't like them at all.

Just plugging in any item with common mode filtering and / or adding series inductance in the form of filters screws the tune of my system completely, even if the filtered item is switched off.

The series inductors in many filters cause a rise in mains impedance that my equipment doesn't like.

Personally I prefer to eliminate the noise at source within the PSU, and avoid it's generation, rather than attempt to filter it using brute force methods. It's a much more elegant solution.

Andy.
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Old 27th July 2001, 12:31 AM   #25
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Geoff,
I haven't tried MOVs--the circuits I threw together were simply 1:1 toroid isolation transformers that I bought surplus, and a few other junk box odds & ends. They look like a train wreck (I used perf board to hold the little parts), but made a far larger difference in sound quality than their humble origins and parts would have led me to believe. One of these days, I'm going to take a more scientific (a word some folks probably don't expect me to use...) approach to the topic and rebuild the poor things as proper circuits.
Petter,
I don't have a decent CD player at present; I just 'borrow' the DVD player from my video system when I want to listen to CDs. However, I do try to keep the DVD player (a mid-line Sony--forget the model) isolated on it's own 'conditioner' so as to stop digital gremlins from sneaking back up the line and down into my other equipment.

Grey
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Old 27th July 2001, 10:45 AM   #26
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Dear Grey,

Thanks for your comments. You might be interested to hear the whole story.

My power supply to the valve pre-amp consists of a transformer, 4, 1N4007 as a bridge rectifier and of course condensers. I did not have caps across the diodes. The transformer failed most liekly due to under power since it was an old one from my spares. So, I bought a replacement and that one has a centre tapping. Since I am using the 4 diodes, I only used half of the secondary coil.

When I decided to change the rectifiers for the pre-amp (since the power amp proved successful), I used two ultra fast rectifiers and the centre tap (this is more economical to do) and the result was disappointing as descrbied previosuly. As I said, I could not figure out the reason behind it as both circuits used were full-wave rectification. Going back to theory, I read from a book explaining that the secondary coil would need to work twice as hard when using two rectifiers and the centre tap.

As suggested, I bought two more of the same ultra fast rectifiers to build a bridge and the expected result re-appear. Yes, this time it illustrates that the component brings improvement to one circuit, it does the same elsewhere. So, the explanation might be that my new transformer is not that powerful too.

Many thanks for you guys participating and commenting.

William, 27/7/2001

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Old 27th July 2001, 04:07 PM   #27
Eric is offline Eric  United States
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Default Screened toroids

Looks like I'm a little late to the discussion on shielding toriods... but I just got back an e-mail from someone at Plitron. Adding an electrostatic screen and magnetic shielding to their 300va toroid adds approximately $10 to the cost and increases lead time to about 4-5 weeks. If you're not in a hurry, it seems that it might be worth the extra cost.
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Old 27th July 2001, 07:01 PM   #28
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Default Screened toroid

2 years ago I had a lengthy design process going with Plitron specifically for Audio transformers for power amplifiers. As you know, they are knowledgeable in this matter and manufacture units for companies such as BAT.

I figured and electrostatic screen would be a good thing. However, their chief designer said that he did not recommend it as it caused other "problems". I was on an international call and chose to pursue other detailed topics at that time.

However, I also note that renowned DIY'er Thorsten Loesch likes electrostatic screens. Now he is a tube man so his needs are different than transistor power amp units.

Now as far as magnetic shielding goes, I usually specify double normal thickness for toroids when I get custom units made.

Petter

[Edited by Petter on 07-27-2001 at 02:41 PM]
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Old 27th July 2001, 07:17 PM   #29
Eric is offline Eric  United States
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Hmmmm. Transformer screens and shields cause other problems? Sounds like this might be worth a phone call to talk to someone. I'll try to give them a call over the next few days and post a brief summary when I'm done. I just made the automatic assumption that adding both would be a good thing...

[Edited by Eric on 07-27-2001 at 02:50 PM]
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Old 27th July 2001, 07:40 PM   #30
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Default Clarification: Screen, not shield ...

My reference to "other problems" were related only to electrostatic screen, not magnetic shield /which you can make up quite easily by yourself (a few turns of core material arond the circumference of the donut).

Petter
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