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Old 8th February 2006, 10:39 PM   #1
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Default back to back isolation transformers:

Hi all:

I live in a country with 220VAC mains.
Recently, I received a BIG, "EI" type, 220VAC to 120 VAC, transformer, as a gift. I put it to use as an "isolation Tx" feeding a cheap, moded Sony DVDP used as a transport for my secondary sytem. This DVD has a SMPS and accepts 120VAC.
The improvement in sound was huge! (I hate this word:huge) I got excited because every aspect of musical reproduction improved.

I have no luck and could not get a true 1:1 isolation Tx afterwards, to feed the rest of my equipment. Then I thought to get another 220 to 120 Tx and put it in series with the former in reverse sens, this way:

mains AC->220VAC(primary)->120VAC(secondary)->120VAC(secondary)->220VAC(primary)->system or DIY filter.

It would work, wouldn't it?
Maybe I'll get more HF isolation.

What do you think?

Thanks.
Mauricio.
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Old 9th February 2006, 01:02 AM   #2
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Yes it will work, regulatioon won't be as good as a single transformer.

Remember that your amp will take a lot more current than a cd player and will need a suitably large transformer.

I have a 3KVA 1:1 240V isolation transformer in my lab and its big, heavy and hums loudly.
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Old 9th February 2006, 01:18 PM   #3
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Thanks!

Mine must be around 2KVA. Only the first one will hum
and I will try a "DC block" before it.
Appart that, I made a heavy wood box to enclose the Tx and it reduces drastically audible humm in a manner that it doesn't boder.
My amps are UCD180, more than 90% efficient. I think the Tx's should have no problems feeding them.

Regards
Mauricio
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Old 17th February 2006, 10:18 PM   #4
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Here's my report on my back to back transformers to imitate an iso-trans:

The second 220VAC to 120VAC I was kindly given was a little smaller than the previous monster. I tried them on my active system. First using my DIY simple filter to plug DVD and both stereo UCD180 amps, living a Behringer active crossover on his own, unfiltered. Then, isolating only the UCD amp that feeds the midwoofer and tweeter (without filter).

I both cases (in my system, with my AC) I heard noticeable improvements. Even my non-audiophile father heard a change at once.

What I hear is a rounder, softer and calmer music presentation, with better clarity and warmth on midrange, and more natural timbres. Also soundstage improved in width and performers positioning.

I don't find a lack of dynamics at all.

Now I have to continue my search of true isolating transformers


I hope this helps.
Mauricio
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Old 18th February 2006, 02:43 PM   #5
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For the ultimate in power source stability you want something like this http://cgi.ebay.com/California-Instr...QQcmdZViewItem
. It converts the incoming power to DC, then makes AC with a linear amplifier. You are completely isolated from the power company and line disturbances.
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Old 18th February 2006, 08:02 PM   #6
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Mauricio,

can not comment on placing 2 transformers in series.
I am using 4 big toroidal 1:1 isolation transformers in combination with high current Shaffner filters, there certainly is quite a difference.

Overhere such totally enclosed 2 KVA donut transformers are used by farmers in the field, and sometimes offered for sale at $40 the piece. No buzzing, and like regular toroidal transformers they can do 4 KVA the piece briefly. Best audio investments i made.
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Old 18th February 2006, 08:42 PM   #7
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Max,

The following URLs might be of interest to you -
http://www.audiocircle.com/circles/v...ic.php?t=18441
http://www.audiocircle.com/circles/v...ic.php?t=18443
as it takes a very similar approach to your experiments.

Some quick comments -
1. Ideally, the output of your 2nd transformer should be a dual winding that would allow you to ground the center tap which would give you balanced (technical) power. The benefits of this is the cancellation of reactive leakage currents on grounded components -
http://www.equitech.com/articles/articles.html
2. Both transformers should have either a shield that can be grounded between the primary and secondary if the windings are overwound, or a split bobbin construction. This minimizes the capacitive coupling between primary and secondary, minimizing noise coupling.
3. While transformers are wonderful for eliminating common mode noise, they're not particularly effective against differential(transverse) noise. Adding capacitors 'accross the line' between the transformers, X caps, will convert much of the differential noise to common mode which will be addressed by the common mode capabilities of the following transformer.

Regards,
Paul (aka Occam on AudioCircle)
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Old 20th February 2006, 01:44 AM   #8
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A thousand thanks to everyone of you that contributed with such good advices!

Dear pmKap:
My own filter uses some varistors followed by one X1 cap and then a special shotgun ferrite wich I think eliminates both kind of noise to some degree:
VDR: RS components catalog nº 289 7301

Capacitor X1: 47nF; RS components cat nº 240 5334.

Double apperture ferrite: 10uH each;
RS components cat nº 212 0617. I use at least 5 of these.

Best wishes.
Mauricio
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Old 21st February 2006, 09:40 AM   #9
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If u have two step down transformers, why not add up the secondary voltages and use for 120 and 240. Connect primaries to mains as usual.

Gajanan Phadte
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Old 22nd February 2006, 02:42 AM   #10
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Hi Gajanan:

Quote:
If u have two step down transformers, why not add up the secondary voltages and use for 120 and 240. Connect primaries to mains as usual.
Do you mean like this???

[IMG]Click the image to open in full size.[/IMG]

What must I do not to burn the house?
Can I attach also primaries like upper winding with upper winding and lower with lower?

Will this give "balanced power"?

Any help welcome

Thanks for your interest
Mauricio
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