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Old 10th October 2006, 12:22 PM   #1
Ropie is offline Ropie  United Kingdom
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Default Step Down Transformer

Hi,

I was toying with the idea of purchasing a Bottleheads phono preamp kit for use in the UK via a step-down transformer.

My question is, with so many SDTs on the market will the quality of the SDT have a direct effect on the sound quality of the preamp? If so, is it even worth running a valve phono stage from a SDT at all?
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Old 13th October 2006, 06:19 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the lack of availability of a 240Vac transformer was the main reason I did not buy a bottlehead preamp.

I never even got as far as thinking about performance.

If you search for the threads on balanced mains input, you will find dual transformers used with a bit of capacitance (as interference suppression) in between.
You could adapt this to halve the supply voltage to the Bottlehead.
I think he went 240 :24 then 24:240.
You could do 240:24 then 24:120. Job done.

The originator claims big things for the improvement in sound quality.
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Old 13th October 2006, 11:34 PM   #3
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Actually, obtain 2 of the same transformer with split 120/120 primaries and split secondaries, for purposes of this discussion, lets say 24/24vac.

Wire the primaries of the first seconday in series for 240vac and the secondaries in parallel for 24vac. Take the output of this transformer and connect it to the secondaries of the 2nd transformer's seconday wired in series, for 48vac. Take the output from that 2nd transformer from its primaries wired in series. Because you are feeding 24vac into the second transformer nominally wired for 48vac, the primary of the 2nd transformer which is wired for 240vac will output 120vac.

Furthermore, ground (to the mains ground) the centertap of the primary of your output transformer (you're using it as the secondary). This will give you balanced (technical) power, (+60vac) and (-60vac) referenced to mains ground. This has the benefit of 'canceling' reactive leakage currents to your grounded chassis-
http://www.equitech.com/articles/widescreen.html

You'll also want to put a capacitor(s) accross the line on those secondaries. This will take differential (transverse) mode noise and convert it to common mode noise and allow the transformers to do that voodoo, that they do so well.... deal with common mode noise. These caps are going to see (in this example) 24vac, so the need for X2 rated caps isn't there. Assuming you use something like Clairtronic 100va transformers, I'd suggest somewhere between 8-15uf of capacitance (you'll ideally experiment), and bypassing works well here. At minimum .1uf.
ClarityCaps will work exceptionally well, with the 630vdc versions the best. The lower voltage and less expensive ones should work quite well.

If your goal is simply to provide 120vac to power your bottlehead, don't bother, as you can get very good performance from a 'site transformer' which you can generally find on ebay locally. The are designed to power US type powertools outside of N.A. in 240vac environments. They usually have shielding between primary and seconday which will provide some noise cancellation, as well as balanced (technical) power. This balanced power isn't intended to cancel reative leakage, but rather for safety. 60vac is far less dangerous than 120vac and that is less dangerous than 240vac...

If you're truly as masochist and want to see what brute force powerconditioning can do, you now must obtain proper transformers. Do not use toroidal transformers. They have very wide bandwidth and high primary to secondary capacitive coupling, exactly what you don't want. Rather, use split bobbin E core transformers, which for this application have the characteristics that you do want. This configuration of cascaded transformers with one **** backwards has wretched regulation; the voltage drops rapidly with increased load. Therefore a slight boost works well. Use a 230vac primary transformer for input and a 120/120 (240vac) primary transformer for output, both with the same voltage, such as you can get from RapidElectronics -
For input -
http://www.rapidonline.com/productin...moduleno=72692
either single 230vac input #88-3460 or 115/115 #88-3511 both with dual 20vac secondaries

For output - all with dual 20vac secondaries and 120/120 primaries
http://www.rapidonline.com/productin...moduleno=75700
#77-0386, very inexpensive! as its not RHOS
or
http://www.rapidonline.com/productin...moduleno=74759
#88-0452

I'll be out of the US for the next 8 days and will check this thread when I return for any questions, damnations, etc...
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Old 15th October 2006, 10:42 AM   #4
Ropie is offline Ropie  United Kingdom
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Quote:
Originally posted by pmkap
If your goal is simply to provide 120vac to power your bottlehead, don't bother, as you can get very good performance from a 'site transformer' which you can generally find on ebay locally. The are designed to power US type powertools outside of N.A. in 240vac environments. They usually have shielding between primary and seconday which will provide some noise cancellation, as well as balanced (technical) power. This balanced power isn't intended to cancel reative leakage, but rather for safety. 60vac is far less dangerous than 120vac and that is less dangerous than 240vac...
Yes, this was what I wanted to know. That sounds encouraging, thanks.
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