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Old 11th December 2003, 08:59 PM   #1
sushi is offline sushi  Norway
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Hi!

I am currently building the Aleph P1.7 preamplifier and I cannot find a transformer with a 2x30V rating. I can only find 2x28V or 2x33V. Will one of these be fine and which should I choose? Also is it better to get bigger transformers over 100VA and more or should I stick with the original rating?

Thanks for any answers.
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Old 12th December 2003, 03:42 AM   #2
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I would assume the preamp uses a regulated power supply so a few volts either way on the transformer could be handled by the regulator and is not a big deal.
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Old 12th December 2003, 10:01 AM   #3
Klaus is offline Klaus  Germany
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Hi,

I get 82V from a 2 x 30V transformer at nominal net voltage. This is regulated to 62V, depending on the accuracy of the Zener stack. To my point of view it would be better to go up with the regulator input voltage – it should work even finer. So if you don't mind the increased heat losses of the reg go for the 2 x 33V. If heat is a problem, 2 x 28V also works fine. The analog part of Aleph P1.7 consumes around 130mA. So the heat dissipation can easily be calculated.

In theory a small power rated transformer should be ok – but to my experiments the sound increases with more dampening of the bigger one. Why ever… I use in my 1.7 a really big one with 250VA and 4 x 10.000uF main buffer. The sound is unbelievable stabile and solid. When I reduced to "just" 4700uF the stage became a bit of nervous. For PSUs there is no to much, I guess.

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Klaus
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Old 12th December 2003, 10:47 AM   #4
Mad_K is offline Mad_K  Norway
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the only "problem" is that the 33V trafo gives you about 105VDC!! (if it's a small one with typically 15% regulation) this translates to big heatsinks on the regulator mosfets and more expensive caps. I used only one 33V trafo, but left the zeners out, so the regulators are floating giving me about 47V "regulated" in my second BZLS.

Go for 50VA 2*28VAC from elfa -they'll give you about 85V unregulated DC
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Old 12th December 2003, 10:56 AM   #5
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Default Re: Transformer Rating

Quote:
Originally posted by sushi
Hi!

I am currently building the Aleph P1.7 preamplifier and I cannot find a transformer with a 2x30V rating. I can only find 2x28V or 2x33V. Will one of these be fine and which should I choose? Also is it better to get bigger transformers over 100VA and more or should I stick with the original rating?

Thanks for any answers.
How much continuios (spelling?) power does the preamp need?
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Old 12th December 2003, 01:06 PM   #6
sushi is offline sushi  Norway
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My friend has a 500VA 2x28V transformer he is willing to sell me for a small amount so I guess this will be ok. 500VA may be a bit overkill but it will work. What do you guys think?
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Old 12th December 2003, 03:25 PM   #7
Cobra2 is offline Cobra2  Norway
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Wink "A little too much"....

-is just fine.
Get it, if the price is right.

I use 2x60V - 200VA custom-ordered trafo, too expensive...

Arne K
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Old 12th December 2003, 05:09 PM   #8
Kermit is offline Kermit  Norway
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I use 2x28v in my boz, that has a similar supply. You will get a sligtly lower regulated voltage unless you change the value of the resistor feeding the zener's.
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Old 14th August 2007, 10:58 AM   #9
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Okay, bringing this thread back from the dead!

Quick question. I'm a bit cash strapped at the moment, so can't afford the recommended 30-0-30 traffo, but I do have a 24-0-24 25VA in the junk box. I'm thinking that I could try it out, and if regulation isn't sufficient with that headroom, bypass one of the zeners to get a lower regulated output. I can't see anything in the circuit that would fail at the lower voltage, so I'm assuming the only drawbacks will be a slightly higher noise level as Nelson describes in the BOSOZ app. note. Comments?
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Old 15th August 2007, 02:25 AM   #10
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I don't even bother with the zener stack. I just let ps transistors remove all the ripple. I think this would be termed a 'capacitance multiplier'.
I've never noticed a problem with noise or with line voltage fluctuations. Currently, on my j-fetCCBZLS (which I had to put back together because the two preamps I bought were noisy), I am using a single 30-0-30 160VA trafo. It gives app. +-39v output and it works great.

JJ
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