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Old 22nd November 2007, 04:36 PM   #1
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Default Converting watt to VA???

Hello,

I have an 110V preamplifier/processor that I am using in the UK. For the moment I am using an external converter but I just noticed that I have a lot of space inside the chassis so maybe I could put a step down transformer in there instead. I still want to keep the original transformers because there are so many conversion inside the unit (for 5V, 30V and something else I don't know what it is for) so the easiest option would be to just step everything down to 110V. Bassically I am just moving the step down inside the chassis.

I know the preamp uses 50 watt, what VA should I choose for the step down transformer? I m thinking if an isolation transformer like this, should it work (is available from 50VA to 500VA):
http://uk.farnell.com/jsp/Electrical...sp?sku=9938443

Thanks!!!
Robert
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Old 22nd November 2007, 05:13 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
you could use 50VA for 50W of consumption.
But running a transformer at full power all the time creates quite a lot of heat.
Usually VA takes account of phase angle of the load, but this is generally ignored by us audio lot.
Instead adopt the de-rating specified by the transformer manufacturers when using a capacitor input filter after the rectifier.
De-rate the transformer to 70% for power output. Since the rectrifier and smoothing will increase the DC output voltage by a factor of sqrt(2) the continuous output DC current becomes half of the rated AC current.
If you need +-25V @ 1A you need 50/0.7=70VA @18+18Vac. A 70VA 18+18Vac transformer is rated @ 1.9Aac (about twice the maximum DC current avialable for your circuit).
Does your pre-amp really consume 50W continuous? or is that a worst case condition that is rarely if ever reached?
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Old 23rd November 2007, 07:30 AM   #3
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Great, thanks for your answer!

I got the power consumtion from the back of the unit so I guess it is the maximum.

So if I go for a 100VA I should be safe? I could fit a 160VA as well, and the price and size difference is not that big. Any suggestions/thoughts?

Thanksfor your time!

Robert
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Old 23rd November 2007, 08:02 AM   #4
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By the way, I guess this transformer is single phase (no center tap), does that mean anything for a standard step down conversion?

It is also an isolation transformer, is that good?

Or should I use something like this (toroidal w 2*55V secondaries, running in series)?
http://uk.farnell.com/jsp/Electrical...sp?sku=9532765


Sorry for all these questions, but I really want to make this work as good as possible!

Robert
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Old 23rd November 2007, 10:05 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
I like toroids, many others don't, particularly for lower power needs.
The EI seems to let less garbage through from the mains and this helps pre-amps to perform better.

If your maximum input power is 50W then a 50VA EI could be the way to go.

Your selected isolator could be a very high quality unit, but it omits to mention regulation and electrostatic screening. Hard to decide if this unit is worth the extra cash.

I did see another poster use a 240/440Vac isolation transformer that weighed about double the normal for it's rating and regulation was exemplary for an EI, even bettering toroids in this respect. I cannot recall who the manufacturer was, but RS or Farnell was the retailer.

Could you measure the power consumption of your pre-amp? It could be as little as 10W!
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Old 23rd November 2007, 02:02 PM   #6
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Thanks again for your answer, really good stuff and I am actually learning something

I can't really measure the wattage of the unit, but I agree that 50 watt seems like a lot (although it is a processor preamp).
http://www.outlawaudio.com/products/990.html

I would really like to try the isolator, do you think it is a too big risk and that I should just grab the toroidal?

I just hate choices that I don't know everything behind, please make the decision for me

Regards
Robert
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Old 29th November 2007, 08:20 AM   #7
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The 50VA isolation transformer will work just fine.
Trenton
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Old 29th November 2007, 08:58 AM   #8
Bobken is offline Bobken  United Kingdom
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Hi,

I would favour Andrew's suggestion and go for a toroid if you are not already committed to purchasing the more costly isolation transformer.

The stray magnetic fields from a toroid are likely to be much lower than any usual 'EI' type of transformer, which I assume this isolation transformer must be.

You say you intend to install this in the existing enclosure, which clearly has a lot of low-level (and therefore probably sensitive to hum pick-up) circuitry within this enclosure. This being the case, you might end up needing to add some internal screening around the 'EI' type, but this is much less likely with a toroid.

Regards,
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Old 29th November 2007, 09:18 PM   #9
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50VA should work just fine.
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