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Old 13th February 2007, 07:16 PM   #1
hc167 is offline hc167  United States
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Wink question with selecting transformer

I have question of selecting a transformer. Which transformer company produce good transformer? Plitron? or AnTek? I read the posting at this site and look like AnTek is having some hum noise when it is used to build aleph 30 amp. I do not want the ignoring hum noise. Should I go for Plitron? What other companies should I consider? also. the service manual for aleph 30 said 25 +/- for the rail voltage and people here discussed that if they get a 22+22 V transformer, they will have about 25+/- volts. why is the value increase from 22 to 25? is it because using 120Hz of electrical power? thanks a lot for all your answers. This is the first time I do DIY.
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Old 13th February 2007, 09:00 PM   #2
paulb is offline paulb  Canada
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Go to this site
http://sound.westhost.com/articles.htm
and read the articles on transformers and power supplies.
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Old 13th February 2007, 09:28 PM   #3
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There was a thread recently in the Solid State forum on what sort of transformers are out there. There were also some indications as to where to buy them. Rather than repeat all the same stuff here, it would be better if you looked through that thread.
As far as the voltage increase, it has to do with the way AC is rectified, meaning turned into DC. The voltage rating of a transformer is usually reported in RMS volts. Assuming that you're using a full wave bridge rectifier like most folks, the voltage that shows up at the capacitor is pretty close to the peak voltage, which is 1.414 times the RMS value. Subtract the voltage drop across the diodes that make up the bridge and you'll end up with a pretty close approximation of the actual voltage.
So, with calculator firmly in hand:
22Vrms * 1.414 = 31.1Vdc
31.1Vdc - 2.5V = 28.6Vdc (subtracting approximate diode losses)
So actually, a 22-0-22Vac transformer will give you a little more than 25Vdc at the rails. Don't worry about the extra three volts or so. It won't hurt the circuit, although you might want to note whether you'll need more heatsinking. There will also be a bit of a drop due to the circuit taking current from the caps, but how much depends on the current capability of the transformer.
So, all in all, a 20 to 22Vac transformer will give you something close to 25V rails.

Grey
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Old 13th February 2007, 10:55 PM   #4
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I am thinking of trying the antek mainly due to the price, but I currently have a Plitron in my A30. This is mainly because they are here in Toronto and I have the option to pickup so I don't pay anything for shipping (although I do pay all local taxes).

I suspect you're building your A30 with Peter's boards and in his thread, he built his with a 20+20V 500VA Plitron - he's getting 27.5V rails (after rectification).

My toroid is 18+18V 625VA and I get 26.5V on my rails measured w/o any load.

I think this is the thread that Grey is talking about:

Source For Toroid Tansformers

Good luck with your parts search - this is part of the fun in DIY as well

Stephen
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Old 13th February 2007, 11:04 PM   #5
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Plitrons tend to deliver a volt or two more than their specified voltage.

Grey
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Old 14th February 2007, 03:00 PM   #6
hc167 is offline hc167  United States
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thanks a lot for all of your answer. these information are very helpful. one more question. if power supply circuit can generate enough current for both L and R channel. why do people bother having individual power supply circuit for each channel? What is this good for?

also, I knew that some early low end mark levinson amplifier (such as ML11) use one transformer and then used 2 set of bridge rectifiers and filters circuits to make it virtually like having 2 power supplies so that one channel can virtually has its own power supply circuit. this design is obviously lower the cost. but is it really virtually having 2 power supply. How is it compare to the actual two individual power supplies?

again, thanks a lot
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Old 14th February 2007, 08:56 PM   #7
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This can go in any of several directions. I'll give you a few possibilities:
--Separate transformers ensure the greatest possible isolation between the channels.
--Separate transformers may allow the designer to allow for more current at the output. For instance, a 2kVA transformer may not be available--or economical--but two 1kVA transformers are a cinch.
--A single transformer can be used in any number of ways. Separate secondaries for each channel, with separate bridges and filtration. A single secondary with bridges and filtration for each channel. A single secondary with one bridge and two sets of filtration...anything you can imagine has been tried.
--Active regulation will ensure nearly perfect isolation between the channels, but involves more circuitry and heat.
--A capacitor multiplier occupies a sort of middle ground between regulation and passive filtration.
If you want to really, really pull out all the stops, you'll use two transformers. Unless you're encroaching on the sort of VA specs that require custom-made transformers, it will be more expensive. Whether it's worth the money, weight, and added parts is something you'll have to decide for yourself.

Grey
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Old 14th September 2011, 03:32 AM   #8
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Where did you get a 625 va rating at 18 volt secondaries? plitron standard 18 dual secondary only goes up to 300va
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Old 14th September 2011, 03:57 AM   #9
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I'm running an ANTEK 400 VA in my F5. No issues. Construction appears to be sound. In fact, I just ordered another ANTEK (300VA) for a LM3886 chip amp. I'd use them again, when the need/urge arises.....
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