First PSU Build help!!!

[Pafi]

your 1kVA 480:120Vac transformer has maximum current values of ~2.1Aac for the Primary and ~8.3Aac for the secondary.
These do not change if you change the supply voltage.

Your 1kVA effectively becomes a 250VA transformer when you change the supply from 480Vac to 120Vac.

This is true if you absolutely don't care about voltage drop, only the thermal limit of the transformer. But if you care about the drop, then you will find that at nominal current the (absolute) drop also will be the nominal. And a nominal drop of for example 6 V (5 % of 120V) is quite good at 120V output, but very bad at 30V. In other terms: the relative copper loss will be 4 times higher then originally. A correctly dimensioned 250VA transformer typically has only about 8 % copper loss, not 20 % like this transformer. So I would rather call it a 125 VA transformer, with 10 % drop. The situation can be even worse if it is not a strongly coupled transformer, inductive drop can further decrease usable power limited by drop, especially if it is loaded by a peak rectifier.

Otherwise it operates as normally as any mains transformer.
The output AC waveform will still be a close replica of the AC waveform that you input.

This is basically right, for light or linear loads.

The waveforms through the rectifier will still be the same.
The waveforms across the smoothing capacitors will still be the same.

Not at all. You didn't take account impedances at all. As usual. The impedances are approximately 2.5 times higher.

 

[Pafi]

Sorry for resurrecting this old thread but I learned a lot from it.

I have been considering buying a couple of the many 480v primary 120v secondary (~1kva) transformers available on ebay for dirt cheap and putting 120V across the primary which would give me about 30v from the secondary. From the article referenced in this thread it appears that putting 120v across the primary designed for 480v most likely won't sufficiently magnetize the core and the resulting 30v from the secondary is going to be a very rough wave instead of the smooth sine wave I want for a quiet amp?

Do I understand this correctly?

Can you post a link to that transformer?

I didnt read that article, but this it not correct at all. If you operate a transformer with lower voltage, it will be more linear. But the load is not linear, so it will generate harmonics. Since a bigger transformer (and especially a 4 times overrated one) has typically higher leakage flux, this can lead to a higher noise level, but this is not arises in the secondary voltage, but in the surrounding area, signal wires, etc...

If I buy transformers with 120v primaries and 40v secondaries then limit the primary voltage with a variac (To lets say 90v or 60v input) am I going to run into the same problem with a noisy output voltage from the secondary coil?

No. This is a very small change for the above mentioned effect to be noticable.

 

[oughtsix]

Well, I will start with what I ended up with:
Powerstat Variac 116U 3Y 230VAC 0 270VAC 7 5A 3 5 KVA 3P Phase Variable Power | eBay

The price was too good to pass up and my current variac is a General Radio lunch box that is rated at 3 amp max. I pulled out the middle variac so I have a double variac instead of a triple now. I figured I might try to just use the double variac to directly supply the two legs of my AC current? I don't know how close 2 variacs linked together will track each others output voltage but the mechanical linkage mechanism between the two makes it very easy to offset one variac from the other in minute amounts. I did lookup the specs on the 116U variacs and see that their efficiency falls off quite quickly under about 30 volts. I don't know if this will be an issue?


There are probably at least a hundred different ebay auctions with 480v:120v isolation transformers for dirt cheap. It takes a good amount of hunting to find an auction with multiple identical 480v:120: transformers. I was thinking of buying 4 of them so I could have isolated left / right channels. But I have kind of abandoned this idea due to the perceived impracticality of using transformers outside their rated envelopes.
Here are a couple of the 480v:120v transformers I was referring to:
Allen Bradley 1 KVA Control Circuit Transformer x 231515 240 480 to 120 Used | eBay
Basler Electric 1KVA Transformer 460 230 Volt Primary 115 Volt Secondary | eBay



My goal is to play around with the many Class A amp boards available on the internet/ebay. My thinking is that I need to start with a solid low noise adjustable power supply. I would like to start playing with the KSA 50 boards which requires dual +/- voltages, one for the drivers and a different voltage for the control circuit. I am not looking to design the ultimate class a amplifier. I am not looking to design the ultimate class a power supply either. Rather I would like to build a nice set of speakers and be able to discern their quality better than my Yamaha receiver will permit.

I still have my ancient Threshold amp (one channel blew) and a pair of Magnepan II's that I used to love. The traces on the Magnepan have de-laminated and one of them has an open some where. Even if I fixed the Maggies they don't really fit in my current house. I have been hunting a good design for a not too intrusive floor standing speaker and would like to use ribbons. I am not a bass freak (quite the opposite), instead I value and enjoy clarity and detail. If I could find an amp / speaker that would come close to my Threshold/Magnepan combo without breaking the bank I would be very happy.

I am an accomplished woodworker with a complete shop and have experience with many electronic projects.