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Author Topic: Need tranformer buying advice  (Read 1940 times)


  • Chipper
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Need tranformer buying advice
« on: October 25, 2018, 11:09:08 AM »
I'm planning on building the 60w SS amp from Redcircuits as the first amp I've build that's >1W.

I know nothing about transformers. The BOM reads
T1__220V Primary, 48-50V Secondary 75 to 150VA  Mains transformer (See Notes)
and the notes say
T1 transformer can be also a 24 + 24V or 25 + 25V type (i.e. 48V or 50V center tapped). Obviously, the center-tap must be left unconnected.
Nothing obvious to me.
I'm in Canada, so i should get 115V primary I assume.

Stuck on which transformer to actually order from digikey.
- single or dual primary windings?
- best VA value?  Teemu's book says 250VA is ideal for 60-100W; this spec says 75-150VA.
- Laminated or toroidal? Tordoidal is 3x the price.
- Current - Output Max 2A or 3A (these are the options that come up in digikey's search results)
Price ranges from C$30 to C$70.

Will this one do the job?


  • Legendary
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Re: Need tranformer buying advice
« Reply #1 on: October 25, 2018, 01:05:17 PM »
SO you need a 48-50v secondary that can provide 75 to 150 VAs,  For this VA and watts is the same.  Just look for 75 watts or VA minimum.

The 48v can either be one 48v winding, or a 48v center tapped winding - just don't use the tap.   Or dual secondaries 24v and 24v can be wired in series to make the 48v.

Whoever designed the thing has 220/240v mains, so they asked for a 240v primary.  If your mains are 120v, then get a 120v primary.   This ain't rocket surgery, so consider primaries of 120v/117v/115v/110v all to be the same.  Close enough.

Primaries can be single or dual.  A plain old single 120v primary is fine.  A dual primary just means there are two 120v primaries.  They can be wired in parallel for 120v operation, or in series for 240v operation.  In other words the same transformer can be used on either 120v or 240v with a simple wire change.

Watts or VA means volts times amps.  so 50v at 2A means 100VA and 50v at 3A means 150VA.  Some transformers are rated by their makers as XXX volt amps, others might be rated at some amps at some volts.  Same thing, just said differently.

Unless you need the low profile of a toroid, why spend three times as much for it?


  • Chipper
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Re: Need tranformer buying advice
« Reply #2 on: October 25, 2018, 11:07:37 PM »
Very helpful, thanks a lot.