Source of Transformer for Power Amp

[keilau]

For a solid stat power amp transformer, I want 2 pairs of secondary windings. For a 100 watts/channel design, they are high current 35 VAC and low current 40 VAC, or similar. The high current winding for the power (current amp) stage and the higher voltage output for the input/voltage amp stage.

Parts Express has a line of Toroidal Power Transformers that are reasonably priced. But these transformers do not have the second winding needed for most power amp design.

Any suggestion?

 

[rayma]

For a solid state power amp transformer, I want 2 pairs of secondary windings.
For a 100 watts/channel design, they are high current 35 VAC and low current 40 VAC.

You're going to have to use two separate transformers. They can be physically stacked, though.
Or use 40V for both stages. It's only 15% more than 35V.

 

[keilau]

You're going to have to use two separate transformers. They can be physically stacked, though.
Or use 40V for both stages. It's only 15% more than 35V.

Yes, the voltage stage needs about 10% higher voltage than the current amp.

I think about stacking the low power transformer too. I would appreciate any comment on the pros and cons of using stacking or completely separate parallel transformer.

A single dual winding toroidal is more ideal.

 

[rayma]

I would appreciate any comment on the pros and cons of using stacking or completely separate parallel transformer.

You could use a single, dual secondary 40V high current transformer, and on one
of the secondaries buck the voltage down to 35V with a small high current 5V-6V transformer.

 

[keilau]

You could use a single, dual secondary 40V high current transformer, and on one
of the secondaries buck the voltage down to 35V with a small high current 5V-6V transformer.

It sounds like a very inefficient way both cost wise and electrical wise.

 

[rayma]

It sounds like a very inefficient way both cost wise and electrical wise.

Then you're back to two toroids.

 

[AndrewT]

buy a 0-35, 0-35Vac dual secondary toroid with an open centre and without a guass band/belly band.

wind on a dozen or so turns to create a low current 5Vac secondary. Wind on a second set of turns carefully placing each extra turn in between the turns of the previous extra secondary.

You now have a dual 35Vac with a dual 5Vac transformer.

Options.
a.) create a multiple tap secondary 0-35-40Vac , 0-35-40Vac
Convert to Centre tapped 40-35-0-35-40Vac secondary.
Add a high power bridge rectifier to the 35,35 tappings.
add a lower power bridge rectifier to the 40,40 tappigs.
Connect the centre point of both sets of smoothing caps back to the Centre Tap.
You now have -58Vdc, -50Vdc, Zero Volts, +50Vdc, +58Vdc

or
b.) attach a bridge rectfier to each secondary winding. Smooth each with it's own capacitor.
You now have 4 isolated supplies two at 49Vdc and two at 7Vdc
Because these are isolated you can series connect them in any arrangement you require.

 

[keilau]

You now have -58Vdc, -50Vdc, Zero Volts, +50Vdc, +58Vdc

I looked at several of the power amplifier schematics from NAD, Adcom, Parasound, Marantz and Nikko, this type of dual DC supply is almost universal. Why is it so difficult to get the transformer for a DIY project?

I would like to use the toroidal type. Once a while, there were a surplus power amp transformer showing up on Ebay. But the seller often wants a premium for it.

I am reluctant to mod the transformer for fear of reducing its long term reliability.