Confused on Correct Transformer Calculation for Aleph60

[PaulyG]

Hi All:

I am buying the parts to build an Aleph 60. However, I am a bit confused on what voltage to specify for the secondary windings for the transformer (I am planning to use a Avel Lindberg or Plitron). The DC voltage for the amp is 35v. I am planning to just use a bridge rectifier and then capacitors (about 120,000mf) as the filter section. I thought that to get the DC voltage required, you would use the following formula: VDC=VAC x 1.414 - 1.4v (for the voltage drop across 2 diodes). This would imply I need 25v secondaries for both the positive and negative supplies.

Now for the confusion...

If you look on a couple of transformer web sites, they use a different formula: VAC = 0.8 x ( VDC + 2 ). This would imply that I should specify 30volt secondaries on the transformer. (see sites below for reference)

http://www.atc-frost.com/products/design/va.htm

http://www.plitron.com/ (see Tech Notes)

OK, which is the correct formula and what have been the experiences of others when specifying transformers? Would hate to spec the wrong item considering the cost....

Thanks,


PaulyG

[BDP]

PaulyG

The formula you give that results in the value of 25 volts is correct. The variations come into play when you start loading the transformer, adding thermistors and using filtering chokes. These variables will give you less DC voltage. As an example I ordered 25 volt AC secondaries ,500 watt transformer to run one channel. I also used filtering chokes. I'm biasing one channel at 2.9amps per rail and my DC voltage is a little over 32 volts. If you were to use a thermister for inrush current you would have to take the voltage drop into consideration, which would be around 1volt depending on how much bias you would be running. The exact value isnt critical to the amps operation unless you have to meet a certain maximium output level.

BDP