Quick Question about Toroid for BOSOZ

[wchick]

I found from Plitron that a 300VA 60+60 toroid transformer is only $9 more than a 50VA 60+60 one. I want to purchase a transformer for my BOSOZ project. Is the 300VA is too big? I would not mind the $9 difference and I heard that more powerful transformer is better. If I go for the 300VA one, should I also increase the size of capacitors (from 1000 uF to 5,000 or even 10,000 uF)?? Thanks for your advice.

Wallace

 

[jwb]

A transformer will provide higher-than-rated voltages if you aren't pulling much current through it. So if you pull 100mA from a 120VCT 300VA torroid, the actual voltages will be more: 140VCT, maybe. This doesn't really matter because BOSOZ regulates the output using a Zener stack, but it will increase the power dissipation in the followers Q101 and Q102.

 

[grataku]

Good question which I have already addressed proviously at length. Be prepared to buy initial filering caps with a voltage rating higher than 100VDC. I found that plitron likes to have output voltages regulation that is higher than the declared by anywhere from 50 to 100%. In my case I chose an 80VA 60+60 which turned out to be 72+72 with no load.
The mosfet end up having to dissipate an enormous amount of power.

 

[wchick]

Thanks

Thanks grataku, I read your previous posts about this issue. I think I'd go for a 80VA 50+50V toroid as you've suggested. By the way, what power resistor you are using now? I saw some Mil 5W wire wound resistor for sale, wonder if they are good for the preamp. I will also increase the heat sink for the mosfets in the power supply. Thanks.