Virtual ct on a trafo with no ct

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I have a PGA2311 Preamp how requires 6 -0- 6 v input. Is it possible to use a 12v ac trafo with no center tap but use 2 100ohm resistor from each tap from the secondary side to form a 6 -0- 6v trafo?
 
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Rundmaus

Member
2005-08-21 10:46 pm
Feed the 12VAC to a bridge rectifier, giving you approx. 18V peak. Afterwards filter the DC with two large capacitors in series. grounding the center point of the two caps will give you a symmetric +-9V supply. Equalizing resistors parallel to the caps may be used, but often are omitted in the low-voltage regime.

If the preamp is sensitive to overvoltage, use a pair of regs to get the 9V down to 6V.

Rundmaus

EDIT: Aaaaah, been reading to quick-and-dirty. Does the preamp need 6-0-6 AC or a symmetric +-6VDC supply?
 

Rundmaus

Member
2005-08-21 10:46 pm
6VAC should give a little less than 9VDC after rectification. Given there are no large mains fluctuations, this should be enough headroom for a 5V reg. If in doubt, use dual 9V transformer or replace regs by low-drop ones.

Rundmaus

PS. 30W seems a lot for a preamp. Does the manual state a current consumption?
 
This will be half-wave rectification for each of the rails with very poor ripple performance. I doubt the regs will manage a nice DC from that.

Rundmaus

The ripple is the double of a full-wave rectifier, but even with a current draw of plus/minus 60 mA (the PGA draws only about plus/minus 10 mA), the ripple is only 1 V with 1000 uF capacitors, and that is no problem for the regulators. And the ripple can be reduced by using larger capacitors....
 

Rundmaus

Member
2005-08-21 10:46 pm
I reread post #1 and still don't see the advantage.

I attached your schematic, changed to bridge rectification. Can you explain what your reference to post #1 meant and why half-wave rectification for each rail is preferable to the bridge rect. version?

Rundmaus
 

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Rundmaus

Member
2005-08-21 10:46 pm
Ah right... One could use equalizing resistors parallel to the caps, but that would make it more complicated and dissipate a lot of power...

Rundmaus

PS. But anyway, as there is rectification, filtering and voltage regulation already implemented on the board the thread opener bought, both solutions are out of the game. Getting an appropriate transformer would be the best way, IMHO.