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Old 25th January 2008, 08:45 PM   #1
geminni is offline geminni  Serbia
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Default Transformer with uneven ac out

I need 2x15v supply for preamplifier. I have a transformer with two output. One is 23v and second is 30v. Two windings are connected together.
Can i connect it to a rectifier? Will this voltage difference bother?
If it is ok then i will also use zener, transistor to lover voltage at 15v dc.
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Old 26th January 2008, 04:44 AM   #2
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You can use 15V zener regulation to get your needed +/-15VDC with no problem as long as both the 23 and 30VAC outputs can deliver the Current you need
Free Schematic and Service Manual downloads, Spare time company (just for fun):
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Old 26th January 2008, 11:50 AM   #3
AndrewT is offline AndrewT  Scotland
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or add the 7Vac winding to the 23Vac to match the 30Vac.

Check which winding has the smallest wire diameter. This becomes the effective limiter of your output current.
regards Andrew T.
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Old 26th January 2008, 12:40 PM   #4
geminni is offline geminni  Serbia
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So, the voltage difference will not make things go bad
Ok, i'll try that and adding that 7 volt in seres with 23v.

Thanks guys!
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Old 31st January 2008, 05:13 PM   #5
geminni is offline geminni  Serbia
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It's working!
Now i have +-40,5V. How can i make +-15V for preamp?
I have been looking in LM317 data sheet, looking on net and found that its maximum input voltage is 40v.
Would the classic zener-transistor combination be enough for preamp?
I ask because i have made same setup for amplifier with TDA1516q and it distorted very bad on higher power output. When i replaced zener-transistor with 7815 it worked very nice. No distortion on louder volume.
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Old 31st January 2008, 08:45 PM   #6
jaycee is online now jaycee  United Kingdom
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Use the transistor-zener as a "pre regulator" to drop down to about 25V or so... then use LM317/337 or 7815/7915 to supply the preamp.

Alternatively build something like the Jung or Sulzer discrete regulators, but that's a lot more complex.
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Old 1st February 2008, 05:52 AM   #7
geminni is offline geminni  Serbia
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Thanks! I'll try that with 2 stage voltage drop
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