[NOOB] Power supply mayhem - diyAudio
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Old 28th May 2009, 03:46 PM   #1
Andreq is offline Andreq  Canada
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Question [NOOB] Power supply mayhem

Hi,

I'm quite new to DiyAudio. I've just got 2 LM3886 and I'm trying to build my own gainclone from scratch. But for now, I'm disparately trying to build a power supply out of this :

12-0-12 10A Transformer (sold as 24VAC 10A -_-!)

It's not toroid...it's the "square" kind.

I want to build a symmetric/bipolar XX-0-XX psu out if it. (XX = I don't know the output dc voltage... should be 24 * 1.4)

My first try resulted in a blown fuse...but anyway.

I did my homework and found some "solution", but I'd like to know if you think any of them could be used.

Solution #1 :
Using 2 rectifier bridge :
Click the image to open in full size.

This is kinda my first attempt, but I didn't put any capacitor... I've only plugged both rectifier in parallel (AC side) and series (DC side)... and it blown my fuse. Maybe my fuse was too weak (some random Fuse the clerk gave me... I didn't even look at it...but I asked for 125V 2A). My new fuse is 125V 3A (I had some in my scrap bin!).

Do you think if I add the capacitor It will work? I didn't use any capacitor as I was only testing the output voltage (without load) on my multimeter.

Solution #2 :
Using 2 Diode
Click the image to open in full size.

I didn't try this solution, but still, are the capacitors necessary for not blowing anything up?



That's pretty much it.

I'm also found regulated solution (with Zener or LM317/137), but I'd like to avoid regulated voltage as it will limit my output current.

As I said, I'm new to Diy Audio, but I've been doing some little Electronic (mostly microcontroller driven board). I'm used to work with 5vdc and 12vcd regulated psu... but I never built one!

And... thanks for this wonderful forum full of information!



p.s. I know I could use the lm3886 in single power supply, but I'd prefer to have a symmetric/bipolar solution
p.p.s I've just realized that my 2 solution are on the same website (I've use google search to find schematic)... anyway I've seen similar schematic on other pages...
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Old 28th May 2009, 10:53 PM   #2
star882 is offline star882  United States
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The correct way is to ground the center tap and connect a bridge rectifier to the two ends. Then filter and regulate the + and - outputs.
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Old 29th May 2009, 02:23 AM   #3
Andreq is offline Andreq  Canada
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Ho... sorry, I didn't say that I want a 24VCD (or more) PSU.

I can't use the 12-0-12 center tap as I will only get 12VCD...the lm3886 will probably output nothing with such a little voltage.
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Old 29th May 2009, 02:23 AM   #4
Andreq is offline Andreq  Canada
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Update :

I've just tryed my solution #2 and it seem to be "ok".

I'm getting 39-0-39 (without load).

I'm using 1000uf 64V capacitor. I got 2 4700uf 35V cap, but seeing the output, I will not try them!

Do you think this solution is "ok" (read as... will produce amplification)?

Should I had some more component to reduce potential hum ?

Thanks
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Old 8th June 2009, 12:33 PM   #5
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I wonder, why you didn't find this.

Quote:
Originally posted by Andreq
I can't use the 12-0-12 center tap as I will only get 12VCD...the lm3886 will probably output nothing with such a little voltage.
Don't guess. Read the datasheet. The LM3886 works from 10 V DC to 42 V DC. A 12-0-12 V transformer gives you ~16 V DC at idle and ~14 V DC loaded, which again gives ~9-12 W into 8 Ohm or ~19-24 W into 4 Ohm.

Quote:
Originally posted by Andreq
I've just tryed my solution #2 and it seem to be "ok".

I'm getting 39-0-39 (without load).
No way. You must have tried something different.
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Old 8th June 2009, 12:50 PM   #6
Andreq is offline Andreq  Canada
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Hi,

I can confirm I'm using solution #2 ans it output ~39VDC without load.

I'm not using the center tap, so I have 24VAC coming out of the transformer.

As I'm going to use 8ohm speaker, I don't think using the center tap (12VAC ouput) will be powerful enough.

Do you think that ~9-12W is a lot of power for general music/movie use? I'm not an audio freak and I'll be using 25yrs old speaker with that rig, but I'd like to have enough power to have powerful mid and bass...

Thanks for your input.

and... I'm still waiting for my LM3886, so any input would be great until I get them


furthermore, can I somehow add a parallel resistor to my speaker to make it 4ohm.

As I said, I'm far from being audio guru, but I'm good with micro controller electronic. Electronically speaking, I can use parallel resistor (2 x 8ohm) to have 4 ohm.
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Old 8th June 2009, 01:36 PM   #7
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You've built a half-wave voltage doubler. This is less than ideal for a power amplifier.

http://ourworld.compuserve.com/homep...t/elect346.htm
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Old 8th June 2009, 04:57 PM   #8
Andreq is offline Andreq  Canada
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Should I use the center tap than?

It's quite a shame as the transformer is 12VAC 10A... It have plenty of power... but little voltage.
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Old 8th June 2009, 07:21 PM   #9
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Quote:
Originally posted by Andreq
Do you think that ~9-12W is a lot of power for general music/movie use?
That depends on many factors, e. g. the speaker sensitivity. Anyhow volume is a very personal experience. What one person perceives as disturbingly loud, is not loud at all for somebody else.

Quote:
Originally posted by Andreq
I'd like to have enough power to have powerful mid and bass...
A lot of that depends on the speaker, but for powerful bass you should add some more capacitance to the power supply.

Quote:
Originally posted by Andreq
furthermore, can I somehow add a parallel resistor to my speaker to make it 4ohm.
You can do that, but the speaker will still be an 8 Ohm speaker. The additional power will be converted into heat in the resistor, so you will make your amplifier work harder without benefit.

Quote:
Originally posted by Andreq
Should I use the center tap than?
Yes. The voltage doubler suffers from high ripple voltage and severe voltage drop under load.

Quote:
Originally posted by Andreq
It's quite a shame as the transformer is 12VAC 10A... It have plenty of power... but little voltage.
Could be good for a multi-channel amplifier.
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Old 8th June 2009, 07:53 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
your 12-0-12Vac transformer will give ~+-18Vdc across the two capacitors.
You can run most of the chipamps off this supply voltage.
When heavily loaded the voltage will probably drop towards +-15Vdc. The 3886 will still run from this range of supply voltage.
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