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Old 18th January 2010, 06:08 PM   #1
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Default Connecting a center tap transformer

Hello!
I''ve got some issues with my BrianGT LM3886 Stereokit.
Problems is, I dont know how I should connect the transformer to the rectifier.

My transformers is one of those center taped ones, and I read in "Chip amp power supply- a beginners guide" that center taped transformers did not work on my rectifier (the one that comes with the kit)

Isnt it possible to do like this? (see picture)

Click the image to open in full size.

Its possible that I totally messed up that schematic since I'm not sure how to write it down (damn, I feel nobish)

When I measure the voltage between pin 1 and 2 I get a 21v result, same with 2 and 3. When I measure the voltage between 1 and 3 I get a 42v result.

Is it possible to do like this? or do I need to either;
1, modify my rectifier.
2, split the center tap on the transformer.
3, Buy a new dual secondary transformer.

Thanks

/Max
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Old 18th January 2010, 07:02 PM   #2
Puffin is offline Puffin  United Kingdom
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There was a post a long time ago on here where someone had modified the PS board to take a CT tranny. I would guess it was 18mths - 2 years ago.

Edit: Found it (2 years ago)

Click the image to open in full size.

Ther is also info on Chipamp.com "Users guide to non-inverted LM3875 kit". Page 3 (3.1)

Last edited by Puffin; 18th January 2010 at 07:14 PM.
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Old 20th January 2010, 11:11 AM   #3
AndrewT is offline AndrewT  Scotland
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Puffins pic shows that 4 rectifier diodes have been removed. This converts the dual rectifier of the standard BrianGT PCB to a single bridge rectifier that suits a centre tapped transformer.

Take care to add the shorting links to the correct pads.
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Old 20th January 2010, 03:13 PM   #4
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Awesome! Thanks alot for the help, I'll look into this for sure.

Just one additional question, is there any downside with using a single bridge rectifier instead of the standard dual configuration?
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Old 20th January 2010, 07:36 PM   #5
AndrewT is offline AndrewT  Scotland
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only if you build a stereo amplifier.
In a monoblock I believe both work equally well.

In a 2channel (or more) amp the grounding becomes more difficult if you use a centre tapped transformer, because that forces one to adopt a single audio ground that must be shared with all the amplifiers and that in turn increases the voltage drops due to longer leads and introduces loop areas that could induce more interference from EM fields.
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Old 22nd January 2010, 12:58 AM   #6
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What about not even hooking up the center tap?

I built an amp with two 12-0-12 transformers and one power supply board and it worked fine.

Should it have even worked?
If it is not the right way then why? Safety?
Are there differences in center tap transformers?

I'm surely no expert just wanting to learn more about transformers and there application.
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Old 22nd January 2010, 10:24 AM   #7
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I'm no expert either, but I GUESS (correct me if I'm wrong) that you're wiring up the +12 rails with eachother and therefor gain a total of 24v from two trafos. Which is about the same as a 2x24v dual secondary trafo (I think).

I also think that you should connect the center tap directly to your star grounding point.

In my case I just have a 21-0-21 trafo which will give me 42v on a single rail, which wont work.

Waiting for someone to correct me about this, so dont trust me...yet
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Old 25th January 2010, 12:29 PM   #8
AndrewT is offline AndrewT  Scotland
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21-0-21Vac is a centre tapped secondary.
It must be used with a 4diode bridge rectifier.
The centre tap must be connected to the middle of the two smoothing capacitors in the PSU.
Take a wire from this Zero Volts line to your central Star Ground (= Audio Ground).
Expect the 21-0-21Vac to give ~ +-29Vdc.
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Old 28th January 2010, 01:20 PM   #9
maurycy is offline maurycy  United States
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I have LM3875 running of the 42Vct 21V- 0 - 21V 80VA CT (from Apex Jr). It works fine. As mentioned above you have to modify the rectifier by removing 4 diodes.

My speakers are small (Mission M30i) but the GC is better than $300 Denon receiver.
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Old 30th January 2010, 03:27 PM   #10
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Quote:
Originally Posted by Puffin View Post
There was a post a long time ago on here where someone had modified the PS board to take a CT tranny. I would guess it was 18mths - 2 years ago.

Edit: Found it (2 years ago)

Click the image to open in full size.

There is also info on Chipamp.com "Users guide to non-inverted LM3875 kit". Page 3 (3.1)
That looks familar! Its mine. I'm so embarrassed.
That's also an awful thing. It just pours out high frequency noise, mainly from the diodes.
Instead, MR756, or ordinary 6A* diodes would work better.

Fortunately, Mark Houston provides a nice diagram of a "no noise" rectifier using ordinary diodes:
Click the image to open in full size.

Although I did it a different way, it would have been MUCH easier to simply have purchased some ordinary 6A05 diodes from the local radio shack. They already come 4 in a package. They also have the 10nF polyester caps too. OH, but that would have been much too easy.

On my chipamp.com power supply (the photo that Puffin found). . . See the "tied together" PG+ and PG-, creating a power star ground?
This tie goes cabled to the CG terminal at the amplifier boards.
This tie also goes cabled to the speaker jacks serving as speaker ground.
From this tie, a "thinner" cable (like 22ga or 24ga solid copper) can provide a unified input circuit ground so you don't get hums.

P.S.
My power supply is also equipped with a 4.7uF big plastic tweeter cap connected directly between V+ and V- at the output of the power supply board, to further block power noise. A mention of this is in the updated publication of the CarlosFM 2005 power supply. It had the amp running cooler (cleaner power does it) and it also sank the mids a bit (almost enough).

P.P.S.
The directions for that kit, although plentiful, are also unclear. The input load is 22k resistor//20k potentiometer, which makes approximately 11k input load. If you omit the potentiometer, then swap the 22k input load for a 10k resistor (decreases DC offset).
The DC blocking cap for the input system goes between your input RCA jack and your potentiometer--put the blocking cap at the RCA jack (in series with the signal +). Also install a 220pF (or nearby value) little ceramic cap as a load (+ to -) at the RCA input jack itself.
Here's where the directions fail: If you were to put a cap between the potentiometer and the amplifier, the DC offset would be unacceptable and the bass poor.
For most amplifiers, you really really should use an input filter cap. It goes at the RCA jack (somewhere between the source and your potentiometer--not between potentiometer and amplifier).
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