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Old 2nd August 2015, 06:42 PM   #1
Kimi is offline Kimi  Finland
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Default Another noob transformer question

Hello everyone,

First of all I have to apologize a bit for this very beginner question. I tried to search and google around for a couple of evenings, but I'm none the wiser...

I'm putting together my first ever diy-amp, the Chipamp 3886 stereo kit to be exact. I don't know anything about electronics as I normally make diy-speakers just for the woodworking, but decided to give this side of things a go...

Anyway, this is my problem:

I have a toroidal transformer with the following markings:
25-0-25 V
Blue - black - blue

The primary wires are red, that's no problem and then out come the two blue ones and a black one.

Am I correct that my transformer has single secondaries?

From the kit manual I understood that I should use a trafo with dual secondaries. Can I use the trafo I have and if so how do I connect it?

Many thanks in advance and sorry if this has been asked and answered before. I couldn't find anything that was clear enough for me..
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Old 2nd August 2015, 07:09 PM   #2
Mooly is offline Mooly  United Kingdom
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Your transformer is fine. Its the equivalent of two 25 volts windings in series with a common centre tapping. So its fine for the chip amp. If you needed an application with two isolated 25 volts windings then it would not be suitable... but you don't

So the black wire is the 'ground' and the two blue connect to the bridge (either way around, it doesn't matter)

(when you power your amp up for the first time, use a bulb tester... it can save expensive mistakes)
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Old 2nd August 2015, 07:24 PM   #3
Kimi is offline Kimi  Finland
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Quote:
Originally Posted by Mooly View Post
Your transformer is fine. Its the equivalent of two 25 volts windings in series with a common centre tapping. So its fine for the chip amp. If you needed an application with two isolated 25 volts windings then it would not be suitable... but you don't

So the black wire is the 'ground' and the two blue connect to the bridge (either way around, it doesn't matter)

(when you power your amp up for the first time, use a bulb tester... it can save expensive mistakes)
Ok thanks!

I want to clarify a bit more..

The Chipamp.com PSU board has places for four secondary wires and the schematic in the manual also has four leads coming from the trafo.

This is from the manual:
Click the image to open in full size.

And also on this picture from the chipamp.com pages I can see four wires going in to the PSU:
Click the image to open in full size.

Do I have to split the two blue ones in to four perhaps?


Oh and yeah the bulb tester is the next thing to put together while I wait for the last parts to arrive for the amp.
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Old 2nd August 2015, 07:28 PM   #4
Mooly is offline Mooly  United Kingdom
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+PGND and -PGND at the right become one line (joined). That joined line goes to the black tranny wire. You replace the two bridge rectifiers with one singe one.

Like this as a wiring example.
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Old 2nd August 2015, 07:33 PM   #5
Mooly is offline Mooly  United Kingdom
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Like this.
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File Type: png Capture2.PNG (63.2 KB, 63 views)
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Old 2nd August 2015, 07:51 PM   #6
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I don't think that your artwork would go for auction anytime soon
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Old 2nd August 2015, 08:21 PM   #7
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Quote:
Originally Posted by Mooly View Post
Like this.
You can use one of these, cheap, bullet-proof, get one that is way over-rated....if you want, bolt it up to a small plate of Aluminium.

----------------------------------------------------------Rick..........
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Last edited by Richard Ellis; 2nd August 2015 at 08:22 PM. Reason: typo
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Old Yesterday, 04:35 AM   #8
Kimi is offline Kimi  Finland
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Quote:
Originally Posted by Mooly View Post
Like this.
Alright, thanks again that looks clear enough I think.

I'll have to take a look at the pcb when I come home from work, but I guess I have to modify it a bit? Or I mean I have to solder wires somewhere.
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Old Yesterday, 04:44 AM   #9
Kimi is offline Kimi  Finland
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One more thing: If I build more of these things in the future I have to get a transformer with dual secondaries of course.

How do I tell if a trafo has them or not? How are they marked in ebay for example?
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Old Yesterday, 06:23 AM   #10
Mooly is offline Mooly  United Kingdom
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You only have the description to go off when buying, along with any manufacturer markings that allow you to look up any data sheets.

The PCB should be easy to rework by removing a bridge. You will have to cut a trace on the bridge that is remaining to isolate it from ground. Use a modelling knive to cut the print twice a few mm apart and lift the print away to get good isolation. Add a link from bridge to tranny and join the two 'grounds' and you should be fine.
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