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Old 29th March 2011, 01:45 PM   #1
andb is offline andb  Czech Republic
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Default Can this trafo be used in a chipamp?

After searching through my parts bin, I came across this transformer. It is printed on the label that its 2x19v / 200VA, which would seem the right one to use for one of BrianGT's 3886 chipamps. However, the secondary wiring isn't the typical 4 colors I see most people using. It seems that 2 have voltage and the center is a ground.

Can I use this toroid? Would the proper way to wire this to take the center wire to chassis ground and the two with voltage both to the AC1's and to the AC2's in parallel?

Thanks for helping me not blow everything up

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Old 29th March 2011, 02:10 PM   #2
AndrewT is online now AndrewT  Scotland
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It is likely to be a centre tapped secondary.

Go and read up on the difference between centre tapped and dual secondary.

You can check if it is centre tapped, using simple cheap tools/equipment.

Do you need an explanation of that procedure?

Notice it is a 230:19-0-19Vac transformer.
It will not measure 19Vac when connected to 230Vac. It will measure differently if it is connected to a 220Vac or 240Vac supply.
regards Andrew T.
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Old 31st March 2011, 09:36 AM   #3
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In the really basic photos below are some possible answers to your questions, because clearly marked bridge rectifiers also happen to work well. Absence of gimmicks can sound good too.

At Decibel Dungeon you can find many more answers and possibly avoid international shipping, because the parts for LM3886 amplifiers are available almost everywhere.

Gainclone chip amp index page

Please also look up bleeder resistors so that the potentially lethal shock in the capacitors can be drained out soon after the amplifier is unplugged.

LED indicator lights are also a good thing.

I couldn't find a clear photo showing the center tap cable meeting up with star ground and speaker ground. So, please use the search feature to research AndrewT's many posts on the subject(s) of grounding and the importance of safety earth.

That will give you the "rest of the story" for your center tap cable. Good layout practices relate to both safety and sound quality.
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Old 1st April 2011, 02:15 PM   #4
andb is offline andb  Czech Republic
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Thanks for the feedback! Andrew, your pointing out that it was center tapped helped a lot, then yesterday I found a number of Daniel's posts getting a center tapped trafo working on the chipamp. I decided to relegate this one back to the parts bin and I ordered a dual secondary trafo from, which look to be pretty nice quality.

I'm using the BrianGT PCB's and I didn't look at the schematics if there is a bleeder resistor there at all. I'm in the habit of using the continuity testing mode on my volt-ohm meter before working on anything, it will make a nice loud beep for the 15 or so seconds it takes to drain the caps.

Thanks for the bold tip on grounding, I definitely appreciate its importance. I've used an anodized aluminum case from Modushop and have filed down the edges on all sides to make sure that the whole case is properly connected to earth (I hope I've done this right ).
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Old 1st April 2011, 06:04 PM   #5
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If you already own that kit, then have a look at this picture.

After good quality layout, including seemly grounding, the best tweak may be part Cc to at least 100pF, and this is listed in the LM3886 datasheet by National Semiconductor. Its listed as an optional part, but healthy amplifiers are certainly more useful.

Also, please notice that the grounding arrangement of that kit is designed for dual mono or monoblocs. It will take some additional work to get it stable in a stereo configuration (no matter what transformer you use).

One possible and easy way to do that for a stereo version is to bond the power grounds and speaker ground all at the "output end" of the power supply board (See Photo!), creating a normal power star ground at that location. Then, your amplifier boards hook up to the power board with the normal 3 conductor arrangement of two for voltage, one for ground (amp board CG goes to power star ground). That may help avoid noisy ground loops. Avoiding haywire ground loops is pretty helpful at keeping the amp stable, and stable sounds good.

If this description was unclear, please see the Decibel Dungeon page on gainclone power supplies and grounding.

EDIT! This picture shows hookup for center tap transformer.
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Old 2nd April 2011, 12:58 PM   #6
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Originally Posted by andb View Post
. . . Thanks for the bold tip on grounding, I definitely appreciate its importance. I've used an anodized aluminum case from Modushop and have filed down the edges on all sides to make sure that the whole case is properly connected to earth (I hope I've done this right ).
The supply has both bleeder resistors and an LED indicator light.

But its very unfortunate that the LED indicator doesn't indicate the either the presence or lack of voltage in the main supply--the led has a private secondary supply of its own with 1n4004, 10uF, 10k, LED.

With the kit, light off doesn't mean safe.
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Old 22nd June 2011, 08:14 PM   #7
andb is offline andb  Czech Republic
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Default Blowing fuses with new trafo

After waiting 7 (yes, seven) weeks for some cloth covered solid core wire to clear customs, I finally sat down to finish building the chipamp. I got what should be a dual secondary trafo from I decided to use the 3875 boards I had sitting around instead of the 3886, maybe I'll swap them in the future to listen to the difference.

Unfortunately I seem to have made some kind of mistake on the wiring. When I turn the amp on, the trafo buzzes a bit then the fuse blows. I thought it might be an issue with something on the PS, since earlier I had blown a diode on the snubberized PS with the old trafo and too big a fuse. So to be careful I replaced all the diodes with BYW29-200's. The plain PS was wired for the center tapped trafo, so I stripped all the wires and diodes, also replacing them with the BYW29-200's. I tried both in the amp to see if it was an issue with one of the PSes

I figure I've probably done something really silly with wiring, and it would be great if someone would be able to either point it out to me or help me to "debug" this thing.

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File Type: jpg amp_3875_plain_ps_back.JPG (400.9 KB, 33 views)
File Type: jpg amp_3875_amp_pcb_bottom.JPG (353.0 KB, 36 views)
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Old 22nd June 2011, 08:29 PM   #8
infinia is offline infinia  United States
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Try reversing the phase one of the XFMR secondaries. ie swap white and black on one winding.
It's normal to test thing in stages before wiring it all up. Using a home brew "light bulb tester" saves damage and hair pulling too.
1) check the transformer AC by itself with out secondaries connected.
2) check the power supply +/- voltage unloaded.
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Old 23rd June 2011, 12:04 AM   #9
PJN is offline PJN  United States
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Check the trafo wiring the if it is a CT then the small diameter wires are probably the secondary's and the fat black and white wired might be the primarys. You may have it wired backwards. Like Infinia suggests just test the trafo hooked to AC first with your DMM to make sure that you have it hooked up properly.

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Old 23rd June 2011, 01:43 AM   #10
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Hi Andb,

I'm a bit confused by your photos. It looks, in the 4th photo, that your replaced the PS filter caps with something much smaller, but you didn't mention this in your text. Also, I assume the orange wire coming from the transformer is ground for the internal shielding. Am I correct? As long as you are willing to consider silly mistakes, check that you have the mains input hooked up correctly. It looks like you are using a combination mains input, on/off switch, and fuse-holder with a ground. Which tab is the ground is not always obvious. Like Infinia stated, test things in stages to isolate the mistake. I recently finished this very same kit, and can attest that if you follow Brian's instructions exactly you will end up with a very nice-sounding amp. What tripped me up was that I didn't know that the outsides of the diodes are not insulated, meaning that if they are touching each other or the case, you will blow a mains fuse. Are your diodes touching anything?
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