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Old 13th March 2009, 07:46 PM   #1
cdl is offline cdl  Denmark
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Default Modify 4-wire NewClassD PSU module for use with centre-tapped transformer

This thread is to describe the modifications I did to the (4-wire) NCD power module to make it work with my centre-tapped transformer. Also showing some pics of my NCD implementation.

Of course it would be better just to use a transformer with separate windings (as the module is intended for). Or - if one must use a centre-tapped transformer - it would be cheaper to build a simple rectifier bridge+ 2 caps.

So why the craziness? I was re-using the enclosure and transformer of my old classAB amp... which I thought was with separate windings, as it had a 4-wire plug and the schematics of the old amp showed separate windings. So I just bought the standard 2 amps + 1 PSU package from NCD.

Here's the class A/B amp with it's guts showing - screws loose already, ready for to be removed...

(In case anyone is wondering why I'd gut my Aura PA-200, one channel was periodically faulty - and I just couldn't get to the bottom of the problem, not to mention having trouble getting spare parts for the thing).
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Old 13th March 2009, 07:55 PM   #2
cdl is offline cdl  Denmark
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Basic layout in place. Already encountered the first problem - PSU module with caps mounted is just too high to fit in the enclosure OK so mount beside PCB instead... boy did I have fun de-soldering those guys from a PCB with full power planes
Ups- next problem: they won't fit next to the PCB, no room because of the transformer Fortunately there was just room to fit them lying on top of the PCB with short wires.
REMEMBER INSULATION BETWEEN CAPS AND BORD!
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Old 13th March 2009, 08:18 PM   #3
cdl is offline cdl  Denmark
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Quick check of the connections from the transformer - the thing is centre-tapped 34V-0V-34V not 2x34V as the schematic shows.
Quick check of diagram: this will not work with the NCD PSU

So what to do? Mod it so that four of the diodes form a regular rectifier bridge (the other 4 are unused). It seems complicated looking at the schem. But actually all that has to be done is to de-solder the two "bottom" legs of the "bottom" rectifier diodes (which can be hard enough, mind you - patience!).
Flip the legs out of the wholes in the PCB, solder on a piece of wire, and cover up with insulation (legs must NOT have connection to the PCB beneath them).
The rectifiers will seem a bit "loose" now that they're only connected with one leg, but don't worry - when you bolt the PCB to the case, heatsink or whatever, they will be fine.

Now attach the two wire ends to the correct power plane as shown - should be easy. It ain't pretty, but it works.

Almost done - next step in the next post.
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Old 13th March 2009, 08:31 PM   #4
cdl is offline cdl  Denmark
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Now all you need to do is:

Connect centre tap of transformer to groundplane somewhere - I used the very handy terminals for off-board mounting of the caps - one leg of each cap goes straight to the groundplane (Measure to make sure you get the right leg!!!). See blue circle. I also connected the two "sides" of the groundplane with an extra wire - certainly not necessary.

Then connect the ends of the transformer coil to the "outer" terminals of the PSU - see red circle.

That's it - you are up and running!!
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Old 13th March 2009, 08:39 PM   #5
cdl is offline cdl  Denmark
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Almost ready for first test of my amp. Yes, those are fuses you see on the small PCB to the left... I am a chicken.
Later I had to exchange the black wiring used to connect the modules to the PSU... they are individual conducters stripped out of a surplus lenght of the thick Audioquest speaker cable - I'll treat my power to the best too, I thought. But alas - the black stuff doesn't insulate Took me a while to figure out the problem - had already mailed Lars because I thought one module wasn't working (thanks for being patient Lars).
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Old 13th March 2009, 08:51 PM   #6
cdl is offline cdl  Denmark
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Final picture of the "entrails" after mounting the EMC-kit today. You can see the blue capacitors and the big ferrites on the speaker cables. Also added a ferrite to the power cables just in case.
The clear things strapped to the amp modules are lightguides that bring the blue light to the front panel.
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Old 13th March 2009, 08:54 PM   #7
cdl is offline cdl  Denmark
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The front panel in dark surroundings. The opening in the front panel from the brigde mode indicator of the original amp came in handy for the 2nd module.
Below the left opening is the power switch, also salvaged from the PA-200 of course.
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Old 13th March 2009, 09:08 PM   #8
cdl is offline cdl  Denmark
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Default Final post

... And here's how she looks re-assembled... very pleasing (to me at least). Only thing missing (suggestion for you, Lars) is a little stylish emblem saying "Powered by NewClassD" or something. The fascia is actually mirror/silver, it's just hard to show on photos.

I found the (slight) turn-on/off clicks disturbing at first but have become accustomed to them now. Only annoyance is that in my setup the amp does cause a bit of disturbance in DAB and FM radio - discussed in this thread.

I have posted about the sound elsewhere, so I will just say that I am very happy with the sound and power of this amp. Even with 88dB sensitivity speakers, I doubt that I have ever come close to the peak output power. Which - even the 33V transformer - should still be some 300W or so on 4 ohms I reckon (? just a guess, didn't calculate or measure). Enjoy
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