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Class D Switching Power Amplifiers and Power D/A conversion

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Old 14th January 2013, 06:30 PM   #5951
pos is offline pos  Europe
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For now I don't feel confortable enough with the idea of removing that damned R141 myself, so I use autoformers.
It is a much better solution than resistors Lpad IMHO, for compression drivers at least : the nc400 sees a higher load, the compression drivers sees a decent damping factor, and almost no power is lost in the process. What not to like?

I use those:
eBay | +Jabo+Autotransformer+für+Hornspeaker++Klipsch+und +Altec+Crossover+

But even the cheap inwall models sold at parts epress do well for the high frequencies, for what I can measure and hear, except for a tiny UHF rolloff that is easy to compensate.
Old 14th January 2013, 06:37 PM   #5952
Julf is offline Julf  Europe
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Originally Posted by pos View Post
What not to like?
Distortion and non-linearity?
Old 14th January 2013, 08:23 PM   #5953
pos is offline pos  Europe
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Distortion? Not likely to be measurable at those frequencies with any half decent autoformer (not to mention the big ones linked above).

non-linearity? The frequency response was unchanged with the big autoformer, and with only a minor rolloff up high with the cheap one.

I have done a lot of measurements on my compression drivers, and could not detect any drawback in using an autoformer vs a lpad.
Old 14th January 2013, 11:38 PM   #5954
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Default gain resistor removal

The thought of surgery on my NCores scared me at first. What gave me the creeps was the idea of tossing around solder all over the place, bridging all kinds of points, or destroying a random component in the vincinity by ways of excess heat.

After a close encounter with our old friend hum, I did not only guess but know that a 400W amp wired straight to a 30/70W (sine/peak) fullrange driver is dangerous to the latter. Guess I was lucky the aluminium cones did not have a facelift that day. So it was decided, those gain resistors had to go.

First up, UcD400ST. Here it is R3 which needs removal for minimum gain. Desoldering can be messy, so I thought, why not slice the thing in half, remove the debris and leave anything stuck to the pads in place? I even considered the dremel with a cutting wheel on the sole condition that the board leave enough room. Eyeballing the wire cutter, I thought maybe the force pushing the pads outwards might become too great if I just cut through. So I tried to nibble away horizontally, maybe get the upper half to come off or something, and the poor thing just crumbled.

So I took some narrow pliers and just squished to dust what remained of the connection between the two pads. Holding the board at an angle makes the dust fall right off the edge. I gave the board a good scrubbing with a toothbrush and a sweep of the compressed air can anyway, just to make sure nothing builds a bridge that should rather be burnt.

What can I say, it powered up neatly and did not eat the canary speaker after that, and would the latter not be so bad, it would have been music that was played, except at a lot lower a volume.

The problem with NCore and its R141 is that it is a different type of component - not a cuboid of black brittleness, but a ceramic tube, almost like ordinary through-hole resistors, and pretty tough. I had to push a lot harder to get that one pulverized, but it went smoothly as well. While slowly increasing the force applied to the part, it is paramount to not pull upwards or sideways against the board. Holding the amp module by the resistor alone looks brutal, but the weight of an NCore or UcD against the pads does not seem to do any harm.

I expected some remains to be stuck to the pads, but apart from a thin layer of solder they were almost even. Looks like it is possible to get those gain resistors clean off without even heating up the soldering iron Just grab the module by the gain resistor and crush - the latter. Squeeze, don't pull - that's what Americans say, huh.

Hope that helps whoever would like to lower the gain and is afraid of the whole desoldering business.
Old 15th January 2013, 12:19 AM   #5955
OllBoll is offline OllBoll  Sweden
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An easy way to remove R141:

First splatter the top and both sides of the resistor with some solder.

Then heat both sides at the same time by laying the soldering iron and when heated just push the resistor away, or push it off with a tweezer.

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Old 15th January 2013, 12:43 AM   #5956
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OllBoll, your method looks manageable as well

As far as I am concerned, whenever there is nothing at stake I am quite well in control of my extremities, but once there's a chance for something to break, like really bad, I am bound to mess up. Glass of water next to the keyboard? What spasm is going through my shoulder to move my elbow not past but right through it when there is not even a reason for moving it there to begin with? Lend a hand, or rather sever an arm for that matter?

Just desoldered probably damaged voltage regulators from the O2 board during PSU debugging, now let's go for the replacements. Placed them in the right position and direction ten times at least, right before soldering they fall out, and then I put them in all the wrong ways possible except they wind up on the right side of the board!? Needless to say, desoldering a second time I fried the PCB pretty bad and I yet have to answer swalter's e-mail how it all went. That time I was just a iota of frustration away from kicking myself in the head for being a stupid f'tard...

Needless to say, when it came to the UcDs and NCores, my first thought was to hit Murphy over the head with a hatchet. Avoid whatever can go wrong the stupid way. Which is also why the dremel ultimately stayed in its box that day
"What could possibly go wrong!?"
*wheeeeeeeeeeeeeeeeeeeeeeeeCCCCHHRRRRRRRRRRRRRRRRR *

Guess I'm just fsck'd that way. No hard feelings unless it's blatant malice or my own fault
Old 15th January 2013, 01:04 AM   #5957
OllBoll is offline OllBoll  Sweden
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An even easier (and much more safe one) would be to just use an L-pad attenuator before the NCore. Not as ideal but in the real world it shouldn't matter.
Old 15th January 2013, 04:36 AM   #5958
ds23man is offline ds23man  Netherlands
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Originally Posted by Julf View Post
And lose the low output impedance / high damping factor of the amp?

is it? I can see the reasons for why pos wants to use the nc400, and lowering the gain would definitely help in his situation.
The damping factor is very important with cone drivers, with compression drivers it is less important. But what is wrong with an att. network before the driver? Most passive speakers use them to match the midrange and highrange.
Old 15th January 2013, 05:09 AM   #5959
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Originally Posted by ds23man View Post
But what is wrong with an att. network before the driver? Most passive speakers use them to match the midrange and highrange.
Well if you are a minimalist ........
Old 15th January 2013, 05:33 AM   #5960
ds23man is offline ds23man  Netherlands
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Originally Posted by mike_mcf View Post
Well if you are a minimalist ........
Then you should use a nice 20 watt class A amp for a 113db driver

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