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-   -   NCD questions (http://www.diyaudio.com/forums/class-d/88117-ncd-questions.html)

Bgt 11th October 2006 07:53 PM

NCD questions
 
Lars, received your modules today and just want to know, are there no coupling caps?:D Nice to have the input amp. on a socket.
I measure 2.4 ohms between input and output ground? Where is the resistor situated? pretty and small pcb;)
Thanx
Bert

classd4sure 12th October 2006 12:17 PM

Re: NCD questions
 
Quote:

Originally posted by Bgt
Lars, received your modules today and just want to know, are there no coupling caps?:D Nice to have the input amp. on a socket.
I measure 2.4 ohms between input and output ground? Where is the resistor situated? pretty and small pcb;)
Thanx
Bert

Doesn't look like you're getting an answer. As you can see, there are no coupling caps! Also, there's no means for adjusting DC offset :xeye:

2.5 ohm groundlift resistor, it's right next to the "D" in the screen printed "NewClassD".... I took mine out, and shorted the pads, makes it a little more accurate.

When you plug it into the supply, you'll be measuring about 12.5ohms instead. That's because there's yet another ground lift resistor in the supply of 10ohms. You can locate that easily enough, it's kind of all by itself on the top of the PCB. I popped that one off too, left open!

Lars Clausen 12th October 2006 12:31 PM

Hi Bert

Sorry i didn't see this thread, but i do appreciate you started it. I kind of think it would be nice to have an all-NewClassD thread again.

You are right, there are no coupling caps. This relies on the signal source to be free of DC comnponents, which it should be in any case. Even in case you have coupling caps, and there is a small DC component on the signal from the preamp, you would still get a loud bang, when you move the plugs. And small crackling noise when you don't. Therefore i think coupling capacitors are a half solution to a problem that is really somewhere else in the system. (And should be solved there). I hope you see the background for this design decision now :)

There is a Ground Lift resistor, with the purpose of being able to connect the input GND's together (in the signal source) without getting unwanted Ground loops. I can not recommend shorting them both out, even though you can short one out in the system, without risking ground loops.

Thanks for the nice comments :)

All the best from

Lars Clausen

Bgt 12th October 2006 12:58 PM

Hi Lars,
The first thing I would do anyway is taking the coupling caps. out, if possible(depends on physical built of the amp. and input chip). I prefer allout DC anyway. Razorsharp bass response:D. If you use DC protection it is not a problem anyway.
What is the gain resistor value? I want to normalize the input sensitivity for my passive pre. About 35db is enough. Is there no offset adjuster? This because of the lm6172 which is an offset sensitive chip.
The groundlift resistor I want to experiment with.
So is there another groundlift resistor in the circuit like Chris said?
Cannot imagine what this would do? After all the only that matters is the 1 between main (supply lines of amp.)incoming 0V and input 0V(of amp.) which goes to the amp. input lines.

soongsc 12th October 2006 08:05 PM

I remember some Tripath amps using chokes fo somne sort between the signal and power grounds.

BWRX 12th October 2006 10:03 PM

Yes, some use a ferrite chip inductor. That's more of a band-aid fix to avoid ground loops and to try and keep high currents out of the signal ground plane.

ghemink 13th October 2006 03:30 AM

Quote:

Originally posted by BWRX
Yes, some use a ferrite chip inductor. That's more of a band-aid fix to avoid ground loops and to try and keep high currents out of the signal ground plane.

The info on GND loops when you use multiple amps on one power supply is one issue that has bothered me also. My issue: what is the best grounding in case of XLR inputs when multiple amps are connected to the same supply?

In general people say that signal GND should be connected to pin 1 of the XLR input and that pin 1 of the XLR should be connected to the case. The same people also say that the supply GND should be floating from the chassis (although this requires double isolated supplies for safety reasons I believe) for best sonic results. I also can see that point.

However, if we have multiple modules and all their signal GND are connected to pin 1 and to the chassis, we have created GND loops. One way to get rid of them is the resistor between power GND and signal GND as in the NCD modules, or likely the same effect can be achieved when connecting the signal GND with a resistor to pin 1 for amps that do not have the GND lift resistor on the module itself (UcD for example). Is there general consensus that this is a good method to prevent GND loops? Best is of course to have a dedicated power supply for each amp, leave the power supplies floating from the chassis and connect signal GND to the chassis, this is the most expensive solution though and whonder whether it is really required.

Advice/comments/ideas please

Best regards

Gertjan

Bgt 13th October 2006 03:23 PM

Quote:

Originally posted by ghemink
The info on GND loops when you use multiple amps on one power supply is one issue that has bothered me also.

Should be no problem:


http://www.grotel.nl/photo/wiring%20...d%20stereo.jpg

ghemink 13th October 2006 03:48 PM

Quote:

Originally posted by Bgt


Should be no problem:


http://www.grotel.nl/photo/wiring%20...d%20stereo.jpg


Hi Bert,

Thanks for the info, however, this does not look like the Hypex recommendation, they recommended to keep the power supply floating. I`m also using XLR balanced inputs while the GNDing scheme in your link is for RCA input. It may work pretty good though as those UcD differential inputs maybe quite forgiving.

Best regards

Gertjan

Lars Clausen 13th October 2006 04:23 PM

The best GND'ing setup for balanced input depends largely on the way the balanced output was implemented in the preamplifier.

Some use two active output with a phase reversal, and GND of both connected to pin 1. Others have a transformer with a center pin connected to pin 1. Others again have only connected pin 1 to the chassis, and have the signal pins more or less floating in relation to pin 1. The professional standard is a further different variant with several twisted pairs in one shielded cable, where the pin 1 carries only shield drain. In this case transformers in the input is more or less a 'must have'.

I can't see there is one GND setup in the power amplifier that can take all cases into account optimally.

So to make the best solution, you must first look at the way your preamplifier has implemented balanced output. But i guess this discussion is not realted to NewClassD, but balanced signalling in general.


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