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Old 27th December 2005, 12:25 PM   #511
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Konnichiwa,

Quote:
Originally posted by ergo
I just made an experiment a few weeks ago with LC filtering on DCX.

My conclusion was that it looks good on paper but was bad in reality. The problem is that there seems to be a lot of RF floating around inside DCX and the L part of the filter will pick it up like antenna.
I rather doubt that, actually, though some types of chokes are more sensitive to noise pickup than others, I suggest fully enclosed types, eg poy core etc.....

Quote:
Originally posted by ergo
I can post measurements later when I get home. I looked at the spectrum of 10Hz-40kHz and the LC type filter created a lot more dirt that raised above the noisefloor.
Are you sure that this was not the result of resonances in the filter? LC filters are invariably critical and need a well defined termination impedance to not cause all sorts of mischief.

Ciao T
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Old 27th December 2005, 01:22 PM   #512
ergo is offline ergo  Estonia
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Red graph is the LCL filter terminated by 1k resistor and the yellow graph is simple RC and all the rest is exactly the same.

Can this be a resonance related problem? If it is it gives awfully wide effect all over the upper measuring range

Ergo
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Old 27th December 2005, 01:31 PM   #513
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Hi Ergo,
First of all, thanks for the XO2/5 clock installation tips you made available for us. I used it on my own modest website (in french) Clock installation About the distortion characteristics of the Behringer here's what I measured on my own DCX with the RC 1st order low-pass filter.
Cheers
Thierry
Click the image to open in full size.
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Old 27th December 2005, 01:39 PM   #514
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Konnichiwa,

Quote:
Originally posted by ergo
Red graph is the LCL filter terminated by 1k resistor and the yellow graph is simple RC and all the rest is exactly the same.

Can this be a resonance related problem? If it is it gives awfully wide effect all over the upper measuring range
Hard to be sure, the screenscrape is fairly low-rez, but it looks to me more like a lot of distortion being present in the inductor, suggesting poor core linearity.

Sayonara
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Old 27th December 2005, 02:27 PM   #515
AR2 is offline AR2  United States
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I do not remember if we already had this discussion before or I 'v seen it at yahoo DCX group. It was about calculating correct cap in order to provide DC blocking as well as HF roll off.

Before I did my mod I consider price for that approach, and with good caps, it still would require some investment. Besides the price there is performance issue, and I would always choose transformer over cap in the signal path.

I completely agree with Kuei - transformers are best choice. Yes they will cost you as much as two Behringer units, but as I mentioned good caps will do as well. I needed balanced out, so in my case it would be minimum 12 caps - for 6 balanced outs.
I am using Lundahl 1674 transformers, and what they do: they block DC out of DACs, they provide balanced input and output, they roll off HF above 50 KHz, as well they provide galvanic insulation. On the top of that they are 1+1:1; 1+1:2 or 1+1:4.

That means that you could gain + 3dB in 1+1:2 set up without having anything else in the path of the signal. Obviously the best way would be 1+ 1:1. I am using mine in 1+1:2 since I need little more gain. Lundahls are $ 90.00 each.

In my set up nothing could replace that clean and wide open top end coming straight from DAC to the amp. There is only transformer and single resistor in the passive volume control before signal hits the amp.

Peace, and Happy Holidays

AR2
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Old 27th December 2005, 02:51 PM   #516
ergo is offline ergo  Estonia
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Hmm perhaps yes - it might be the inductors were not good enough.

This is the type I used
Inductor
I think I will try to get by without inductors. Seems to be a more predictable way to go.

Anyway, this is the current state of my analog stages. I have two channels built and each channel has one OPA134 and it's own shunt regulator.

I also have a PCB design ready for all 6 channels but the next step is to get it etched and it will take a bit of time before I can do that

Click the image to open in full size.
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Old 27th December 2005, 03:03 PM   #517
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I noticed that the inductor link you provided was of a "ferrite core" type, which are never recommended for audio signals. I would use only air core inductors, however, I have never seen them in such a small package.
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Old 27th December 2005, 03:51 PM   #518
Davey is offline Davey  United States
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Quote:
Originally posted by Kuei Yang Wang
Konnichiwa,



It is not surprising AT ALL. The circuit you cite is aimed at the exact opposite problem to the one faced wwhen directly interfacing the DCX2496 DAC's, that is it is meant to give a Pseudo-balanced output from a single ended source, while we need one that gives a SE source from a true balanced output.

Sayonara
Well, the transformer allows a configuration for pretty much any solution needed. How 'bout this simple circuit:

http://www.jensentransformers.com/as/as089.pdf

An appropriate filter in addition and it should work fine.

Davey.
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Old 27th December 2005, 03:54 PM   #519
Davey is offline Davey  United States
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Quote:
Originally posted by ergo
Red graph is the LCL filter terminated by 1k resistor and the yellow graph is simple RC and all the rest is exactly the same.

Can this be a resonance related problem? If it is it gives awfully wide effect all over the upper measuring range

Ergo
Ergo,

Set the SpectraPlus horizontal scale to linear vice log and the start to somewhere just below your excitation frequency and you can visualize much better.

Cheers,

Davey.
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Old 27th December 2005, 04:05 PM   #520
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You know what, this is funny but the "Mute outs" is still bloody working even if T1 to T6 trannies are out of business !

Still can't figure out what Behringer has done there


I suppose it's possible that they activate "muting" at various points along the signal path....like maybe the DAC itself doesn't "mute" or "unmute" very cleanly so they threw in an mute/unmute downstream.. maybe it's just a quiescing or stabilizing feature for that filter circuit when no signal is feeding it.

Anyway, just hypothosising since I don't have the rest of the circuit or the design engineer here with me. That circuit is most definitely a shunt straight to ground though. Someone must have thought is necessary..
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