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Old 8th February 2011, 03:34 PM   #2951
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Thanks for the advice sendler, sek and AR2, I'll be sure to keep it in mind once I get around to swapping DAC chips.

For now I've only done the direct out mod, as I wanted to see how much difference the direct out mod would make... I ended up making a passive low-pass filter on some stripboard, using 0.033uF caps and 100R resistors, and I can easily bypass this if I choose to once I get around to swapping DAC chips.

It has made a big difference sound, the sound is more refined, and the bass and treble both seem to be more defined and overall better. I'm glad I've done this mod and I found it to be easy to do, in the end I used PCB mounted plugs then I didn't need to cut into the ribbon cable....

Here are some pics... Here is the stripboard which has the passive low pass filter in, and it also grounds the inputs to all the op-amps to avoid any noise issues:

Click the image to open in full size.
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I used 100ohm resistors and 0.033uF caps.

It's not exactly a work of art but it seems to work so I'm happy with it, I was worried the tracks may pick up noise but the outputs on my DCX seem very quiet and much better than before so it's all good.

My DC blocking caps are on the left, I used a mixture of values 6.8uF for Low channels, 4.7uF for mids, and 2.2uF caps for highs. I seemed to just about have enough room:
Click the image to open in full size.

Testing it out, all the channels work and I'm no longer getting the infamous egg frying sound on any of the channels.
Click the image to open in full size.

Last edited by MikeHunt79; 8th February 2011 at 03:37 PM.
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Old 8th February 2011, 06:06 PM   #2952
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hmm, NOW I'm hungry

btw, I love perf board work. bravo for that. so many people are of the mind 'etched board or nothing'.
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Old 8th February 2011, 06:56 PM   #2953
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OK I finally got to try out my new DCX. Trying it drive it with a new Gemini CDMP-1400 player. Using the XLR outs from the player I hardly get enough power to light the -30DB lights on the input to the DCX. Assuming I what to run the inputs to the DCX as high as posible what would be the best way to fix this, without adding a preamp? Andy
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Old 8th February 2011, 08:11 PM   #2954
sendler is offline sendler  United States
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Default 100R

Quote:
Originally Posted by MikeHunt79 View Post
Thanks for the advice sendler, sek and AR2, I'll be sure to keep it in mind once I get around to swapping DAC chips.

For now I've only done the direct out mod, as I wanted to see how much difference the direct out mod would make... I ended up making a passive low-pass filter on some stripboard, using 0.033uF caps and 100R resistors
100R is a lot of load on the dac. Did you compare the sound without the filter?
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Old 8th February 2011, 09:10 PM   #2955
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Very nice direct out mod, Mike.

I also modded my DCX yesterday. Direct balanced output with Audiophiler-polypropylene caps according to these instructions: Blanced passive output stage for the Behringer DCX2496 (also used unbalanced instructions where applicable)

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Old 9th February 2011, 07:19 PM   #2956
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I measured the balanced passive out modded DCX today. It looks quite good compared to stock-DCX, which distorts mainly thr 3rd and 5th harmonics. With passive mod the distortion spectrum is mainly 2nd harmonic, and the total THD level is lower than stock.

The DCX was measured with EMU 0404 USB (from which I have measured THD of approx. 0,0005%), with digital input and unbalanced output. I will measure the DCX with balanced interconnects next week. I have understood that balanced signal reduces even order harmonics, so one could hope that DCX distorts even less with balanced connection.

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Old 9th February 2011, 07:40 PM   #2957
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Legis,

Interesting measurements, thanks for that!
Can I ask you a favor? I am in a discussion in the yahoo dcx group and one member has good distortion results as you have, BUT he has quite low crosstalk between channels, about -60dB.
Could you measure the xtalk on your unit? You could for instance sweep a signal into one channel and measure in the other channel. If possible, both analog and digital input.

thanks in advance,

jan didden
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Old 9th February 2011, 09:08 PM   #2958
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Quote:
Originally Posted by janneman View Post
Legis,

Interesting measurements, thanks for that!
Can I ask you a favor? I am in a discussion in the yahoo dcx group and one member has good distortion results as you have, BUT he has quite low crosstalk between channels, about -60dB.
Could you measure the xtalk on your unit? You could for instance sweep a signal into one channel and measure in the other channel. If possible, both analog and digital input.

thanks in advance,

jan didden
Sure! I'm glad to be able to contribute.


All measurements are made with unbalanced connections.



Analog in



EMU 0404 USB's crosstalk for reference (RMAA):

Click the image to open in full size.



DCX couple of dB's behind (RMAA):

Click the image to open in full size.




Digital in



Normal measurement for level reference because of my uncalibrated measurements

Click the image to open in full size.



Second reference measurement with cables disconnected:

Click the image to open in full size.


And the real DCX crosstalk measutement with digital in. I did the measurement by muting opposite input and output and measured the leakage between unmuted channels. I don't know about this method's accuracy...

Click the image to open in full size.


The digital in's crosstalk seems to fade to noise floor mostly, if measured correctly. Low end's crosstalk is decreased over 20dB's to over -80dB compared to analog in -situation.


-Legis

Last edited by Legis; 9th February 2011 at 09:28 PM.
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Old 9th February 2011, 10:07 PM   #2959
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Forgot to measure xtalk from left input (input no. 1 in DCX) to right output (output no. 4 in DCX) with digital in.

Click the image to open in full size.



I noticed also that I should have done the first digital in graph with same vertical scale that I used in the other graphs, to ease the direct comparison.

Last edited by Legis; 9th February 2011 at 10:09 PM.
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Old 9th February 2011, 10:25 PM   #2960
Legis is offline Legis  Finland
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By the way, my measurements of EMU 0404 seems to be somewhat worse than it "should" be. Here's another measurement of 0404 with unbalanced connections and same sampling freq I used: RightMark Audio Analyzer test : E-MU 0404 USB loopback unbal 24b 48k

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