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DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496
DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496
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Old 13th June 2013, 10:46 PM   #11
tomtom is offline tomtom  Slovakia
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Quote:
Originally Posted by kuribo View Post
Why not replace the dac chips with the es9018 and add usb input???

I can dream, can't he?
Impossible.

No way that behringer PCB survive. And besides AKM 4393 are quite good.

some links:

Ergo Audio :: Oettle digi-in mod
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Old 14th June 2013, 11:50 AM   #12
BuildMeSomething is offline BuildMeSomething  United Kingdom
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I'm liking where this is all leading... The two down sides for me regarding this unit are the limited digital inputs and the 10VRMS output... Having a choice of USB / Optical input would be nice options and the proposed 2VRMS output level very nice...

Paul
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Old 15th June 2013, 09:42 AM   #13
rikkitikkitavi is offline rikkitikkitavi
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I did modify my DX2496 by gutting the analouge parts.
Replaced them with a simple single ended - differential buffer with Vcom offset for the ADC input, and a simple 2nd order differential filter on each DAC out, cutting at 70 kHz aboutish. In total 5 OPA2134. All built on Veroboard (but with some consideration to layout)

Gain structure is designed that 2 Volt peaks ADC input.

Sonically it was a great improvement of course with far less noise. I really encourage you to proceed in this.

In progress is a linear powersupply with external transformer box , both for noise reasons but also because I want to remove the chassis from protective earth due to ground loops, and in that I will construct a new chassis and run the DSP main board bare (no display front panel card) Second to that is a 6 channel volume control and I am home free.
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Old 17th June 2013, 08:47 PM   #14
opc is offline opc  Canada
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DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496
Alright... this is the last of the measurements for the stock unit that I wanted to get posted. These are mostly a mishmash of things I measured while I had everything set up.

1. SNR @ 2VRMS output, 20kHz BW

- I wanted to measure this to get an idea of what the baseline SNR is with a 2VRMS output. This is more in-line with what most hifi users will actually be getting from the unit since most people can't make use of the 10VRMS output. As expected, it's not great and ranges anywhere from 91dB to 98dB.

2. Distortion Product Ratio - 1kHz 2VRMS

- This just makes it easy to see the distortion signature of the stock output stage. It has a dominant 3rd harmonic that is about 88dB down, and a second harmonic that is about 97dB down.

3. FFT of Noise Floor - Full Band 20Hz to 80kHz

- I wanted to see if there was anything particularly nasty a little farther out, but everything looks alright here. The noise floor starts to climb after 40kHz, but nothing too scary.

4. Output noise level - 20kHz BW

- Self explanatory

5. Output noise level - 90kHz BW

- Self explanatory

6. THD VS FR with XO on and off.

- I wanted to make sure that enabling the crossovers didn't impact distortion. Thankfully, there is no increase in distortion levels with the filters on or off. This is very good news, as it indicates that the DSP section itself is not contributing to distortion.

7. THD VS Level - Analog Input

- Self explanatory

8. THD VS Level - Digital Input

- Self explanatory

Next up - a plan on how I'd like to address some of the issues found in the measurements, and hopefully a discussion of what people would like to keep in the DCX, and what they can live without.

Cheers,
Owen
Attached Images
File Type: png Signal to Noise Ratio 2VRMS.png (60.2 KB, 773 views)
File Type: png Distortion Product Ratio 2VRMS.png (49.5 KB, 760 views)
File Type: png FFT Spectrum Monitor - noise floor full band.png (204.3 KB, 761 views)
File Type: png Output Noise Floor - 22k.png (77.6 KB, 756 views)
File Type: png Output Noise Floor - full band.png (76.3 KB, 746 views)
File Type: png THD+N Level - Crossover vs no xo.png (80.2 KB, 86 views)
File Type: png THD Ratio VS Level.png (105.4 KB, 100 views)
File Type: png THD Ratio VS Level DIGI.png (66.4 KB, 95 views)
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Old 17th June 2013, 10:09 PM   #15
opc is offline opc  Canada
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DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496
So here's the rough initial plan, broken down into things that will definitely be done, and things I would like to do, but that may require a little more effort to implement.

Everything here is open to debate and suggestions, so don't be shy.

Replace the entire analog output and input section

I would propose the use of a single OPA1632 per channel for all 6 outputs and the two inputs. The balanced outputs from the DAC would run directly into the OPA1632 and then directly to the output connectors. The gain would be set to give 2VRMS at 0dBFS for the output and 0dBFS @2VRMS for the input.

The above has the following advantages:
1. It maintains a fully differential signal path, which is what the DAC wants.
2. It allows the use of a single phase of the output for SE, while still maintaining the CMRR the DAC wants to see.
3. It brings the gain in-line with what we need for hifi use
4. The MUTE functionality can be maintained by connecting to the enable pin on the OPA.
5. Performance of the OPA is worlds better than the stock op-amps.
6. Can be DC coupled, and allows for an optional LPF to be populated.

Replace the entire SMPS with a linear supply

I would like to strip out the existing SMPS entirely and replace it with a fully linear supply. A trio of small 10VA toroids (one for digital and two for analog) could be used to feed a dedicated PSU section with a good rectifier section and full suite of low noise regulators. The TPS7A3301 in a TO-220 pack could be used for all first stage supplies (+/-15VA, +5VA +3.3VA +5VD +3.3VD) and then smaller positive voltage regs could be used for any subsequent supplies.

All of the above would have to fit inside the chassis as I'm not a big fan of making anything external. The toroids themselves are small enough that they can me made to fit and be kept a good distance from the sensitive output circuitry.

Overall, the above PSU changes would dramatically improve the noise floor of the unit, and reduce EMI/EMC concerns.

Remove all input circuitry from input C and make it a dedicated AES/EBU input

Since I have never in my life connected a MIC to the DCX, I would propose the removal of the input circuitry for input C, and instead dedicate it to be the AES/EBU input which would run directly to the digital board. The UI would remain exactly as is (switching between analog and digital input would still work as it did before) but the option to plug in a MIC would no longer be there. This should reduce some of the noise being coupled in from digital input to the analog outputs.

More to come...

Cheers,
Owen
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Old 17th June 2013, 10:15 PM   #16
kuribo is offline kuribo
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usb input, balanced outputs would be high on my list.
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Old 17th June 2013, 11:07 PM   #17
tomtom is offline tomtom  Slovakia
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Linear supply will be nice.

I dont care about mic. But i think most people want SPDIF /or better USB/ instead Aes/Ebu. Also without changing ASRC and clock almost not worth to do.
Please if you can measure also balanced direct voltage out - with simple passive low pass and DC cap. Load ca. 1k - very easy mod and maybe good enough /for out section only/.

thank you very much
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Old 17th June 2013, 11:39 PM   #18
SY is offline SY  United States
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DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496
What's the advantage of opamps in the output vs plain jane passive RC?
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Old 18th June 2013, 09:14 AM   #19
jan.didden is offline jan.didden  Europe
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Stuart,

Assuming that the output R-C is part of the reconstruction filter, the load influences the filter characteristics. You's only use the opamp as buffer.

jan
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Old 18th June 2013, 09:17 AM   #20
hochopeper is offline hochopeper  Australia
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Isn't the DAC output quite low also? So needs some gain to get to 2V RMS.
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