DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496

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Linear supply will be nice.

The subject of this thread is "objectively tackling the improvement of a stock dcx2496". To me that implies first mapping out what needs to be improved, then making a change, and then verifying if it actually improved the situation.

Things that need to be improved could include things like SNR, THD and gain structure / output voltage. I don't really see "lack of linear supply" as an issue. There has to be some problem that the linear supply supposedly solves, and it has to be verified that the supply actually solves that problem. Otherwise it belongs in the "has to be coloured pink" category.

So, what specific issue do you want to solve with a linear supply?
 
I was going off this ...
The DAC itself also has a rather low differential output of 0.85VRMS at 0dBFS, so I think I would opt for a single OPA1632 per channel with a gain of 2.4 to get the differential output up to 2VRMS and the SE output up to 1VRMS.

Are you using balanced or SE outputs? If you're using SE then you're giving up the CMRR needed to maximize the DAC performance and you're only getting 0.425VRMS at 0dBFS :eek:
 
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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.

Putting aside that, unless you're an inveterate equipment swapper, the values can just be set for your preamp or passive control, how much difference is there between loading your circuit with (say) 10k and 100k using "normal" cables (say, 75pF)?

Output voltage from the DAC is +/-2.4V.
 
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Transformer outputs work best for me in blind listening tests. There is enough output level for almost any amp.

On one unit I modified the Spdif was routed to an RCA input over by the multipin connector in the back. In fact it was 2 RCA with a switch. Worked very well.
Cleaning up the PSU helps, so does replacing the electrolytic caps that decouple the DAC chips. A larger value and lower ESR cap seems to work well.

Looking forward to seeing what you do with the jitter.
 
there are those that take the opportunity provided by DSP to do away with the preamp altogether, which can only be a source of noise, as the interconnecting line level cables are. makes for pretty large output impedance too, not so hot for some amps.

@ Pano, large electros of any type for decoupling a DAC? not much chop for any HF or RFI. SMD X7R FTW
 
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Nothing specific - DCX is nice hardware but PS is unreliable joke.

I guess the right description would be "cost-optimized" :)

Reliability is definitely a valid concern.

I dont have false hope in PS. :)

In my experience the PS is something to think about when everything else has been taken care of - and even then a linear supply is not necessarily better than a switcher from a sound quality point of view.
 
I reckon a cheapo linear will win out against a cheapo switcher though. switchers can indeed be excellent if done well, with keeping residue/sidebands out of the audio band part of the design spec, but if its careless application, or simply designed without any of that in mind, it can be horrid and completely unsuitable for audio.
 

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Hi Julf,

I agree with you entirely that a well designed and implemented SMPS can provide excellent performance, and that not all linear supplies are good just by virtue of the fact that they're linear.

For a medium to high power amplifier, I would say that hands down, a good regulated SMPS is the best solution. For a low power amplifier, or for line level applications, the arguments for switching supplies start to fall apart a bit since it's significantly harder to design and implement a switching supply that has lower output noise than a well implemented linear supply. There are also no real advantages at lower power levels, as efficiency no longer plays a significant role.

All the above is besides the point, however, since the SMPS in the DCX is neither well designed, nor low noise. They also use a plethora of cheap fixed voltage regulators that have abysmal performance when it comes to rejecting the noise from the SMPS itself.

The evidence of this is in the FFT plots I showed earlier. The harmonics caused but the PSU are only 10dB lower than the second harmonic, and unlike the second harmonic, they're entirely unrelated to the fundamental which makes them far more audible. I don't think anyone here would disagree that the outputs on the DCX are noisy, and the best way to address that noise is by eliminating it at the source, and the source is almost certainly the PSU.

I'll try and get a few measurements of the just the PSU rails to make sure, but I can't think of anything else that would be causing those spikes starting at ~88Hz.

I would also strongly disagree about the PSU being the last thing to address. I would argue that it's one of the first as its impacts every other functional aspect in the DCX.

Cheers,
Owen
 
I am afraid that can apply to a cheap linear supply too.

it can yes of course, but I think its not quite as abhorrent, like switching noise right in the middle of the mid-highs (and here apparently ravishing the IC regs PSRR). to be clear i'm in favor of good switchers, I think they are in many aspects better for poweramps, you can have fully regulated high current much more easily and i'm using them as you know.

but as Owen says above, it starts to become not such a worthwhile proposition for line level applications. its still doable of course, but the question of 'why?' becomes much more valid.
 
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Owen,

All the above is besides the point, however, since the SMPS in the DCX is neither well designed, nor low noise. They also use a plethora of cheap fixed voltage regulators that have abysmal performance when it comes to rejecting the noise from the SMPS itself.

A very valid point. And looking at your plots I agree a better PS is a good idea - my point is simply that a *better* PS is needed - linear or switching.

I would also strongly disagree about the PSU being the last thing to address. I would argue that it's one of the first as its impacts every other functional aspect in the DCX.

Fair enough - my comment was a more general one - in the case of the DCX2496 a PS upgrade definitely looks justified.
 
it can yes of course, but I think its not quite as abhorrent, like switching noise right in the middle of the mid-highs.

I have seen some rather horrible linear regulators too...

but as Owen says above, it starts to become not such a worthwhile proposition for line level applications. its still doable of course, but the question of 'why?' becomes much more valid.

I agree in the case of line-level linear stuff, but if there are digital circuits and processing involved, linear supplies often don't have the stable regulation needed.
 
I have seen some rather horrible linear regulators too...

sure, but can we stay on topic please? :D I think its probably easier to do linear 'blind' ie. without properly measuring the output and just following a datasheet most of the way, to an average level, not even necessarily a high level. with switchers as pre-regs like here, its going to be very unpredictable exactly how and where they might impact the following stages. but the main point here is, this switcher in particular seems a bit evil.


I agree in the case of line-level linear stuff, but if there are digital circuits and processing involved, linear supplies often don't have the stable regulation needed.

thats what good local decoupling caps and techniques are for.
 
sure, but can we stay on topic please? :D

Trying to... :)

I think its probably easier to do linear 'blind' ie. without properly measuring the output and just following a datasheet most of the way, to an average level, not even necessarily a high level.

I don't disagree.

the main point here is, this switcher in particular seems a bit evil.

As you might have seen from my earlier message, I concur...
 
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@ Pano, large electros of any type for decoupling a DAC? not much chop for any HF or RFI. SMD X7R FTW
There are a whole bunch of cheap and cheerful 10uF caps at the DAC chips and near the 3 leg regulators. I just swap them out for 40uF Panasonic FM series caps of the same size. The Panny FMs are low ESR across a wide range. Works great. Not all need to be changed, the caps on the analog rails seem to yield the best results. Subjective enhancement = more dynamics, more grunt.

The chips are also decoupled with SMD caps nearby.
 
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