κDCX fully-differential analog board development

kappaDCX

Re: κDCX fully-differential analog board development

Postby BesPav » August 9th, 2016, 10:30 pm

amb wrote:what does this project have to do with the γ3 or Audio Widget?


Not in this project, Ti!

As i suppose, the main goal of using DCX - is to provide comprehensive digital filtering in bi/tri-amp systems.

When using DCX we put in system additional ADC-DAC transformation with digital processing in the digital domain.

But, as usual, now our music are in digital domain and we already can complete all digital processing without performing first DAC transformation.

So, most of multiband users just need 6+1 (tri-band+sub) syncronous DAC channels (based on one masterclock for excluding time shifting between channels) and good software for digital processing.
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Re: κDCX fully-differential analog board development

Postby amb » August 10th, 2016, 2:21 am

BesPav wrote:
amb wrote:what does this project have to do with the γ3 or Audio Widget?


Not in this project, Ti!

OK, so it probably belongs in a different thread. Anyway...

As i suppose, the main goal of using DCX - is to provide comprehensive digital filtering in bi/tri-amp systems.
When using DCX we put in system additional ADC-DAC transformation with digital processing in the digital domain.
But, as usual, now our music are in digital domain and we already can complete all digital processing without performing first DAC transformation.

Yes, this is all true and understood.

So, most of multiband users just need 6+1 (tri-band+sub) syncronous DAC channels (based on one masterclock for excluding time shifting between channels) and good software for digital processing.

Perhaps, but many of us already have one or more DCX2496 units, or plan to purchase one, and would like to improve upon its performance in an easy and unintrusive way. That's the target of this project. There are a lot of DCX2496s out there, and their owners have set their systems up specifically for them. As you can imagine, something like the DCX2496 would serve as an important centerpiece of an audio system. Maybe some of them would be willing to drastically rebuild their system just to remove the extra A/D and D/A conversion, but I suspect not many, especially if it requires a sychronized multi-channel DAC (does such a thing exist?), and a dedicated computer to do real-time processing.

In my case, I am using the DCX2496 for the Audio Artistry CBT36 line array speakers which needs specific crossover characteristics and equalization to perform properly, and Audio Artistry provides a preset file for the DCX2496. To duplicate that on a different system would be non-trivial. We had discussed this in the CBT36 thread.

My own testing has revealed that the A/D and D/A in the DCX2496 performs very well. Even though such a conversion might offend an audio purist, the reality is that the process is quite transparent. As I said in the opening post, it is not a weak point in the DCX2496.
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Re: κDCX fully-differential analog board development

Postby amb » August 10th, 2016, 2:46 am

The schematic diagram in the first post of this thread has been updated to v0.2. Please take a look.

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Re: κDCX fully-differential analog board development

Postby poseidonsvoice » August 11th, 2016, 2:52 pm

Ti,

Thanks for starting this, my interest is definitely peaked! Currently my DCX serves as the center and cornerstone for a multisub system and my mains which are a controlled directivity design uses passive crossovers (but done well ;) ).

I'll definitely do your proposed DCX upgrade and then consider a full active upgrade for my mains as well. Moreover there are a number of enthusiasts who use Tom Danley's Synergy Horn and an upgraded DCX would work for them as well like Red Spade Audio, etc...

Best,
Anand.
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Re: κDCX fully-differential analog board development

Postby amb » August 11th, 2016, 3:05 pm

Cool! Help me spread the word that this project is in the works. :)
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Re: κDCX fully-differential analog board development

Postby amb » August 11th, 2016, 10:48 pm

As I mentioned in the first post, the original Behringer analog board uses muting transistors to shunt the signal to ground when mute is needed, but such transistors are non-linear loads on the signal and is best avoided. Also, the muting transistors are before the final output opamps, so any turn on/off transients emitted by those opamps will not be muted.

I built a test circuit on a α24 PCB to check the behavior of the OPA1632's "Enable" pin, in order to find out whether this could be used as a "mute" control.

I am happy to say that the OPA1632 did not disappoint! I soldered the OPA1632 to the α24 PCB with the enable pin lifted, and then a switched is wired between the lifted pin and V- to allow me to manually toggle the enable function. I connected the α24 to a Mini³ headphone amp (with gain of 5) to listen for any noise when toggling the enable state. With the Mini³ volume pot turned to maximum, I heard only the faintest of a click on my HD580 headphones.

On the κDCX, there will be control logic to mute the outputs during power up, and also when the DCX2496 firmware sends the "mute" signal. Since the OPA1632 is the final output device on all six of the κDCX's output channels, this scheme will provide a true mute, avoiding the shortcoming of the original Behringer design mentioned above.

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Re: κDCX fully-differential analog board development

Postby amb » August 12th, 2016, 12:31 am

Well, I posted the above "success" story too quickly. While there is negligible amount of noise when the OPA1632 Enable pin is toggled, it doesn't work well as a mute feature. When disabled, some audio still comes through the output. So it doesn't "mute" very effectively. I'm back to the drawing board on this issue.
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Re: κDCX fully-differential analog board development

Postby Theolean » August 12th, 2016, 4:39 am

Yes, this is exactly the behaviour I would expect. Because you only disable OPA1632 and not short any signal. So there is a way over R10 / R11 / C4 / C5
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κDCX fully-differential analog board development

Postby amb » August 12th, 2016, 7:06 am

You're right. The Enable pin, when connected to V-, disables the output stage within the OPA1632. It doesn't shut off the power completely, but a disabled output stage is enough to cause the negative feedback paths to act as leak-through resistors from input to output.

I have a few possible solutions to test. I will talk more about them in another update.

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Re: κDCX fully-differential analog board development

Postby whomper » August 14th, 2016, 11:56 pm

Speaking of DCX upgrade, is there a need to review the PSU? Although the analog board will have a dedicated set of regulators, will the digital section benefit from a more stable and clean PSU?
κDC2496, γ1.5, γ2, γ3, α10 (α24 based), balanced β22, 2 x β24, M³, Grado ES2E, GS2000E, AGK K700, CBT36 Speakers
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