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κDCX fully-differential analog board development

kappaDCX

κDCX fully-differential analog board development

Postby amb » August 6th, 2016, 7:36 am

Last update: October 13, 2016
Current status: κDCX now released, official website is online.

Introduction

κDCX ("kappa DCX") is an upgrade for the Behringer DCX2496 "Ultradrive Pro" and DCX2496LE "Ultradrive" loudspeaker management systems. It is designed to be a drop-in replacement for the original analog board, with full compatibility with the unit's rear panel cutouts and internal connections. It requires no other modification or hacks to the DCX2496/DCX2496LE, and an upgraded unit can be easily reverted to its stock form. Unlike other similar products, installing the κDCX does not cause any loss of features or functionality. Everything on the original analog board is implemented on the κDCX, including the RS232 port and the two RS485 (RJ45) "Link" ports for daisy-chaining multiple DCX2496s.

κDCX improves upon the original analog board in major key areas, and elevate the DCX2496/DCX2496LE to a true high-end unit with superior measured performance and sound quality.

Behringer DCX2496LE is a cost-reduced version of DCX2496, it does not have input C, the RS232 port and the two RS485 "Link" ports. Its input A does not support the AES/EBU digital input option. Otherwise it is the same as DCX2496.

Background & overview

DCX2496
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DCX2496LE
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The Behringer DCX2496 is a Sharc DSP-based electronic crossover unit with advanced equalization/filtering capabilities. It is a popular solution for bi-amp or tri-amp systems where the woofer(s), midrange(s) and tweeters(s) are each driven by separate power amplifiers. Despite the low price, it has impressive capabilities. As a Behringer, which is a pro-audio company, the unit is designed primarily for pro-sound use rather than in-home audio. Nevertheless many people use DCX2496 in their home hi-fi systems, including myself. See my Audio Artistry CBT36 line array thread to see how the DCX2496 serves an all-important role in my speaker rig.

DCX2496
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DCX2496LE
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DCX2496 has 3 inputs and 6 outputs, DCX2496LE has 2 inputs and 6 outputs, all with XLR balanced connectors. A common configuration is to use input A and B as the stereo left and right inputs, outputs 1, 2 and 3 for the left channel low, mid and high outputs, and outputs 4, 5 and 6 for the right channel low, mid and high outputs. On DCX2496, Input A can alternatively be set up as a digital input (AES/EBU), and input C can be used as an extra input channel, or as a microphone input for auto-delay correction measurements. The input to output mapping/routing/linking is all user-configurable, not to mention the crossover filter type/slope, level, static and dynamic equalization, delay, phase and other parameters (any of these on a per-port basis). There are a number of presets to choose from, and parameters can be altered via pushbuttons and the knob on the front panel. On DCX2496, additional presets can be loaded via the RS232 serial port connected to a Windows PC running the Behringer DCX2496 software. Older DCX2496 units with the PCMCIA slot (deleted in the newer version) can also load presets stored on PCMCIA flash cards.

Analog input signals are converted to 24 bit, 96KHz digital PCM stream with two Cirrus Logic CS5381 ADCs (older DCX2496s used Asahi Kasei AK5393s) for processing in the DSP, and the post-processed signals are converted to analog with three Asahi Kasei AK4393 DACs.

The ADC, DSP and DAC sections of the DCX2496 are quite good and (in my opinion) need no modification. There are mods out there for the digital sections but it is not clear whether meaningful improvements can be achieved in those areas. At any rate they are beyond the scope of this project.

The main shortcomings in the DCX2496/DCX2496LE is in the analog input and output sections. I wish I could post the DCXC2496 schematic diagram here to illustrate them, but I am not doing that for obvious reasons. But the schematic can be found on the 'net if you look. Anyway, here are the points of interest:

  • Uses cheap 4580 opamps everywhere.
  • Several non-audio grade electrolytic coupling capacitors in the signal path, in the input as well as the output sections.
  • Turn-on muting bipolar transistors that could introduce non-linearities/add distortion to the signal. Also, the muting transistors are before the final output opamps, so any turn-on/off noise from them will not be suppressed.
  • Too many balanced-to-unbalanced conversions and vice versa: Each input channel converts the balanced signal from the XLR input jack to unbalanced, then converts back to balanced before sending the signal to the ADC. Each output channel converts the balanced signal from the DAC to unbalanced, then converts back to balanced for the XLR output jack.
  • The DCX2496 specifications state that the maximum analog input and output levels are +22dBu (about 9.76V RMS, and I assume it's referring to 0dBFS), which may be appropriate for pro-audio equipment, but for consumer gear it is way too high. Most home audio power amps require no more than 1V-2V to reach maximum output power. The result of this is that at reasonable listening levels, the volume will have to be set so low, that the DCX2496's onboard ADCs and DACs are operating nowhere near full scale. The resolution, linearity and dynamic range are all adversely affected. Indeed, even when playing at moderately loud levels, I see only one or two LED segments of my stock DCX2496 front panel level meters flash on and off.
I had previously made some simple modifications to my DCX2496's analog board to address the first three issues. I replaced all the opamps with OPA1612, jumpered across some unnecessary coupling capacitors and replaced the rest with audio grade caps. I also removed all the turn-on muting transistors. While those mods did make a tangible sound quality improvement, it did not address the two remaining issues, which are important. That mod also made it necessary to turn on the DCX2496 before powering up the power amps, and to turn off the DCX2496 after the power amps, in order to avoid a thump. Or leave the DCX2496 powered up all the time. Clearly, there should be a better, more elegant solution.

Fortunately, Behringer designed the unit such that the entire analog section is on its own PCB. It is secured to the chassis by the rear panel connectors' mounting screws and connected to the rest of the circuitry via two ribbon cables with dual row IDC (insulation displacement connector) receptacles. This makes it easy to remove and replace the analog board with an upgraded one, without affecting the other boards.

DCX2496 internal view (the analog board is the long board along the rear panel):
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DCX2496 internal view (old version with PCMCIA slot)
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DCX2496LE internal view
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κDCX will fit both the old and new DCX2496 versions, as well as the DCX2496LE.

I had looked at other DCX2496 upgrade options out there for some time, but I am not happy with any of them for various reasons. Thus I set out to design my own. What you see here is the result, and any refinements as we journey to turn a concept into reality.

DCX2496 upgrade - the AMB way

0. Full compatibility with the DCX2496 and DCX2496LE

No hacking, no lost features or functionality, perfect fit, reversible. Same functionality as the original Behringer analog board but with high-end performance.

1. Fully-differential topology, zero coupling capacitors

Since the DCX2496 has balanced inputs and outputs, and the analog side of the internal ADCs and DACs are also balanced, there is no reason not to keep the entire internal signal path balanced. In fact, not just balanced, but fully-differential. I explained the difference (ha!) between balanced and fully-differential in this post of the α24 development thread, and the beauty of the fully-differential concept. The α24's modified instrumentation amplifier topology fits here perfectly, for both the input and output sections. The α24's 3rd stage (balanced-to-unbalanced opamp) is not needed here, so it is omitted. What's taken almost verbatim from the α24 is the OPA1612 input stage and the OPA1632 fully-differential 2nd stage. For the DCX2496 input section, setting the OPA1632's Vocm pin to 1/2 AVDD (2.5V) establishes the common mode voltage to the ADC. For the output section, connecting the Vocm pin to signal ground assures a common mode output potential at 0. Despite the 1/2 AVDD common mode DC offset from the DAC, it will be canceled due to differential action as well as a 0V Vocm, so there should be negligible output DC offset. This is demonstrated with the α24 operating in the γ3 DAC. Hence, there will be no coupling capacitors in the signal path at all. The only exception is on input C when it is used as a microphone input, because such caps are needed to block the microphone phantom power from entering the opamp.

2. Opamps specifically designed for audio

As a consequence of the above, we're using great-sounding OPA1612 and OPA1632 opamps instead of the dreadful 4580. The OPA1612 and OPA1632 offer much lower noise and distortion, higher bandwidth and faster slew rate than the 4580. I did not consider a discrete solution because of the limited PCB real estate available, and also due to the fantastic performance achieved in the α24. On each input and output channel, the κDCX has two opamp stages (the OPA1612 and OPA1632), compared to three 4580 opamp stages on the original Behringer analog board.

3. Enhanced turn-on/off muting

κDCX will use two reverse-parallel connected JFETs across the hot and cold signals for output muting. These JFETs are chosen for their low Rds(on) (on resistance), and are located at the output jacks. This arrangement will provide -40dB of muting (1/100 attenuation) which is adequate for the purpose, and does not add any distortion within the intended output voltage swing range. This solution has the advantage that the mode is "muted" when no power is applied, and will not un-mute until the muting control circuit pulls their gate voltage to the negative rail, turning off the JFETs. It is also simple and inexpensive to implement, draws virtually no current, and has a small PCB footprint.

4. Configurable input and output gain

I will provide resistor values for both the pro-audio and the consumer (home) levels of operation. Thus, one could upgrade a DCX2496 with a κDCX built for either environment (or a mixed environment). This also allows optimization of the DCX2496 to work best with your source/preamp and your power amps.

The maximum input and output voltages are set at build time. Recommended resistor values are provided for the following (balanced): 1Vrms (+2.2dBu), 2Vrms (+8.2dBu), 4Vrms (+14.3dBu), 8Vrms (+20.3dBu)

Each of the 3 input and 6 output channels can be separately configured. I will discuss about this in more detail in a future post.

5. Ultra low noise, high PSRR local power supply regulation

In the DCX2496, the power supply unit (PSU) provides +15V and -15V (regulated with 7815 and 7915 regulators) to the analog board via the ribbon cable. There is no local voltage regulation. On the κDCX, I will add the σ78 and σ79 regulators to provide additional regulation in order to provide the cleanest DC power possible to the opamps. The rails will be regulated down to +14V and -14V, to keep power dissipation low and to minimize loss of voltage swing.

6. Ease of build

All passive parts (resistors, capacitors, relays connectors, switches) are through-hole. The only surface-mount devices are the opamps and transistors which are in easy-to-solder SOIC-8 and SOT23-3 packages, respectively. All parts are readily available at the usual major distributors.

κDCX schematic diagram, PCB layout, 3D rendering, parts list

Please see the official κDCX website for the most up-to-date version of these.

Board photos

Click the photo for a larger version.

A blank κDCX PCB in the foreground, and the stock DCX2496 analog board in the background.
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κDCX populated with parts and mounted in the DCX2496 chassis.
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Ribbon cables connected.
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3 people like this post.
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amb
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Re: κDCX fully-differential analog board development

Postby whomper » August 6th, 2016, 1:09 pm

Looking forward for this project to upgrade my unit. Meanwhile I'm building my second Beta24 to create a bi-amped config.


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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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Re: κDCX fully-differential analog board development

Postby Nisbeth » August 7th, 2016, 9:01 am

Interesting. I owned a Pilgham-upgraded DCX some years ago but sold it because I ditched my DIY-speakers and bought a pair of single-wired songs fabers instead. The improvement over stock was quite significant with the upgrade.

Note that (as far as I can see) the LE-version doesn't have the AES/EBU option either. That's a major feature loss IMHO, but not everyone will agree I guess.

/U.
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Re: κDCX fully-differential analog board development

Postby amb » August 7th, 2016, 2:21 pm

A little off-topic, but there is a known bug in the CS8420 DIR/ASRC chip used in the DCX2496 that sometimes goes into a mode where the treble becomes rolled off. The only solution is to power down and up again. I don't know if this bug has ever been fixed, but it would only occur when using input A as a digital (AES/EBU) input.

Linux-works' digital-output hack would require you to use multiple DACs at the outputs and have a way to synchronize their volume settings. Not something that's easily done. If you have multiple S/PDIF or AES/EBU sources, then you'll have to build an additional switcher, because the DCX2496 has only one digital input. It also does not give you USB audio capability.
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Re: κDCX fully-differential analog board development

Postby amb » August 7th, 2016, 5:08 pm

Whomper, if you're still considering the CBT36 line arrays, don't forget that subwoofer(s) are highly recommended. That makes it a 3-way system and would require 3 stereo DACs (and volume synchronization for all three) if you go with linux-works' digital out mod. Not only does it get expensive, but it violates the first goal outlined in the first post of this thread. It fundamentally changes the way DCX2496 was intended to work. Some people have no problem with this, but that's not what the κDCX project is about.
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Re: κDCX fully-differential analog board development

Postby amb » August 7th, 2016, 11:52 pm

I edited the first post of this thread to fill in more information, including the schematic diagram. Please take a look and comment.
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Re: κDCX fully-differential analog board development

Postby whomper » August 9th, 2016, 2:55 am

amb wrote:Whomper, if you're still considering the CBT36 line arrays, don't forget that subwoofer(s) are highly recommended. That makes it a 3-way system and would require 3 stereo DACs (and volume synchronization for all three) if you go with linux-works' digital out mod. Not only does it get expensive, but it violates the first goal outlined in the first post of this thread. It fundamentally changes the way DCX2496 was intended to work. Some people have no problem with this, but that's not what the κDCX project is about.


Yes, I am considering a pair of CBT36, so will probably want to leave a 3 way option relevant. Need to figure out how I'm going to travel back home with that kit with me...
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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Re: κDCX fully-differential analog board development

Postby BesPav » August 9th, 2016, 10:50 am

Hi, Ti!
Useful project!

Why you did not go to 6-channel Gamma3 (or8-channel)? You only need to provide common masterclock between boards and upgrade your "Audio widget" to 6-channel (or use 3 of them in slave mode).

Then each can go to fabfilter.com and take "Pro Q-2".
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Re: κDCX fully-differential analog board development

Postby amb » August 9th, 2016, 11:26 am

Linux-works, please re-read the goal of the κDCX project. This is not about making a DCX2496 do something that it wasn't designed to do, nor is it about perfboards or air-wiring to IC pins or PCB traces.

Please stop the thread crapping and contribute meaningful discussions about the κDCX instead.

BestPav. what does this project have to do with the γ3 or Audio Widget?
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Re: κDCX fully-differential analog board development

Postby linux-works » August 9th, 2016, 1:07 pm

sorry to intrude.

all my comments have been removed from this thread.
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