Improved digital input stage for the Behringer DCX2496

The digital input stage of the Behringer DCX2496 has three disadvantages:

  • The maximum sample rate of the input signal is 96kHz.

  • With the CS8420 a first generation SRC was used. It has a minor jitter rejection leading to jitter components in its digital output.

  • The PCB-layout around the CS8420 is not compatible with the recommendations of Cirrus Logic leading to a high jitter at sample rates of 96kHz.

  • There is no correction of emphasis leading to playback errors with emphassis CDs.

This inspired me to design a new digital input stage (incl. PCB) with the following features:

  • Low noise power supply with LM2936 and Black-Gate capacitors.

  • Usage of the CS8416 AES-S/PDIF receiver to allow input sample rates up to 192kHz.

  • Optimized PCB layout for minimal jitter during clock regeneration for the CS8416.

  • Usage of a second generation SRC (AD1896) with delay line and digital PLL. This leads to a jitter rejection down to less than 5pS!

  • High SNR (AD18961- 142db).

  • Switchable deemphasis correction (Auto/Off).

  • Simple installation in the DCX2496.

Schaltplan Ansicht der eingebauten Platine

Description:
The PCB was designed on a strict separation of digital and analog power supplies. Both the analog und digital supply voltages (5V each) were taken from the CS8420 via ribbon cable and are reduced to  3.3V with low noise power regulators (LM2936) in combination with Black-Gate capacitors. The AD1896 is configured the way that its output signals reaching the DCX2496, deliver standard TTL-level (5V). This is necessary to be compatible with the signals of the CS8420.

The input to the CS8416 is fed via two 100nF capacitors. Leaving the original CS8420 in its place both units operate in sync and stay both full functional. Therefore a simple A/B-comparison is possible. The layout of the PCB around the CS8416 exactly suits the specification in the datasheet.

The outputs of the CS8416 (26,27,28) are connected with the input of the SRC AD1896. The SRC operates completely asynchronous. A XTAL-oscillator (>= 28MHz) supplies the clock for the DSP. The frequency must not be below 28MHz. A lower frequency causes output errors at an input sample rate of 192kHz. The output of the AD1896 is synchronized via pin 24 and 25 by the DCX2496.

Assembly:
Die circuit is built-on a double layer PCB with large ground planes. All signals were supplied via ribbon cable coming from the CS8420 (see photos). The CS8420 could stay in its place. Only pin 18 must be disconnected from the PCB of the DCX2496. With a spiky soldering iron and a needle this is quite easy. Attaching the needle below the pin just heat the pin and bend the pin up. The now free pad below pin 18 has to be connected to the SDOUT signal delivered by the ribbon cable. This disables the CS8420 and delivers the signals from the AD1896 to the DCX2496.

Listening:
Caused by the better FIR-Filter of the AD1896 the DCX2496 sounds more fresh and open after the modification. Best you can verify this with an A/B-test. Therefore insert a switch between the output of the CS8420 pin 18 and the pad below it. The other input of the switch should be connected to SDOUT delivered by the ribbon cable. Because both SRCs are running in sync the switch can be operated at any time.
Really extreme was the difference between both units feeding the DCX2496 with a signal delivered via a telephone line from my working room (see "Different ways to transport SPDIF Audio signals"). Caused by the sub optimal wire the jitter was clearly visible in the eye-diagram. In case of the original CS8420 this minor quality signal was clearly audible. In case of the new input stage there was no audible difference between the telephone line and a short 75Ohm line.
With the new input stage also differences between different wires (low cost - high end) were no more audible. In my opinion the AD1896 with its delay line and digital PLL delivers the best jitter rejection (<5pS) I have ever heard. 

Summary:
This mod I can really recommend for those who like to get most out of their DCX2496.

Links
   
Photos Additional informations Manufacturer aprx. price
Images of the digital input stage      
PCB data with viewer. The viewer allows the creation of GERBER-files. Download GS-Elektronik produces perfect PCBs directly out of the .lay file.

Price for 2PCBs aprx. 80Euro.

SRC AD1896 Datasheet    
AES - S/PDIF receiver CS8416 up to 192kHz Datasheet

 

Manufacturer: Cirrus Logic

 

 
Original ASRC of the DCX2496 (CS8420) Datasheet Manufacturer: Cirrus Logic  

All prices april 2005