NOS PCM1794

[Redshift187]

I present my DAC design, The Shanelle V1.0. It has taken several months to research, design and build this first version. I would like to thank Doede for the inspiration from his DDDAC 1794, Ross for his modifications to the DDDAC 1794 (especially the "pin 20 mod" JFET CCS), Sérgio for his analysis of the PCM1794 distortion in various output configurations and Owen for introducing me to the OPA1632, differential circuits and just generally inspiring me. Please excuse the lackluster SMD soldering, there were a lot of parts to solder and I really wanted to get through it so I could listen 🙂

• Dual mono 24/192 PCM1794a DAC
• Differential buffered and single ended outputs
• DC coupled throughout
• Header for I2S input matches DIYINHK USB to I2S interfaces
• Integrated low noise LDO power supplies
  • +13.5V & -13.5V for analogue filters and buffers
  • 7V for DAC ICs analogue
  • 3.3V for DAC ICs digital and off-board USB isolator
  • 5V for logic circuitry (the logic gates switch faster at 5V than 3.3V and the PCM1794 accepts 5V digital inputs)
• Integrated buffers to drive differential headphones
• No digital filter or upsampling/oversampling (NOS or non-oversampling)
• 35kHz 2nd order multi-feedback analogue low-pass filter (-40dB @ 362kHz)
• Synchronous data reclock after conversion to Right Justified for dual mono
• JFET constant current supply for DAC ICs current reference (datasheet uses a resistor)
  • Through holes for off-board potentiometers for determining correct value for the desired current
  • Surface mount pads for fixed resistor after value is determined
• Solid ground plane under sensitive circuits
• Power plane for analogue voltage to the DAC ICs
• Recommended parts are non-boutique, mostly surface mount for improved noise rejection (no smaller than 0805 for ease of soldering except I/V resistors)
  • Susumu RG resistors for analogue circuits
  • PPS and acrylic capacitors for analogue circuits
  • polymer capacitors for local bulk decoupling
• Additional outputs for use with external I/V or analogue output section
• Passive I/V with the choice of through hole or 0603 for I/V resistors

There are a few mistakes, but nothing that requires cutting traces:

• The differential to single ended converters were originally intended to be standard op-amps, but that imbalances the I/V resistance, so I switched to instrumentation op-amps
• The 7DIG LED is backwards
• The dual dip switches were intended to enable/disable the digital filtering/oversampling, but won't work because SCK is tied to BCK

 

[Bas Horneman]

d I really wanted to get through it so I could listen

And what was listening like?

 

[Redshift187]

First, a little background on how I use it:

Foobar2000 with SOX DSP to upsample 44.1k -> 176.4k and 48k -> 192k, Passband = 90%, Allow aliasing/imaging enabled, Phase response set to 30% (gave the least amount of post ringing with no pre ringing when square wave tested).

With headphones, I listen with a pair of Sennheiser HD600's with a balanced, shielded cable connected directly to the balanced outputs. For non headphones, it's connected to a single ended DC coupled preamp followed by a DC coupled power amp via the single ended outputs.

I'm not great with describing the sound of equipment, so bear with me. The background is incredibly black. The sound is very full and revealing with excellent tonal balance, but not fatiguing. The stereo imaging is excellent; with great recordings, it sounds like the musicians are right there in my living room.

Unfortunately, I don't have any equipment I can test it with that's better than 100dB dynamic range.

 

[Bas Horneman]

Sounds like it was worth the work and expense you've put into it. 🙂

 

[Redshift187]

Absolutely! Originally, I designed it for headphones at work, but after my girlfriend heard it hooked up to the main home system, I wasn't allowed to take it back to work LOL. I'm trying to sell my HeadRoom Micro DAC that it replaced so I can buy parts to build a second 🙂

 

[Bas Horneman]

but after my girlfriend heard it hooked up to the main home system, I wasn't allowed to take it back to work LOL

You can't get a better endorsement than that!

 

[zman01]

Congrats Tyler! impressive stuff.