Diy DAC/amp with PCM2707, PCM1794, OPA1612

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r4pt0r4

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Joined 2018
Hi guys,

I'm currently fiddling around since I want to build my second DAC/amp board for semi mobile use.

My plan is to use a PCM2707C for USB to I2S converstion, a PCM1794A will be used as the DAC and the amplifier stage will contain a pair of OPA1612 and a OPA1622.
 The plan is to build the amplifier stage directly as described by TI in their reference design tidu672c


Voltage conversion for the PCM2707 and PCM1794 will be done by two LP5907, voltage inversion for the opamps will be done by two LM2662


[...]
Two transimpedance amplifiers are used to convert the differential output current from the audio DAC to a differential output voltage.
Once the differential output current from the DAC has been converted to a differential voltage by the transimpedance amplifiers,
the differential voltage is converted to a single-ended voltage by the difference amplifier. The amplifier must also be capable
of driving the headphone impedance.
[...]
Click to expand...
Attached is a flow diagram I threw together to illustrate my plans.

Furthermore the KiCAD schematic is attached as well. I'm currently optimizing the design and will head over to the PCB layout.

Any thoughts of you fine folks regarding my idea? Feasible? Any traps I could encounter? Generelly a bad idea?

The board will be used most of the time in combination with my notebook. Headpones will be Shure SE315 and a AKG Q701.


[edit] Messed up the title, I know. Already reported it to the moderator :D
 

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Markw4

Member
Joined 2016
Don't much care for any kind of SMPS for hi fi unless very carefully engineered. PSRR is never good enough to prevent some switching noise or music signal modulation in the audio output, IME. In particular, switched capacitor inverters have output droop unless steps are taken to mitigate it. Others don't seem to be bothered much by it, so YMMV.
 
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r4pt0r4

Member
Joined 2018
Markw4 said:
Don't much care for any kind of SMPS for hi fi unless very carefully engineered. PSRR is never good enough to prevent some switching noise or music signal modulation in the audio output, IME. In particular, switched capacitor inverters have output droop unless steps are taken to mitigate it. Others don't seem to be bothered much by it, so YMMV.
Click to expand...



Thank you for the input. I chose the LP5907 since TI states

with ultra-low noise and high PSRR ideal for RF and analog circuits
Click to expand...
But yeah, I get your point. Will look for an alternative.



Lingwendil said:
I would go with a more modern USB chip, like the CM6631A or similar, the PCM2707 is a bit older design that wont match the full capabilities of the PCM1794.
Click to expand...


Yes, the 2707 is quiet an old beast. Looks like a good option, thank you, I'll look into it. It's cheaper than the 2707 as well. But since I'll only build one or two, price isn't that much of a deal.
 
abraxalito

abraxalito

Member
Joined 2007
Having used an LM2662 to feed the analog rails of my DAC I'd say you'll definitely need filtering on its output, LC. That's before sending it to a regulator with good line rejection at 100kHz. Also watch your grounding, switching devices put hash on the GND so any common ground impedance with your analogue stage spells trouble.
 
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r4pt0r4

Member
Joined 2018
abraxalito said:
Having used an LM2662 to feed the analog rails of my DAC I'd say you'll definitely need filtering on its output, LC. That's before sending it to a regulator with good line rejection at 100kHz. Also watch your grounding, switching devices put hash on the GND so any common ground impedance with your analogue stage spells trouble.
Click to expand...


Hm yeah, filtering looks like to be a must. Maybe I can find a "quieter" inverter.


weissi said:
I recommend to leave out the PCM2707 completely and to get a readymade solution like Amanero/jlsounds/waveio.
Click to expand...


I get you point, but since this thingy won't have the goal to be super hifi more than this is a fun little project I wanna design myself, an already prebuild solution would be against my intention.



Although the boards you mentioned do look awesome for their purpose.
 
metalman

metalman

Member
Joined 2003
Back (way back) when the PCM2707 was one of the better usb audio inteface chips that was diyable, I designed and built a DAC that went PCM2707 to SRC4192 to PCM1798 to a D1 style discrete IV stage. The 4192 and 1798 are slaved to a very low jitter clock, and I hard set the 4192 to upsample everything to 24/192. I probably have builders bias but I think it sounds quite good and I haven’t heard another DAC that sounds sufficiently better to make me want to move on. However a goodly portion of the quality also comes from the separate discrete shunt regulators for the 3.3V, 5V digital, 5V analog, and the two +/-22V rails for the IV stage.

The important note for this thread is that it won’t be problematic to make the 2707 and the 1794 play well together.

Cheers,
Terry
 
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