D1 output stage and PCM1738

[Holger Barske]

I am thinking of adding a Pass D1 output stage to my PCM 1738-equipped CD-Player.
I have read a lot of threads covering the adoption of the Pass I/V stage to PCM 63 and PCM 1704 types of DACs – no problem so far.

What bothers me a little is: The PCM 1738 has fully differential outputs (2.4 mA p-p) NOT referenced to ground, but referenced to a pin called Vcom, which seems to be set to half of the supply voltage (2.5V, see attached schematic taken from the PCM 1738 datasheet). Any ideas of how to implement the D1 stage under these circumstances?

PCM1738 datasheet

[relder]

Assuming Vcom2 is stable I'm guessing it's just a matter of adjusting the pot (P4 and P5) to get the voltage you want at the PCM1738 current output pin. Normaly you'd adjust the pot to get 0 volts at the output, now you'll adjust until it matches Vcom2.

If Vcom2 isn't a stable voltage then that might be a different story, requiring a little more work.

[acaudio]

Hi Holger,
I finished so far the audio part of my D1 output stage in connection with Shanling CD-S100MK2 (PCM1738). The DAC accepts a wide range ot Vcom2 Voltages from -0,3V to Vcc+0,3V. My first test was adjusting the D1 In to 0V, but it gives a bit hum at max. volume. Second I connected GNDA CD player to the GNDA D1. Hum is out, but the dynamic range was compressed. At last I justified the current inputs D1 up to 2,50V. It´s the best choice, no hum and no compression. Fully dynamic.

Best Regards
Adam

[Ila]

Hi HBarske,

I would recommend to choose the DC voltage as close as possible to the vcom2 voltage. A current source has a finite output resistance, especially when done in MOS. So it is wise to have the specifies voltage over the switching devices on the chip.

What about a servo circuit ?

BTW: is it worth the effort of building a discrete output stage when working with a PCM1738 ? Just a question.

Greatings

Ila

[Holger Barske]

Nice to see there's some movement on this thing.

@Adam: Received your email concerning this, I will answer a little later, not much time these days, but thank you for bringing this up again.

@Ila: Is it worth the hassle? I really can't tell you yet, but since that Shanling player is quite a nice piece of equipment (I started this thread 2 years ago with exactly that player sitting in front of me) it is certainly worth of giving it a try - I don't think the PCM1738 is that bad at all.

[Ila]

Hi HBarske !

PCM1738: It seems you wanted to build something from scratch, but to upgrade an otherwise good player with the D1 output stage is another story.

Ila

[acaudio]

Originally posted by acaudio
...Second I connected GNDA CD player to the GNDA D1. Hum is out, but the dynamic range was compressed. At last I justified the current inputs D1 up to 2,50V...

I reached a pretty improvement.
If you want to use the PCM1738 with D1, you should not connecting the Vcom2 Pin to anything. Only the Pins Iout- and Iout+ should be connected. AGND-D1 and AGND-CDP sholuld be conncted too. The PCM1738 is fully balanced in point-to-point Mode, the Vcom2 Voltage is only required if OP-amps are used. The inputs of the D1 I/V Stage should be adjusted to 2,50V.
The sound becomes wonderful drive and detailfocus, it´s a lot´s better as before.
Adam

[poolorpond]

acaudio,

I wanted to do this(single ended) with a tda1545a nos dac (diyparadise) and had the same concerns about the reference voltage on the DAC and the 0v on the D1. Please correct me if I am wrong.

You connected the D1 +in to dac +out.
You leave Vcom unconnected.
You biased the D1 to read 2.5v at input.
You connected the +D1 input ground to the cd player's ground.

I did the same connecting the balanced D1 stage to an ak4524vf DAC with the exception that I set the D1 to 0v. It sounds great. Do you know why you had to set yours to 2.5v? Thanks

[georgehifi]

This is a very good sounding output stage for the PCM1738, discrete and heaps of drive, if asked for it can give 6watts!

Cheers George

[Holger Barske]

Opamp I/V conversion, unbalanced, complex. Maybe good, but a completely different cup of tea.