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
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
Attachments
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.
If Vcom2 isn't a stable voltage then that might be a different story, requiring a little more work.
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
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
Attachments
D1 output stage for PCM1738
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
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
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.
@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.
acaudio said:
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
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
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
Hi guys,
here is my last version, built as test for my friend.
We tested both at PCM1738 and PCM1704U-K (Hoerwege PCM Board, cut direct after PCM) Both PCMs sounds marvelous with this D1-Modification. It´s no a big difference between 1738 and 1704U-K. After long listening I mean, the 1704 sounds a small amount better, but for "low cost high end" is the 1738 a very good choice.
Best Regards, Adam
here is my last version, built as test for my friend.
We tested both at PCM1738 and PCM1704U-K (Hoerwege PCM Board, cut direct after PCM) Both PCMs sounds marvelous with this D1-Modification. It´s no a big difference between 1738 and 1704U-K. After long listening I mean, the 1704 sounds a small amount better, but for "low cost high end" is the 1738 a very good choice.
Best Regards, Adam
Attachments
acaudio,
What did you do for volume control? I tried a stereo pot, each deck coupling one balanced channel at the output and got some nasty ringing. It looks like you tapped your volume at a different location.
Did you couple the channels with a pot?
What value?
Did you need to bypass the pot with a cap? What size? in series or parallel? I haven't played around too much with this as I cant seem to calculate the values for the resonant circuit to get rid of the ringing. I will try your vol location to see if that makes a difference.
What did you do for volume control? I tried a stereo pot, each deck coupling one balanced channel at the output and got some nasty ringing. It looks like you tapped your volume at a different location.
Did you couple the channels with a pot?
What value?
Did you need to bypass the pot with a cap? What size? in series or parallel? I haven't played around too much with this as I cant seem to calculate the values for the resonant circuit to get rid of the ringing. I will try your vol location to see if that makes a difference.
poolorpond said:
Hi poolorpond,
in balanced mode a pot around 5k could be used, like in original d1 between VOL+ an VOL-. In my stage the pot was AC coupled with 100µ cap in series (not bypassed).
In unbalanced mode we tested it after the output coupling cap (220µ) with 100R in series and 1k pot to ground as shunt (voltage divider), it works fine.
Adam
Hi acaudio,
i have put your schematic into a sim and it seems to work fine with a AC current source on input. It produces a 6Vpp on the output with a 4mA amplitude on the input. There is only one exception, the heavy DC offset on the output. Is there a possibility to get rid of this without putting capacitators in the signal way?
I intend to use 2 AD1955 in Mono so i need a bias current of
-6.48mA on the DAC outputs and a Vref of 2.8 V.
I have pulled up the the input with a resistor as told on page 13 in the AD1955 datasheet.
Rpullup = (Vsupply - Vbias) / (Ibias +(Vbias / Ri/v))
Should the input be at 0V or 2.8V.
I am not shure if this will work propperly.
Regards
Stephan
http://www.siebensinne.com/DAC_DA.jpg
i have put your schematic into a sim and it seems to work fine with a AC current source on input. It produces a 6Vpp on the output with a 4mA amplitude on the input. There is only one exception, the heavy DC offset on the output. Is there a possibility to get rid of this without putting capacitators in the signal way?
I intend to use 2 AD1955 in Mono so i need a bias current of
-6.48mA on the DAC outputs and a Vref of 2.8 V.
I have pulled up the the input with a resistor as told on page 13 in the AD1955 datasheet.
Rpullup = (Vsupply - Vbias) / (Ibias +(Vbias / Ri/v))
Should the input be at 0V or 2.8V.
I am not shure if this will work propperly.
Regards
Stephan
http://www.siebensinne.com/DAC_DA.jpg
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