DAC Schematic Review and Layout Suggestions - WM8804 and PCM5102A - Page 9 - diyAudio
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Old 19th September 2014, 10:03 PM   #81
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Ok, just did a test with a headphone volume control attached inline and turned it down until RMAA reported that there was no clipping. The voltage after the pot was 1.4VAC. Distortion has been reduced, but take the results below with a grain of salt - the volume control is after all a cheap and crappy piece of equipment.

Click the image to open in full size.

Honestly, (while I'm preparing version 2 of the board) I'm also wondering if there are any layout/schematic issues in my design that are hindering performance.

And BTW I do have boards avaliable, but you may want to wait until Version 2 first.
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Old 20th September 2014, 03:32 AM   #82
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Hi, what would be the night and day differences that would make worth building a V2?
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Old 20th September 2014, 11:47 AM   #83
kgoulet is offline kgoulet  Canada
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Using the volume control I think you showed that full scale output is not clipping. Checking the audio spectrum might provide some more clues.

The multimeter you have looks to be a solid meter for home and auto measurements, but audio signal measurements might be a little off. The most sensitive scale for AC is 250Vac. I doubt that last decimal point is guaranteed to be that accurate. Especially for non 60Hz measurements.

Some other tests to check the accuracy of the Realtek measurements would be to measure the noise with the line inputs shorted together. Then measure noise from the DAC playing silence (the DAC is supposed to have auto-mute). Then measure noise playing the smallest signal possible.

Trying to improve the circuit without being sure of the measurements would be difficult.

If you really want a version 2, maybe tackle the problem you already encountered: not all devices are aligned to a 2.1vrms = 0dB line level.

If you can provide a scalable output, that could be a useful feature. If you add some sort of peak detector or rms to DC converter (LTC1968 for example), then you could use the more sensitive DC ranges of your meter to match the output to whatever input you connect to.
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Old 20th September 2014, 03:17 PM   #84
Eldam is offline Eldam  France
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I have two Sure kits . What could be the cursor setting for a TDA1541 please ?
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Old 20th September 2014, 04:11 PM   #85
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To be honest, my second revision will probably include a 3.6V regulator for the charge pump, better part placement, and a better 3.5mm jack (the current one requires me to route the signals in loops, which may be affecting my sound). I also want to remove the trace running under the PCM5102A and create a ground plane on the top layer underneath both chips.

I'm looking at recording interfaces right now, which have much higher input ranges as well as lower THD and noise measurement ranges than my Realtek. When they go on sale, I'll pick one up and use it to do some proper measurements.

To be honest, I don't think the measured noise will be much lower with the DAC on mute. I'm already pushing the Realtek ADC (which by the way is only 16-bit) to its limits already.
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Old 9th October 2014, 10:43 PM   #86
Rochey is offline Rochey  United States
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Might I make a suggestion? I believe the clipping at full scale is caused by gain error in the line driver. (shown in the datasheet, it's one of the joys of silicon resistors...).

If you are relaying out a board, the PCM5122, even though it's mainly an I2C part can also do HW control with more GPIO pins. One of the pins can do an analog gain switch from 2.1VRMS to ~1VRMS. This *should* maintain the SNR, whilst reducing the output from getting near the power supply rails.

Good luck,

Rochey
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Old 11th October 2014, 03:11 PM   #87
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Quote:
Originally Posted by Rochey View Post
Might I make a suggestion? I believe the clipping at full scale is caused by gain error in the line driver. (shown in the datasheet, it's one of the joys of silicon resistors...).

If you are relaying out a board, the PCM5122, even though it's mainly an I2C part can also do HW control with more GPIO pins. One of the pins can do an analog gain switch from 2.1VRMS to ~1VRMS. This *should* maintain the SNR, whilst reducing the output from getting near the power supply rails.

Good luck,

Rochey
Would using a 3.6V regulator on the charge pump provide the same effect? I feel that I'm kind of wasting the PCM5122 by using it like a hardware-controlled device...
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Old 13th October 2014, 08:46 PM   #88
Rochey is offline Rochey  United States
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Quote:
Originally Posted by df audio View Post
Would using a 3.6V regulator on the charge pump provide the same effect? I feel that I'm kind of wasting the PCM5122 by using it like a hardware-controlled device...
I see where your coming from. Running at 1VRMS gives you WAY more headroom though... plus, you can hook up I2C for a rainy day and put the HW vs I2C control pins on jumpers
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Old 12th January 2015, 10:18 PM   #89
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BTW, this project is not dead (I've gotten messages & PMs asking me about this). While I've been busy with schoolwork, studying for exams, and such, I'm still thinking of making a V2 board and am laying out the PCB in my spare time.

Basically, the board will include more effective routing and part placement (I've been looking at many DAC designs as well as the EVM boards), ground planes under the chips, support for film caps on the output, a 3.6V regulator for the charge pump, and possible headers for people who don't plan to use the board-mounted 3.5mm and RCA jacks.

Possible ideas on the horizon (which I might implement) include the use of the better performing CS4351 DAC chip instead of the PCM5102A as I now have a linear power supply and can supply the 9-12V required for the chip. I'm also thinking of using a remote (relay) switch using a pushbutton in the front rather than having the switch on the side as it currently is.

I might as well say this now, but I've been listening to my DAC for quite a while and probably has been fully "broken in", if that makes a difference or not. When compared to my other equipment, SQ is rather nice but not completely neutral, with a slightly warmer sound. Clipping is not evident during normal listening, and is barely noticeable at 0db points. I have some more listening impressions here.

Stay tuned.
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