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Just how bad is it? Audio mixer with headphone amp.
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Audioguru:
Paul, your simulation does not say and does not show a volume control but now you say it is TI's "active" volume control circuit.
Medical experts say that sound levels over 85dB cause hearing loss and you are talking about 120dB that is going to destroy your hearing.
If your headphones survive 120dB, their power is 35dB more that the 85dB that begins to cause hearing loss. 35dB is about 4000 times the power. Hit yourself with a hammer that causes a small wound. Then hit yourself 4000 times harder and see what happens.

I protected my hearing with earplugs when in loud sounds but my hearing is normal for my age (72). The high frequencies are reduced to almost nothing so I got hearing aids which create normal hearing for me up to 8kHz. The missing top octave cannot be boosted anymore without destroying the remains of my high frequency hearing and/or there will be acoustical feedback whistling if there is more boost.
paulca:
So, taking a break from the opamps for a few days, I looked at the USB DAC.

I'm currently thinking of the PCM2906C.  The typical application circuit doesn't look too challenging to implement.  There are a few design decisions to make, but I'm currently thinking of just tying the HID inputs low as I don't want/need buttons.

The issues I am pondering is power.  Obviously TI are making these "Bus powered" as 5V is available off the USB.  There is an extended schematic "For high performance" which appears to use a linear 3.3V regulator, but that's about the only change.  I'm not sure what advantage one gets from running the chip on 3.3V instead of 5V I might do some digging there.

My issue with power is that it creates a route for EMI noise from my PC.  The DAC being digital will not have noise on the audio, but it will have on the power.  Now obviously I could stick a dummy load, like a 100K resistor on the VCC of the USB port and run the DAC off a 5V regulator off the +15V rail of the project (I know there are caveats, unbalanced load on the rails and I have not thought that through yet). 

The problem is then the ground.  I can't leave it off as it is needed for the D+ D- (I believe).  But once connected to the board it will then potentially pollute the rest of the board, including the analogue amplifier grounds with PC PSU and GFX Card noise.  I can still hear it on my current USB DAC, although very quiet, however it IS bus powered.

How do you filter a ground?  After a quick googling, this is as I suspected non-nonsensical.  However I did figure out why the "High performance" circuit runs off 3.3V.  The LDO and associated filtering caps will cut most of the noise out of the USB power. I might take that option. 

This has also made me wonder if I would get rid of the noise on the power rails from the XP Power +-15V PSU if I dropped them to 12V with LDOs after ferrite and cap filters.  I probably don't need the 15V rails.
BrianHG:
With analog DAC
https://www.ebay.com/itm/CM6631A-digital-interface-USB-to-I2S-SPDIF-coaxial-32-24Bit-192K-sound-card/132272012272?epid=887353946&hash=item1ecc06bbf0:g:wH4AAOSw3ZNZdgsT

https://www.ebay.com/itm/DAC-384KHZ-32bit-Decoding-Module-XMOS-PCM5102-TDA1308-USB-Decoder-Board/232547127833?hash=item3624e39219:g:hbMAAOSw09xZ-sV~

Without Analog DAC, just I2S interface.  (you can opto isolate (use a high speed 8 channel digital galvanic isolator) these and place your DAC IC on the analog side of your preamp, no more connection to your PC's GND)
https://www.ebay.com/itm/CM6631A-USB-2-0-to-SPDIF-Coaxial-Optical-24bit-192kHz-Sound-Card-DAC-Board/322769627550?epid=1339946835&hash=item4b2691a99e:g:omsAAOSwkklZpQ6B

https://www.ebay.com/itm/XMOS-USB-Digital-Interface-I2S-SPDIF-Output-Board-Module-DSD256-32bit-384KHz/272891906642?epid=17007417855&hash=item3f89a00a52:g:mP8AAOSwevdZ5srx

https://www.ebay.com/itm/Italy-Amanero-USB-IIS-Combo384-module-USB-to-IIS-adapter-DSD512-PCM384K/272563718243?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D50073%26meid%3D61b938851ef9422b8621e9c6035a28b0%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D272563722080%26itm%3D272563718243&_trksid=p2047675.c100005.m1851 (new chipset with full list of OS support)

Both I2S and DAC with optional case:
https://www.aliexpress.com/item/HIFI-CM6631A-Digital-interface-32-24Bit-192K-Sound-Card-USB-to-I2S-SPDIF-Coaxial-Output/32855527872.html?spm=2114.search0104.3.231.6e5f4d70Qt73Ho&ws_ab_test=searchweb0_0%2Csearchweb201602_4_10152_10151_10065_10344_10068_10130_10342_10547_10325_10343_10546_10340_10548_10341_10084_10083_10618_10139_10307_5711211_10313_10059_10534_100031_10103_10627_10626_10624_10623_10622_10621_10620_5711311_5722415%2Csearchweb201603_2%2CppcSwitch_5&algo_expid=c9a89ce6-7582-41c6-b9ec-6810a48f3dd7-36&algo_pvid=c9a89ce6-7582-41c6-b9ec-6810a48f3dd7&priceBeautifyAB=0
paulca:
Interesting boards, but I was kind of hoping to get everything onto one board.  Some of those board smell of audiophool too.  32bit?

Further, what are the chances of these boards being class compliant USB audio devices that work without specialist drivers/firmware?

But the idea of dividing the DAC into two stages is an interesting one, but don't opto-couplers for USB cost $$$$?  Due to the higher data rates and timing?  Is SPDIF easier to opto couple?  Does I2S not need a ground connection?

The other option is an actual optical, but I haven't checked how much they cost.
BrianHG:

--- Quote from: paulca on March 04, 2018, 10:10:46 pm ---Interesting boards, but I was kind of hoping to get everything onto one board.  Some of those board smell of audiophool too.  32bit?

Further, what are the chances of these boards being class compliant USB audio devices that work without specialist drivers/firmware?

But the idea of dividing the DAC into two stages is an interesting one, but don't opto-couplers for USB cost $$$$?  Due to the higher data rates and timing?  Is SPDIF easier to opto couple?  Does I2S not need a ground connection?

The other option is an actual optical, but I haven't checked how much they cost.

--- End quote ---
Look at the SPDIF, it is already coupled.  That black cube isolation transformer beside the SPDIF jack.  SPDIF is difficult to interface compared to I2S.  I2S is on every audio DAC you can buy today.  Example: http://www.ti.com/audio-ic/converters/dac/products.html#p1021=2;2&p84=32;32&p1339=2;2&p348max=384;384

These 32bit 384khz stereo DAC start at 1$.  High end ones at 3$, differential output ones at 4.25$.  The top ones have over 114db SNR.  This is not audiophool.  What if in windows, you turn down the volume to -50db.  With this, you still get you source 16 bits going right through at low volume because of the depth of 32 bits.  As for sample rate, what if you are playing DVD audio at 48Khz, or CD audio at 44.1Khz, or, HD-DVDA/SACD/TrueHD Bluray at 96/192Khz, 24 bit audio.  Keeping your system at 384Khz, or even slowing it down to the standard 192KHz makes for a non-wobbly output like when you tried to play my .wav files which played perfectly clean like a function generator on my old 192Khz 24 bit PC motherboard.  Also, the differential/balanced DAC versions outputs a killer 4.2vrms levels.  No noise here if in your preamp you wire that input channel to receive the differential audio.  Compare that to your 1vrms single-ended old 48khz 16bit usb dac.  Run TI's PCM5242 at 384Khz in differential out, you will get 114db, for 4$.

If you want separation, just use the one of the simple I2S boards.  Mount one sound board in your preamp isolated from the rest of everything.  Those sound boards are self powered from the USB power.  Next, get one of these: https://www.digikey.com/product-detail/en/maxim-integrated/MAX14930CAWE/MAX14930CAWE-ND/5396463  Yes, this is basically a 4 input, 4 output 150MHz optocoupler with 2ns rise and fall time.

I2S only needs 4 digital outputs.  On you preamp, one side of the isolator is powered by the 3.3v output on the USB sound board, & gnd & data fed.  On the other side, you power from your preamp the 3.3v & gnd & the 4 I2S data lines go to your 3$ ti DAC.  The output of the TI dac goes to an input channel on your preamp.  Keep the isolator at the edge of you pre-amp PCB and do not GND or VCC fill under it.  Make the connector off the PCB going to the USB sound board as far to the edge as safely possible.
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