Dongle DAC as balanced output

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Tubetec

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Nov 18, 2015
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Ive been checking the phone and computer shops for these things ,
its mainly generic China interfaces , 5-10 euros
I found the Apple USB C headset adapter for around 15 euros ,
It gets a good review over in ASR and stands up well to DACs many times its pricetag ,
it puts out a volt into 30 ohms
Very good distortion figures , noise around -113db , upto 48khz/24bit ,
Works with Windows .
I was going to try and make a pair of balanced outputs from two dongles
instead of feeding the dongle the usual L,R signal arrange for the DAW to send a mono signal with +/- phase to each pair of outputs ,

ok it doesnt drive full studio level , maybe just a passive attenuator box followed by a power
amp/active monitor or variable line input .

On the subject of USB power , a single cell lipo USB power bank needs a dc-dc convertor , that makes way more noise than the battery itself ,
I recently tested a DC-DC switcher I got off Ebay , around -44dbu on the Fluke with the output unloaded.
Nicad cells measure the lowest in terms of noise,
Some where around -200db off load
4x1.2v =4.8v its a good number , its within the spec for USB 5v, no switching means no added noise, no regulation is required and nicads can handle amps of current and not even break a sweat ,
it would require charge management a USB C hub and a box with a couple of XLR's on it


https://www.audiosciencereview.com/...pple-vs-google-usb-c-headphone-adapters.5541/
 
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By my calculation max output is +8dbu(2vrms) , that seems entirely reasonable to me, allowing for some gain in the equipment that follows it .
At least the signal is balanced and at very low impedence , if it needs to drive a long cable it can alternatively the USB cable could be extended and short patch leads used .
 
The Apple ones have 3.5mm TRRS, so two single ended HP outs and a microphone in and sheild, there are a few dedicated hifi USB dac/HP amps that have fully balanced topologies they tend to be a bit bigger .
The DAC and drive circuitry are contained in the USB plug , its no bigger than a fingernail.

Heres another guy who does a good evaluation on the apple dongle with windows ,
https://www.head-fi.org/showcase/apple-usb-c-to-3-5-mm-headphone-jack-adapter.23420/reviewsIt also allows vol up/down (100steps)and play/pause commands to be transmitted down from the headset cable , thats very usefull indeed
One very nice feature is when theres nothing plugged in the device vanishes from the audio tray ,likewise if you only have TRS phones the devices sences it and the mic option in windows audio isnt available .
 
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Tubetec said:
On the subject of USB power , a single cell lipo USB power bank needs a dc-dc convertor , that makes way more noise than the battery itself ,
I recently tested a DC-DC switcher I got off Ebay , around -44dbu on the Fluke with the output unloaded.
Nicad cells measure the lowest in terms of noise,
Some where around -200db off load
4x1.2v =4.8v its a good number , its within the spec for USB 5v, no switching means no added noise, no regulation is required and nicads can handle amps of current and not even break a sweat ,
it would require charge management a USB C hub and a box with a couple of XLR's on it
Click to expand...
I'm surprized at your comment regarding power banks.
I use them with my Zoom recorder and my AKAI MPC keyboard and I have zero noise issues, although there's a built-in converter.
Not all of them are equal, probably...
 
Tubetec said:
The Apple ones have 3.5mm TRRS, so two single ended HP outs and a microphone in and sheild, there are a few dedicated hifi USB dac/HP amps that have fully balanced topologies they tend to be a bit bigger .
The DAC and drive circuitry are contained in the USB plug , its no bigger than a fingernail.
Click to expand...
I was utterly unaware of the existence of such things.
 
The inline headphone remote works like this ,
It uses the electret power rail shorted to ground through resistances to communicate up/down volume to the software ,dead short is play/ pause.
If it gives smoothly stepped volume with minimal artifacts and no bloops, blips or chirps in the audio out I will be very happy ,
seeing as I'll use the device differentially instead of taking signal from L/R and common , hopefully any disturbances on the ground line cancel from the audio .
1674222491581.png
1674223207940.png
 
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I appropriated a set of the Apple TRRS headphones with the remote from a friend who had upgraded their Iphone.
I was able to audition the sound, using the volume control is seamless , flawless with perfect interchannel balance on the Iphone , performance under windows remains to be seen ,
The really cool thing is I only need to cut off the phones and attach a male XLR plug and my new device is ready to rock .

A nearly free balanced audio output courtesy of Apple corp , I'll have two of them Sir, TYVM ,
Im going to grab a coffee and head off down the road to haggle with the man in the phone shop,
more to follow.....
 
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Tubetec said:
At least the signal is balanced and at very low impedence , if it needs to drive a long cable it can alternatively
Click to expand...

Are you sure about that? I'd imagine the TRRS plug is also for a mic input? I haven't got an Apple USB-C adapter handy to check, but I'd be kind of amazed if it was balanced.

Tubetec said:
the USB cable could be extended and short patch leads used .
Click to expand...

Max. 5 meter, possibly less.
 
I think you might have missed the point ,
Im not sending out L/R signal, Im sending out a single channel on one dongle with opposite phases in place of L/R signal.
obviously I need two dongles to get a stereo mix from the DAW .



He didnt have the genuine Apple parts so I wasnt wasting my time ,
At any rate I can get the proper parts at Harvey Normans for a tenner ,so I'll wait ,

The simple inline remote has a very tactile feel about it , with a momentary press it increments the volume by the smallest division ,with medium and longer button press times the attenuation curve accelerates and it behaves like a very natural steady fade out or fade in over duration of around a second ,

Incorporating momentary on-off-on rocker switches on a panel in place of the tiny apple inline remote would give things a very much more solid feel.
A 12v automotive electric window up/down switch fits the job description perfectly , not a very vintage look though .
1674257672697.png 1674257565798.png
 
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Tubetec said:
I think you might have missed the point ,
Im not sending out L/R signal, Im sending out a single channel on one dongle with opposite phases in place of L/R signal.
Click to expand...

You're right. I missed that part.

Tubetec said:
obviously I need two dongles to get a stereo mix from the DAW .
Click to expand...

Which could be a bit tricky as most software can't use two (or more) output devices. But I don't know how you're counting on driving them.
 
This dongle requires the use of Asio4all or other third party Asio workaround , I was hoping to amalgamate things that way ,
In terms of using the DAW it might just be a matter of setting up extra input tracks fed by the same source but with the phase flipped and then feeding them to the correct channel of the relevant dongle output .

CS or one of its subsidiaries apparently make the chips for apple , an extra 6db drive would be a definate advantage ,
but the low cost and good quality of the Apple and the fact thats its easy to get makes life nice and simple .
The proper HIFI dongles with TRRRS balanced outs also dont usually have remote capabillity ,which I definately want to have .

I ran the meter across the Apple headphones, the play pause button connects a 1 ohm resistor across the mic input to ground ,
the volume up/down buttons didnt have any effect on the resistance there so there must be some thing more going on.
I might try and find a teardown of the of the remote to find out whats going on inside without smashing one up ,for now I just have an XLR plug soldered onto where the headphones buds used be ,the TRRS plugs into the dac 75cm away ,
The mic input can be reused as an Aux input to the DAW , it probably sences if electret power is required or the mic is externally powered as might be the case with noise cancelling phones , the mic input might also have gain adjustment via the Windows audio control panel , could make a very cool talkback mic input .

I want to thank everyone for their interest and ideas ,the more the merrier as they say ,
 
Heres The inside of the remote ,
and the teardown on it,
https://www.ifixit.com/Teardown/Apple+EarPods+Teardown/10501
Maybe it needs to be no more complicated than a modular approach , the apple dac and remote PCB mounted up in a metal box with USB in and XLR , even an XLR to USB barrel jack adapter housing is perfect for a low profile test bench set up where you want cables neat and tidy.
You have very fine control of output level steps and your plugging in directly into the DUT input .

The mems mic probably doesnt sound bad and the mic gain has adjustment in software from the audio panel ,
adding a PTT switch for the mic would be the icing on the cake and of course positioning the mic so it has a clear shot through the front panel or barrel.
A hard mute on the the line out would also be and excellent addition .



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thor.zmt said:
There are some units with 4V Balanced output based around the Cirrus Logic CS43131 or CS43198. They cost a trife more but are a way better choice.

Thor
Click to expand...
Very good idea (choice) ! But the PSU problem greates of all ! I not found best souce, than Ni - Mh accus ! ((I'm an 67 years old electric and sound engineer from Hungary) My "craze" the noise ! I found that the lower the noise level, the more beautiful and natural the sound!
 

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