Another Headphone amplifier solution that I just found

Started by kitor, September 19, 2014, 01:14:54 PM

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kitor

I just bought myself MEDUSA NX Core Gaming Stereo Headset by Speedlink:
http://www.speedlink.com/?p=2&cat=4316&pid=25572&paus=1 (don't know retail price, got mine used for less than $10)
Note that's stereo, not 5.1 version.

This headset comes in two parts - one is headphones itself, other one is kind of USB soundcard. And we are interested with this part, as you can plug any headphones there.
It looks like this:

And have about 4 meters of USB cable... but not only with USB connector, also two RCA ones.
Why? It works also with PlayStation, etc - you connect USB to console as power supply and "inject" RCA into audio cable that goes from console to your TV.

And here comes the magic (that I found by accident) - in this "console" mode device works as... just headphone amplifier. I connected USB into my phone charger, RCA directly into Canon AV cable (as headphones got both male and female ones), turned on headphone monitoring in ML on my 550d... and it just works. Oh, and you got even volume control on "game" slider  :) Of course you can connect USB into some powerbank, I just don't have any with me now - that's why I used phone charger.

But, you know, "we need do dig deeper".
Cable is just too long, and I don't need USB soundcard (I bought this for good, cheap headphones, not for card itself). I disassembled device, found there just three microcontrollers - one is USB soundcard itself, other one kind of eeprom that stores card firmware (we don't need them for this purpose) - and third one is amplifier itself. After some checks using multimeter I belive that RCA input is independent from USB soundcard and goes directly to ampiflier and headphones.
And here's it's datasheet:
http://www.gmt.com.tw/product/datasheet/EDS-1402.pdf

Now TL;DR quote from PDF above:
QuoteIt can operate from 3.0V to 7V single power supply (...)  The quiescent current is typically 1.5mA per amplifier @ 5V

That means that most probably (I need to confirm that) it can be powered directly from any cellphone battery (they are rated at about 3.7v), and at about 4mA current (two channels and smaller voltage) it would be enough for few hours of operation. Sadly, all Canon DSLRs seems to have 7.2V battery packs (fully charged goes over 8v) so taking power directly from battery grip is not an option.
So "to be continued"  ;)
Too many Canon cameras.
If you have a dead R, RP, 250D mainboard (e.g. after camera repair) and want to donate for experiments, I'll cover shipping costs.

dpjpandone

Tapping power from a battery grip would be the best option. You can even mount it inside the grip (they have a lot of extra room) you might be able to get away with powering directly off the battery as the 1v overvoltage probably won't hurt it, but to be safe you can add a 5v regulator. Lot's of headphone amp schematics online, mostly very simple, (just a few resistors, caps, and op-amp)

kitor

I'll try tomorrow, as I already made power jack in my battery grip. I know about schematics online, and even wanted to make one, but this got ready to use case etc, and it's already assembled ;)

I know that they are many schematics of simple ampifliers, but this one is ready to use and have nice case :)
Too many Canon cameras.
If you have a dead R, RP, 250D mainboard (e.g. after camera repair) and want to donate for experiments, I'll cover shipping costs.

kitor

Did it, sadly there's a problem - camera is giving me weird noises on headphones. This only happens if amplifier is powered from grip. If I take grip tray off camera (with batteries) and insert another one to camera (so amp and camera have independent power sources) there's no such noise.
That's weird for me, as in 550d ground is common for power and audio (and this amp application also have common ground) - from my knowledge if something in camera is making those noises, due to common ground this should happen also with amp powered from another source.
I have no time to investigate it now, as I need this for today shooting - I'm making quick'n'dirty external battery for this amplifier, and later I'll give it to someone with more electronic knowledge than I have.

This is what should it look like:
Too many Canon cameras.
If you have a dead R, RP, 250D mainboard (e.g. after camera repair) and want to donate for experiments, I'll cover shipping costs.

dpjpandone

you are probably hearing high frequency noise from the camera's processor, so you will need to add some filtering capacitors, try some small ceramic caps