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Topic: Realtek ALC888 distortion problem (Read 6009 times) previous topic - next topic
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Realtek ALC888 distortion problem

Hi,
I am feeding a single stereo output from my computer (which has a Realtek ALC888 sound card on board) into my Behringer 1204FX mixer.



Using the two mono inputs A + B works fine, and I can adjust the gain using knobs C + D.
When I use a stereo input (E + F), the sound is fine as long as the gain button (G) is raised (ie low gain, the volume slider has to be all the way up leaving me no headroom or control, and I suspect making the signal noiser than it could be).
When I depress the button for higher gain, it initially works fine, but after about 15 seconds, the sound badly deteriorates.
distorted music

At 5 seconds, I press the button, at 20 seconds, the distortion can be heard.

The inputs H + I, exhibit the same problems. I have used a few other sound cards without any problems...
Sound Blaster 24 Live USB sound card
Another motherboard which has a Realtek ALC883 chip on board.
Another onboard soundcard on an older motherboard

I have tried the problematic ALC888 output with a similar Behringer 1002FX mixer, and have no problems. Which could suggest some problem with the 1204fx mixer.

I have tried using a different jack socket for the output(they are configurable), and this hasnt helped.

Once the distortion starts, pressing the button again restores sound quality after say 0.5 second. If I then quickly press the button again, the sound goes distorted after about a second.

It appears to me on evidence that the impedance of the mixer is too low, and the output amps in the chip are working too hard, getting hot, and starting to sound rubbish.

Has anyone any ideas whats happenning here, and how to fix it?. Due to my application, I dont have space to put in a PCI card. I could use an external USB sound card if I have to. It seems that the onboard sound should be better than my usb sound blaster live, so if I have to go for an external usb sound card, can someone point me at something a bit better?.

Any help appreciated.

Realtek ALC888 distortion problem

Reply #1 –
The distortion is from using the +4db input.  Pro gear is (semi) calibrated for +4db while consumer gear is much hotter and needs -10db. 

On another note, I also have a Berhinger mixer that does some funky things on the stereo channels.  Using separate channel strips works fine.

"You can fight without ever winning, but never win without a fight."  Neil Peart  'Resist'

Realtek ALC888 distortion problem

Reply #2 –
This sounds like a case of something with a DC offset. Initially the blocking capacitor handles the difference in voltage, but gradually it charges up until something reaches its DC limit, causing it to distort.
 

Realtek ALC888 distortion problem

Reply #3 –
Quote from: pdq on
This sounds like a case of something with a DC offset. Initially the blocking capacitor handles the difference in voltage, but gradually it charges up until something reaches its DC limit, causing it to distort.


That's my feeling too. When the audio fails, there is nearly nothing in the negative direction, only positive peaks. I'd like to see a scope connected via 'Y' cord to watch the computer output when it fails and then unplug the feed to the mixer to see if there is any change coming from the computer. I have more doubts about the mixer than the computer. This is one problem where a 'scope will be invaluable.

G²

Realtek ALC888 distortion problem

Reply #4 –
Quote from: dreamliner77 on
The distortion is from using the +4db input.  Pro gear is (semi) calibrated for +4db while consumer gear is much hotter and needs -10db. 

On another note, I also have a Berhinger mixer that does some funky things on the stereo channels.  Using separate channel strips works fine.



I always thought that to be the opposite -> Commodity hardware to output a lower signal than pro hardware.

Also, following OP's explanation, it seems that the distortion comes when pressing the button, so selecting -10dB.  And i guess we can consider the output of the soundcard as consumer, not pro hardware.

Realtek ALC888 distortion problem

Reply #5 –
Thanks guys,
There is indeed a roughly 5 volt DC offset with the button pushed down. With no input connected this offset is 5.75V.
I set it up/ pack it away in a rush regularly, so I *may* have wired it up funny recently, and damaged something( I vaguely remember pulling out 2 jack plugs from the inputs in question because they were in the wrong holes! ).
Unless one of you guys can suggest an easy fix, I'll be retiring it, keeping it as a back up or for spares.

dreamliner77, Jaz,
The -10db setting has a higher gain than the +4db setting. It is the -10db setting that makes it go initially louder at 5 seconds in the audio clip, and this setting that soon after starts sounding really distorted. The sound card is not pro gear, its just the onboard integrated sound.

Realtek ALC888 distortion problem

Reply #6 –
Quote
There is indeed a roughly 5 volt DC offset with the button pushed down. With no input connected this offset is 5.75V.
  Is that offset coming out of the soundcard, or is it present at the mixer input with nothing connected? 

It could simply be leakage through a capacitor.  If that's the case, a resistor between the input and ground (maybe 10K) should knock-out the DC voltage.  (Since you know how to measure voltage, I assume you can figure-out how to rig-up a resistor  )

P.S.
I just remembered that when I hooked-up my soundcard output to an oscilloscope (probably 10 Megohms) and no other load, I had an offset.  When I added a resistor it came back to zero.

Realtek ALC888 distortion problem

Reply #7 –
Wow,
Cheers, thats just the easy fix I was after!!.
The voltage at the input is 5.75 volts when nothing is connected. A 10k resistor drops the DC voltage to 0.01V, and a 100k resistor drops it to 0.02V.
Bonus!

OK, its confession time now...
I have a degree in Electronics, but I'm not the best service engineer!. A leaky capacitor never occurred to me, but that seems to fit the description spot on. Its great having other eyes on the problem .

I'll open it up, and try and find a capacitor that looks suspect + replace it
I'll report back

Realtek ALC888 distortion problem

Reply #8 –
Lots of screws + nuts needed removing to look at the side of the board with the capacitors on, so I decided to put 4 10k ohm resistors across these inputs.

The distortion is fixed!.
Thanks again

Realtek ALC888 distortion problem

Reply #9 –
Quote from: dreamliner77 on
The distortion is from using the +4db input.  Pro gear is (semi) calibrated for +4db while consumer gear is much hotter and needs -10db.


You've got things reversed. -10 dBv is a far lower voltage than +4. dBu  A -10 dBv input must have far more gain than a +4 dBu input, all other things being equal.

In the cosmic scheme of things the reference voltages for the two sensitivities are different, so their actual voltages are not exactly 14 dB apart.

+4 dBu is  1.227652988 volts
-10 dBv is  0.316227766 volts

A CD player can put out up to about 2.5 volts, so an input with a sensitivity of +4 dBu can clip when hooked to a CD player. However, the -10 dBv input will clip even harder, all other things being equal.