Logitech x-540 Pulsating/woofing/tremolo type volume problem

[lordvader88]

These are very cheap 5.1 surround system for PC. Inside is pretty basic, as in it has 1 big chip doing most everything. The STA540 4-channel, class AB audio amplifier
https://www.sparkfun.com/datasheets/Components/General/STA540.pdf
Had this problem for years, on may system setups, it's the Logitech for 100% for sure.

My problem is at certain volumes on it's external knob, I get a pulsating/woofing/tremolo type volume effect @ ~2 Hertz. It's super noticeable when white noise, or a rain storm, waves, certain things in music like holding big notes. I can't notice it if just speech is playing.

It's on all all 5 speakers separately, when doing speaker static test. Not sure about the subW.

I just finally looked at the right-out on a scope, and I can't get clean triggering at all on anything I tried, a 1kHz sinewave on utube looks like a dirty mess at only 10's of mV. So the scope is rolling. With it on the white noise, I think the base signal was about 20-40mVpp and @2Hz I'm seeing narrow peaks jumping to 60mVpp (at this random volume).



When it's doing it, I usually just tweak it's volume knob back/forth a bit and with luck it stops somewhere and doesn't do it. I usually listen from 1/4-1/2 volume, I haven't checked upper volumes. The knob is a potentiometer.

Another thing I just noticed when playing a 1kHz sine wave was that changing the volume back/forth, on the external knob AND/OR the PC, made brief popping/static type sound that seems to drown
out the sine wave as it happens.

I never worked on audio before, any ideas ?

[lordvader88]

I think it's giving me a brain sickness, and oxygen deprivation from breathing real slow and holding my breath to hear if it's doing it. And I hear it in my imagination sometimes, or my blood moving in my ears 

[lordvader88]

I've ordered a new chip

Lately is seems the volume is splitting between the front/L/R speakers in a way I don't expect 0or remember, maybe it's win7 IDK

fOR ABOUT $1.50 I have 2 new chips.

I hope that fixes the problem, if not, I think there's just random caps' to test,...pretty much everything is in the multipin IC (idr maybe 12-16)

China to farthest east side of Canada is so slow

[LateLesley]

I'm not so sure a new chip will fix the problem, As all it is doing is amplifying the incoming signal. I'd be tempted to tell you to look at the powerline with the scope, while it's doing it's thing, and see if the power rail is stable or not. I'd also check for the noise on the input to the chip channels. The two things you need is stable power, and then to pick a channel, and follow it back through the circuit, or follow from the input to the output, and see where the noise is being introduced.

I found a pic of what seems to be the internals - https://www.electronicspoint.com/attachments/100_8868-jpg.4608/

It seems to be using 2 STA 540's for amplifier duties, but it also has 2 R2S15201 (the two DIP packages to the left, looking from the rear panel) - these are 3 channel Electronic volume controls, so I'd be having a look around them.  ( found on page 20 of this PDF, all I could find quickly - http://datasheet.datasheetarchive.com/originals/scans/Scans-008/Scans-00160796.pdf)

But for audio, the first rule is stable power, which doesn't shift when under load, and looking at the very cheap PSU section, I think this is where you are going to find your problem, with the supply rails dipping, and introducing oscillations at certain power levels.
 
So i'd get the scope watching the STA540's power rails, and the R2S15201 power rails, and watch what is going on. Any large ripple or dips in power, is going to affect your output audio.

If they check out, then we need to look further back at the small chip op-amps (the 14 pin and 10 pin devices - I can't see their model numbers)

[b_force]

Sounds like a stability issue and therefor the system goes into oscillation or something.

Could be power supply, a bad chip or a bad design.

[chickenHeadKnob]

Is the volume knob a simple pot? It is speaking to me saying "I am a dirty boy"