Sunday, September 6, 2009

Epic fail on the X-240 diagnosis

OK so I it turns out I'm wrong about the remote being the source of the noise. After a big session of connection tracing in the remote today, I came to the conclusion that I could just de-solder the audio wires in the remote, and see what happens.

So after doing this, the hum/buzz was still there. Further tracing showed that there is a direct connection between the headphone socket and the subwoofer enclosure and this is not an audio cable. It is probably flipping a bit in an integrated circuit inside the amp which disables the pre-amp (and as a result my previous diagnosis of the amp still being turned on when the headphones are connected is wrong).

Anyway the status therefore is that the noise is being produced within the subwoofer enclosure, not the remote. This makes it a lot harder to fix because this is where the "brains" of the amp are and there are a lot of components which could cause the problem. I still believe it's a design fault or the quality of the components used to hit their price point, but it may be beyond my abilities. I'm going to leave it a few days and see if I get inspired to keep digging.

Oh, there isomething else discovered today: when turned "off" the speakers consume 6 watts of power, and when turned "on" they consume 7 watts. This is pretty poor for standby power consumption, and does not speak well for the environment friendliness of their circuitry. By comparison, my ancient set of Altec Lansing ACS45's (the original plastic-subby version) pulls 4 watts when "off" and 8 watts when on. That's a much nicer difference in consumption - and it even has an "auto off" feature.

Friday, September 4, 2009

X-240 architecture

Well I pulled apart the X-240 last night and - while I didn't fix the problem - I think I discovered the source.

So, the "remote" where the volume control lives is acting as the pre-amplifier (kinda, it's more that it's restricting the "volume" of the input source). Signals come from the PC here and a potentiometer (the volume control wheel) determines how much this signal is restricted - full volume I am assuming is "no reduction in input power" . It's also where the majority of the brains are located, including some additional circuitry which allows for the speakers to be shut down when headphones are connected. And most importantly, when this happens the hum goes away.

This "speakers turn off when headphones are connected" function interests me. It's not like power is being cut to the speakers, just the input source. If you put the speakers close to your ears you can hear a faint hiss, which shows that the power amp is still powering the speakers but is not amplifying any signal.

Further testing also showed that the headphone socket will work even when the speaker system has no power to it at all (even at the mains wall socket). This shows that the headphone connector is mechanically switching the audio input source from the PC sound card to the headphones. Only when the headphones are removed from this socket does the mechanical switch go back to passing the audio input source to the X-240 power amp, via the volume control potentiometer.

Therefore, my deduction is that the noise is coming from the circuitry in the remote. The power amp in the subwoofer box has filtering capacitors on it and shows - by itself - no external noise source. At the moment, I believe that the circuitry in the remote is the source of the noise, or there is noise coming up the power cable from the woofer box.

There are a number of ways I can try to test this, and my ideas at the moment are:
  • Solder in my own separate power lines between the subwoofer box and the remote
  • Hard-wire the power circuit and use the switch on the remote just for the audio connection
  • Hard-wire the audio connection between the PC, the potentiometer, and the output back to the power amp (bypassing the power switch)
  • Hard-wire the power, and also use my own potentiometer for volume control rather than the surface-mounted one in the remote.
Well, that's it so far. Now all I have to do is find some time to run the tests!

Wednesday, September 2, 2009

Logitech X-240 speakers, and humming.

This is way off the usual RC topic, but recently I purchased some Logitech X-240 2.1 speakers for my main PC in the study. They sound fine, but they hum like crazy as soon as the amp is switched on.

For those of you with the same speakers annoyed by the hum, allow me to let you know I'm about to pull this sucker apart and find the source. I've already had a peek inside the box by removing the subwoofer and I can see a very cheap job of cabling, and a separate AC-DC transformer. The transformer will be my first port of call, following by the amplifier circuit itself.

I may not fix it, but I haven't seen anyone else yet trying to fix their X-240. Sure it may well be because they're so cheap and folks feel like "they get what they pay for", but I've got a 10-year old Altec Lansing 2.1 system on my wife's PC which doesn't hum at all, and I recall paying a similar price. I am determined to track down and fix the hum, unlike Logitech who don't seem to care a whisker (the problem looks to be have been in the product for a long time).