Sound Devices 633 channel 2 making a 20-30hz persistent noise.

[jimmiplex]

I just purchased a used 633, and I noticed the channel 2 meter was registering a signal with nothing plugged in, about -15dBFS.  When I engage the HPF, it disappeared.  I made some recordings so I could look in Protools, and it looked like there was a 20-30hz noise not present in the other channels.  With the HPF on, my recordings sounded fine.  I was wondering if anyone has run into this problem before, what caused that, and ultimately if I should just return the 633.  Any help would be appreciated!

[Philip Perkins]

Have you asked Sound Devices support about this?  Does this sound continue when you turn P48 off?  If you switch the input to line level?

[Conor]

Sounds like DC offset, likely a hardware issue. That being said you could just continue to use it with the HPF engaged - you'll never need those frequencies anyway

[jimmiplex]

Yes, it continued regardless of mic/line or phantom power settings.  The weirdest part was when I adjusted the trim, the meter would read up or down a little in the direction I was turning the knob, but would then go back to where it was.  Sound Devices didn't give me an answer further than it needed repairs that were estimated to cost $950.  My biggest concern was if it was water damage or something like that, I had an MM1 preamp get water damaged years ago, and it worked for a while after it got wet, but weird things started to happen before it just stopped working.  I ended up returning the 633 just because the return period was pretty short and I couldn't be sure there weren't more issues about to surface.

 

That's good to know about DC offset, the preamp did seem to sound fine otherwise with the HPF engaged.  Does the presence of DC offset indicate something more sinister with the preamp?  I don't believe I have ever encountered this before.

 

Thanks for the responses, this was a weird one and I appreciate the help!

[The Documentary Sound Guy]

I'd imagine the level staying the same is largely because the DC offset is being dumped into the signal downstream of the trim pot.

Some component in the input chain has failed, and is caused in a voltage offset in the input.  Unless you have other reason to blame water damage, there's no way to know what caused that component to fail ... heat, high voltage, temperature swings ... electronic parts fail for all sorts of reasons, and you'll probably never know the exact reason.

Someone handy with electronics could probably fix it for less than Sound Devices wants, but then you'd be relying on a third party and you lose the 'factory certified' certainty.  At this point, the 633 is well out of warranty anyway.  Personally, I'd have no qualms giving it to someone who knows electronics, but it's up to you how you want to handle it.