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Buzz / Hum recording on H120 - plug in power issue?

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davehouse:
Hi,

I recently aquired a new 2nd hand H120 having fried my other one (doh!) I rockboxed the new one and am using the build offered in the last page of this thread: http://forums.rockbox.org/index.php?topic=2513.0

Now, something strange is happening to my recordings that wasn't happening before. I am getting a short, intermittent, bassy buzz/hum ranging from about .5 seconds to 3 seconds that shows up on the peakmeter, through headphones when monitoring and in the recording itself. It is only apparent at high gain but most of my recordings require a high gain so decreasing gain isnt an option.

I've done some experimenting with various mics and realised something strange. If I use my Sony ECM-DS70P, no buzz. If I use my Soundman OKM Binaural mics with their little amp, buzz. If I use the OKMs without the amp, plugged directly into Line In, no buzz. If I monitor the recording screen with no mic at all but Source set to Line In, buzz. This is the case for every combination of recording settings - format, bit rate, clipping light on/off, etc etc. And using the iRiver firmware - no buzz under any circumstances as far as I can tell.

This led me to believe that perhaps there's an issue with the plug in power supply? The Sony and the amp-less OKMs draw on the H120's plug in power and I get no buzz. As far as I know, the OKM amp 'bypasses' the plug in power supply and provides its own voltage, and this way I get the buzz. And if there's no mic at all I get the buzz, too. So it seems like if the plug in power supply isn't 'going anywhere' then the buzz is present, but if a mic makes use of it, all is good.

Any ideas what might be causing it? Like I say I had no such issues with my old H120. I'd assume it was a hardware issue but the fact that the iRiver firmware works fine suggests otherwise.

Many thanks,

Dave

AlexP:
Firstly, that build is both well out of date, and a custom build.  Before asking for support we ask you to test with a clean, current build.  Please do so.  As for plug power supply - I believe the only difference between line in and mic in is that one of them (mic) supplies the power.  If you are using a self-powered mic, then line in might be better.

So, current build please, and try that out.

davehouse:
Hi, thanks for the reply.

I tried the current H120 build and the buzz seems to have dissapeared. I am currently only able to test with no mic plugged in, monitoring the line in source. Before, however, this produced the buzzing. I am getting a high pitched ring now, though.

I've uploaded 2 examples below. Both are recordings of line in with no mic input but are representitive of the issue explained above.

http://www.carbonbaseddesign.co.uk/abc/R_LINE_0001.wav - Using current build, high pitched ringing

http://www.carbonbaseddesign.co.uk/abc/rec_0001.mp3 - Using REP build above, buzz and high pitched ringing

Re. using Line In or Mic source, the only option I have is Line In other than build in mic...

dreamlayers:
This is analog noise.  It's extremely unlikely that Rockbox is corrupting digital data in this way.

The electrical noise is probably coming from variations in power usage by other parts of the H120, such as hard drive activity or PWM backlight fading.  Check if the noises are correlated with these things.

petur:
could it not also be from a bad ground connection somewhere (jack, cable,...) ? I admit I haven't yet listed to the soundfile (I'm at work atm)

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