Topic: BIZARRE PROBLEM! HELP!!

[SmokinJoe]

I don't know that this is your problem, but I'll throw this out as brainstorming possibilities... 

I have an ST-9100 and an R-09.  I've noticed that there is an RF type interference when the two items are close together...  i.e. I don't put them in the same pocket! 

Seriously, plug in them in with a patch cord, no mics, turn them on like you were recording, and put them face to face.  Then move them apart a few inches and back together, you can get a little bit of noise when they are together.  It's fairly steady, kind of like a 60 cycle hum, but higher pitched than that.  You know all those electronic devices which say "complies with FCC part 15 subpart B"?  That's what this is all about... some electronics generate noise that bother other electronics.

That said, the noise isn't very loud, in my case, down around -30 or -40 db, but if you superimpose it on your audio signal, it might create this kind of thing.  And to rationalize the fact that it comes and goes, I would guess that as you move around in your seat a little, the items move and aren't quite in the same face to face location... they move around a little.

Again... just food for thought...

Seperately, with slow writing to flash cards... won't that show up as skips, rather than noise?

[jmz93]

I don't know that this is your problem, but I'll throw this out as brainstorming possibilities... 

I have an ST-9100 and an R-09.  I've noticed that there is an RF type interference when the two items are close together...  i.e. I don't put them in the same pocket! 

I've noticed that too. But, the R9 and preamp were in separate jeans pockets.

But yes, the R9 does pick up RF interfearance fairly easily.  Either that, or it's the R9's processor generating noise, that is picked up by the ST9100 and fed back to the R9.

Still messing around with everything - will let you guys know what I discover.

[jmz93]

Still messing around with everything - will let you guys know what I discover.

Update! Problem solved, but the mystery continues.

1. Ran several recording tests with the R9's internal mics, and with a Sony ECM-907 ... no problem, no clipping or wird digital distortion,
including when running at 24/48. So, it's not the recorder, and the line in and mic in jacks are both still solid.

2.  Attached AT853's, ST9100 preamp, Nickspicks silver cable with right-angle Neutrik connectors between preamp and R9. 
Go into REC/Pause, monitoring through headphones.  I hear whisper-quiet ambiant noise from the room,
computer fan noise off to one side, etc. nice stereo spread. I start talking quietly and approaching the mics.  Once I get
right on top of them, a louder word from me sends them right into that weird digital distortion.  I wait, and oddly, the ambiant room noise is
no longer spread out in stereo, but sounds more mono,and is quietly pulsing/wavering. Oddly, any noise louder than
a whisper is all distorted.  Not just loud plausives, but everything. 

3.  I touch the mics slightly, and once my hand contacts them, bumping them a little, the digi-noise goes away. Nice ambient soundfield again.

4.  I turn the preamp down, the R9 down a bit, etc. Still, although the overall level being recorded is down,
once I hit the mics with a loud sound they switch from behaving to crazy distorted.  Wait a few seconds, tap the mics with my hand
and noise goes away..  Repeat test over and over again baffled.

5. Fresh battery in preamp, same results as before.

6.  Although the cable from Nick looks sound, conducts a stereo signal just fine or appears to anyway ...
I decide to swap it for an old Radioshack 6' 1/8"-1/8" chord. Cheap construction and straight ends.

Repeat test and ... amazingly, I can stick one of the mics right in my mouth
and yell, and nothing happens! Things clip a little, but it's regular-sounding
brickwalling, not the weird digi-noise of before.  it's just regular clipping, the result of things being too high for the
hot level being captured by the mics.  Whatever I do, turning various things up far too high, jostling the mics around, etc.
i cannot get them to do the odd things they were doing when Nick's silver cable was in line. 

so, could this be some sort of grounding issue, introduced by the more expensive cable?  Like I said, it
functions, it conducts a signal... but it acts up with loud signals, then behaves once I physically touch the mics. 

I'll use the cheap Ratshack cable for now, and things should work fine!

Glad to have narrowed it down to one component, but I'm still at a loss as to
why things go haywire with what appears to b a much better cable, and why a sound source beond a certain threshold sends some part of the chain
into weird digital distortion or ossilation.

[Professor chaos]

an ounce of prevetion is worth a pound of cure. the input jack is flaky, its a matter of time before it goes. i've seen the mods done by schoepsnbox and they are worth it. you'll never have a problem w/ that jack. also, i'd get a cable with a right angle plug.

-Mike W.

[guysonic]

One possibility not yet mentioned is the external mic preamplifier itself becoming unstable (oscillating at very high frequency) with different output cable capacitance/inductance loading of the pre's output amplifier. 

Hitting the preamp with a fast strong signal is usually a good way to make an amplifier circuit show instability modes (ringing/oscillation).  This seems what is occurring with mic signals that suddenly get stronger, maybe causing the preamp to go unstable.

This can creat a very large inaudible signal with chance of slamming deck inputs as AC/DC overload.

No easy way to test for this as usually not audible, being at >20,000Hz high frequency, short of attaching an oscilloscope or some other (non-interfering) way to see the pre's output signal while connected to a particularly troublesome interconnection load. 

Another quick check is to solder an in-series 50-to-100 ohm resister to each amplifier output that 99% of the time solves an instability problem with little other affect on the preamp's usual performance.  If the problem goes away, then amp instability gone with fix already in-place. 

Otherwise, if still having a problem, you can leave resisters in place anyway as these are standard on many amps driving cable, and a good design-in for honoring 'Murphy.'

[SmokinJoe]

What Guy says make sense....  here is my take, which may be the same thing, or something different, I'm not sure.

When an electrical signal travels down a length of wire and hits a high impedence endpoint it has a certain amount of reflectance back up the wire... and possibly the signal builds on top of itself in a resonant fashion.  On some types of communication cables we put resisters at the end to absorb that energy.   As a ham radio operator I vaguely remember that you cut coax to certain lengths, which are typically an even multiple of the appropriate wavelength, not just "this ought to be long enough to reach".  Ever hear the the min length of a digital coax patch cable is 1/2 meter?  I suspect this is a similar situation.

Basically this is the electrical equivalent of "my car shimmies at about xxx mph... less than that OK, more than that OK".  At just the right frequency everything builds.  Kind of like that famous bridge (Tacoma Narrows???) which swings and rolls and shook itself apart... a little bit of energy builds in a resonant fashion and gets out of hand.

Like the bridge, it's not an easy thing to predict or calculate.  99% of the time it's not a problem.  Maybe the engineers at some of the big cable manufacturers have a simulation which helps them avoid these resonances, but when one of us makes a cable "Oh, I want it just long enough to go from this pocket to that pocket, with RA connectors", etc., there is that 1% chance you will get that situation. It doesn't mean Nick makes crap cables, or that Chris makes bad preamps, it's just that unfortunately once in a while we plug combinations together and get things that no one could predict.

Bottom line: Shit happens.