Marty Michener wrote:
> At 03:25 AM 10/19/02 -0400, you wrote:
>
>>Spent another frustrating evening trying to get the whine out of the
>>pairing. It took considerable time to make up a connecting cable where
>>each line was in it's own separate braid shield. And, unfortunately,
>>while it subdued most of the unwanted oscillation, some remains.
>>
>>Walt
>>
>
>
> Walt:
>
> I have not been following absolutely everything you have said about this
> whine problem, but it does smell more and more of unwanted RF, or at least
> ultra-sonic frequencies interacting. Some of my early (1953?) tube
> contraptions would never work because I had pentodes oscillating at RF, or
> whatever . . .
>
> Have you tried simple RF-chokes in series at one or both cable ends? They
> should not affect audio freqs.
>
> Maybe this is a totally red herring, or however that should be said . . .
> ;^) (I think its probably funnier wrong.)
Well, it can't be killed by hanging chokes and caps on everything, nor
by running each line through a ferrite connector. I've pretty much
exhausted that route.
I have looked with a oscilloscope, while they are not the ideal for
looking at mics due to the low signal levels, I have found that a small
trace of the 8 mhz RF from the mic is everywhere. If I was getting a
single frequency then I'd think it was a interaction of the crystal
controlled frequency being slightly different. But this is evenly spaced
bands all over the entire audio spectrum. According to the oscilloscope
it's only in the audio frequencies too. Nothing at all from 20 khz up to
8 mhz.
It would be hard to remove the 8mhz, one side of the capsule is
connected to the shield. They do have internal chokes at two points, and
of course the RF circuit and mic have transformers that are tuneable.
I have also Built a special connecting cable for each mic that has every
conductor inside it's own shield. That did reduce the oscillation quite
a bit but not eliminate it.
I've also tried shielding the two power supplies from each other,
shielding the components of the power supply from each other. Those have
no effect.
The stuff I've done should have at least changed the tune if it were a
RF problem. So I'm leaning away from that.
The oscillation requires two mics, disconnect one and the other will run
perfectly. I personally believe the output amp in each mic combine in
some way to form a multivibrator. Just need to figure some way to get
the feedback loop either cut or change the parameters enough to make it
latch. The problem is the shield is in common everywhere, and hard to
break that up. Though disconnecting the shield at the power supply end
does not stop it.
The power supply diagram from Sennheiser is here:
http://frogrecordist.home.mindspring.com/images/Wiring-110.jpg
I could not come up with 8 volt zenars and left them out. Power is from
9 volt NIMH, which actually are closer to 10 volts at full charge. I
used trimpots for the resistor and set the voltage to the mic with a
voltmeter. I did not build the "audio compensation" supply that they
show somehow connected into the signal line. Bringing the signal out
through 4.7 mf caps, currently both lines have caps. Cap in the power
supply is a 470mf, which seems to be what sennheiser used in all it's
power supplies of the period. Their multi mic power supplies just use a
resistor and cap setup like mine, nothing special.
I do have extras of this mic, and two mics do the same.
Walt
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