Re: "hearbeatings" in my schoeps




On 10/9/09 6:26 AM, in article
0001HW.C6F4B2350040826AF0284530@xxxxxxxxxxxxxxxxxxx, "Ty Ford"
<tyreeford@xxxxxxxxxxx> wrote:

On Fri, 9 Oct 2009 04:02:17 -0400, yepthisismyemail@xxxxxxxxx wrote
(in article
<793ff9b4-5677-4a70-8e9f-4446ad128099@xxxxxxxxxxxxxxxxxxxxxxxxxxx>):

On Oct 8, 2:17 pm, ben lowry <blo...@xxxxxxx> wrote:
If you were using wireless boom: a Lectro um400a can add a thumping
heartbeat sound as its battery dies, while the on-mic audio sounds
thin.
ben lowry

Just noticed the same thing when the batteries were dieing on my mix
pre with my CMIT 5u plugged in. It took me a minute to figure out and
I was really scared when my KMR 81 worked fine with the Mix Pre. good
thing I plugged the CMIT 5u into the 744t and it worked fine!
ian

I'm thinking I remember motorboating with the MixPre when batteries were down
or when used with a phantom-powered mic if the headphone amp (draw on
batteries) was greater than the converter could handle.

I think using the high-pass filter reduced the motorboating, as did turning
down the headphones.

Not a Schoeps issue, though.

Ah, remember the SD note about motorboating?


http://www.sounddevices.com/notes/mixers/oscillation-xfmr-inputs/

Some of the text from the note:

Microphones with low rejection to common-mode phantom voltages fluctuate
their output level based on the phantom voltage supplied. This variation
³wiggles² the bipolar +/-15 volt audio rails ? which subsequently fluctuates
the phantom voltage rails, which fluctuates the output level and sets up the
oscillation.

Sound Devices field mixers and preamps are very portable products and have
compact, efficient power supplies. The AA batteries in the product supply
several voltages including, ±15 volts for audio, 48 or 15 V for phantom
power, and 5 V for LED¹s. The power supply is suitable for most microphones,
but is susceptible to oscillation with microphones prone to drawing current
based on output level.

Remedy
With microphones prone to this condition, several things can be done to
minimize and/or prevent the condition.
* Engage the high-pass filter.
* Use 48 V phantom even if not required.
* Power the microphone(s) from an external phantom supply.
* Power the mixer from an external, DC power supply.



Jim



.

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