[grc] A bit of trouble...

[UnderCurrents Radio]

Not at all an expert in this, but once solved a problem like this by patching all the gear into a unit supplying clean balanced power.

http://www.sweetwater.com/store/detail/P3600ARG <http://www.sweetwater.com/store/detail/P3600ARG>

Gregg McVicar
www.undercurrentsradio.net <http://www.undercurrentsradio.net/>



> On Nov 16, 2016, at 2:11 PM, al davis via grc <grc at maillist.peak.org> wrote:
> 
> On Sun, 30 Oct 2016 19:33:12 -0700
> Lanny Cotler via grc <grc at maillist.peak.org> wrote:
> 
>> 1. I heard it.  soft voices and music (I clearly understood the
>> voices, recognized them.) And a loud (louder than the voices) buzz.
>> (it did not sound like just AC hum.)
> 
>> 2. when I placed my hand near (a couple of inches) the Amp power
>> cord; the buzz changed.
> 
>> 3. The only remedy I had was to turn off our transmitter.
>>      a. that reduced the buzz to almost not noticeable, and
>>     b. no KLLG signal was heard at all.
> 
>> 4. Later, after the transmitter was off, their sound guy was cleaning
>> up the stage (before the musicians appeared.)  When he bundled all
>> the power cords together, the buzz disappeared.
> 
>> This sounds (to me & the sound guy) like a bad ground.
> 
> 
> Wiring, grounding, bad equipment design ..  could be any or all of
> that.  It could be any of the equipment in the place, including
> something that doesn't seem to be bothered.
> 
> The common wisdom of grounding one end of a shield to avoid ground
> loops isn't always so wise.  At low frequencies (60 Hz) the wavelength
> is long, a ground loop can cause hum because of induced current, so
> break the loop to fix it.  At 100 MHz, the wavelength is 3 meters, so
> shields grounded at one end act as antennas, so ground it everywhere
> and make as many loops as you can.
> 
> It could be the transmitter's grounding.  Poor transmitter ground could
> lead to RF all over, and also it would be prone to lightning damage.
> 
> Some equipment does not have proper RF bypassing on the inputs.  This
> is common on consumer stuff, even more common on niche market gadgets.
> Even some equipment marketed for broadcast does not have RF bypassing
> on the inputs.  Look at the schematic.
> 
> The very common one-op-amp "differential" input stage, with no
> bypassing, is especially prone to this problem.  The variant with a
> gain trim that shunts the input (with no bypass) is really asking for
> trouble.
> 
> 5532 op-amps are more prone to RF problems than the FET opamps like
> LF353 or TL072.  If the input stage is a 5532, you may be able to fix
> the RF problem by changing the opamp to a LF353.
> 
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