'If you believe - no proof is necessary; if you don't believe- no proof is
possible"! So it is with many discussions on this reflector!
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Tuesday, January 28, 2014 10:28 PM
To: jim@audiosystemsgroup.com; topband@contesting.com
Subject: Re: Topband: Chassis Bonding
Jim,
You are giving terrible advice.
I'm not going to get into a prolonged debate about life in the real consumer
world, but the hard cold fact is no one is going to run a huge copper buss
across a room just to meet some unreasonable fixation with eliminating a $2
transformer.
> How does a proper 6 ft long #10 copper bond strangely "develop"
> resistance?
1.) Equipment is not always 6ft length from something else
2.) #10, ignoring connection imperfections, has about .01 ohms per 10 ft.
That's up to 0.2volts with 20 amps of power supply loop current.
3.) From vast manufacturing experience and dealing with customers, good luck
on getting the consumer world to do that properly 100% of the time.
> A ham station is not a broadcast station. In a BC station, equipment is
> widely separated, interconnects are BALANCED, the signal to noise ratios
> required differ by at least 40 dB. In a ham station, interconnected
> equipment is quite close together, chassis bonding can be 6 ft or less.
Sorry, that just isn't true. We often had to run unbalanced equipment into
audio consoles. With that equipment, we floated the shield at the consoles
with isolation transformers. Virtually every studio had some unbalanced
lines.
Also, almost every Ham radio manufacturer in the world isolates the low
level audio shields inside radios except at one point because they are well
aware of issues with the large currents in 12V solid state high power
systems.
>> 20A DC, and, as we discussed several years ago, 20A DC modulated if it's
> SSB, but NOT 20A of hum or buzz.
>
You better take some time and put a scope on a power supply lead with a SSB
transmitter. While the supply outputs DC, the power supply loading by the
radio is a time-varying resistance that varies between 5-10 ohms at zero
audio crossings to about .5 ohms (for a 100W radio) at audio peak. If you
couple into the PS line with an audio amp, it sounds almost exactly like a
SSB signal with no BFO except a bit bassier.
>> If the bond lead has .01 ohms resistance, there is the potential for over
>> .2 volts hum or noise.
>
> The source of the hum and buzz is LEAKAGE current, not power supply
> current. Leakage current rarely exceeds 10-20 mA unless some piece of
> gear has developed a fault.
>
You are considering power line AC to the safety ground in commercial
equipment. There are lots of cases of Ham gear where people ground things to
cabinets, as wrong as that is, but the major unfixable issues are utility
company ground loops and power supply currents.
Ham stations have grounds, and those grounds cannot be bonded perfectly to
the mains, nor will many people bond them at all. This is just the real
world we live in. The power mains try to use the Ham gear grounds as a
ground. If I read the neutral to ground bond at my house, I typically have 1
amp or more showing from the mains ground (the primary return is always
bonded to the transformer secondary center tap at the pole) trying to ground
to my tower grounds and radials. The voltage driving that current isn't
high, it is caused by the neutral voltage drop back to the substation, but
it is a real voltage and current that exists.
Besides that, we have power supply negative lead currents that are also NOT
DC, because the leads have a time-varying load resistance.
It's an imperfect world, and a simple $1-$2 isolation transformer fixes it
all so far as audio line ingress is concerned.
Telling people to not use transformers and to bond things instead, will
eventually cause nothing but long term grief. I can assure you no
manufacturer in their right mind would listen to that advice. ANY good
interface by almost any manufacturer has shield isolation, with a chassis
bond only at one point. If they don't do that, the help lines get busy with
complaints.
73 Tom
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