[TenTec] Running a Century 21 on external 12V power

[Jim Brown]

On Fri,6/2/2017 7:41 AM, Dukes HiFi wrote:
>> On Jun 2, 2017, at 2:09 AM, Jim Brown <k9yc at audiosystemsgroup.com> wrote:
>>
>> Electrical codes (like NEC) are written by a bunch of EXCELLENT electrical engineers, and are based on solid engineering. Codes are written to take a VERY wide variety of conditions into account -- everything from lightning protection, electrical safety, to the interconnection of all sorts of low voltage systems (like home entertainment systems, telephone, CATV, alarm and security systems, etc.). Anyone who thinks they're wrong needs to learn some more engineering, or to learn more about those multiple systems types. And anyone who thinks there is a difference between theory and practice needs to learn more about one or the other.
>
> Codes regarding my bathroom project were also written by a bunch of EXCELLENT engineers. These codes forbid me from building my bathroom without a sit-down tub as opposed to what I want, a large standup shower… Codes are well-meaning, but, as you yourself state, are the result of trying to address every possible situation. My suggestion of TEMPORARILY opening the ground on the C21 to determine if this affected hum was intended as a diagnostic. Had this corrected the problem, I would have suggested such possible things as an isolation transformer (fully legal and safe) as a permanent corrective action.

Isolation transformers are NOT solutions if legally used. By code, 
neutral must be bonded to the green wire on the secondary of the 
transformer, and that must be the same ground as on the primary. So 
ground goes right past the transformer.  And if it does not, the system 
is unsafe, because there's no reliable return path to blow a fuse or 
breaker.

> As far as ground loops versus what you call “bonding” goes, the end result is the same. Improper grounding in electronics does cause hum. It is ALWAY 60 Hz (in USA and Canada, 50 Hz elsewhere) and no amount of added filter capacitance in a power supply will reduce it.

You are confusing HUM (pure 60 Hz) with BUZZ (triplen harmonics of 60 
Hz). Most HUM is magnetically coupled -- yes, loops are involved. 
Virtually all BUZZ is the result of leakage currents on the AC line. 
Those harmonics are capacity coupled from phase (hot) to ground (green 
wire) and create an IR drop on the green wire, raising the chassis of 
each piece of gear above "ground." The currents, and the lengths of the 
green wire, are different for each piece of equipment.  It is the 
DIFFERENCE between the interconnected chassis that causes that noise 
current  (BUZZ) to flow on the shield of the audio (and video) cable, 
and because that signal wiring is unbalanced, it's added to the signal.

There is another important mechanism, known as "the Pin One Problem," 
whereby cable shields are NOT properly bonded to the chassis where they 
enter equipment, but instead go to a connector mounted on a circuit 
board, then wander around the "ground" bus until they eventually find 
the chassis. And in that wandering, create IR drops that are injected in 
the signal flow. This mechanism couples all current, both audio and RF, 
flowing on cable shields. This is why ferrite chokes work on signal 
wiring to kill RFI.

> Now that it appears that the actual problem might be magnetic coupling from the transformer to circuit wiring (i said MAY be, it’s not a done deal yet), the cure is simple, magnetic shielding. I assume you believe in magnetic shielding… No amount of ground alteration will correct for magnetic coupling - different origin, different cure.

There are many solutions for magnetic coupling.  Magnetic coupling is 
proportional to the area of the loop coupling the signal/noise. 
Minimizing the loop area is an excellent (and well known) means of 
minimizing magnetic coupling.  The use of twisted pair is well known to 
minimize magnetic coupling.

> I am fully aware of double insulation and the relaxation of the ground requirement. Again, the REASON these audio products use double insulation is exactly hum elimination by elimination of ground loops and the associated hum thereto.
>
> Call it what you like, bonding or ground loop elimination, the outcome is the same, hum reduction.
>
> My cable isolators are Jensen…
>
> Without them, I get very strong hum in my A/V system. With them, the hum goes away.
>
> 73
>
> Gary
>
> W0DVN
>
> PS: You have strong knowledge of codes and reasons therefor. I have personal experience in hum elimination spanning 40+ years. I am not an electrical engineer and I do not know everything. I am an applied physicist with good observational skills and may use an incorrect term to describe what I see (from an EE perspective), but, as I said, the outcome is the same. I use diagnostic methods to determine root causes and then try various means to correct problems safely and permanently (such as legal and safe isolation devices and magnetic shielding where appropriate). Operation of a radio with the ground pin disconnected for 5 minutes as a diagnostic is not going start a fire or harm anyone, nor will it result in lightening damage, unless done during a lightening storm. Every principle that a applies to hum elimination in high end audio equipment applies to ham radio equipment as well, since the causes are exactly the same, regardless of what name you give them.

Gary,

I graduated from EE in 1964, and I've been trying to continue learning 
something new every day. Some about EE, some about other "stuff," some 
about "life," including who's screwing who, how, and for how much. Much 
of what I've learned is the result of not thinking I know it all, but 
instead working in tech committees with other smart engineers, and 
always trying to keep my mind open for new "stuff."

Most of what I now know about hum, buzz, RFI, I learned  by working with 
some really smart guys (including Bill Whitlock, Neil Muncy, guys from 
ABC-TV, BBC, and some very good audio mfrs) on the EMC Working Group of 
the Standards Committee of the Audio Engineering Society, and I started 
learning that stuff around 2002. And what I learned was nothing more 
than the fundamentals we'd all learned in school, but forgotten or moved 
to the back of our minds.

The tutorials on my website all include development of whatever the 
topic by going back to the fundamentals. A guy with your background 
should appreciate them, and hopefully learn from them. I knew NOTHING 
about ferrites until I started extensive research around 2002 to address 
RFI to audio systems. That research was published as peer-reviewed 
technical papers over the next four years. They're on my website.

When I retired, I continued that work, applied to ham radio.

73, Jim K9YC