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

[Dukes HiFi]

Sorry Jim, ground loops are real. That is why many high end audio components are sold with IEC cords that do NOT have the grounds connected. In fact, there is a version of the “standard” IEC three prong chassis connector that has no ground post at all.


I have three Paragon Audio examples in my own living room, two rear channel powered surround speakers and a very nice center front channel powered speaker. They came from the factory that way and are NOT illegal.


Ground loops DO cause hum. That is the sole reason for what is known as “Star grounding” in sophisticated electronic components. Every competent electronic designer employs “Star grounding” for just the purpose of eliminating ground loops, especially in products that involve audio.

I personally corrected the design of a very costly high voltage regulated 60 Hz AC power supply from a prominent vendor because I identified that they had depended upon a PAINTED hole as the electrical connection for their well-intended star ground. They forgot that paint is an insulator and this “AC” power supply suddenly became immensely polarity dependent as a result. An AC supply should NEVER be polarity dependent!

A prominent high end audio equipment manufacturer, Audio Research, made the same mistake in their D115 vacuum tube power amplifier, only it this case, the hole they were using for “Star” was anodized. Different cause, same result, hum.

This is also the reason why several companies sell a useful device (of which I own two) that isolates the cable company’s coax shield from electrical ground - these DO eliminate ground loop hum that arises from the multiple grounds that are involved with a massive cable system and a home audio system.

This is not theory or legal mumbo jumbo written by lawyers, this is personal experience with hum elimination and some very solid science.

With all due respect,

Gary

W0DVN


PS: I did not say eliminating the ground pin on the C21 WOULD resolve the hum, only that it COULD solve the problem. It seems that it did not so there must be another path to ground or another mechanism of 60 Hz coupling that is the offending root cause of the hum in the C21 In question. The fact that the gentleman was able to worsen the hum by moving his hand near some capacitor suggests that there is a high impedance “antenna” for 60 Hz in this radio, and a source of significant 60 AC energy within. I say this because operating this radio with the ground pin removed is the electrical equivalent of operating the radio from an external 12 volt DC supply ELECTRICALLY. Something else is coupling 60 Hz AC into the audio line of the radio. There is NO Way that he could affect power supply 120 Hz ripple by moving his hands around some capacitors inside the radio. He is only affecting coupling of 60 Hz energy into the audio line, not power supply regulation. 

It is even possible that the hum coupling is magnetic, coupling from the magnetic transformer core, which is inside the C21 cabinet,  directly to the audio circuit, and thus the use of an eternal supply would most likely eliminate this. This was the exact reason why Counterpoint Audio chose to put the power transformer for their esteemed SA-3000 preamplifier in a separate case from the main electronics cabinet. Even though Counterpoint still sent the AC power from the transformer into the preamplifier cabinet to be rectified, filtered and regulated there, the magnetic coupling aspect of hum generation (by the transformer) was totally eliminated by 6 feet of separation between the transformer and the electronics.It may well be that the C21 audio circuit has so much gain (for the dual direct conversion design to work) and such high impedance, combined with the possibility that the transformer has so much magnetic leakage that this presents itself as the hum that is reported. A simple piece of Nickel foil between the transformer and the rest of the radio would fix this.

I still contend that there is no reason why operation of a C21 should require an external supply. I assume Ten Tec felt that the amount of coupling was acceptable for most users and that headphones would only be used with an attenuator.



> On Jun 1, 2017, at 10:42 PM, Jim Brown <k9yc at audiosystemsgroup.com> wrote:
> 
> On Thu,6/1/2017 8:02 PM, Dukes HiFi wrote:
>> The internal supply is pretty good. If it is introducing hum of the ground loop type, why not just lift the ground (green wire). That should eliminate ground loops.
> 
> NO, NO, NO. This both illegal and unsafe. NEVER do this.
> 
> Ground loops are a myth.
> 
> 73, Jim K9YC
> 
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