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Input impedance vs hum/buzz
Input impedance vs hum/buzz
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Old 14th October 2021, 02:39 AM   #21
jxdking is offline jxdking  China
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Quote:
Originally Posted by stephe View Post
They probably have Hot and neutral reversed. The narrow slot to the ground pin should read 120V.
I tested, and the polarity is correct. Based on computer's UPS indicates, the ground wire has voltage present. My AC test pen also confirms that. I think the ground is just open, because I regularly touch my computer case and never got shock.
Even though, it is not related to my hum problem. My receiver and the st-70 are all 2 prong plug.
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Old 14th October 2021, 02:43 AM   #22
stephe is offline stephe  United States
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Quote:
Originally Posted by jxdking View Post
You probably are correct. Probably I will change it to 3 prong cord.
Side story, I don’t trust the ground coming out of my house wall, which is built in 1960s. My computer’s UPS reports “Faulty Wiring” on those outlets.
Quote:
Originally Posted by jxdking View Post
I tested, and the polarity is correct. Based on computer's UPS indicates, the ground wire has voltage present. My AC test pen also confirms that. I think the ground is just open, because I regularly touch my computer case and never got shock.
Even though, it is not related to my hum problem. My receiver and the st-70 are all 2 prong plug.
That sounds safe... How did you determine the "polarity" is correct with no ground reference at the outlet?
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Old 14th October 2021, 02:51 AM   #23
jxdking is offline jxdking  China
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I did some test with different audio source with my dynaco st-70.
1. Costco Yamaha 7.1 audio receiver, HUM
2. Laptop with AC adapter plugged in to the wall, NO HUM.
3. Bluetooth receiver with USB power adapter plugged in to the wall, NO HUM.

Here is my observation.
When it hums, it is somehow ground looped without grounding any of my device. All of my devices used are just 2 prong plug. How? Here is my hypothesis. The big power transformer like the one in dynaco st-70 has some stray capacitance across primary and secondary. Thus, they can ground loop through the neutral and live rail in side the wall.
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Old 14th October 2021, 02:55 AM   #24
rayma is offline rayma  United States
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Measure the stray AC voltage between the ST70 chassis (with no input) and the AC line safety ground.
If you measure any AC voltage, try reversing the ST70 AC plug in the wall socket and check again.
The leakage can be asymmetrical.
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Old 14th October 2021, 03:33 AM   #25
stephe is offline stephe  United States
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The neutral is connected to earth regardless of whether the cord also has a safety ground or not.
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Old 14th October 2021, 03:41 AM   #26
jxdking is offline jxdking  China
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Originally Posted by stephe View Post
That sounds safe... How did you determine the "polarity" is correct with no ground reference at the outlet?
I stick my ac tester pen in the longer slot from the wall socket. It does not light up. It will light up in the shorter slot and the ground hole.
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Old 14th October 2021, 10:20 AM   #27
Depanatoru is offline Depanatoru  Romania
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It doesn't have to be a ground loop ... just bad grounding in that Yamaha .
If the metal case , which is probably touching the output RCA ground , is not connected properly it will hum ... probably is good enough for other amplifiers but for not for ST70

Last edited by Depanatoru; 14th October 2021 at 10:23 AM.
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Old 15th October 2021, 12:06 AM   #28
Chris Hornbeck is offline Chris Hornbeck  United States
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Originally Posted by jxdking View Post
I stick my ac tester pen in the longer slot from the wall socket. It does not light up. It will light up in the shorter slot and the ground hole.

This sounds like the wall socket is incorrectly wired. Find and open the breaker for that outlet and observe the socket wiring by colors. If you can't see the error, it may be at another socket (typically in the same room, but nearby). Do not use this socket until the error is corrected.


Generally, you can trust the $10 plug-in three-light testers to tell you the truth, so one is a great investment.


Be safe, and all good fortune,
Chris
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Old 15th October 2021, 12:09 AM   #29
rayma is offline rayma  United States
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Originally Posted by jxdking View Post
It will light up in the shorter slot and the ground hole.
Very bad and dangerous. You need an electrician.
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Old 16th October 2021, 11:02 PM   #30
PRR is offline PRR  United States
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Input impedance vs hum/buzz
Quote:
Originally Posted by Chris Hornbeck View Post
...Generally, you can trust the $10 plug-in three-light testers to tell you the truth, so one is a great investment...
They will not detect an N/G swap. Or a bootleg ground.

i.e.: they will tell you an outlet is wrong, but can not prove it is right. ENTIRELY worth the $9. I have several. But if I need to know, I have an expert trace the wiring from the street to the listening room.

John at another forum worked on a smarter tester.
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