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RCA input noise problem
RCA input noise problem
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Old 15th June 2019, 06:21 AM   #1
barber02 is offline barber02
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Default RCA input noise problem

Why do some amplifiers develop a buzz at the RCA input jacks when you unplug and plug in right and left channels individually. Itís loud when one channel is unplugged and barely noticable but still there when both are plugged in. I repaired and amp the other day where the power supply was destroyed when the owner tried to connect an external capacitor to the +12 volt power line.
I had to replace all of the power supply mosfets and some gate resistors and two power supply capacitors.

How do you eliminate this problem? Connect a resistor across the inputs? Send it to ground with a capacitor or resistor? Not sure what to do.

David
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Old 15th June 2019, 06:30 AM   #2
Perry Babin is offline Perry Babin  United States
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Why are they plugging/unplugging the RCAs without switching the amp off?
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Old 15th June 2019, 06:34 AM   #3
wiseoldtech is offline wiseoldtech  United States
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Was the amp operating properly on the repair bench after servicing?
Are all grounds tight and solid?
Any comprimise of grounding can cause hum issues.
Also, plugging things in under power is not wise to do.
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Old 15th June 2019, 06:49 AM   #4
barber02 is offline barber02
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Oh my goodness. Iíve seen people do crazy stuff in the backs of their cars a trucks. Blows me away sometimes. Thatís another story.

I donít remember hearing the hum when I repaired the amp but when it came back a few days later there was a slightly noticeable hum with both RCAs connected and no music playing through it. I touched the cone of my test speaker and I could feel it vibrating. I was wiggling and spinning the RCAs while they were plugged in and the left channel pulled out and the hum was really loud and I plugged it back in and the very low volume hum was back. Then I pulled the right channel to see if both channels were affected and it did the same thing. Very loud hum until both were plugged in, then back to low volume hum.

David
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Old 15th June 2019, 07:06 AM   #5
Perry Babin is offline Perry Babin  United States
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The grounding scheme varies but there is always, at least some, isolation from ground.

Did you try more than one source to see if the problem existed with all sources?

Do those sources work perfectly with all other amps?

Is there noise in the vehicle with both RCAs plugged in?

If you're using a mains powered source (even through a 12v supply), is the signal source ground connected to the ground of the power supply that's powering the amp?
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Old 15th June 2019, 07:20 AM   #6
barber02 is offline barber02
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I tried three different sources. A laptop computer, my cell phone, and my signal generator.
All had the same issue. All sources work great with other amps.

Not sure about the ground. I know they are plugged into different outlets on different walls.

David
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Old 15th June 2019, 07:22 AM   #7
barber02 is offline barber02
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I am using a Pyramid linear power supply.

I didn’t hear it in the car.

David
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Old 15th June 2019, 07:27 AM   #8
Perry Babin is offline Perry Babin  United States
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The same issue? Do you mean all had the same hum with the RCAs fully plugged in?

If there is no problem in the vehicle, is there really a problem that needs to be resolved?

As a side note, the hum is caused by a 60 cycle mains source. It could be source related. It could also be related to close high voltage sources (fluorescent lighting...).
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Old 15th June 2019, 07:49 AM   #9
barber02 is offline barber02
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Yes to the same issue question. They all had the same low volume hum.

I’ll install it in my vehicle tomorrow and check for the hum.
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Old 15th June 2019, 08:06 AM   #10
Perry Babin is offline Perry Babin  United States
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In the vehicle, there probably won't be a 60Hz hum. Engine noise is more likely to be the problem, if there is one.
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