Why do some amplifiers develop a buzz at the RCA input jacks when you unplug and plug in right and left channels individually. It’s 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
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
Oh my goodness. I’ve seen people do crazy stuff in the backs of their cars a trucks. Blows me away sometimes. That’s another story.
I don’t 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
I don’t 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
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?
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?
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...).
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...).
Any type of amplifer, preamp, will show hum when its inputs are disconnected because now the inputs have no source loading on them - they're "wide open".
To assume that an amp should be dead "silent" with wide open inputs is false thinking.
And bench testing of an amp with an AC powered supply, unless the supply is regulated and sufficiently filtered, will naturally be a source of background hum.
How do I know these things? - after formal electronics training and 45 years in the service business, I think I ought to know.
To assume that an amp should be dead "silent" with wide open inputs is false thinking.
And bench testing of an amp with an AC powered supply, unless the supply is regulated and sufficiently filtered, will naturally be a source of background hum.
How do I know these things? - after formal electronics training and 45 years in the service business, I think I ought to know.
Well. I am a little curious as to why some amps seem to be affected by this hum and some aren’t. It’s only every so often that I get an amp with this low volume hum with both RCAs plugged in on the work bench. It’s very low volume, but it’s there. Most amps don’t seem to mind the setup that I am currently using. It could be two amps from the same manufacturer identical in every way and one will exhibit this issue and one won’t. It is curious to me. That’s why I assumed it was a problem with the amp’s input.
Thanks to all for helping me understand this issue and sharing your knowledge. I really appreciate it. There is nothing more time consuming that chasing a problem that isn’t there.
David
Thanks to all for helping me understand this issue and sharing your knowledge. I really appreciate it. There is nothing more time consuming that chasing a problem that isn’t there.
David
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