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120Hz hum in my amp, and here is a scope picture.
120Hz hum in my amp, and here is a scope picture.
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Old 25th March 2019, 04:38 AM   #1
Hextejas is offline Hextejas
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Default 120Hz hum in my amp, and here is a scope picture.

I am trying to find the source of the 120Hz buzz coming out of my speakers. At least I think it is 120. Anyhow I disconnected all input to the amp and connected the end of the speaker cable to one of the inputs to the scope. The other input to the scope has a 120Hz from a signal generator. Be aware that I am a total newbie to this and I don't really know what I am seeing on the scope from the amp. I expected a much cleaner picture.
So, am I going about this in the correct manner ?
If not, which is likely, please guide me.
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Old 25th March 2019, 04:44 AM   #2
gabdx is offline gabdx  Canada
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It's easy.

60hz is your AC. This can pollute your ground and your amplifier circuit.

120hz is a byproduct of rectification. Rectification is turning the 60hz into a 0hz signal, pure + and ground. or - and ground.

So this comes from the amplifier power supply or a power supply nearby.
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Old 25th March 2019, 05:29 AM   #3
1audio is offline 1audio  United States
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120Hz hum in my amp, and here is a scope picture.
It looks as though either the input of the amp is open or the amp is oscillating. You must short the input to get a meaningful reading of the hum.
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Old 25th March 2019, 10:53 AM   #4
Hextejas is offline Hextejas
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Thank you audio, but being the newbie that I am, I am not clear exactly how to do this. I have had it suggested to me before and it had me scratching my head. I have 2 posts dealing with the signal in.
They are:
1) SIG IN
And
2) SIG GND
Am I to run a jumper between these 2 or run a jumper from SIG IN and the chassis ?
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Old 25th March 2019, 10:56 AM   #5
Hextejas is offline Hextejas
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Quote:
Originally Posted by gabdx View Post
It's easy.

60hz is your AC. This can pollute your ground and your amplifier circuit.

120hz is a byproduct of rectification. Rectification is turning the 60hz into a 0hz signal, pure + and ground. or - and ground.

So this comes from the amplifier power supply or a power supply nearby.
Thank you gabdx. Can a nearby power supply induce a hum into my circuit ?
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Old 25th March 2019, 11:19 AM   #6
Hextejas is offline Hextejas
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Default Which piece of equipment is doing this ?

The picture shows a 1kHz test tone, except part of it is missing. If I fool around with the scope dials, I can see the missing segment of the curve about 2 inches above the rest of the curve.
Is this a problem with the scope or the signal generator.
Both devices were bought 2nd hand at swap meets.
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Old 25th March 2019, 11:21 AM   #7
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Probably the Y amplifier of your scope, has severe distortion in it to cause that.
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Old 25th March 2019, 12:00 PM   #8
DF96 is offline DF96  England
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Every voltage is between two points. The scope displays the voltage between sig in and sig gnd. If you want to measure 'a voltage' (although there is no such thing) then you need to ensure that the sig gnd is connected to whatever is the reference point for 'the voltage'. This may be signal ground in the circuit, or sometimes the chassis.
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Old 25th March 2019, 01:23 PM   #9
jazbo8 is offline jazbo8
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Old 25th March 2019, 03:35 PM   #10
BSST is offline BSST  United States
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Hi Hextejas,

I'll be a bit more specific in technique: I suggest acquiring a RCA phone plug and soldering a short between center pin and ground sleeve. That represents the best possible source--- 0 impedance, no noise, no ground loops.

I also suggest connecting a speaker to the amp. Then you'll be able to hear if the simple act of connecting your scope with its attendant safety ground introduces added hum. A last tip: your scope likely has a Line Trigger option that will ensure the scope sweep is synchronized to the power line.

Good luck!
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