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-   -   120Hz hum in my amp, and here is a scope picture. (https://www.diyaudio.com/forums/equipment-and-tools/335717-120hz-hum-amp-scope-picture.html)

Hextejas 25th March 2019 04:38 AM

120Hz hum in my amp, and here is a scope picture.
 
1 Attachment(s)
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.

gabdx 25th March 2019 04:44 AM

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.

1audio 25th March 2019 05:29 AM

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.

Hextejas 25th March 2019 10:53 AM

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 ?

Hextejas 25th March 2019 10:56 AM

Quote:

Originally Posted by gabdx (https://www.diyaudio.com/forums/equipment-and-tools/335717-120hz-hum-amp-scope-picture-post5740029.html#post5740029)
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 ?

Hextejas 25th March 2019 11:19 AM

Which piece of equipment is doing this ?
 
1 Attachment(s)
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.

JonSnell Electronic 25th March 2019 11:21 AM

Probably the Y amplifier of your scope, has severe distortion in it to cause that.

DF96 25th March 2019 12:00 PM

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.

jazbo8 25th March 2019 01:23 PM

:cop: Threads merged.

BSST 25th March 2019 03:35 PM

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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