120Hz hum in my amp, and here is a scope picture.

Hextejas · 25th March 2019, 04:38 AM

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

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

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

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!

25th March 2019, 04:44 AM	#2
gabdx diyAudio Member Join Date: Jan 2008	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. __________________ amplificateurslegrand.ca

25th March 2019, 05:29 AM	#3
1audio diyAudio Member Join Date: Mar 2004 Location: SF Bay Area	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. __________________ Demian Martin Product Design Services

25th March 2019, 11:21 AM	#7
JonSnell Electronic diyAudio Member Join Date: Dec 2012 Location: The Jurassic Coast, England. GB	Probably the Y amplifier of your scope, has severe distortion in it to cause that. __________________ Support for Fender, Marshall and all Valve Equipment; Audio Innovations, Audiorama FU29, Quad and Leak. www.jonsnell.co.uk

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25th March 2019, 10:53 AM	#4
Hextejas diyAudio Member Join Date: Jan 2018	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 ?

25th March 2019, 12:00 PM	#8
DF96 R.I.P. Join Date: May 2007	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.

25th March 2019, 01:23 PM	#9
jazbo8 diyAudio Moderator Emeritus Join Date: Jan 2011 Location: In Transient	Threads merged.

25th March 2019, 03:35 PM	#10
BSST diyAudio Member Join Date: Jul 2018	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!