I have a little amplifier that works perfectly fine when not in sleep mode(auto on), but when it goes to sleep after roughly 15 seconds of no signal, it emits a pretty loud hum through both channels.

Here is how it looks on the oscope. 120hz hum.



And here are the insides. Pretty simple amp.



Any ideas what could be causing it?

 

Humming in sleep mode, I think that is called snoring.

 

120hz noise, seems like you have a leaking capacitor

 

120hz noise, seems like you have a leaking capacitor

Some of your noises get amplified you have to find the source.

 

I have a little amplifier that works perfectly fine when not in sleep mode(auto on), but when it goes to sleep after roughly 15 seconds of no signal, it emits a pretty loud hum through both channels.

Here is how it looks on the oscope. 120hz hum.

View attachment 162792

And here are the insides. Pretty simple amp.

View attachment 162793

Any ideas what could be causing it?

I would suspect the 8 pin chip Njm2072 as not shutting down the muting mode...
Mains hum is usually caused by faulty smoothing capacitors in the psu..

Try probing around these pins..

https://www.njr.com/semicon/products/NJM2072.html

 

Probably it is not a leaky capacitor, but rather that in the alleged sleep mode it is not really sleeping. So the logical approach is to locate a circuit schematic and discover what stage is switched off for the alleged sleep mode. Next you would determine if the voltage change for the sleep mode is actually happening correctly. Since there is a 15 second delay to the hum arriving it seems like part of the circuit is coming back on.
Does the same thing happen if you disconnect the audio source? Do you have a circuit drawing??

 

Also if you solve the problem, please post the results, so we can know for future references.

 

Based on the description of the sleep mode controller IC it seems that it is not adequately forcing the sleep mode. Thus the power supply voltage rises and the amplifier starts to amplify. My suggestio is to manually switch it off when not using it, since it is obviously consuming power when it is not being used.
OR, did it previously shut down completely but now it does not? AND, have you tried disconnecting the audio signal source when it is in the sleep mode and humming? You might be getting the hum from the audio source device.

 

13 years ago I had this DVD player magnavox. There was a horizontal noise rolling on video. When I checked where that noise was coming from, it was from filter capacitor in power supply where the noise was at 120hz.

 

My guess is that the horizontal noise was constant at all times, and that capacitor had lost capacitance, or increased internal resistance, rather than developing leakage.

 

I would suspect the 8 pin chip Njm2072 as not shutting down the muting mode...
Mains hum is usually caused by faulty smoothing capacitors in the psu..

Try probing around these pins..

https://www.njr.com/semicon/products/NJM2072.html

So I did a few measurements and that chip runs at 10.5v rail voltage. Which is above the 7v max voltage according to the spec sheet.

It has OUT1 and OUT2 pins.

OUT1 is at ground when the amplifier hums or plays sound, and at rail voltage when the amp is off with a physical switch.

OUT2 is reversed. At rail when humming or playing, and at ground when amp is off.

I'm guessing the OUT pin should change state when going from normal to sleep?

On the INPUT pin I read 1.3v when playing sound or when the amp is in sleep humming. (with a multi-meter)

 

Humming in sleep mode, called snoring

I think it is probably trying to remember the notes.!
Max.

 

Given that the data sheet tells us that the abs max supply voltage is 8 volts, and if your meter is reading right, that goes along with the waveform looking like a voltage breakdown waveform. AND it seems like there is some ripple as well. so the first thing to do would be to check out the power supply circuit and see if some part has become short circuited or electrically leaky. One more thing is to listen carefully and hear if the amplifier still sounds as good as it did prior to this problem appearing. Also, is it possible that now it is connected for the wrong line voltage, such as 120 volts when the mains are 220 volts.

 

if your meter is reading right

Yup, it's a Fluke and I check it against AD584 voltage ref. So it does seem like the chip is running over spec. It's plugged into 120V and is designed for 120V.

I guess next step would be to probe the power supply. The amp does sound just fine when it's not snoring.

 

Yup, it's a Fluke and I check it against AD584 voltage ref. So it does seem like the chip is running over spec. It's plugged into 120V and is designed for 120V.

I guess next step would be to probe the power supply. The amp does sound just fine when it's not snoring.

OK, things get interesting at this point, since a higher voltage is seldom what happens, usually the voltage drops when something fails.So it is indeed a puzzle here.
So one more question about the amplifier is, with it on and no signal, is the output quiet until the sleep mode switches on? I am wondering if it is just noise that is no longer being switched off. And does the power supply voltage change when the sleep mode switches in?

 

Schematic shows pin8 of that IC (U303) ties to a +12 volt bus.


I don't think it is humming because it is not muting, but rather it is not muting because it is humming. Take a look at the +12 volt bus (pin 8 of that 2072 is on +12 volts) with your scope and see how much ripple there is. Then check HV+ (CN306_1 Pin3) and HV- (CN306_1 pin1) for ripple. Use caution on the last two as I don't see a value listed for HV+ or HV-. Probably 35 - 50 volts.

 

For you experts out there, I think this is the unit in question:

 

Thanks YLLi, and now I am quite confused because that circuit is a lot more than what shows in the photo at the start of this thread. There is a very large difference indeed. So once again we have missed arriving at much useful advice because of not knowing enough about the question. That does happen a lot around these threads.

Without the the whole question it is difficult to provide a whole answer.

 

After second look, it may not be the v3, but instead an earlier version. The V1 has the stereo-mono switch as per the TS pictures, but does not have input activated auto on. V3 has the auto-on, but the stereo-mono switch is in the wrong place compared to the TS picture.

Instead of guessing, perhaps the TS can tell us what model it actually is.

 

So one more question about the amplifier is, with it on and no signal, is the output quiet until the sleep mode switches on?

Yup, dead silent until sleep.

And does the power supply voltage change when the sleep mode switches in?

Where would I measure this?