Help Troubleshooting Hum in Audio Circuit

[acoles1619]

Hey Guys,

Having an issue with a power circuit I built for some audio equipment. Essentially what the circuit does is take a 12V input, and converts it to 5V using a switching regulator to one output. The other output is just supposed to be a passthrough, whatever the input is, is what the output of the second jack will be. Everything works properly when using either the 12V output or 5V independently. However, when using both at the same time there seems to be some hum introduced into the circuit. I am posting my wiring diagram, any help is greatly appreciated.

 

[Resqueline]

Ho hum.. Can you please expand on the "some audio equipment" & the "some hum introduced"?

 

[davenn]

so many questions.....

what is the resistor going to the relay ? value and its use, I cant see any use for it?
what contact of the relay is it connected to N.O. , N.C. , COM ?

what is controlling the operation of the relay ?
if its manually controlled why not just use a SPDT switch ?

in fact you need to label ALL the relay contacts and where the different lines go to on it, else it makes no sense to us.

what are the values of C1 and C2?
what is the switching Reg?
what is the source of the 12VDC and is it already regulated and capacitor smoothed ?
Do you realise that a diode in series with the input is going to cause a voltage drop and if not rated to handle the current requirements its gonna get very hot ?

If you really want reverse polarity protection, put the diode between the + and - rails after the fuse. (bar end of the diode to + rail) That way the diode doesnt affect the normal operation of the power supply

ok that will do for a start.... show us a new schematic with lots of labelling and a new
D1 position

see how ya go with all that
cheers
Dave

 

[acoles1619]

Haha, looks like I have a little work to do. Wasn't sure how in depth I needed to get with my wiring diagram, but I can address everything you were asking about. It's getting a tad late over here so I will post a new diagram tomorrow, fully updated. I was told that this could be a ground loop issue, but after doing some research I don't see how this is possible. It should all be connected to a common ground. Maybe I am missing something? At any rate thanks for quick replies and I will provide more info tomorrow.

 

[acoles1619]

The audio equipment being powered is a little recorder on the 5V output and a receiver on the 12V output. The receiver is hooked up to the recorder to feed audio, and when both the receiver and recorder are powered from my circuit, there is a humming noise coming through on the recorder.



"what is the resistor going to the relay ? value and its use, I cant see any use for it?"

The relay is actually a PhotoMos relay, so I have a 1.2K resistor at the input so the Photo Diode inside is not damaged.

"what is controlling the operation of the relay ?
if its manually controlled why not just use a SPDT switch ?"

The toggle switch is controlling the operation of the relay, the purpose of the relay is because I need to have a small toggle to control this circuit, but most small toggles can't handle the large amount of current. Thus the relay.

"what are the values of C1 and C2?"

I labeled these in my new diagram.

"what is the switching Reg?"

It is an MIC4680 switching regulator

"what is the source of the 12VDC and is it already regulated and capacitor smoothed?"

The source is from a lithium ion battery pack, I am not sure if it is regulated or smoothed.

"Do you realise that a diode in series with the input is going to cause a voltage drop and if not rated to handle the current requirements its gonna get very hot ?"

The diode I am using is within my current limits and it has an extremely low voltage drop.

"If you really want reverse polarity protection, put the diode between the + and - rails after the fuse. (bar end of the diode to + rail) That way the diode doesnt affect the normal operation of the power supply"

I moved the diode after the fuse. Rookie question but how will this not affect the normal operation of the supply?


Hopefully I addressed the questions. So what does everyone think?

 

[Resqueline]

Dave's suggestion for the diode & the fuse configuration works like this: if you hook up the battery the wrong way around, the diode conducts and the fuse blows.
The advantage is you don't get the diode drop on the power line, the disadvantage is you have to replace the fuse after a polarity mistake.

About the audio hum; is there anything else wired to the receiver & recorder? What's the recorder output? Is the hum recognizable as being a mains frequency?

 

[acoles1619]

Oh ok, I see what you are saying now about the diode. The fuse is actually a PTC resettable fuse, the configuration you mentioned should still work with this right?

It is set up like this, receiver has power from the circuit, and one output cable for audio to the recorder, the recorder has 5V power from the circuit. The only other thing here is a set of headphones plugged into the recorder, this is where I am hearing the hum. How would I recognize the hum as being a mains frequency?

There is another scenario that I have tried that eliminates the hum, not really sure why though. Same setup as before, but I power the receiver off of batteries, and use the 12V out to power something else, like a little mixer. This eliminates the hum, but this is not how the circuit will be used normally, hope I am making sense. Thanks!

 

[acoles1619]

I also wanted to add that I am reading a lot about ground loops causing problems with hum in audio. Could I have done this somehow? Also, I have a ground lift adapter that can be put inline with the XLR cable from the receiver to the recorder, using this also eliminates the hum.

 

[acoles1619]

Just bumping the post. Thoughts? Opinions?

 

[davenn]

Dave's suggestion for the diode & the fuse configuration works like this: if you hook up the battery the wrong way around, the diode conducts and the fuse blows.
The advantage is you don't get the diode drop on the power line, the disadvantage is you have to replace the fuse after a polarity mistake.

True, but its much easier to replace a fuse than unsoldering/replacing a bar-b-qued crumbling diode

Oh ok, I see what you are saying now about the diode. The fuse is actually a PTC resettable fuse, the configuration you mentioned should still work with this right?

yes but you didnt change the way I wanted you to place the fuse .... like this....

Cheers
Dave

 

[davenn]

I cant read the type of PhotoMos relay from the circuit
do you have a link to a datasheet for it ?

Dave

 

[acoles1619]

Thanks for the fuse advice Dave. I will be sure to do that. But any opinion on the hum? It is driving me crazy.

 

[acoles1619]

Sorry didn't see your post about the relay. It is an AQV252G. Here is a link to the datasheet.

http://www.panasonic-electric-works.com/catalogues/downloads/photomos/ds_x615_en_aqv252g.pdf

 

[davenn]

Thanks for the fuse advice Dave. I will be sure to do that. But any opinion on the hum? It is driving me crazy.

thanks for the link in your next post. havent even got to thinking abt the hum yet haha
have been just trying to figure out your power supply and switching circuit.
spent the last 30 mins or so reading through the application notes for that MOS switch
I didnt know those things existed

Dave

 

[acoles1619]

Haha, yeah I stumbled upon them by accident one day. They are pretty neat, very compact and can withstand high currents.