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60 Cycle Hum Driving Me Batty

royaaron1

royaaron1

New Member
I have a circuit which takes a 15VDC supply from a wall adapter.
Most connections are made by shielded cables and I have employed the manufacturer's (and others) recommendations for hum reduction.
Is it possible to insert a 60-cycle notch filter or other high-pass that'll attenuate the hum?
 
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royaaron1 said:
Is it possible to insert a 60-cycle notch filter or other high-pass that'll attenuate the hum?
Click to expand...
Hard to say without knowing what the circuit is about. For basic analogue audio the answer would be no, you would wipe out too much of the surrounds.

60Hz hum (assume you have measured freq) are caused by a GND loop, a schematic/sketch showing the grounding within the circuit would be a start.
 
The wall wart itself will list AC or DC output from it in that very tiny lettering they put of them.
 
Are you sure it's 60Hz and not 120 Hz? I'm betting the adapter just has too much ripple for the 220uf cap to absorb. Do you have a regulated bench supply you could try?
 
Only ground thing I don't like is if you do have multiple grounds on the lm386, they should be summed before actual ground I think.

My first suspect would be the wall wart as well
 
Circuit appears to be purely DC, any 60/120Hz would be coming from your walwart. Schematic shows circuit can be powerd by 9VDC battery. LM386 datasheet shows absolute maximum supply of 15V or 22V depending on chip version.

Maybe try with a 9V battery?
Maybe you should find a 9-12Vdc supply?

Following concerns me...
royaaron1 said:
it's surely a DC output, something like 15VDC
Click to expand...
 
mbz said:
Circuit appears to be purely DC, any 60/120Hz would be coming from your walwart. Schematic shows circuit can be powerd by 9VDC battery. LM386 datasheet shows absolute maximum supply of 15V or 22V depending on chip version.

Maybe try with a 9V battery?
Maybe you should find a 9-12Vdc supply?

Following concerns me...
Click to expand...
with a 9V battery there is no hum. and
loudnoises said:
Only ground thing I don't like is if you do have multiple grounds on the lm386, they should be summed before actual ground I think.

My first suspect would be the wall warBy "summed"
loudnoises said:
Only ground thing I don't like is if you do have multiple grounds on the lm386, they should be summed before actual ground I think.

My first suspect would be the wall wart as well
Click to expand...
By "summed" do you mean that all connections that are to be grounded should be connected, and only one wire be connected to the chasis?
Click to expand...
 
loudnoises said:
Only ground thing I don't like is if you do have multiple grounds on the lm386, they should be summed before actual ground I think.

My first suspect would be the wall wart as well
Click to expand...
I'm sure you're right; with a 9 volt battery there's no hum at all.

is there a schematic for a 120VAC input/15VDC output that heavily suppresses 60/120Hz?
 
I sympathize with your hum issue. I had one a while back that involved my computer. I tried everything in the book to fix it and had no luck. Finally I stuck my neck out a little bit and ordered a cheap Chinese USB ground isolator which immediately fixed my issue. It might be worth it to see if you can find an isolation transformer for your system. I know I have used a XLR style one at wok in the sound booth. It was way appreciated for the little that it cost. XLR isolaters can easily be adapted to RCA style connectors. It would be worth a try.
 
TomSc said:
Are you sure it's 60Hz and not 120 Hz?
Click to expand...
Can use mobile phone ap like spectroid or others, also pc freeware like audacity, 120Hz will be more of a buzz.

royaaron1 said:
Any suggestions regarding that capacitor?
Click to expand...
Main psu cap but I think it's a ca of bs in so bs out, The other possiblilty is that you are pushing the IC too hard at somewhere 15Vdc, have you measured the walwart output? probably slightly higher than nominal value. Find a 9V, xxmA wallwart
 
royaaron1 said:
with a 9V battery there is no hum. and
Click to expand...
Yes, gather then connect. Or at least same point. But it really may not matter, I'm grasping for ideas other than just the wall wart.

You can get really nice standalone dedicated power supplies. Metal cage and fixed voltage. Wall warts are hit or miss.

Why are you going for 15v if you only need 9v?
 
The LM386s can take as much as 18V. And, having used a 9V battery - to isolate the hum source - there's a direct correlation between supply voltage and volume.
But I'm going to look into a standalone supply.
Thank you!
 
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