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AMP kits that has a low frequency buzz/hum

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mramos1

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I bought and built a 18 watt stereo amplifier kit, built it and dropped it in a metal hammond box. For a desktop ipod amp. I used RCA females for the input and solid core wirewrap wire to connect the inputs to the board (GND LEFT and RIGHT). The kit has screw down connectors on it.

I am getting a lot of (I guess 60Hz) low frequency humming/buzzing. If I grab or move it, the buzz level goes up and down.

Anyone have a clever idea how to eleminate the buzz.

It might be from the power supply (wall wartl) or the unsheilded input wires. There is no ground to the metal case or caps across the inputs either.

Looking for ideas on where to start.
 
Are you getting noise if you don't connect the input signals? I think it's from the power supply, a bypass capacitor will reduce noise coupling.
Can you power the circuit from a battery?
 
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eng1 said:
Are you getting noise if you don't connect the input signals? I think it's from the power supply, a bypass capacitor will reduce noise coupling.
Can you power the circuit from a battery?

Pretty sure the noise was there with nothing connected.

I will put a cap in and see what I get. If nothing else, get a good 12VDC wall wart if they exists.

I left the thing at the office and can not test it.
 
Could you provide more information about the circuit you built and how it's connected, what's powering it etc..? Power line buzz can come from a lot of places, a good one to check for is ground loops, improperly grounded shielding, or a power supply with bad ripple rejection. Considering the buzz changes when you get near the unit I'm gonna hazzard a guess it's a ground loop maybe from insufficiant issolation at the power/signal sources.
 
You must use shielded RCA audio cables. Unshielded speaker cables also have RCA plugs and would pick up hum.

The wires from the input jacks to the circuit board make perfect antennas for picking up hum because they aren't shielded. Connect one input jack's ground to the metal box so that the box becomes the shield.
 
It is a Kitsrus.com kit88 the paper states.

https://kitsrus.com/kits.html search for 88, but it only has PCB picture and amp final datasheet.

I have it in a metal box (hamond 1590) and it is grounded to the case only via the connectors I drilled and mounted to the case.

I can jumper ground on power supply to ground in the inputs, and think I will also try a cap to filter power supply ripple to ground. Also I want thinking about shielded cables on the input and grounding the shield. I hate the fact I left it at work. Wanted to get it working this weekend.
 
audioguru said:
You must use shielded RCA audio cables. Unshielded speaker cables also have RCA plugs and would pick up hum.

The wires from the input jacks to the circuit board make perfect antennas for picking up hum because they aren't shielded. Connect one input jack's ground to the metal box so that the box becomes the shield.

I was going to attempt that at work today, but all the shielded cable there was hard to flex (75 ohm cable TV cable, and the length connector to circuit in maybe 1.5 inches)..

They make very thin coax tv cable now, I plan to get one from a local store and cut it up. Or search the garage for old RCA cables I cut off old TV tuners, they are braded and not insulated.

I have plenty of ideas now. Going after inputs picking up noise first and ripple from power supply second.

Thanks for the input.
 
I bet the amp is overloading the wall-wart and it is causing a lot of hum. I've never seen a wall-wart with an output of 24VDC/2A.

Don't use cable TV cable, use shielded audio cable. Buy shielded cables with RCA connectors on the ends at The Dollar Store then cut off the connectors and use the shielded cable.

The kit's instructions need to be followed:
 

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I used a walwart to supply the FM transmitter for my MP3 player, and it transmitted the same noise you describe to the radio.(the noise didn't exsist when batteries were used) First I tried a couple different electrolytics with no improvements, then I passed the wire (positive and negative) through a torroid about seven times and the noise is gone. I hope this helps.

I believe this indicates that my problem was common mode noise, so if you still have problems after trying shielded cables you might want to try a snap on choke or something (I already had the torroid).
 
audioguru said:
I bet the amp is overloading the wall-wart and it is causing a lot of hum. I've never seen a wall-wart with an output of 24VDC/2A.

Don't use cable TV cable, use shielded audio cable. Buy shielded cables with RCA connectors on the ends at The Dollar Store then cut off the connectors and use the shielded cable.

The kit's instructions need to be followed:
I have RCA cables. And they flex nice.. The cheap ones do.

I am running it at 12V 1AMP. But I was not pushing anything, so I think power is OK. I was worried about noise from it. But I will get an RCA cable and shield the inputs first.

Sig239:
Not a bad idea. I agree with sceadwain and audioguru on grounding and shielding. If that does not work 100%, I will try that too. It can not hurt.
 
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My post mentioned a power supply with "bad ripple rejection" A heavily loaded wallwart is just that. I'd try another 'wall wart' with a significantly higher than needed current rating. Wallwarts are not typically rated for driving audio amplifiers directly, and 60hz transfers quiet well if you're not using capacitive coupling stages.
 
With only 12V, its output into 8 ohm speakers is only 1.8W per channel at clipping. Into 4 ohm speakers its output is 3.0W per channel and the wall-wart will be at its max current.
 
An old PC power supply will work but you'll probably need to add an extra noise filter.
 
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