loading

How do I get rid of the Hummmmmmm?

In my latest project I've installed an iVO-Sound m1050 4GB portable media player in to a Magnavox D8443 Power Players cassette bay.

You can hear a distinct Hummmmm in the speakers. If you touch the control buttons for the MP3 player the Hummmmm gets louder. The Hummmmm increases when you turn up the volume. Sometimes there is almost like a static-like sound to the higher audio frequencies. Sometimes the sound is more tinny. It's almost like a charge is building up maybe. When the music is playing the hum is less noticeable. The hum seems to get worse progressively the more you use the displaced controls.

All controls that were displaced have been mounted in the old cassette players buttons. The tape circuits are off. The MP3 player switches with the Radio to the Amp. There is a charger but it was disconnected for now. The displaced USB port is extended to the back of the box where it also switches to the charger.

What I have tried so far:

- I have hooked up an Alternative MP3 player to the new Aux Input RCAs that also switch with the Radio to the Amp and the External MP3 player sounds great!

- I made sure the tape heads and wires were all removed from the cassette compartment area.

- I originally connected the audio with a headphone jack but now it is hardwired.

- I removed the Metal back case of the MP3 player and that helped a bit. All the wiring was strung through the metal casing.

- I have Tried an Audio System Ground Loop Isolator between the MP3 player and the Amp. That took away most of the hum but made the sound very "tinny" with almost no Bass.

- I tried Ferrites on the Audio cable between the MP3 player and the Amp and the Power Cord and a number of other places. There was no discernible change. The cables weren't long enough to loop so I used several ferrites in series.

- I have order a couple Audio Transformers so I can try and use them like the ground loop isolator if it is an impedance problem but I don't expect much different a result.

- I have plugged the AC cord into it's own circuit on my breaker box and that seems to help a little.

(My original MP3 player - boombox installation turned out great. The main difference is that most of the buttons were on the face of the MP3 player and only two buttons were displaced to the front of the box and that was the power and reset. The charger made horrible interference racket but you could just listen to the radio while it charged.)

Picture of How do I get rid of the Hummmmmmm?
magD8443-b.jpg
Mod-Diag-a.JPG
sort by: active | newest | oldest
That's definitely 60 Hz. The 420 Hz that your tuner is hearing is the seventh multiple of 60. Can you run a wire from that Aux connector to a good ground, such as a metal pipe?
hum.png
Kepster (author)  NobodyInParticular8 years ago
Ok I ran a nice thick copper wire from the Aux ground to a pipe. The hum was gone. When you touch the buttons you get some RF interference from a local radio station. So how do I solve this? I can't run a stake in the ground everywhere I go. When I ran the Audio lines through the Ground Loop Isolator I lost the hum but I also lost a lot of the sound from the music track.
gmoon Kepster8 years ago
That does indeed sound like a ground-loop problem. The middle (green wire) prong on a three-prong plug is an earth ground, just like your pipe. You could add a three-prong cord, and connect the green wire to your AUX ground. The chassis (earth) ground on older 2-prong plugs was often connected directly to the "neutral" wire (white) on the plug. The user would switch the plug polarity depending on the noise being produced. Not a safe setup--you could easily make the chassis "hot", which was a problem if you touched the chassis and were grounded elsewhere. Like bare feet, for instance.
A couple more thoughts:

Electrically isolate the components, so your power wires are not even in the same circuit as the MP3 player. Or,

Skip the AC entirely. Could a lantern battery or old laptop power pack supply enough power?
Kepster (author)  NobodyInParticular8 years ago
geez an Opto-isolator? Where do I get the components? And how much is that going to set me back? If it's relatively inexpensive that sounds like the best route to go. Then the amp still gets the audio without directly being connected to the MP3 player. *poof* any ground or impedance problem solved.
Kepster (author)  NobodyInParticular8 years ago
I got a couple audio transformers and they had the same adverse effect that the ground loop isolator did.

So I decided to try out the Opto-isolators.

I was able to pick up a couple Opto-isolators assembled on breakout boards from Sparkfun.com http://www.sparkfun.com/commerce/product_info.php?products_id=9118

I'm not quite sure how to wire it up yet. But I'm that much closer.
I hope you are still considering using that three-prong plug. As gmoon pointed out, there are some good reasons we switched.

http://en.wikipedia.org/wiki/Electric_shock#Avoiding_danger_of_shock
http://en.wikipedia.org/wiki/Ground_loop_(electricity)
NachoMahma8 years ago
. I'm guessing it's a shielding/grounding problem. . Check the shields on all you wiring. . If it's a two-prong plug for the wall, unplug it and flip the plug over so the prong that was in the left side is now in the right side.
Would it be dangerous to replace the two-prong power cord with a 3-prong, and connect the badly-grounded components to the third?
Kepster (author)  NobodyInParticular8 years ago
That's not something I'd like to do but I don't think I have much choice. I can probably get a connector like they use for a computer or a monitor. The trouble then comes up again if I'm ever plugged into an ungrounded circuit. The more major modifications I need to make then the less practical it is for someone else to try this. I wish there was a simpler solution. It's definitely the ground. It's funny because I've tried all kinds of different configurations to make this work. If I isolate just the ground without the left and right channels I get reverb. If I use it on all three audio lines I lose bass and mid tones. If I wire directly to the board I get the hum. If I ground the all the ground wires the hum goes away.
Kepster (author)  NachoMahma8 years ago
Shielding is good...I even use that heat shrink tube, that's great stuff _
...great guess, I initially thought it might be bad shielding on the audio cord so I replaced the whole thing.

Funny thing about the old skool boommers is that the power cords are fully reversible where it connects to the boombox and where it plugs into the wall.

Try recording the hum and viewing its spectrum (or uploading to a sound-sharing site and posting the link). I suspect it is from the power supply, as ShutterBugger said. If its spectrum has peaks at multiples of 60Hz, that would prove it.
Kepster (author)  NobodyInParticular8 years ago
Here is audio clip of the hum you can hear me touching the buttons on the top of the boombox.
http://www.furocious.com/hummmm.wav

I just discovered that when I touch the Aux In RCA connectors the hum goes dead.
Hum could come from bad filter capacitors in the Magnavox power supply. The amplifier would then amplify the power line hum (60 cycle). ~bob~
Kepster (author)  ShutterBugger8 years ago
When I switch to radio or a different input the hum isn't there. What I did discover is that when I touch the Aux In RCAs on the side the hum goes away. I hadn't noticed that before. That is something I did not expect to find because that is on a different circuit. When I switch to the Aux In the hum is gone.