Signal hum ideas please.

[losacco]

I have the Walton 300b monos that are very quiet until you connect a source. What causes this? Is it RF? Is there an optimal wiring scheme or wire type for the input signal?

Patrick

[20to20]

Wouldn't be RF (Radio Frequency), but EMI (Electromagnetic) from amp transformers too close to the source signal. Magnetic cartridges are very sensitive to transformer EMI and can pick up EMI fron 2 feet. Just a couple of ideas. Use the turntable method of hum kill by tying a ground to the amp from your source. Alligator clip leads will work for testing. Walton no help? Good luck.

[TheGimp]

Hum is often times caused by ground loops. When you connect a source you are connecting the source ground to the Walton 300b ground. Each of these has a ground to the power recepticle. You create a large (Ground)loop antenna. The solution is to break the ground between the source and Walton 300B by not connecting the shield between the two.

[20to20]

Originally Posted by TheGimp

The solution is to break the ground between the source and Walton 300B by not connecting the shield between the two.

Gimp, how do you break the shield on covered, molded, RCA cables? If that's the solution then there is another problem because the cables should not be the source of a ground loop hum.

[TheGimp]

To test it I would go to Radio Shack and get a short female to male RCA adapter (maybe a Y). You can experiment iwth it to veirfy that this is (or is not) your problem.

Peel back the insulation, break the shield all the way around, and insert in line with your source and amp.

If this eliminates the hum, then I would consider lifting the ground at the RCA recepticle in the amp.

Others might have alternate suggestions.

[kevinkr]

I was going to suggest ground loop breakers in each amplifier, but decided that perhaps this is not such a great idea after all.

I would recommend placing a 1 - 4.7 ohm resistor shunted with a 0.01uF disk ceramic cap (parallel RC) in series with the ground connection at each amplifier RCA input jack and see if that does it. Should break the loop and still provide a good ground wrt to external EMI..

[20to20]

I've just polished my crystal and I see Aunty Em with an MP3 player laying on a desktop under a flourescent light, hooked up through a 6 ft. miniplug cable into an adapter jack plugged into an RCA cable into the amp. Or, maybe not.. it could be flying monkeys singing into open RCA cables.

[kevinkr]

Originally Posted by 20to20

I've just polished my crystal and I see Aunty Em with an MP3 player laying on a desktop under a flourescent light, hooked up through a 6 ft. miniplug cable into an adapter jack plugged into an RCA cable into the amp. Or, maybe not.. it could be flying monkeys singing into open RCA cables.

Now that's quite the image..

[Mooly]

Do you get the noise if you first of all fit just shorting plugs to the inputs with no leads, and then if that's OK what happens when the two input grounds (l and R) are connected together with the shorting plugs still in place ?

Is the hum a deep pure fundamental of 50 or 60 hz or is it "harsh" with harmonics at 100/120 hz ?

[atmasphere]

Originally Posted by TheGimp

To test it I would go to Radio Shack and get a short female to male RCA adapter (maybe a Y). You can experiment iwth it to veirfy that this is (or is not) your problem.

Peel back the insulation, break the shield all the way around, and insert in line with your source and amp.

If this eliminates the hum, then I would consider lifting the ground at the RCA recepticle in the amp.

Others might have alternate suggestions.

Certainly! You won't get anywhere lifting the ground on the audio signal!

I am in agreement that it is a ground loop, caused by the fact that the chassis is grounding (using a flawed technique) into the wall ground.

Now for safety purposes you want that ground, but for ground loops it sucks.

You can test this by floating the ground (3rd prong of the AC cord) using cheaters available from the hardware store. If that takes care of it, then you will want to do some mods to prevent the ground loop.

The chassis needs to be at AC ground potential, so the fuses will blow in case of a bad malfunction. The audio circuitry should 'float' at the same potential. The easiest way to do this is to ground the chassis to the AC ground, and have the audio path not ground to the chassis except through a 100 ohm (or thereabouts) resistor (the 100 ohms is enough resistance that the ground loop currents will be significantly reduced, often to inaudibility). If it was built well (star grounded), this will be easy to do, as all the power supply grounds will not be the same as the chassis. OTOH if you have a lot of can-style filter caps that are directly grounded, this could be a nightmare, in which case there are other approaches.

Get back and let us know....