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.
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.
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.
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.
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.
Ground Loop Breaker Suggestion Deleted
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..
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..
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 ?
Is the hum a deep pure fundamental of 50 or 60 hz or is it "harsh" with harmonics at 100/120 hz ?
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....
IF it's a ground loop...
The chassis green safety ground is always good to have, of course. If your shield grounds between units are chassis grounded then you only need ONE end of the system to be safety grounded. That will eliminate a ground loop created by multiple units with 3-wire cables and the RCA cable shield grounds. Simply use one of those 3 prong/2 prong cheaters on everything but one unit and any A/C ground loop will be broken, and you will still have a safety ground circuit. If the RCA jacks are not chassis grounded, then this is all mute.
Uh...get your VTVM (not a LCD display DVM, a real honest to goodness VTVM) and measure for voltage between chassis A and chassis B and what you are calling your 'ground'. Start high, then progress down to the mV range on your VTVM.
For unbalanced input devices, like your RCA cable connected hi-fi equipment, the presence of potential differences between the two chassis is a sure hum source. Any voltage there, even a few hundred millivolts, is a warning of grounding problems.
Don't overlook the obvious. Many people make grand assumptions about the quality and uniformity of the neutrals/ground relationship of their AC mains source. Do not presume that your receptacles' neutral and ground are at zero potential. Check them. (measure between the wide blade to "u" ground with your VTVM) Any stray voltage here and you have a bad earth ground.
Finally, speaking of "obvious", are the two devices plugged into the same receptacle? See above for "grand assumptions" extended to the ground potential between duplex receptacle A and duplex receptacle B
Cheers!
cbg
For unbalanced input devices, like your RCA cable connected hi-fi equipment, the presence of potential differences between the two chassis is a sure hum source. Any voltage there, even a few hundred millivolts, is a warning of grounding problems.
Don't overlook the obvious. Many people make grand assumptions about the quality and uniformity of the neutrals/ground relationship of their AC mains source. Do not presume that your receptacles' neutral and ground are at zero potential. Check them. (measure between the wide blade to "u" ground with your VTVM) Any stray voltage here and you have a bad earth ground.
Finally, speaking of "obvious", are the two devices plugged into the same receptacle? See above for "grand assumptions" extended to the ground potential between duplex receptacle A and duplex receptacle B
Cheers!
cbg
I had considered mentioning the average home's ground system quality but thought it would just blow minds. I was going to suggest starting with going into the panel and tightening all of the neutral ground lugs and the safety ground lugs and the primary line in ground, and the breaker box copper ground lug and the meter pan copper lug, but WHEEWWWW!!!
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This ground lift generally works well for me...
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
Yeah, 20to20 makes a good point about the "daunting" part...
Still, checking grounds isn't totally out of the skill range for a reasonably skilled DIY.
Whatever one's skill level, one can certainly check the quality of one's earth rod connection with perfect safety. It is external to the mains box and can be found, checked, and screws tightened with absolute safety. (at least the end outside the mains box)
If you are not comfortable inside your mains box, then hire an electrician for an hour and simply ask him to do a routine cleaning/tightening of the neutral and ground connections inside your box. Explain that you are an old school hi-fi nut and you need really good ground purity for your "old fashioned" tube equipment.
In almost every case a qualified electrician could do this work in 1/2 hour. It really isn't complicated...just loosen, spray clean, retighten each neutral and ground connection at the bus bar inside the mains box. I've done mine in 20 minutes, and it is a 200 amp service with 24 breakers.
Of course, if one doesn't know what one is doing, one can end up dead...just as one can when probing VTVM leads into live receptacles. Be wise and be cautious. If you're not sure, stop, research, ask, or hire.
Cheers!
CBG
Still, checking grounds isn't totally out of the skill range for a reasonably skilled DIY.
Whatever one's skill level, one can certainly check the quality of one's earth rod connection with perfect safety. It is external to the mains box and can be found, checked, and screws tightened with absolute safety. (at least the end outside the mains box)
If you are not comfortable inside your mains box, then hire an electrician for an hour and simply ask him to do a routine cleaning/tightening of the neutral and ground connections inside your box. Explain that you are an old school hi-fi nut and you need really good ground purity for your "old fashioned" tube equipment.
In almost every case a qualified electrician could do this work in 1/2 hour. It really isn't complicated...just loosen, spray clean, retighten each neutral and ground connection at the bus bar inside the mains box. I've done mine in 20 minutes, and it is a 200 amp service with 24 breakers.
Of course, if one doesn't know what one is doing, one can end up dead...just as one can when probing VTVM leads into live receptacles. Be wise and be cautious. If you're not sure, stop, research, ask, or hire.
Cheers!
CBG
Along with checking the return for all the receptical circuits goes the big aluminum service neutral. If that has flowed in the breaker panel or the meter box then everything gets tossed to the copper ground instead of going back to the pole. I just hesitate to suggest to anyone to get close to the service lugs to tighten that. Even the commons and ground buss is a PIA to get to in my panel behind all the *******' spaghetti.
An infrared photo of the inside of a panel would be interesting with all the appliances going. That would tell a lot.
One other possibility that is suggested by hum being present only when a short or source is connected across the input is that the input conductor and its ground return conductor, between the input jack and the first amplification component's input resistor and its ground reference, have loop area formed between them, which would allow AC fields in the air to induce AC currents in them (see Faraday's Law), which would induce AC voltages across the amp input impedance (and also across the source impedance if a source is connected instead of a short). I would expect this to be more common in DIY amps than in commercial units. But you could check the wiring. The same thing could also occur if the input jack's outer "shield" part is not isolated from the chassis, if it carries the signal ground.
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