A while back I built a Pass F5T and it was delightful - except for a small hum I was getting from my speakers. I've gotten a hum before on a different amp (Akitika GT-102, LM386 chip amp) but recall that hum was a physical one from the transformer. This hum, from the F5T was decidedly from the speaker, not the physical amp.
What are the chances the hum was coming in through the wall AC? Now that the amp is gone, and I never did bring my speakers and amp to someone else's house to see if I had the same issue; is there a way to see if my house AC has got a hum? Like an oscilloscope? or something?
Now that I'm thinking back on it, I wish I would have shut off every breaker in my house, then only turned on the one with the power for the amp: see if that does anything. I'd like to get another sweet amp situation going, but I really don't want to run into this infamous hum again.
Any ideas?
What are the chances the hum was coming in through the wall AC? Now that the amp is gone, and I never did bring my speakers and amp to someone else's house to see if I had the same issue; is there a way to see if my house AC has got a hum? Like an oscilloscope? or something?
Now that I'm thinking back on it, I wish I would have shut off every breaker in my house, then only turned on the one with the power for the amp: see if that does anything. I'd like to get another sweet amp situation going, but I really don't want to run into this infamous hum again.
Any ideas?
A clipped AC voltage can cause an EI transformer to hum or buzz as a result of the dirty waveform.
Could be a ground loop in the amplifier, or elsewhere in the system or AC line wiring.
First try reversing one at a time the AC plugs for each of the system components, and see if there is any change.
If not, insert shorting plugs into both amplifier RCA inputs, and see if there is still hum in the speakers.
First try reversing one at a time the AC plugs for each of the system components, and see if there is any change.
If not, insert shorting plugs into both amplifier RCA inputs, and see if there is still hum in the speakers.
Yup. If the hum goes away with a shorted input, it's an audio ground loop most likely. If it doesn't, and it's line frequency, it's still most likely a loop. If it's linex2 (100Hz, 120Hz) it's bad filtering in the amp PSU IMHO.
The amplifier's original wiring could have a problem. Or another component could be the problem.
Or the house AC power ground panel could be corroded.
I've seen houses with the entire AC line hot/cold wired backwards.
Or the house AC power ground panel could be corroded.
I've seen houses with the entire AC line hot/cold wired backwards.
I think it was a sarcastic comment implying that the 120VAC and 230VAC is 100% hum... as opposed to DC...
If the amplifier and for instance a source both have Audio GND connected directly to PE these things happen. If it is a ground loop you are experiencing so 50 or 60 Hz. It is a good habit to only connect the amplifiers Audio GND to PE directly and give the other devices a lifted connection to PE aka a 100 Ohm 2W resistor from Audio GND to PE. In other words: things will be OK if only 1 device has a straight connection from Audio GND to PE. Please don't do the opposite which is none of them connected to a PE wall socket. Coincidentally I just solved such a riddle with a PSU of a DAC which had Audio GND connected straight to PE (why?). Severe hum was the result.
As others pointed out AC mains voltage is 50 or 60 Hz and therefor pure hum but normally not heard 🙂
For testing you can connect only the amplifier and just 1 source and put one of them in a wall socket/distributor that has no PE. If the hum is gone you had a ground loop.
As others pointed out AC mains voltage is 50 or 60 Hz and therefor pure hum but normally not heard 🙂
For testing you can connect only the amplifier and just 1 source and put one of them in a wall socket/distributor that has no PE. If the hum is gone you had a ground loop.
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Or if it's a build like mine, there's so much wiring the entire building radiates 60Hz (an AM radio is useless here).
I've encountered ground loop problems when the A/V system included a cable TV connection. That can be solved by wiring a pair of 75-to-300ohm adapters (coax to twin-lead) back-to-back. And when sources and amps were plugged into different electrical circuits in an old building. And when I failed to make use of the ground lug on an RCA socket and trusted it to be grounded through the chassis.
But, the OP says the amp is gone, so, all we can do is speculate. Bottom line, the AC itself is pure hum, so it has to be something related to grounding and/or poor shielding. It is possible for AC to be "polluted" with electrical noise from motors or light dimmers, but if that is audible, it'll be static or whines, not a low frequency hum. And normally, the power supply filter caps will do a fine job of filtering that out.
But, the OP says the amp is gone, so, all we can do is speculate. Bottom line, the AC itself is pure hum, so it has to be something related to grounding and/or poor shielding. It is possible for AC to be "polluted" with electrical noise from motors or light dimmers, but if that is audible, it'll be static or whines, not a low frequency hum. And normally, the power supply filter caps will do a fine job of filtering that out.
There's actually a transformer FOR that purpose. Might be less loss (if it matters) than a b-b connection. Jensen will $ell you a very excellent one. Amaz/eBay have cheapos.
If the situation is simple, like my house, the solution may be simple. My power line from the street is long. There's a few Volts drop in the neutral. So the cable-box has one voltage on chassis and another on the cable input. While this must be commonplace in real life, I think the box is only tested in the lab. I found my cable has a proper ground-block at the meter-pole, but there's another 50' to the house. So another fraction Volt. I put a cable ground-block AT my main fuse-box (after testing for no hazardous stray voltages). All the hum-bars went away. Ground-blocks should be as nearly lossless as any connection can be.
Sounds like I'm just going to get a sweet Pass Class A amp back in my house and go from there. Thanks for all the feedback!
That is life on the east coast. We all suffer single bushing distribution transformers. Meaning, every second or third pole has a grounded wire into the dirt that goes up to the neutral conductor at the transformer, and the hv feed to the transformer is a wire at the tippy top of the pole, the house feeds are at the level of the return for that hv as well as the house neutrals..
There will always be "stray" currents in the actual earth, as roughly 5% of the distribution grid current goes through the earth, not the wire being used as neutral/hv return.
It sounds like you identified an earth 60 hz voltage drop and fixed it. I would find it daunting to measure the actual neutral drop, as any way to measure it requires having a return which causes a loop. But it sounds like you got both of those suckers under control..nice
John
You are correct on supply hum.
Ground loop hum can be 50/60 hz as a result of induced voltage from the wall wiring.
If a ground loop is picking up the current draw of a big amplifier, it will be heavy 60hz, and then odds, 180, 300, 480...haversine components.
Typically, if your system is picking up the haversines of the amp in the ground loop, it will be quiet during the low passages, and pick up during the loud ones. That is a very large confounder in understanding why a system doesn't appear to sound right. Since no draw during silence produces no hum, one can think there is no ground loop issue.
John
A hi-Z meter won't make a significant loop.
I did not seek global truths, only the drops I could measure around the house and yard. I had some discrimination because the earth currents go a different way than the line currents (the line follows the road south of me while some dirt current may head north toward the substation). The observed voltage changes 0-4V with my load current. But by bypassing cable to electric within a few feet of the TV/box the hum-bars essentially vanished.