Oldpinkman
28-06-2013, 08:52
Yet again I am plagued by hum issues. My life in Hifi has been plagued by hum. I should be a leading world authority, but it always gets me again. It is now solved to an acceptable level, but I feel a reference manual is needed. I understand the theory (idea) of earth loops. I am still owed a proper explanation, with a schematic and current values for the hum I first fixed in this system by Owen. But I need to leave him in peace a bit, so anybody clever here want to comment on hum? (How to eliminate it, how to identify its cause by testing, how to explain it with values)
The hum is on the AV input to the PIP only. Its the AV hook-up, not the input. CD player and record player are silent. I failed to notice it before, because it is quite low level and was masked by the internal hum in the 405, which is why the 405 is in the hospital atm. It is a signal hum (audible in the speakers, and increases with amplifier gain). All equipment is earthed via its mains lead to a common point on the ring main. Lifting earths has no effect. The hum is from the AV amp even with all its inputs and other connections removed. I have a box which puts a 100hm resistor across the earth connection (adds 100 ohm in series to the screen) on the AV signal leads. It makes no difference whether it is used or not. It is worse (but not as dramatically as it was a few months ago before round 1 of earthing) when any dimmer switch is used in the house. But, with or without dimmer switches it is quite low level. We had been living with it until breakfast today.
To shorten the story, I have an earth lead from AV amp to power amp, and from power amp to preamp. The preamp connection had come loose. But even with no preamp to power amp earth - no hum on phono or cd. The arcam P10 has no chassis earth terminal I can find, and tiny screws for securing the case which I can't get a wire to stay under. It had a damaged front cover, so I shoved the wires in the front chassis. Realising this was a bit of a bodge, even for me, I found a self-tapper today and screwed it into the chassis. So I joined the AV earth to the preamp earth by twisting them together. Bugger me - no hum (very low level hum, inaudible at the gain setting I currently have for the pip when using the AV system, but still faintly audible if I crank up the gain to full). So break the loop by earthing AV to Preamp - and earth power amp via the mains connections only. How does that work? Worse - honest - would I lie to you guys? - as I move the twisted earth wires close to the screw on the power amp to earth both devices to that - the hum audibly increases! If I connect the preamp/av wire to the power amp I add hum (not much, but its there) but I don't even have to make a physical connection - just get the wire close, for that hum to start. (Breakfast was ready, so I didn't fiddle about with which bits of what my body might have been touching, but I was basically wearing trousers and kneeling on carpet)
Hum is now at such a low level, and it is only the AV set up, not my music system, I can control it by dropping the gain setting on the pip while reconfiguring the 5.1 so the signal level is high at the input to the pip.
Any thoughts. The AV to pip connection is via about 5m in total of signal cable (joined in the middle by rca to rca connectors), and runs very close to mains cables (domestic compromises). Am I at the limits (given the hum is now VERY low - inaudible at the settings I use) - am I at the limits of what I can expect from unshielded signal cables? What is the solution if I want perfection, and don't want to change the equipment to used balanced connectors? Run the cables up an earthed central heating pipe?
It frustrated the hell out of me, so I needed to rant. Any thoughts welcomed.:(
The hum is on the AV input to the PIP only. Its the AV hook-up, not the input. CD player and record player are silent. I failed to notice it before, because it is quite low level and was masked by the internal hum in the 405, which is why the 405 is in the hospital atm. It is a signal hum (audible in the speakers, and increases with amplifier gain). All equipment is earthed via its mains lead to a common point on the ring main. Lifting earths has no effect. The hum is from the AV amp even with all its inputs and other connections removed. I have a box which puts a 100hm resistor across the earth connection (adds 100 ohm in series to the screen) on the AV signal leads. It makes no difference whether it is used or not. It is worse (but not as dramatically as it was a few months ago before round 1 of earthing) when any dimmer switch is used in the house. But, with or without dimmer switches it is quite low level. We had been living with it until breakfast today.
To shorten the story, I have an earth lead from AV amp to power amp, and from power amp to preamp. The preamp connection had come loose. But even with no preamp to power amp earth - no hum on phono or cd. The arcam P10 has no chassis earth terminal I can find, and tiny screws for securing the case which I can't get a wire to stay under. It had a damaged front cover, so I shoved the wires in the front chassis. Realising this was a bit of a bodge, even for me, I found a self-tapper today and screwed it into the chassis. So I joined the AV earth to the preamp earth by twisting them together. Bugger me - no hum (very low level hum, inaudible at the gain setting I currently have for the pip when using the AV system, but still faintly audible if I crank up the gain to full). So break the loop by earthing AV to Preamp - and earth power amp via the mains connections only. How does that work? Worse - honest - would I lie to you guys? - as I move the twisted earth wires close to the screw on the power amp to earth both devices to that - the hum audibly increases! If I connect the preamp/av wire to the power amp I add hum (not much, but its there) but I don't even have to make a physical connection - just get the wire close, for that hum to start. (Breakfast was ready, so I didn't fiddle about with which bits of what my body might have been touching, but I was basically wearing trousers and kneeling on carpet)
Hum is now at such a low level, and it is only the AV set up, not my music system, I can control it by dropping the gain setting on the pip while reconfiguring the 5.1 so the signal level is high at the input to the pip.
Any thoughts. The AV to pip connection is via about 5m in total of signal cable (joined in the middle by rca to rca connectors), and runs very close to mains cables (domestic compromises). Am I at the limits (given the hum is now VERY low - inaudible at the settings I use) - am I at the limits of what I can expect from unshielded signal cables? What is the solution if I want perfection, and don't want to change the equipment to used balanced connectors? Run the cables up an earthed central heating pipe?
It frustrated the hell out of me, so I needed to rant. Any thoughts welcomed.:(