Post by jon on Jun 30, 2021 17:50:24 GMT 12
Ghastly topic, however for the first time in my life I have a ground loop issue. Ha! Sheer dumb luck. Anyway buy a new bit of kit and here we are.
Current system with ground loop issues Turntable Phono stage Streamer (Raspberry Pi) Thomas Meyer 10Y VT25 preamp Quad 405-2 power amps JBL L300 Summits
All interconnects are IBM type 1 shielded of 1 metre length. Speaker cable is CAT6 cable. around 3 metres in length.
Turning on gets me a loud buzzing sound! Fab - not!
I have excluded the turntable and phono stage from this, the phono stage has an issue and I'm waiting on some replacement CV2492 valves. so they are are turned off!
Checking the power supply I have two plugboards (6 socket and 4 socket) going to a single wall socket. (8 power units in total) I know the single wall socket is running back to the mains fuse board on a single spur. It should not be that. Since I could not accommodate all 8 plugs on a single board that may be an issue. Consider getting an 8 socket power board.
Switch on procedure
1) Wall Socket 2) Preamp back power switch (transformer warm up) high pitch buzz heard 3) Preamp front power switch (valve turn on), loud low pitch buzz 4) Access Raspberry Pi, select music and play - buzz vanishes.
So let's eliminate some things:
Turn off all sources, and preamp. Leaving amplifiers. Turn on and we have no buzz. therefore not the power amplifiers
So likely the preamp.
However what has me puzzled is the fact that as soon as music is playing the buzz literally disappears. How do I know? I start the Pi playing and the lead in, as it buffers, creates silence.
I am NOT, repeat, NOT replacing the preamp. It is utterly bewitching when playing.
Any further ideas?
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Post by colinf on Jun 30, 2021 17:59:01 GMT 12
If you turn volume to 0 on the preamp are you still getting hum? If you disconnect the RCA cables from the RPi to the preamp what happens to the hum?
AMR-iFi R&D
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Post by jon on Jun 30, 2021 18:30:49 GMT 12
Volume to 0 still hums. However thank you for the suggestion to disconnect the interconnects. Hum has gone.
I must admit I had assumed (nasty word) that leaving the Rpi powered off would suffice. Then it dawned on me that your test was a little different to turning it off.
OK, so that leads me to conclude a new 8 socket plug board is called for... I think.
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Post by colinf on Jun 30, 2021 22:47:14 GMT 12
Ok good, that shows there to be a genuine ground loop, not just hum from the preamp power supply or heater circuit etc. Next look at how many mains earths there are in the system. Each Quad would (or should!) have an earth, the preamp may have an earth and the RPi might have one as well! Unplug each component and measure them disconnected from each other with a multimeter on low ohms resistance range. You’re looking for continuity between the mains plug earth pin and the RCA socket ground on each unit. If the resistance is about an ohm or so on more than one component, you’re likely getting a ground loop through the mains earthing and the power board(s). Ideally only one component would be earthed for no ground loop, but this can lead to safety issues with mains-powered components. If there’s more than one ground (apart from the 2 Quads) you can use a ground-lift resistor, like 10 to 47 ohms 5w wirewound, to lift the earth of the preamp and the RPi, along with anything else plugged in that might be hard-earthed to the mains. I know I shouldn’t plug their products on the forum here (as I’m their chief tech in U.K.!) but iFi make a Gnd Defender for these situations. It’s a small IEC unit that plugs in series with the power inlet of a hifi component. It has the ground lift resistor along with extra safety circuitry in case of a fault with the component that requires earthing to be connected.
AMR-iFi R&D
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Post by michaelw on Jul 1, 2021 17:03:37 GMT 12
That's a fancy solution. In the old days we used cheater plugs. As an industry participant can we have a signature ?
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Post by jon on Jul 1, 2021 18:48:51 GMT 12
Thanks, Colin, for the advice. I will check, probably on the weekend. The builders are in. Central heating!!!
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Post by colinf on Jul 1, 2021 18:59:33 GMT 12
We are having central heating issues as well! It’s summer here so a good time to upgrade.
AMR-iFi R&D
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