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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 20th Oct 2020, 2:27 pm   #1
bikerhifinut
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Default Quad405 earth returns

Quick question. I'm returning these 405 monoblocs back to stereo whilst the boards of one amp are in drydock.
So when I reinstall a new input socket, I am assuming that because I will be fitting a pair of Phono RCA sockets that I need to take a ground return from these to the star earth by the Bridge rectifier. The 4pin din socket soldered on the original mono input board has its chassis earth connected across signal earth and as far as I can see this one tiny nut and bolt is the only earth return for the whole amp board. i.e down the input screens. I wouldn't have thought this was very good practice, not least because it relies on a mechanical return to chassis for all the circuitry.
So on that basis would I be better keeping the input sockets isolated from earth and making an extra earth return with a decent gauge wire at the board input tags? or just fit a tag to chassis near the plugs using a serrated washer etc and rely on that for the ground return. This also looks very much like the only return path for the mains safety earth.

Regards.

Andy.
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Old 20th Oct 2020, 4:14 pm   #2
Trigon.
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Default Re: Quad405 earth returns

The boards are each effectively split into two ground sections. One section is connected to the heatsink at the 'extra' screw, and therefore to the mass of metalwork. The other section is connected via the screen braid to the input socket and back panel.

R2 on each board connects the sections together but only comes into play when the board is tested in isolation, and is effectively shorted out when the boards are installed, with both grounds connected to case.

The original wiring is as follows:-

Star point at rectifier screw (3 green wires):-
1-> PSU cap(-) -> PSU cap(+)
2-> Left speaker output socket
3-> Right speaker output socket

Panel screw connection at input DIN:-
-> connector shell tag -> input gnd connection

Connector shell:-
1-> Left signal screen -> left board input gnd tag
2-> Right signal screen -> right board input gnd tag

If you look closely at the circuit diagrams, the two grounding sections are indicated by the use of rectangular and triangular grounding symbols.

Cheers

Last edited by Trigon.; 20th Oct 2020 at 4:24 pm.
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Old 20th Oct 2020, 5:55 pm   #3
bikerhifinut
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Default Re: Quad405 earth returns

Gotcha.
It does seem rather inadequate at first sight which is what threw me.
So I do need to make one small earth tag as I have fitted RCA phonos rather than the DIN input socket so I guess a short wire to a tag screwed to the panel where the DIN socket was earthed will complete the circuit.
Hope it all works as planned.

Andy.
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