Quad 405 Hum when BOTH inputs connected

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06-Nov-2019 06:21 PM

Fergster

Fergster

Posts: 2

Pulling my hair (what's left of it!) out with this issue... 

405 restoration project - early boards 12368 v6 with a few mods (OPA604 with caps, reduced gain, 15V zeners, higher-spec caps, D13 added). Already had a torodial fitted. 22kuF caps (fine - tested on another unit). Boards fully recapped. RCA gnd inputs connected to chassis at input, as is mains earth, & smoothing caps 0V bar. Output gnd is direct to 0V bar.

With no input, there's a very tiny hum from speakers (tiny, but audible if very close, but could probably live with it).

Connecting either RCA input (e.g. to phone output) - no change. However, connecting BOTH L & R, and I get the earth hum - very noticable. The cable is fine (as is the phone output) - both tested on a 606 fine.

I'm at a loss to see where the issue is coming from. Any help/suggestions would be greatly appreciated! Thanks!

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19-Nov-2019 11:04 AM

FBatista

FBatista

Posts: 127

Interesting topic. Any progress?

It seems to be a ground loop issue!
I'd remove the earth connection from the 405 and uprate the IC cable from the preamplifier to the 405, for a start. Not just a thick cable, but a decent copper mesh on it's shield.

Then I'd put all ground points to the 0v bar, plus an extra wire from each pcb's dissipator also to the 0V bar.

Lastly I'd release the centre bolt from the trafo, to rotate it whilst listening the output of each board (work your way to the lowest possible hum, if any). There's a fair chance the toroidal is also radiating.

My 405 is very similar to yours, according to your initial post, except for the 0v bar and the toroidal. I get very residual hum with the ear next to the speaker.

Good luck!


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20-Nov-2019 01:42 AM

EJP

EJP

Posts: 1676

I can't agree with that. It isn't safe. Leave the mains ground alone and insulate the RCA connector grounds. They are supposed to be ground-lifted from the chassis ground.
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20-Nov-2019 03:28 PM

Fergster

Fergster

Posts: 2

Thanks for the suggestions - but I did finally get to the source of the issue.

It turns out that the input earth pin on one of the boards was not connected to the central earth point on the board (e.g. R23, R19, R13 etc). Looking from the component side of the board, there's a link wire going from one end of C4 towards the input signal pin. At some stage the previous owner must have disconnected this from the solder pad nearest the input pin, and re-soldered over the pad (so it looked like there was something soldered!). They then placed the link wire back in the hole (on the component side) so it looked like it was connected!! Madness! I can only assume it was a really large horse that kicked him in the head earlier on the day he did that!

Anyway - now sounding good! Thanks for the suggestions!


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