Best practice for pre amp rewire, help pls

[evozero]

Hi All,
Can you guys advise me on the best way to rewire my preamp to reduce hum?
I have an ASL 2004dt wave pre amp,and its noisy! I have tried changing the valves, diodes, caps, vol pot and resistors in the signal path, and Diodes in the PSU.
The sound quality has improved but the hum remains. i am currently using an DIY passive pre to drive my wave 8 mono blocks, the hum is so much better but the dynamics have reduced from the phonostage, CD is better.
My plan is to rewire the signal cables and try and organise the grounding to reduce hum. I will shorten the cap leads, they didn't make it any worse when I added them. My long term plan is to build a pre3 and phono3, but want to try and resolve this in the meantime. I have added some photos of the current wiring and would really appreciate your input.
Many thanks
Ian

[evozero]

Another pic showing the three inputs on the left and output on right. Notice all the RCA's are joined via bus wire, but both input and output have ground wires joined to the bus and back too the main chassis ground. Also the power connector is earthed onto the chassis, not the main ground.
Is this correct.
Many Thanks
Ian

[soulminer]

Originally Posted by evozero

Is this correct.

Yes

[Richard]

Hi,

I can't see if the circuit is grounded to the mains/chassis earth at all. Usually the case is solidly connected to main earth. The signal ground is then floated inside the case and connected from its earthiest point to the mains earth at one point only via 100R resistor. Earth currents should flow just one way from the sensitive input to the earthier psu gnd point.

Attached is how I'd instinctively wire it. The pcbs each have a gnd connection point. The one on the smaller board is the psu and should be earthier. Connect that to the mains/chassis earth tag via 100R resistor. Connect the 2 pcb gnd points.

Break the input/output ground connection. The output ground goes to the earthier gnd on the small board. Break the pot ground wires and take them to the gnd on the larger board.

When done, use your meter to check the mains earth is solidly bonded to the chassis parts and that the signal ground reads 100R to chassis. Pay particular attention to the way the screen braid is used on the coax wires. On the inputs and outputs I'd expect it to be connected at one end only to make a screen but check it's not been used as a ground connection.

Rich

[evozero]

Hi All / Richard,
Thanks for the advice, much appreciated. I will try all the sugestions and report back. Can I clarify the grounding cables you have question marked are wrapped around the signal wires and back to the main grounding point. the grounding point is one of the transformer bolts, nearest the dark blue cap on the PSU board. You are correct the screen braid is only connected to ground at the RCA end on the signal cables. on the alps pot the screen braid is connected to the pot ground pins.
Does this change your advice?

Someone else on diyAudio pointed me in the direction of this article;
http://www.tubecad.com/april_may2001/page28.html
Bottom line is its a bit of a rubbish design.
I was planning on rewiring with SONIQS interconnect, but probably not worth the cost, my as well put the money into a PREIII.
What do you think?
Many thanks
Ian

[Richard]

Hi Ian,

I think I can see where the earth wires connect to the case; picture top left of the small pcb?

It seems not to have an earth lift resistor. I would follow the pink connections in the pic. It would be easy to try without much surgery.

Break the input/output ground connection. Disconnect the in/out question marked earth wires at the chassis tag end and solder them onto the large and small pcb gnd points shown.

The pot wires too look like they go to that chassis earth point at the moment so take them instead to the gnd on the larger pcb as shown.

That should be easy using the existing wires. Link the 2 pcb gnds. Then run a new separate wire via 100R resistor (3W or bigger wirewound) to the chassis earth post where the mains earth bonds.

In/out screen braids sound correct, so just check the ones from pot to pcb are only connected at one end also. Lastly, check with a probe on a pcb gnd and another on the chassis earth post for a reading of 100R. If it's 0R or a reading lower than 100R there will be another connection to chassis somewhere.

Sorry for the pedantic instructions but try it exactly as drawn first (ie make the connections at the points shown rather than commoning them elsewhere first or somesuch which might seem like the same thing). The reason is to make a route for earth currents so larger ones don't circulate in sensitive areas. Think the water analogy with big pipes/flow and small ones. Just the position of a connection on a bus bar can make a difference sometimes, relative impedance and all that

Rich

[evozero]

Hi Richard,
I have followed your advice,rewired the grounds as per your diagram and cleaned up all the solder joints. I also replaced some of the wiring and separated the signal and power wires.. Much better! reduced the noise by a huge margin. I am now going to strip the wave 8 mono blocks and do the same. I will give you an update in a day or so.
Thank you so much for your invaluable advice.
Ian