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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EAR834p clone with hum. Would appreciate some help

Aweloi - One addition: the earthing of this board has a flaw, and what I did was disconnect one layer of the earth circuit feom the other channel. I know it can’t be the source of a big hum, but I do not have the link here. However I showed a picture of where I cut a trace , on the Lencoheaven.
I see you have with a lot of dilligence made a metal chassis.
An alternative is thin copper or alu foil that can be glued to the wood . .
Hi. Tin aluminum plate as you used are good for RF shielding, but won’t do anything for line AC signal 50-60hz. You need much beefier alu shielding to be effective at AC line frequency, probably at least 1/4 inch thickness, read this from John Curl years ago.
for effective shielding you need steel or mu-metal. An other thing you can try is to rotate your power transformer, had to fine tuned my toroid transformer for the lowest noise in one of my tube preamp, even if the transfo had shield.

As some other said this big switch to select the inputs is a very bad idea on a phono preamp, by the look at it is a main power on switch. You’ll need at least a low signal relay to switch these very low signal inputs. Your large switch can also degrade the sound.

By using the Step up transformer on the highest gain setting may also contribute to your noise. With the preamp gain, the lowest setting on your step up transformer is probably more than adequate. past a certain gain from the phono section, it is better to increase the preamp volume, than to boost the phono gain too much, because the phono gain will also amplify the noise, which won’t be an issue if you rather amplify the phono output with the preamp.

from ‘All about circuits’ forum:

If you are looking for reduction from hum - that is hum caused by local magnetic fields - you need to provide a path for the magnetic fields causing the hum to be shunted around or away from the sensitive circuit that is susceptible to magnetic pickup. To do that effectively the 'shield' needs to be of a material that has high permeability or low reluctance (they are reciprocal like resistance and conductance). The permeability of iron is .25 and the permeabilitu of aluminum is 1.25 x 10>-6. Nearly a million times less able to reduce magnetically induced hum. Bottom line? Aluminum works fairly well on electric fields (RF), but it's very weak about magnetic fields (ac)

So alu is nearly a million time less effective to reduce ac magnetic field than steel…
Even if the separation plate is steel, any mechanical vibrations from the transformers will be heard on the output. This is because they are mounted on the same chassis. Such vibrations are typically at the mains frequency.

This is one of the main reasons I moved over to toroidal transformers (for Phono amps) which could be mounted with some softer rubber insulator. Another option is to build the power supply in a separate box.

The OP can try temporarily re-locating the mains transformer outside the phono box.
Wait a moment.
1) There is a lot of info on hum loops. These are created because there are large 'openings' in the loops that are in the circuit.
- you might very well ahve such loops. Check out the info.
2) you now have an earthed shield on the input and output; but I would advise that the output only has one connection to the main circuit ground, so the shield is left floating at the RCA terminal. I would do the same with the input.
[I have easytalking. I have a power amp in which one channel connected is OK; connect the second one, hum. I understand your frustration :-|)
And turning the transformer 90 degrees (or almost) is not hard; it might just make the magnetic coupling with some circuit loops less. Hopefully less than your threshold.

I agree with Algar and so the next tip is: take a 'conservenblik' (you might naturally understand our word) and push that over the transformer . . .