It isn't weird at all. As long as an active (powered) source of low output impedance is connected, it shorts the noise source when you turn the volume control fully cw.
Best regards!
Best regards!
I just wanna provide updates to the project. I think I have kinda fixed the problem, which is grounding. Here's what I've done.
1. I check for ground short circuit to chassis. I found that the input RCAs and two ALPS pots are short to chassis.
2. Also, the two ALPS pots metal parts are not grounded. They introduce more hum & hiss when touch.
Then I tried the following fix.
3. I've removed all other inputs, removed tape mon feature (never use) and left only one input.
4. I also changed the cables between input -> volume/balance PCB -> preamp PCB, using mic shielded coax cable.
5. I've grounded the volume and balance pots (to signal ground) and ensure the pots and input RCA ground are not short to chassis.
6. I ensure that there's only one point that circuit ground (signal ground) is connected to chassis.
The hum/hiss is certainly reduced. However, I investigated further and found out that there is weird ground linking wires underneath the PCB which link all grounds at the middle of the PCB (power supply rails, input signal, output signal ground). This "star" ground then wire to rear chassis binding post. I don't remember doing this. I guess it could be the service guy might have done this (I took it to service once in the past few year).
This seem to be OK but I experiment by disconnecting input signal ground from the power supply ground. Voila! The noise is almost gone! Only slight hiss at 3 o clock mark, which is acceptable.
The noise pick up again when large appliance, such as, refrigerator or water heater came on, but it much less than before. I guess I can try some EMI filter on main AC next.
1. I check for ground short circuit to chassis. I found that the input RCAs and two ALPS pots are short to chassis.
2. Also, the two ALPS pots metal parts are not grounded. They introduce more hum & hiss when touch.
Then I tried the following fix.
3. I've removed all other inputs, removed tape mon feature (never use) and left only one input.
4. I also changed the cables between input -> volume/balance PCB -> preamp PCB, using mic shielded coax cable.
5. I've grounded the volume and balance pots (to signal ground) and ensure the pots and input RCA ground are not short to chassis.
6. I ensure that there's only one point that circuit ground (signal ground) is connected to chassis.
The hum/hiss is certainly reduced. However, I investigated further and found out that there is weird ground linking wires underneath the PCB which link all grounds at the middle of the PCB (power supply rails, input signal, output signal ground). This "star" ground then wire to rear chassis binding post. I don't remember doing this. I guess it could be the service guy might have done this (I took it to service once in the past few year).
This seem to be OK but I experiment by disconnecting input signal ground from the power supply ground. Voila! The noise is almost gone! Only slight hiss at 3 o clock mark, which is acceptable.
The noise pick up again when large appliance, such as, refrigerator or water heater came on, but it much less than before. I guess I can try some EMI filter on main AC next.
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