I tried that but it hummed very loudly. The amp PCBs are only grounded via the screened cable inputs running from the phono sockets, so the green wires need to be there to ground the sockets.
I think there should be a rubber insulating pad under the metal plate attaching the toroid to the top plate. Otherwise the heater wiring could be routed around the edges of the case.
What happens to that green wire from the phono plugs? The sheathed cable for the signal input is likely grounded on the PCB.
It goes to the ground point on the chassis. It's hard to see on the picture but its on a solder tag that also connects to the earth of the IEC connector, on the bottom right nut that holds the righthand output transformer.
There is a rubber pad under the toroid's mounting plate - you just can't see it in the pic.
There's also an ally screen around the PSU board now.
There is a rubber pad under the toroid's mounting plate - you just can't see it in the pic.
There's also an ally screen around the PSU board now.
Last edited:
Try disconnecting the wire from the phono plug to the chassis earth. If it is also grounded on the PCB then there is the potential for a ground loop.
I have, but I think the PCB is only grounded via the screen of the coax from the phono plug. It hummed like hell when I disconnected that.
That sounds a bit odd. Presumably the ground of the plug goes to the sheath, and that must be attached to some form of ground on the PCB. How is the PCB grounded?
I think that when you omitted the volume control, you have upset the grounding scheme in some way. Personally I would have kept the volume control because it is a good way to adjust the gain of the amplifier, and it makes it more convenient to link it to other components, like a CD player, if you do not have a pre-amplifier to control the volume.
Looking at the wiring diagram the PCB is, as far as I can tell, only grounded via the sheath of the coax that feeds the phono input to it. The phono input is grounded to chassis earth.
I think it is because the volume control presented the 100k input impedance to the source. If you omit it then I think you have to add a 100k resistor across the phone input.
I had that thought too. But I’ve grounded both inputs and it makes no difference to the hum so, sadly, 100k resistors wouldn’t get rid of it either.
Are those Green wires from the RCA Phono Input Connectors Ground Lug going to a ground point on the amplifier?
Those shielded coax going from the RCA connectors to the PCB . . .
Are the shields at the PCB end connected to the PCB ground?
Those 2 ground connections, one at the RCA connector, and one at the PCB are
what I call a Ground Loop
(that includes the wiring from the Green wire coming from the RCA connector and what ground point it connects to; versus the PCB ground and what point it connects to. All that wiring is a Ground loop that traverses the amplifier at least 2 times the length of the preamp. Loopy Loop anyone?
A Ground loop that includes the input circuitry gets amplified more than any other ground loop. Hmmm.
Check it out.
1. Wires are 1/2 of an Air Transformer.
2. Wires are Resistors.
3. Wires are Inductors.
Any current, and any Magnetic Fields that impinges on the 3 items above, will create a Voltage, which according to Murphy's Law will be amplified.
Grounds are Commonly Misunderstood" - Me
Those shielded coax going from the RCA connectors to the PCB . . .
Are the shields at the PCB end connected to the PCB ground?
Those 2 ground connections, one at the RCA connector, and one at the PCB are
what I call a Ground Loop
(that includes the wiring from the Green wire coming from the RCA connector and what ground point it connects to; versus the PCB ground and what point it connects to. All that wiring is a Ground loop that traverses the amplifier at least 2 times the length of the preamp. Loopy Loop anyone?
A Ground loop that includes the input circuitry gets amplified more than any other ground loop. Hmmm.
Check it out.
1. Wires are 1/2 of an Air Transformer.
2. Wires are Resistors.
3. Wires are Inductors.
Any current, and any Magnetic Fields that impinges on the 3 items above, will create a Voltage, which according to Murphy's Law will be amplified.
Grounds are Commonly Misunderstood" - Me
Last edited:
Hi 6A3sUMMER, if you look at the wiring schema in the construction guide then it does look like the only grounding is from the RCA phono plugs, i.e. the PCB grounds through the shielded cable to the chassis ground.
I understand what you're saying but I can't see how else to wire it that might improve things. The amp PCBs are only grounded at one point, via the screen of the coax input leads, which are connected to the single chassis ground point via the green cables. If I remove any of these leads, the hum becomes much louder, not quieter.
is there any mileage in my thought that the PSU for the relay switching board - or the board itself - could be anything to do with this or am I clutching at straws?
is there any mileage in my thought that the PSU for the relay switching board - or the board itself - could be anything to do with this or am I clutching at straws?
Assume that the original documentation would result in a fully functional amplifier, and any mods you did could have introduced the hum. Then you need to start from the closest circuit matching the original spec, and test from there.
Try to use just ONE of the green wires, so you break the ground loop. If that works, keep one green wire and connect the grounds of the two RCA input plugs with 10 ohms.
Jan
Thanks Jan. I've just removed the green wire link between the left (as you look at the diagram) phono socket and the right one. Makes no difference whatsoever - so I've left it off. However, removing the link between the righthand socket and chassis ground introduces a load more hum, so I've put that back.
I don't understand why removing the link from the lefthand phono ground connection makes no difference. Surely if the righthand one needs connecting to chassis ground the left does too!?
I don't understand why removing the link from the lefthand phono ground connection makes no difference. Surely if the righthand one needs connecting to chassis ground the left does too!?
What you have done does not look correct. There is a good link somewhere on diyaudio for solving hum loops. There should be no green wires on your phono inputs. The power supply 0V should go straight to each PA board not through the phono inputs. Make the power supply 0V the star point and take mains earth and the PA 0V to that.