I built a 6J1/LM1875 amp kit from Aliexpress. I built it into my own wooden chassis with a metal backplate from an old amp.
In the left channel for some songs, quiet ones, for example the start of "The A Team" by Ed Sheeran, there is a static sound along with some of the song, but most songs, and later in this song, that disappears.
The issue goes away if I unplug the other RCA lead (the one for the right channel). The right channel seems to have the problem as well but immeasurably less.
Things checked
1. That the PCB ground goes to the metal backplate
2. That this is the only common ground
3. That the RCA shields are isolated from the backplate
4. That the binding post speaker terminals are isolated from the backplate
5. That the LM1875 chips are isolated from the heatsink (that is part of the backplate).
I have created my own DPDT switch system to switch between two inputs. So from a 2-input RCA (four ports) set of inputs the shields are connected between both sets of inputs. The left and right channels are kept separate and all of this goes to the DPDT switch which allows switching between channels.
The problem seems to be better, but not gone, if I subsitute just a two port RCA jack that connects directly to the input (bypassing the switch system).
I have tried connecting a 10nf ceramic capacitor from the RCA shield to the backplate (which is shared by ground) but this just created a ground loop
It's weird that the problem goes away when one RCA cable is removed.
In the left channel for some songs, quiet ones, for example the start of "The A Team" by Ed Sheeran, there is a static sound along with some of the song, but most songs, and later in this song, that disappears.
The issue goes away if I unplug the other RCA lead (the one for the right channel). The right channel seems to have the problem as well but immeasurably less.
Things checked
1. That the PCB ground goes to the metal backplate
2. That this is the only common ground
3. That the RCA shields are isolated from the backplate
4. That the binding post speaker terminals are isolated from the backplate
5. That the LM1875 chips are isolated from the heatsink (that is part of the backplate).
I have created my own DPDT switch system to switch between two inputs. So from a 2-input RCA (four ports) set of inputs the shields are connected between both sets of inputs. The left and right channels are kept separate and all of this goes to the DPDT switch which allows switching between channels.
The problem seems to be better, but not gone, if I subsitute just a two port RCA jack that connects directly to the input (bypassing the switch system).
I have tried connecting a 10nf ceramic capacitor from the RCA shield to the backplate (which is shared by ground) but this just created a ground loop
It's weird that the problem goes away when one RCA cable is removed.
Hi, I checked the voltage from the input using a multimeter and it was about 0.1mV - this was putting the negative and positive probes on the negative and positive inputs respectively.
I also checked the DC offset going out of the amp by unplugging the speakers, shorting the right+ground inputs and then the left+ground inputs and testing each speaker terminal pair with the multimeter. Each speaker output pair is around 2mV
I also checked the DC offset going out of the amp by unplugging the speakers, shorting the right+ground inputs and then the left+ground inputs and testing each speaker terminal pair with the multimeter. Each speaker output pair is around 2mV
In a wooden box everything could happen , for example it is picking any RF field from outside and auto-oscillate combined with music ... You can't do a good job without a proper case .
I seem to have solved it by a way i cannot understand. I was holding the back RCA ports to see if there was some issue with them being loose and realised if i held both the outsides of the negative RCA cables and then touched by hand to the heatsink (which is part of the metal backplate) the static in the left channel went away.
I then folded some aluminium foil and wrapped it around the two negative rca cables where they enter and attached this to the heatsink.
Is this some sort of RF Shield?
I then folded some aluminium foil and wrapped it around the two negative rca cables where they enter and attached this to the heatsink.
Is this some sort of RF Shield?