I'm trying to puzzle out how 0V is summed in the SSL 4k.
Taking a look at the mix bus card 82E26, "mixbus 0V" isn't referenced directly to local ground. It's got 16k (or a 21k trimmer) between the non-inverting input and ground. I believe this is for balancing bias current, as 16k is also the value of the feedback resistor.
Interestingly, none of the summing resistors seem to be shown in the service manual. As best I can tell from other threads, signal hot uses 15k summing resistors. What I can't figure out is whether summing resistors are used in the 0v line as well (either 15k, or some smaller value for quasi-impedance balanced).
The only clue I can find is on the 82E30 Echo Return card, where Rin of the output buffer (R14, etc) appears to be referenced to the 0V bus (pin 15) rather than a local ground.
A few other references: Douglas Self has a version with 10R between channel ground and the ground bus and 100R to ground at the non-inverting input. Fred Forssell's summing bus design uses 1k between channel ground and the ground bus and 1k to ground at the non-inverting input.
Taking a look at the mix bus card 82E26, "mixbus 0V" isn't referenced directly to local ground. It's got 16k (or a 21k trimmer) between the non-inverting input and ground. I believe this is for balancing bias current, as 16k is also the value of the feedback resistor.
Interestingly, none of the summing resistors seem to be shown in the service manual. As best I can tell from other threads, signal hot uses 15k summing resistors. What I can't figure out is whether summing resistors are used in the 0v line as well (either 15k, or some smaller value for quasi-impedance balanced).
The only clue I can find is on the 82E30 Echo Return card, where Rin of the output buffer (R14, etc) appears to be referenced to the 0V bus (pin 15) rather than a local ground.
A few other references: Douglas Self has a version with 10R between channel ground and the ground bus and 100R to ground at the non-inverting input. Fred Forssell's summing bus design uses 1k between channel ground and the ground bus and 1k to ground at the non-inverting input.
