Hi,
I made some analog crossover and I need to choose only one output from all 8 toutputs I have. So I designed well known circuit with one j112 in series with inverting op amp and one shunt j112 to the ground. The controll circuit is based on Soundcraft GB-8.
When I forget on the op amp resistor values (can be higher), the DC offset on the end of previous stage, separate ground for reference (the difference is constant and really small), distortion (which is not as low as I want, but still very low), I ges some magic on this. The relay (left in the schematic) is driven from separate 12V source, which is conected to analog +-15V in one point (ground to ground).
When the stage is muted, I get normal DC offset on the output of the mute stage (0,5mV) , but ... when I get UNMUTE the stage, the output offset is not something like DC one stage + DC (inverted) second stage. The result is about 2 - 6mV and it moves (!!!) when I MOVE the switch with 8 + 1 wires, which are driving the relays...
I dont understand it. I understand noise from AC line, noise from bad layout... but DC i don't understant.
Note that the control voltages are ok, for the series fet it is about 15mV when it is open, -14,8X when it is off. For the shunt fet is is similar, signal goes thru it well when the circuit is ON, nothing get thru it when OFF, the traces are realy short, all smd, j-fet op amp...
Function OK, but I want to get from all pops and clics. I know i need to block the stages and so... but I dont realy understand the fu**** DC movement on the output with relationship to moving the relay cables... Help me please :-[
I made some analog crossover and I need to choose only one output from all 8 toutputs I have. So I designed well known circuit with one j112 in series with inverting op amp and one shunt j112 to the ground. The controll circuit is based on Soundcraft GB-8.
When I forget on the op amp resistor values (can be higher), the DC offset on the end of previous stage, separate ground for reference (the difference is constant and really small), distortion (which is not as low as I want, but still very low), I ges some magic on this. The relay (left in the schematic) is driven from separate 12V source, which is conected to analog +-15V in one point (ground to ground).
When the stage is muted, I get normal DC offset on the output of the mute stage (0,5mV) , but ... when I get UNMUTE the stage, the output offset is not something like DC one stage + DC (inverted) second stage. The result is about 2 - 6mV and it moves (!!!) when I MOVE the switch with 8 + 1 wires, which are driving the relays...
I dont understand it. I understand noise from AC line, noise from bad layout... but DC i don't understant.
Note that the control voltages are ok, for the series fet it is about 15mV when it is open, -14,8X when it is off. For the shunt fet is is similar, signal goes thru it well when the circuit is ON, nothing get thru it when OFF, the traces are realy short, all smd, j-fet op amp...
Function OK, but I want to get from all pops and clics. I know i need to block the stages and so... but I dont realy understand the fu**** DC movement on the output with relationship to moving the relay cables... Help me please :-[