Hello,
I am having issues with my Rolls MX51s mixer and static on channel 1 (unused channel). Essentially, when the unit is powered, there is static on channel 1, if I turn the potentiometer the sound increases. Since this channel is not currently being used, I would like to “delete it” from the circuit and just use inputs 2, 3, and 4.
Rolls provides their circuit diagrams free of charge in the user manuals (free online download), however, I find it hard to follow the one for the MX51s so I’ve opened it up and traced the tracks and created my own diagram (attached). I have also attached the public Rolls MX51s schematic. I feel like theirs has some mistakes, however, I could also have made the mistake on a few resistor values during my visual inspection.
I am hoping to get some information on how these two op amps are interacting with one another and if possible, a way to “delete” channel 1 from the output signal while leaving the rest of the 3 channels intact. If needed, I can only use 2 channels rather than 3, if for some reason this is required.
I have put a cap on the XLR input, as well as removed the XLR input incase that was causing static, however, that did not work. I tried a few aftermarket choke and capacitor noise filters on the input power line with no change.
Word of Caution: I searched BA4560 and found the pin layout as shown in my diagram. I do not think there are other layouts and it seems to make sense as shown in my diagram. Please correct me if you think I made an error and I can re check the pins with my multimeter.
Summary of Questions:
1) Is it possible to “delete” only the channel 1 (XLR input) circuit from the mixer signal while leaving channels 2, 3, and 4 alone?
a. If I removed Pin 1 from Ch1 Potentiometer, R24, R17, Pin 8 from BA4560 (2), and Pin C from the transistor, and Pin 1 and 2 from BA4560 (2), would that isolate the circuit and allow BA4560 (1) to mix the first 3 channels? I am concerned with PIN 3 and 5 on BA4560 (1) with this approach. What would this do?
2) What is the purpose of having the polarized capacitors C8 and C11 (and C10 and C13) positive to positive? I didn’t think that was possible?
3) Similar to #2, why would C1 and C6 be neg to Pos in the flow? What does it do?
4) Any thoughts on why I am hearing this static? Anything I can do to get rid of the static?
a. Could it be the DC input to the unit?
5) If someone could explain how the two op amps / this circuit is operating it would be greatly appreciated.
Thank you!
Michael

