Does the surface mounted version of the MXL990 have the same circuit as the through-hole version?

[Paul W]

R6 is already wired in series with R6a and R7 likewise in series with R7a. I would think that rather than replacing each pair with an equivalent single resistor I could just leave them that way, and disconnect the switch which optionally shorts across the a member of each pair. (Which I want to do anyway so I can use it for a proper pad.) Is there an advantage to a single resistor vs. a series pair? ( I don’t understand audio circuits very well and don’t know if that’s noisier or something.)

 

[jp8]

The idea of my proposal was to leave the pad switch on the PCB, which is then held on the PCB by the lower three pins. As the lower three pins still connect to the pad resistors, the resistor divider can be undone by shunting the 1k5 an replacing the 680R by 1k5 + 680 = 2k2. Because the upper section of the switch was disconnected from the pad circuit, so replacing the resistors in this branch would not be requiered, but this keeps everything visually symmetrical, for what it's worth. I think the resistor swap is the easiest part of the mod, so why not do it?

The resistor swap will not change the Johnson (thermal) noise, unless you want to be nitpicking about the 20R difference between the 2k2 resistor and the combined 680R + 1k5 resistors. The difference is just 0.039 dBV, unweighted, and can be ignored.

Jan

 

[Paul W]

The idea of my proposal was to leave the pad switch on the PCB, which is then held on the PCB by the lower three pins. As the lower three pins still connect to the pad resistors, the resistor divider can be undone by shunting the 1k5 an replacing the 680R by 1k5 + 680 = 2k2. Because the upper section of the switch was disconnected from the pad circuit, so replacing the resistors in this branch would not be requiered, but this keeps everything visually symmetrical, for what it's worth. I think the resistor swap is the easiest part of the mod, so why not do it?

Mine is the surface mount version and I don’t have mad soldering/desoldering skills, so leaving the resistors alone is appealing.

I was planning to leave the switch in place physically (it is soldered into through holes) but hoping I could cut the relevant traces to disconnect it electrically from the resistors, and rewire it to the new capacitor. I haven’t figured that out in detail, though.

 

[jp8]

Ah, that's right. My bad. I should have known you have the SMT version. Then I guess cutting traces is indeed the easiest solution. Good luck with the modification!

Jan

 

[dbonin]

You could either shunt the capsule with a capacitor, or lower the capsule polarisation voltage. Took a quick glance on the PCBA and schematic and I think the easiest way is to shunt the capsule with a capacitor. Value would depend on the amount of attenuation required and capsule capacity. I would suggest to use an NP0/C0G type ceramic capacitor.

R6 and R7 (680R) should be changed to 2k2. Remove R6a an R7a (1k5) and replace by a jumper wire. Cut the upper three pins of the pad switch. Connect the left pin of the switch to ground. Connect the capacitor between the middle pin of the switch and the left insulated ptfe turret pin, which connects to the capsule. If my description is not clear, I can draw up the schematic and make some pictures and drawings if you like. Sometime next week, when I have more time.

Jan

I just want to say, I f'ing love that a random dude needs help and some other random dude responds with such nerd love that I can't get the smile off of my face! Props to you both. Long live groupdiy!