If I wanted to use the cap and resistor, which values would be suitable?
I don't know; you likely know more about relay power supplies than I do. But there's a possibly bigger issue, which is why I asked for voltages and whether there was ripple on that relay voltage (forget those tests for now...).
Setting aside the possible cap & resistor,
try this:
- Pull the White, Gray & Black wires from their positions on the PSU PCB header.
- Add a jumper from "POLV" to "POLRtRN"
- Connect the White wire to "PolToMic"
- Connect the Black wire to the "GND" header
- Connect the Gray wire to the "POLV" or "POLRtRN" header
The goal here is to send ~48v to the mic on the "PAT" wire when the switch is in the Omni position, while sending a solid 0v when the switch is in the Cardioid position.
When looking through
all the build pictures (one schematic or board layout would have been invaluable), something odd stood out. The polarity switch appears the select among voltages output by the psu board, derived by R7 & R8. These resistors appear to create a voltage divider from the cleanest B+ node to ground.
Except that's not how the switch and pcb is arranged: there is no connection from the 14kΩ to the 12kΩ resistors until the switch makes the connection. This doesn't make sense to me if the only thing needed by the relay is 0v (off) or 48v (on). I went a little cross-eyed looking at the PSU pcb, mic pcb and assembly photos trying to sort out what went where.
Pictures 12 and 19 (PSU PCB folder) show R7 (14kΩ) runs from B+ (at C5) to the "POLV" output of the pcb header for the polarity switch, and picture 169 shows the White polarity switch wire connects here. R7 (12kΩ) runs from "POLRtRN" output of the header to ground; the Black polarity switch wire connects here. The Gray wire connects to "PolToMic", and there is a board trace which runs this point over to the XLR header labeled "PAT" (to set the mic's polar pattern).
Picture 129 (Build the Power Supply folder) the White wire is on the common terminal on the switch. The Black and Gray wires are on the two terminals selected by the switch for connection to the White wire. This seems odd because the Gray wire sends the relay voltage;it appears pegged to B+ in Omni position, but it appears to float in the Cardioid position. And this relay is a 48v relay according to some web research.
It looks like they dropped the ball for this kit; the Pattern Switch might otherwise be used to select steps along a voltage divider to send different polarizing voltages to a backplate or rear diaphragm, hence R7 and R8 set upper & lower limits in the voltage divider built on the switch terminals. But here, all you need is 48v (ON - Omni) or 0v (OFF - Cardioid) to switch the relay. They chose "no voltage" as the Off setting instead of a defined 0v (which is not the same thing).
I'm thinking the floating setting for cardioid is picking up buzz out of the air and applying it to the rear diaphragm, because the switch wiring did not connect to a solid known voltage for that setting.
Topic: Replica Neumann U47 tube microphone build project finished (Read 16270 times)