Hi Guys,
It's my first post here. I am normally building tube guitar amps but I also enjoy building microphones, preamps, compressors etc.
I ordered the mic and mod (vintage microphones) diy U47 kit last week, built it and had a 60Hz buzz/hum in cardioid mode. That's a pretty big bummer for a $1400 mic kit. (that's what my total cost was in Canada)
After scratching my head and going cross-eyed looking over the pcb's with a buddy on another tube amp forum I am part of, I am super happy to share with you a fix for this mic kit.
I cannot take the credit for this fix, "Hot Blue Plates" on the Hoffman Tube Amplifiers forum figured this out...thus I will dub this the "HBP MOD"
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.
If you do that, your microphone will be silent in both cardioid and omni.
It's my first post here. I am normally building tube guitar amps but I also enjoy building microphones, preamps, compressors etc.
I ordered the mic and mod (vintage microphones) diy U47 kit last week, built it and had a 60Hz buzz/hum in cardioid mode. That's a pretty big bummer for a $1400 mic kit. (that's what my total cost was in Canada)
After scratching my head and going cross-eyed looking over the pcb's with a buddy on another tube amp forum I am part of, I am super happy to share with you a fix for this mic kit.
I cannot take the credit for this fix, "Hot Blue Plates" on the Hoffman Tube Amplifiers forum figured this out...thus I will dub this the "HBP MOD"
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.
If you do that, your microphone will be silent in both cardioid and omni.
