Mic and Mod D-47 kit cardioid noise/buzz issue figured out

[Glacier]

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.

 

[Pip]

First of all welcome and come on in the water is fine!

I must first inform you with all due respect to the fact that you are new! We at groupdiy have no love for M&M they,  to use a phrase, are banned!

So glad you got your mic working though. Please explore the site it is filled with amazing information!

 

[Glacier]

Thanks for the heads up on the "situation" with M&M.  I did not know of any of the drama up until I had built the kit and I began looking into the issue I was having a little further.  That is when I started reading and got worried I had wasted my coin.  I don't want to judge because overall I am not disappointed with my purchase but what it has taught me is to next time do a little more research before I buy another diy kit.  Had I known of the general dislike of M&M, I may not have purchased the kit.

I look forward to being a part of the group here.

 

[micaddict]

Good to have you. 

 

[poctop]

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).

This is exactly the intention when i designed this PCB, they messed up the instruction......  in addition to that I would like to mention that i do not have any affiliation whatsoever with M&M anymore for those reason and  Many other reason.


Best,
Dan,

 

[orangechili]

I just wanted to chime in since I recently built a U47 clone off of the D-47 pcb. I have the switch wired up like the instructions and notice no noise difference between omni and cardioid modes. I have even tried 2 capsules in this mic(chinese capsule and Heiserman HK-47).  Luckily you having this issue got you digging deeper and saw the mistake in the instructions so Thank You! I'm going to wire the switch the correct way now.

On another note:

I bought the D-47 pcb separate from mic n mod a couple years ago when he first had them but he only sent the mic pcb, not the psu board. I asked what happened and if he could sell me a psu board, even offered to pay for one, etc. Silence... He takes the extra mic board on the pcb kit and sells them separate apparently. Of course it wan't advertised like this back then when i bought the 'set'.

Dan was kind enough to sell me a separate psu board and give me some useful tips, i will be buying from him from now on.

Life is short, no time for shenanigans

 

[Glacier]

OK so after a few weeks of using the microphone I have learned a bit more...

The buzz was due to the lack of shield connection in the mic cable.  Seems obvious right?  The build guide wasn't exactly clear.

After I tried the mod I listed in my first post, the buzz did go away.  Someone from this group emailed me with his findings and I found that the mod I listed is not necessary.  I noticed that the voltage from cardioid to omni changed when switching. It went from 105 to 103.  Not a big deal but it was creating some extra current draw in omni mode and if it's not designed that way, it shouldn't happen.

My listed mod doesn't do any harm, it could be left alone.  I have removed the mod for now because the mic is quiet as is.

I loaned the mic to a few recording studio's and they were reporting issues with their lights causing interference in the mic.  This is when it became clear to me that the shield needs to be connected to the ground of the power supply.  Once I connected the shield, the interference issue's disappeared.

I recall when assembling the microphone cable, thinking to myself...why wouldn't the shield be connected.  I knew better.  I figured maybe there was a reason why.

 

[hugo]

OK so after a few weeks of using the microphone I have learned a bit more...

The buzz was due to the lack of shield connection in the mic cable.  Seems obvious right?  The build guide wasn't exactly clear.

After I tried the mod I listed in my first post, the buzz did go away.  Someone from this group emailed me with his findings and I found that the mod I listed is not necessary.  I noticed that the voltage from cardioid to omni changed when switching. It went from 105 to 103.  Not a big deal but it was creating some extra current draw in omni mode and if it's not designed that way, it shouldn't happen.

My listed mod doesn't do any harm, it could be left alone.  I have removed the mod for now because the mic is quiet as is.

I loaned the mic to a few recording studio's and they were reporting issues with their lights causing interference in the mic.  This is when it became clear to me that the shield needs to be connected to the ground of the power supply.  Once I connected the shield, the interference issue's disappeared.

I recall when assembling the microphone cable, thinking to myself...why wouldn't the shield be connected.  I knew better.  I figured maybe there was a reason why.

Thank you for this post and this update