ELA M251 Inspired Microphone - Build Thread

[ribor92]

Hello Guys! Trying to sort out a build I did a few years that never really made it over the finish line.

First and foremost I didn't build this mic, I had help from a friend that has the skills. But I'm trying now to see what may be wrong so I can fix it.

I'm starting to suspect that the Capsule is wired wrong based on the info from build guides etc.
I have signal and it sounds good but there is significant noise and it's pretty low output (into ribbon mic territory).
Polar patterns work as intended, and the transformer is wired the right way (confirmed after measuring).

I will post a few pictures of the wiring and you may see for yourselves and maybe can I get some much needed clarity and hopefully a solution! To me it looks like backplate and capsule connections are swapped, am I wrong?

Kind regards
Ritchie

 

[Matador]

BP nodes should be bridged together.

 

[ribor92]

RC and FC should be bridged together.

Good to hear from you Matador!

So just to clarify.
To my (limited) knowledge, shouldn’t the black wires be the backplate?

The black wires are now wired as FC and RC to the pcb right? And the blue wires are bridged and used as backplates. Seems reversed to me from the instructions in the C12 build guide where the CT12 is used.

Sorry for being such a noob!

 

[Tim Campbell]

Wow that is a very old capsule of mine and the membranes look extremely dirty. I have never written on any of my capsules. Where did you source this capsule from? In your photos it almost looks wet.
FC should connect to the front membrane, RC to the rear membrane and the bridged connection to the backplates

 

[ribor92]

Wow that is a very old capsule of mine and the membranes look extremely dirty. I have never written on any of my capsules. Where did you source this capsule from? In your photos it almost looks wet.
FC should connect to the front membrane, RC to the rear membrane and the bridged connection to the backplates

Hello Tim!

Yes it seems to have collected some dust laying in a drawer the last couple of years unfortunately! I got it straight from you somewhere around 2018 so it's been a while. I hope it's just as good though

 

[Matador]

Good to hear from you Matador!

Disregard what I said: my post has been corrected.

RC goes to rear membrane, and FC goes to front, as Tim said. The BP node goes to the backplate. However if your capsule has two backplates, they need to be tied together. You can do this in one of two ways:

1) Two wires, one from each backplate, get tied to the same "BP" node
2) One wire goes to one of the backplates, and the other backplate is jumpered to the first with a short wire header

Since it seems you have separate wires, option 1 is probably easier.

Based on what I see from your pictures, the two capsule membranes are tied together, and the backplates are separate. This will face the two sides of the capsule "inward", meaning you have two cardioid capsules pointing to the "inside" of the capsule. You'll only ever get the "rear" response which will be muffled and dark.

This definitely needs to be swapped.

 

[ribor92]

Disregard what I said: my post has been corrected.

RC goes to rear membrane, and FC goes to front, as Tim said. The BP node goes to the backplate. However if your capsule has two backplates, they need to be tied together. You can do this in one of two ways:

1) Two wires, one from each backplate, get tied to the same "BP" node
2) One wire goes to one of the backplates, and the other backplate is jumpered to the first with a short wire header

Since it seems you have separate wires, option 1 is probably easier.

Based on what I see from your pictures, the two capsule membranes are tied together, and the backplates are separate. This will face the two sides of the capsule "inward", meaning you have two cardioid capsules pointing to the "inside" of the capsule. You'll only ever get the "rear" response which will be muffled and dark.

This definitely needs to be swapped.

Many thanks for the clarification!

I will have my tech guy fix this next week, and hopefully this will also fix the noise issues I have had, is this a plausible cause you think?

 

[paulcheeba]

Will these boards ever be back in stock? Thanks

 

[ribor92]

So for an update I have now corrected that faulty wiring on the capsule and it works much better now of course!

One question though, I was looking at the B+ and the Heater and B+ is solid at 120 now so that's good.
When I check the Heater in the PSU it's perfectly at 6.3V showing "006.3" on my multimeter. But when I check in the mic body it says "065.5".

What should I make of that? Normal?
If not, what could it be?

Kind regards
Ritchie

 

[MagnetoSound]

One question though, I was looking at the B+ and the Heater and B+ is solid at 120 now so that's good.
When I check the Heater in the PSU it's perfectly at 6.3V showing "006.3" on my multimeter. But when I check in the mic body it says "065.5".

What should I make of that? Normal?
If not, what could it be?

Probably depends where you put your negative probe for 0v reference. The ground voltage at the mic body end of the 0v wire or screen will be at a slightly higher potential than the PSU chassis due to the resistance in the cable.

The heater voltage falls but the 0v rises. Depends which is the thicker wire.

 

[ribor92]

Probably depends where you put your negative probe for 0v reference. The ground voltage at the mic body end of the 0v wire or screen will be at a slightly higher potential than the PSU chassis due to the resistance in the cable.

The heater voltage falls but the 0v rises. Depends which is the thicker wire.

Thank you for the answer! I use the mic body as ground when measuring as in the build guide. But I read it as 10x higher at 065.5 as opposed to 006.3 at the PSU, from those number but it sound unreasonably high to me? Or is it just the decimal point being thrown around in my multimeter?

 

[rock soderstrom]

I use the mic body as ground when measuring as in the build guide. But I read it as 10x higher at 065.5 as opposed to 006.3 at the PSU, from those number but it sound unreasonably high to me?

The high heater voltage makes no sense to me, unless you have intentionally elevated the heater voltage.. Where are you measuring it? One measuring tip is on the housing and the other is where?

Or is it just the decimal point being thrown around in my multimeter?

That would not be a particularly trustworthy multimeter

 

[Matador]

If you measure resistance between the mic shell and PSU ground, what do you see?

 

[MagnetoSound]

But I read it as 10x higher at 065.5 as opposed to 006.3 at the PSU, from those number but it sound unreasonably high to me? Or is it just the decimal point being thrown around in my multimeter?

Yeah, I totally missed that. I thought it was a typo and read it as 006.5v.

I’m suspicious of where the other 59 volts is coming from!

 

[ribor92]

If you measure resistance between the mic shell and PSU ground, what do you see?

I’m reading 0.025 from PSU ground to mic shell ground.

 

[ribor92]

One measuring tip is on the housing and the other is where?

On the tube connection like the build guide.

 

[rock soderstrom]

On the tube connection like the build guide.View attachment 122620

Please tell me which pin that is on the tube, it's hard to see in the photo. I think you are measuring the anode voltage on pin 6? Then the voltage you mentioned would make more sense.

 

[rock soderstrom]

I’m reading 0.025 from PSU ground to mic shell ground.

Numbers without information about the units you are measuring make no sense in our field.

What are you measuring? Volts, Ampere, Ohm or Watts?

 

[ribor92]

Numbers without information about the units you are measuring make no sense in our field.

What are you measuring? Volts, Ampere, Ohm or Watts?

Sorry I meassured Volts!

 

[rock soderstrom]

Sorry I meassured Volts!

But Matador was asking for the resistance between the mic shell and the PSU ground.
Deal, from now on always with units after the number. That makes communication much easier.

Check the pinout of the tube, this gives you exact info about where you are measuring...