OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

[AdamFrisen]

Hi @homero.leal

Could you explain how to use TXC in v1.1 of your PCB?
I see it is not connected but could be tied to VCC with a jumper.
I thought it was for the shield/ground wire of the transformer and I read the wire should go to the ground somewhere.

 

[pasarski]

It's a center tap of the transformer. Phantom power is taken from it.

 

[AdamFrisen]

It's a center tap of the transformer. Phantom power is taken from it.

Thanks.

So if the transformer has a center tap wire, I can skip 2K2 resistors and connect that wire to phantom power by bridging that jumper, J1-J2.

What is the use of TXC when the J1-J2 is not connected to phantom power?

 

[AdamFrisen]

UTM 0587
Connect the wires like this:
Red at C7
Black and Yellow to GND
Purple and Green at R18, R19
If the microphone is not in phase, then reverse Green with Violet.
And of course the magnetic/metallic core to GND

What is the yellow wire connected to in the transformer?

I measured it with all other wires and the core itself with voltmeter.

 

[pasarski]

There's 48v at the center tap so you just power the mic from there. Center tap is labeled TXC so if no center tap no TXC and no use for J1-J2.

I already wrote that you can skip the resistors if you have a center tapped transformer but I'm not actually sure.

It seems you have this transformer? No center tap. Yellow wire is for grounding the core.

 

[AdamFrisen]

There's 48v at the center tap so you just power the mic from there. Center tap is labeled TXC so if no center tap no TXC and no use for J1-J2.

It seems you have this transformer? No center tap. Yellow wire is for grounding the core.

Thanks.

So the yellow wire is attached to the core? But my voltmeter doesn't indicate any connection to that chunk of metal in the middle?

 

[pasarski]

I think the laminations may have some none conducting stuff on them and you can't measure the connection because of that(?)

 

[AdamFrisen]

I think the laminations may have some none conducting stuff on them and you can't measure the connection because of that(?)

There are two screw holes in the core which I measured has connection using voltmeter.

I guess I just ground it just as I should ground the metal core.

 

[Khron]

What is the use of TXC when the J1-J2 is not connected to phantom power?

Pretty sure there is none.

 

[Jedias]

Hey guys, I built my first atempt today, and theres no sound at all, I checked the circuit and everithing is right. I can read 10.9volts on the J-Fet Drain. Tried to put 1khz sine on J-Fets's gate and I can read it on my scope, but realy low. Someone knows what can I check?

PS: Checked pins 2 and 3 of the xlr out, theres 48v on both, 0 volts on ground, 0 volts on P+ of the transformer, 10v on J-fet's Drain and 11v on J-Fet's gate. ALso 0v on the BP. (Tested with the capsule disconected).

 

[untamedfrontier]

We probably need more info and pictures to help more. Which jfet, what volt zener diode and BOM did you use?

 

[Jedias]

We probably need more info and pictures to help more. Which jfet, what volt zener diode and BOM did you use?

Hey untamedfrotier, thanks for reply.
Here's the BOM.
R7: 1G
R8: 1M
R9: 7k5
R10: 20K Trimmer
R12: 47k
R13: 330k (5%)
R14: 10k
R15: 300k
R16: 56k (5%)
R18 & R19: 2k2 (closely matched)

C5: 33n
C6: 220p
C7: 1u
C8: 22u 50v
C9: 470n
C10: 4u7 35V
C11: 4u7 63V
C12: 10u 63V
C13: 10n

Q1: 2N3819
GR1: 1N4749 24v

Heres the circuit layout:
Transformer: T-13 (used Blackwires as primary and yellow as secondary)

 

[Jedias]

Just to be sure...
The capsule goes like this, right?

 

[Khron]

Heres the circuit layout:

Is there a picture missing?

 

[AdamFrisen]

Did you already solder the capsule and where did you inject the test signal?
I created the messeingang as suggested by kingkorg after I soldered the capsule at which I input the signal.

 

[AdamFrisen]

Another question (sorry if I am interrupting Jedias question).

I am eagerly expecting capsules from 797 Audio and Arienne's K87.

I read here that 797 is little brighter and wonder where and what values I should use to bring it down to the desired freq. response?

https://groupdiy.com/threads/soliloqueens-k87-k67-and-k47-capsules.81968/

 

[AdamFrisen]

This was interesting read about different capacitor types in audio circuits.

https://www.ti.com/lit/an/slyt796a/slyt796a.pdf?ts=1722851023091&ref_url=https%3A%2F%2Fwww.google.com%2F

so... where C0G/NP0 can be used, they should be inaudible, otherwise and other types, both voltage swings and type might cause audible distortions?

 

[mrchapp]

Hey untamedfrotier, thanks for reply.
Here's the BOM.
[...]
Heres the circuit layout:
Transformer: T-13 (used Blackwires as primary and yellow as secondary)

I'm on the same boat with a very similar BOM, as you can see. I need to do more in-depth analysis with oscilloscope and multimeter, but mine didn't work. One thing that was definitely not good from the outset was that my 1G resistor was 1/4 W, which I later learned may not have been enough. I'll keep digging while keeping an eye on your mic. Good luck!

 

[Khron]

One thing that was definitely not good from the outset was that my 1G resistor was 1/4 W, which I later learned may not have been enough.

How was that "not enough"? Does someone need to brush up on their Ohm's law math?

 

[mrchapp]

How was that "not enough"?

I'm glad you ask, because that means that I'm wrong. My assumption, as I've seen in BOMs shared around is that it calls for a higher wattage resistor. Homero's BOM in Mouser has a 1.2W resistor, so that was the first thing I was going to check. Thanks for your input!