U87 (Revision A) in BM800 microphone donor body

[jp8]

Nice idea! And are there any considerations regarding stiffness? You're essentially creating a dampened resonant mass-spring combination. Does it make sense to aim for a certain resonant frequency? Or should one just take the softest available?

FWIW, I use those rubber fan vibration isolators to mount an electret capsule holder on in a end-addressed mic. I've yet to test it in real life use, though. Maybe they are too floppy, I don't know. Mechanics are not my "core competence".

Jan

 

[mrgrooves666]

Just get any rc hobby servo grommets

 

[RBeTT]

Hi @jp8

Nice idea!

That's not my idea! It has been shared a build on this thread with it. I actually thought this was a standard that's why I was asking.

And are there any considerations regarding stiffness? You're essentially creating a dampened resonant mass-spring combination. Does it make sense to aim for a certain resonant frequency? Or should one just take the softest available?

For me I see as a attenuator of any sort of movement dampening to the capsule, which inherently will help I guess. About any mechanics on it, we are on the same boat...somebody might know better to answer than me, I'm just following directions

 

[theom]

Im nearing final assembly of my 87 boards!
I got an alternative T-13 9.5 to 1 transformer and Im not sure how to terminate it to the board.
There are 2 pairs of windings, wrapped together yellow - black, and yellow - black.
Yellow wires show continuity on a VOM meter.

here is a picture of the docs that came with the transformer.
How would this connect to the U87 fet PCB A V1_7 ?
@mihi_fuchs ?

 

[micolas]

Im nearing final assembly of my 87 boards!
I got an alternative T-13 9.5 to 1 transformer and Im not sure how to terminate it to the board.
There are 2 pairs of windings, wrapped together yellow - black, and yellow - black.
Yellow wires show continuity on a VOM meter.

here is a picture of the docs that came with the transformer.
How would this connect to the U87 fet PCB A V1_7 ?
@mihi_fuchs ?
View attachment 143258

Hi @theom !

ASTDS T-13:
Black - primary
Yellow - secondary

Just plug it in, respecting:

Primary(black) - to
pin1 of j2 conn transformer (C8=1uF)
pin2 of j2 conn transformer (GND)

and

Secondary(yellow)- to Output:
pin3 of j2 conn transformer
pin4 of j2 conn transformer

*
If the microphone is not in phase with any other commercial microphone(condenser or dynamic),
reverse the primary wires,
or secondary wires,
or xlr2 with xlr3.

*
To be sure that the manufacturer did not mistake the color of the wires, measure the ohmic resistance with the DMM
Between the wires that measure more, (about 395...483 ohms), is the Primary.
The other two wires measure less (16.74... 20.46ohms) represent the Secondary.

 

[mihi_fuchs]

Hi @theom !

ASTDS T-13:
Black - primary
Yellow - secondary

Just plug it in, respecting:

Primary(black) - to
pin1 of j2 conn transformer (C8=1uF)
pin2 of j2 conn transformer (GND)

and

Secondary(yellow)- to Output:
pin3 of j2 conn transformer
pin4 of j2 conn transformer

*
If the microphone is not in phase with any other commercial microphone(condenser or dynamic),
reverse the primary wires,
or secondary wires,
or xlr2 with xlr3.

*
To be sure that the manufacturer did not mistake the color of the wires, measure the ohmic resistance with the DMM
Between the wires that measure more, (about 395...483 ohms), is the Primary.
The other two wires measure less (16.74... 20.46ohms) represent the Secondary.

exactly. i would do the same. only thing to mention is that the data gives you in the top right corner which wire is black one/ yellos one. start with that configuration and check phase alignment to be sure the data was fine. maybe spares you a few trials.

BR
Michael

 

[theom]

exactly. i would do the same. only thing to mention is that the data gives you in the top right corner which wire is black one/ yellos one. start with that configuration and check phase alignment to be sure the data was fine. maybe spares you a few trials.

BR
Michael

Thanks folks.
I was wondering how to determine which loop is primary.
Now I know,
theo

 

[mrgrooves666]

Has anyone experience noise with some preamps? Having a weird behavior with my 3 Mihi 87. With my interface preamps they perform as expected. But with a link audio SSL based preamp I built im having some weird noise/hiss. Here's the catch, same link audio preamp with my other mics (all diy) do not present such noise/hiss, including tube m49 diy build, 2 km84 clones, and various opa alice and opic mics. One of the mihi 87's I built for a friend, and weird enough she has the same issue while using an outboard preamp (cranborne camden 500), but not with her interface ( UA apollo). This is breaking my head.

 

[Khron]

Has anyone experience noise with some preamps? Having a weird behavior with my 3 Mihi 87. With my interface preamps they perform as expected. But with a link audio SSL based preamp I built im having some weird noise/hiss. Here's the catch, same link audio preamp with my other mics (all diy) do not present such noise/hiss, including tube m49 diy build, 2 km84 clones, and various opa alice and opic mics. One of the mihi 87's I built for a friend, and weird enough she has the same issue while using an outboard preamp (cranborne camden 500), but not with her interface ( UA apollo). This is breaking my head.

What's the entire chain of equipment between mic and headphones / speakers (or wherever the noise is observed)?

 

[micolas]

What's the entire chain of equipment between mic and headphones / speakers (or wherever the noise is observed)?

+1! Here is an excellent question. Noise can be caused by anything in the entire audio chain. Different modules interact differently.

 

[mrgrooves666]

Hey Khron!
2 chains, A) mic - quad star cable properly done - Steinberg AXR4 - headphones. B) mic - quad star cable - XLR connector rack panel, properly done - Radial workhorse with Link SSL preamp - Samson patchbay - Steinberg AXR4 - headphones. However as I mentioned, the same B chain works without the strange hiss with all my other mics. And the mihi mics do not present the issue with Chain A, only with B. There's some weird interaction going on... I'll try to make a recording of the noise later on tonight. Worth mention, I enabled full operation of Mihi's design, with switches residing inside a 3D printed holder, above the pcb, and wiring from them to the corresponding PCB's leads, including a wire to properly ground the switches with the chassis. This strange hiss is very low in level during normal use, but its there, kind of like a fizzy hiss, and with some compression following up, it obviously starts to become more apparent. I did cleaned as best as I could (do to proximity with the polystirene caps), the floating hi z junction of the fet.

 

[micolas]

Hi @mrgrooves666 !
You could try some things:
In series with the zener diode D1 Z33v from + to R12, put a 100ohm resistor. Increases C10 from 10uF to 220uF/50v.
(In the case of Rode it filters the noise produced by the zener)
In series with the zener diode D25 1N4748A put a 100ohm resistor from + to pin1 of RV24. From pin1 RV24 to GND add an additional 220uF/35v capacitor.
* What output transformer did you use? You could try reversing the primary wires at the same time as reversing the secondary wires. Possibly shielding of the transformer and make electrical connection of the magnetic core to GND.
* Very careful cleaning of the high-impedance input area, the switches area, including the 1Gohm resistors. Beware of styro capacitors, affected ones produce noise.
*The restoration of solders, in the mentioned areas. A cold solder can cause noise.
*Testing the correct connection of XLR Pin1 to GND in the microphone. Likewise in the microphone cable (no/yes?)

 

[mrgrooves666]

Gonna try your suggestions mate thanks! I'm using 3U 87 traffos btw..

 

[micolas]

Gonna try your suggestions mate thanks! I'm using 3U 87 traffos btw..

On the other hand you have the additional sections:

cable - XLR connector rack panel - Radial with Link SSL preamp - Samson patchbay

Each stage must be checked. GND loops?
How is the quality of the Phantom power reaching the mic.

 

[jp8]

What output transformer did you use? You could try reversing the primary wires at the same time as reversing the secondary wires. Possibly shielding of the transformer and make electrical connection of the magnetic core to GND.

I'd give this a chance of success, because this has interaction with the load.

From the description of the noise, it could be the same I experienced with my KM84++ design. I solved this by replacing the bead on the JFET drain by a 470R resistor, isolating reactive loads from the amplifier stage. Swapping the transformer wires could have the same effect.

Jan

 

[mihi_fuchs]

Hey Khron!
2 chains, A) mic - quad star cable properly done - Steinberg AXR4 - headphones. B) mic - quad star cable - XLR connector rack panel, properly done - Radial workhorse with Link SSL preamp - Samson patchbay - Steinberg AXR4 - headphones. However as I mentioned, the same B chain works without the strange hiss with all my other mics. And the mihi mics do not present the issue with Chain A, only with B. There's some weird interaction going on... I'll try to make a recording of the noise later on tonight. Worth mention, I enabled full operation of Mihi's design, with switches residing inside a 3D printed holder, above the pcb, and wiring from them to the corresponding PCB's leads, including a wire to properly ground the switches with the chassis. This strange hiss is very low in level during normal use, but its there, kind of like a fizzy hiss, and with some compression following up, it obviously starts to become more apparent. I did cleaned as best as I could (do to proximity with the polystirene caps), the floating hi z junction of the fet.

I'd start with removing potential equipment form the chain. Use - if possible - without Patchbay first, then maybe try a different lunchbox.

How did you solder your cables (XLR connectors have chassis too. Is that one connected/jumpered to GND pin 1? Your BM800 donor body is metallic, is it? Maybe measure if you have GND connection from mic to preamp within Setup B)

Hope you get it working - it sounds really good on Kevin preamps ... I got the Link SL4K-E in an api Lunchbox here and no issue.

Of course it could be something with the mic itself, but as it works on the A) setup properly I would rule out any other culprit first.

Best regards and a nice evening to everyone,
Michael

 

[mrgrooves666]

Pardon my ignorance but what is the goal or idea of reversing both the primary and secondary wires?

 

[micolas]

Pardon my ignorance but what is the goal or idea of reversing both the primary and secondary wires?

I have talked about this somewhere before. I prefer to connect the beginning of the primary coil (which is sometimes marked on schematics) to the hot part of the audio signal, and the end of the primary coil to GND, because the primary coil (especially the last layers) behaves like an electro-magnetic shield connected to GND (like a Faraday cage) This reduces hum, RF interference, etc.
I speak from my decades of experience.
In order for the microphone output signal to remain in phase, if we reverse the primary wires, we must also reverse the secondary wires. The secondary has low inductance => low impedance so it is more immune to electromagnetic interference, hum, etc.

The beginning of the primary coil
is marked on most schemes
Depending on the circuit topology or how the ground connections were actually made, etc., reversing these wires can produce positive results

 

[Khron]

Pardon my ignorance but what is the goal or idea of reversing both the primary and secondary wires?

My best guess is, at least for the primary, connecting to ground (low-impedance) whichever end of the winding is closest to the outside can shield whatever's closer to the core.