FET847 Microphone Project Circuit By Jonathan Burtner PCB Layout By Poctop

[rock soderstrom]

I figured I wouId buiId this using oId Beyer mic input transformers in reverse, specificaIIy TR45 BV35536 with a 1:5 ratio.

Keep us posted about the outcome with these Beyer transformer, please. I am very interested because I have some of them laying around here.

 

[karloff70]

Keep us posted about the outcome with these Beyer transformer, please. I am very interested because I have some of them laying around here.

Sure. Might take me a coupIe of weeks or so though as a Iot of other stuff on atm.

ShouId be a very cooI mic. It's going into a beat up oId Neumann U87 body and basket that Andreas Grosser kindIy gifted me years ago as it was too beat up for him to use. But perfectIy vibey to me. With FIat47, FET847 circuit and beyer tranny I expect to caII it the Trans67. It Iooks Iike a beat up oId 67, sort of tries to sound in the direction of a 67, but has no vaIve and reaIIy it's a km84 with capsuIe surgery. Perfect mic for 2023 and onwards.

 

[rock soderstrom]

ShouId be a very cooI mic. It's going into a beat up oId Neumann U87 body and basket that Andreas Grosser kindIy gifted me years ago as it was too beat up for him to use. But perfectIy vibey to me. With FIat47, FET847 circuit and beyer tranny I expect to caII it the Trans67. It Iooks Iike a beat up oId 67, sort of tries to sound in the direction of a 67, but has no vaIve and reaIIy it's a km84 with capsuIe surgery. Perfect mic for 2023 and onwards.

Sounds like a good plan!

 

[karloff70]

To check that I am grasping things correctIy, the three hoIes for the switch that are over the trimmer are onIy for switching between the trimmer and the optionaI set resistor, correct? So if I go with using the trimmer I can Ieave both the resistor but aIso the switch off?

And how does one do a switch for cardioid/omni?

 

[Ricardus]

And how does one do a switch for cardioid/omni?

You should be able to simply add an on/off toggle that when it's ON connects the rear capsule to the polarizing voltage that's hooked op to the front capsule.

 

[doqmemory]

And how does one do a switch for cardioid/omni?

You have to make your own provision for that, since it is not included, as stated in the original post:

As designed can be hardwired Cardioid or Omni.
● Cardioid wiring only uses one side of the capsule while omni uses both sides.
● Floating Hi-Z
jumper or switch may be added for switched card/omni operation or rear
diaphragm can be soldered to the FET gate for permanent omni operation.

The simplest way would be to connect a switch as shown in the original schematic:

Schematic JB
https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a088e3.pdf

 

[doqmemory]

You should be able to simply add an on/off toggle that when it's ON connects the rear capsule to the polarizing voltage that's hooked op to the front capsule.

The schematic shows the polarization is on the backplate, not the capsule.

 

[Ricardus]

The schematic shows the polarization is on the backplate, not the capsule.

I don't have the schematic in front of me but if you wire the switch so that that both diaphrams are common when trhe switch is on, and not when it's off, it should be omni, no?

 

[doqmemory]

I don't have the schematic in front of me but if you wire the switch so that that both diaphrams are common when trhe switch is on, and not when it's off, it should be omni, no?

Yes, an SPST switch connecting rear diaphragm to the FET gate (where the front diaphragm is already wired) will give an omni pattern when ON and cardioid when OFF. The point is that only the backplate is polarized, so there is essentially no voltage on such a switch (although it is still hi-Z).

 

[karloff70]

WouId the switch that comes in the BOM work?

 

[Khron]

WouId the switch that comes in the BOM work?

Pretty sure that's why it's there, but on one hand, it's expensive as *beeeeeep*, and on the other hand, looks like you're very much on your own regarding the mechanical mounting.

 

[doqmemory]

WouId the switch that comes in the BOM work?

Yes, it should work.

 

[karloff70]

Pretty sure that's why it's there, but on one hand, it's expensive as *beeeeeep*, and on the other hand, looks like you're very much on your own regarding the mechanical mounting.

Seems it couId get homeIy on the underside, outside of the daIe resistor, with its side to the board and a smaII drop of supergIue.....

 

[karloff70]

Just checking if I am going mad here. The transformer gets wired step up, right? Not step down. So why does it say on page 1 that a 10:1 would be quieter output than a 7:1? Or is it wired step down?

 

[elskardio]

Just checking if I am going mad here. The transformer gets wired step up, right? Not step down. So why does it say on page 1 that a 10:1 would be quieter output than a 7:1? Or is it wired step down?

The transformer is in a step-down configuration... hence the 7:1

 

[karloff70]

The transformer is in a step-down configuration... hence the 7:1

Thank you. What I thought. Here's the thing.....

I was finished assembling but couldn't get a sound. Thought it probably was the a little damaged (open knees) leads from the beyer transformer which I tried to keep full length forst attempt at least, but not sure, and since I was taking a couple of other bits to the tech's I took the mic to him as well. Although I already told him to start the transformer wants to be step down (so with beyer 5:1) he says bullshit, has to be step up, and has apparently made it work, sort of. But says ideally wants a higher ratio transformer, so must be quiet output. So, basically, something else is wrong, right? And the beyer needs go back to step down.

 

[Khron]

Find another tech, first of all...

 

[TwentyTrees]

he says bullshit, has to be step up

Definitely get a new tech. You're right, it should be step-down. He may have confused a microphone output transformer with a mic preamp input transformer, but the mic transformer is primarily stepping down the output impedance of the mic. A 5:1 mic transformer reversed will turn a 200 ohm mic into a 125k ohm mic, which is unlikely to play nicely with standard mic preamps (which of course exist because of the relatively quiet output of most mics).

Do the Beyers test ok?

 

[karloff70]

Definitely get a new tech. You're right, it should be step-down. He may have confused a microphone output transformer with a mic preamp input transformer, but the mic transformer is primarily stepping down the output impedance of the mic. A 5:1 mic transformer reversed will turn a 200 ohm mic into a 125k ohm mic, which is unlikely to play nicely with standard mic preamps (which of course exist because of the relatively quiet output of most mics).

Do the Beyers test ok?

Thank you for your time. The beyer is a mic input transformer in reverse, normally 1:5. One side measures about 350 ohm, the other about 50 ohm. This not being near the impedances of the standard transformer here, would that be a problem, or is the only important part it being wired 5:1, i.e. step down?

I had figured since the standard one is 7:1 and it says 10:1 works but makes it a bit quieter in output, the beyer 5:1 would be loud and proud.

 

[TwentyTrees]

The resistance you've measured don't really correlate to the impedances the transformer is designed to work at, unfortunately. That said, it should work well enough I'd think - output impedance would be a bit higher than most mics, probably not enough to make a difference with most preamps. Let us know how it sounds once it's all up and running!